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peracetic acid
Accepted Manuscript Antibiotic contaminated water treated by photo driven advanced oxidation processes: Ultraviolet/H2O2 vs ultraviolet/peracetic acid Luigi Rizzo, Giusy Lofrano, Carmen Gago, Tatiana Bredneva, Patrizia Iannece, Marta Pazos, Nataliya Krasnogorskaya, Maurizio Carotenuto PII:
S0959-6526(18)32821-X
DOI:
10.1016/j.jclepro.2018.09.101
Reference:
JCLP 14236
To appear in:
Journal of Cleaner Production
Received Date: 26 March 2018 Revised Date:
11 September 2018
Accepted Date: 12 September 2018
Please cite this article as: Rizzo L, Lofrano G, Gago C, Bredneva T, Iannece P, Pazos M, Krasnogorskaya N, Carotenuto M, Antibiotic contaminated water treated by photo driven advanced oxidation processes: Ultraviolet/H2O2 vs ultraviolet/peracetic acid, Journal of Cleaner Production (2018), doi: https://doi.org/10.1016/j.jclepro.2018.09.101. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
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Antibiotic contaminated water treated by photo driven advanced oxidation processes:
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ultraviolet/H2O2 Vs ultraviolet/peracetic acid
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Luigi Rizzoa*, Giusy Lofranob, Carmen Gagoc, Tatiana Brednevad, Patrizia Ianneceb, Marta
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Pazosc, Nataliya Krasnogorskayad, Maurizio Carotenutob
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a
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Fisciano (SA), Italy.
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b
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Department of Civil Engineering, University of Salerno, Via Giovanni Paolo II 132, 84084
Department of Chemistry and Biology “Adolfo Zambelli”, University of Salerno, Via
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Giovanni Paolo II 132, 84084 Fisciano (SA), Italy.
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c
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d
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University, Ufa, Republic of Bashkortostan, Russian Federation.
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Department Production Safety and Industrial Ecology, Ufa State Aviation Technical
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Department of Chemical Engineering, University of Vigo, 36310 Vigo, Spain.
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*Corresponding author: Tel.: +39089969334; fax: +39089969620
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E-mail address: [email protected]
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ACCEPTED MANUSCRIPT Abstract
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The release of antibiotics in aquatic ecosystems from the effluents of wastewater treatment
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plants (WTPs) is of great concern due to possible chronic toxic effects as well as contribution
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to antibiotic resistance spread. In the present work, the degradation of the antibiotic
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chloramphenicol (CAP) by ultraviolet (UV)/peracetic acid (PAA) (an advanced oxidation
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process that has been poorly investigated so far) and UV/H2O2 processes as well as its
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transformation products were studied under different light sources. UV-C/PAA process was
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found to be effective in the degradation of CAP (half life time (t1/2) = 20 min, initial CAP
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concentration 25 mg/L), but not effective when solar radiation was used as light source. It is
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worthy to note that the presence of H2O2 in the commercial PAA solution significantly
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affected the removal efficiency of CAP by UV-C/PAA process. When H2O2 was quenched by
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catalase addition, t1/2 increased to 99 min, meaning that PAA can still produce hydroxyl
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radicals but at a lower rate compared to H2O2. Moreover, process efficiency further decreased
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in the presence of both solar simulated and natural solar irradiation, being CAP removal also
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the result of thermal decomposition. The transformation products detected during CAP
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degradation by UV-C/PAA were different respect to those produced during UV-C/H2O2
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process. In particular, after 120 min most of the compounds detected presented a molecular
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weight > 300 m/z., meaning that UV-C/PAA process needs more time to degrade the
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transformation products.
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Keywords: chloramphenicol, oxidation intermediates, pharmaceutical wastewater,
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photodegradation, solar-driven processes.
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ACCEPTED MANUSCRIPT 1.
Introduction
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The intensive use of antibiotics for human (domestic and hospital use), veterinary and
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agriculture purposes, results in a continuous release into the environment. Wastewater
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treatment plants (WTPs) are among the main anthropogenic sources of antibiotic
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environmental pollution (Michael et al., 2013; Krzeminski et al., 2019) and they are
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considered as one of the main ‘hotspots’ for antibiotic resistance spread into the environment
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(Rizzo et al., 2013; Fiorentino et al., 2018). Although antibiotics are typically detected at low
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concentrations (from a few to some hundreds ng/L) in the effluents of urban WTPs (Michael
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et al., 2013), significantly higher concentrations (up to some tens of mg/L) were detected in
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the effluent of pharmaceutical WTPs (Larsson et al., 2007; Hou et al., 2015; Guo et al., 2018).
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Conventional biological processes are not or poor effective in the removal of a so high
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concentration of antibiotics, therefore a pre-treatment (to improve the biodegradability of the
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wastewater) or a post-treatment step should be used to effectively minimize the release of
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antibiotics into the environment.
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Advanced Oxidation Processes (AOPs), among which ultraviolet (UV)/H2O2, photo-Fenton
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and TiO2 photocatalysis, have been widely investigated for the removal of recalcitrant
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contaminants (RCs) from wastewater (Rizzo, 2011). The capacity of AOPs to effectively
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degrade refractory pollutants is due to their ability to produce reactive oxygen species (ROS),
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such as hydroxyl radicals (HO•). Moreover, in photo driven AOPs, energy costs can be saved
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by using solar light (Sacco et al., 2018; Foteinis et al., 2018). Among AOPs, UV/peracetic
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acid (PAA) process has been poorly explored so far, and basically focused on bacteria
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inactivation (Koivunen and Heinonen-Tanski, 2005; de Souza et al. 2015); only recently its
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effect on pharmaceuticals has been investigated (Cai et al., 2017). PAA is used as alternative
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option to chlorination in wastewater disinfection (Antonelli et al., 2013; Formisano et al.,
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2016). However, disinfection efficiency can be improved by coupling PAA with UV radiation
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(Formisano et al., 2016) due to the formation of HO• according to the following reaction
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(Caretti and Lubello, 2003):
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CH3CO3H + hν → CH3CO2• + HO•
(Eq.1)
Commercially available PAA aqueous solutions include hydrogen peroxide which contributes
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not only to the formation of new PAA as soon as it is consumed, but also to the formation of
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new HO• under UV radiation.
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To our knowledge, no data are available in scientific literature on the effect of UV/PAA
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process on antibiotics and on the formation of their oxidation intermediates. Therefore, in the
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present work, the effect of UV/PAA process on a model antibiotic, namely chloramphenicol
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(CAP), was investigated and compared to UV/H2O2 process. This is an AOP widely used for
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the removal of RCs from water and wastewater (Andreozzi et al., 2003; Baumgarten et al.,
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2007; Ferro et al., 2015) and it has an higher sustainability potential compared to other solar
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driven AOPs (Foteinis et al., 2018). CAP is an antibiotic applied in the treatment of various
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infectious diseases such as meningitis, plague, cholera, and typhoid fever. Despite CAP has
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been banned from several countries due to its carcinogenic effects and other serious adverse
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reactions (i.e., bone marrow depression, aplastic anemia and severe blood disorders), it has
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been detected in surface waters as well as in WTPs effluents (Peng et al. 2006; Choi et al.,
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2008; Liu et al., 2009).
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In this work the degradation of CAP by UV/PAA and UV/H2O2 processes was investigated by
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evaluating the effect of (i) initial CAP concentration, (ii) initial oxidant dose (PAA and H2O2,
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respectively), (iii) different light sources (UV-C and (simulated) solar radiation), (iv) radicals
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effect, (v) water matrix effect (distilled water (DW) Vs wastewater (WW)) as well as (v) the
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formation of oxidation intermediates. Moreover, the effect of natural sunlight on photo-
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oxidation of CAP by H2O2 and PAA, respectively, was also evaluated in outdoor experiments
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through a solar compound triangular collector (CTC) based reactor.
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2.
Material and methods
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2.1
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H2O2 (30% w/w) was purchased from Sigma-Aldrich (Switzerland) while PAA aqueous
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solution (15% w/w and including H2O2, PAA:H2O2 ≅ 2:1) was purchased from Lenntech
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(Italy). Sodium thiosulfate (95% w/w), ammonium molybdate, KI and N,N-diethyl-p-
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phenylenediamine were used for the measurement of residual PAA and H2O2. Catalase from
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bovine liver (2000-5000 units/mg protein) was purchased as hydrolyzed powder from Sigma-
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Aldrich and used as obtained. Chloramphenicol sodium succinate was purchased by Sigma
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Aldrich (CAS Number 982-57-0). Figure S1 (in supplementary material) shows the HPLC
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chromatogram of CAP (molecular weight of 445.18 g/mol) and its chemical structure.
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Chemicals
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2.2
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Wastewater samples were taken from a large WTP (700,000 population equivalent) placed in
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Salerno (Italy), from the effluent of the biological process (activated sludge) and upstream of
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disinfection unit. Average values for some parameters are: pH 7.6, 20.0 mg BOD5/L, 48.0 mg
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COD/L,18.0 mg TSS/L, 7.9 mg Total N/L, 1.8 mg Total P/L. As possible (micro)biological
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effect on CAP removal is of concern, no significant biodegradation contribution is expected in
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parallel to AOPs experiments, taking into account the high initial CAP concentration, the slow
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reaction time for biological process (Krzeminski et al., 2019) and, most important, the
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disinfection effect of the investigated AOPs on the indigenous bacteria occurring in the
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wastewater samples (Caretti and Lubello, 2003; Formisano et al., 2016).
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2.3
AOPs and control experiments
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2.3.1 Lab scale experiments
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The experimental set-up for lab scale AOPs tests includes a light source and an open glass
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cylindrical vessel (total volume of 0.5 L), with aqueous solutions (DW and WW)
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continuously mixed by a magnetic stirrer. UV-C driven AOPs experiments were carried out
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with a 16 W low-pressure mercury vapour lamp (Novus, Italy). The lamp was fixed at 15 cm
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from the upper water level in the reactor. The incident radiation intensity (0.7×10-8 Einstein/s
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at 253.7 nm, 0.77 W/m2) was measured by actinometry. Simulated solar radiation AOPs
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experiments were carried out with a wide spectrum 250 W lamp equipped with a filter
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(Procomat, Italy) (main radiation emission in the range 320–450 nm). The lamp was fixed at
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40 cm from the upper water level in the reactor and aqueous solutions were exposed to a
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range of UV doses (0-12.5 W/m2, spectrometer model HR-2000,Ocean Optics, Florida, USA)
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by varying the exposure time from 0 to 120 min. The effect of oxidant dose, initial
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concentration of CAP, radicals and water matrix (DW Vs. WW) were also investigated.
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Moreover, taking into account that commercial PAA solution also includes H2O2, the
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contribution of PAA to the formation of radicals was evaluated by adding catalase to remove
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H2O2 from the aqueous solution. Control tests were also carried out by investigating the
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contribution of the light source and the oxidant as standalone processes, respectively. Finally,
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taking into account that high temperature (>40°C) can be reached by aqueous matrix during
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outdoor experiments under sunlight, temperature effect on the degradation of CAP was also
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evaluated through the incubation at different temperature of the CAP aqueous solution, under
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dark conditions and in absence of the oxidant.
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2.3.2 Outdoor sunlight experiments
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Laboratory of Sanitary and Environmental Engineering (latitude 40°N, longitude 14°E), in a
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CTC solar reactor. The CTC reactor (10.24 L irradiated volume) consists of 8 acrylic glass
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tubes with an external diameter of 3.3 cm and a length of 150 cm each (Sacco et al., 2018).
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The tubes are housed in the mid of triangular shaped aluminum collectors. The reactor is
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mounted on a mobile and inclinable platform, which was inclined of 40° during the
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experiments, according to the latitude of the Laboratory. A pyrex vessel, filled with the CAP
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DW solution, was connected to a peristaltic pump, which allowed to operate the system in a
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recirculation mode. Only one tube of the reactor was used for the solar driven AOPs
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experiments, so that the total water volume (tube + vessel + connections) was 2.15 L. The
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solution was recirculated at a flow rate of 625 mL/min and the experiments were operated for
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300 min from the morning (typical start time 11.00) to the afternoon (16.00), in June 2017. In
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solar driven AOPs experiments, after the reactor was filled in with the CAP water solution,
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the oxidant (H2O2 and PAA, respectively) was added and the solution was circulated in the
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system under dark for 5 min to allow the oxidant to mix with the water solution. Then the
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cover was removed from the tube and t0 sample was collected to measure initial CAP
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concentration and oxidant dose. To make experiments performed in different days
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comparable, CAP degradation rate was plotted as a function of the cumulative energy per unit
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of volume (QUV) received in the photoreactor, which was calculated according to the
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following Eq.2:
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QUV,n= QUV,n−1 +∆tn·UVG,n · Ar/(1000·Vt)
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QUV,n and QUV,n-1 are the UV energy accumulated per litre (kJ/L) at times n and n-1,
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respectively; UVG,n is the average incident radiation on the irradiated area (W/m2), ∆tn is the
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experimental time of the sample, Ar is the illuminated area of the reactor (m2) and Vt is the
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(Eq.2)
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total volume of the water treated (L). The light intensity was measured by a radiometer (Black
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Comet, StellarNet Inc.) which probe was fixed close to the irradiated tube.
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2.4
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PAA and H2O2 concentrations were measured according to the method from HACH (2014).
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CAP concentration was measured by HPLC-UV (Finnigan Surveyer) equipped with a
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reversed phase C18 analytical column (Vydac, 5µm, 150 mm ×3.0 mm). CAP was separated
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using as mobile phase a mixture of methanol/ultrapure water (30%/70%) at flow rate of 1
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mL/min. The injection volume was 30µL and the wavelength was set at 275 nm according to
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the maximum light absorption of the CAP. The limit of quantification (LOQ) of this method
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was 0.5µg/mL.
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Mass spectra identification of oxidation intermediates and by-products was achieved through
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a Bruker solarix XR Fourier transform mass spectrometer (Bruker Daltonik GmbH, Bremen,
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Germany) equipped with a 7T refrigerated actively shielded superconducting magnet (Bruker
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Biospin, Wissembourg, France). The samples were ionized in negative ion mode using ESI
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(Bruker Daltonik GmbH, Bremen, Germany). Sample solutions were continuously supplied
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using a syringe pump at a flow rate of 120 µL/h. The detection mass range was set at 150-
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3000 m/z. The mass spectra were calibrated externally with NaTFA clusters in negative ion
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mode using a linear calibration. The source voltage was set to -3.9 kV, the dry gas (nitrogen)
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flow rate to 4L/min at 200 °C.
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Analytical measurements
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3.
Results and discussion
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3.1
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The results of CAP degradation fit quite well the pseudo-first order kinetic (Eq.3):
Degradation of CAP by AOPs
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(Eq.3)
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where [CAP] and [CAP0] are the concentrations of CAP (mg/L) at time t and time zero,
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respectively, and k is the pseudo-first-order rate constant for the degradation (1/min). In the
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next subparagraphs the effect of (i) oxidant initial concentration, (ii) CAP initial
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concentration, (iii) water matrix, (iv) residual oxidant concentration, (v) ROS species and (vi)
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solar (simulated) radiation on CAP degradation is explained and discussed according to the
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scientific literature. Kinetic parameters (namely k, half life time (t1/2) and R2) are given in
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table 1.
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Table 1
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3.1.1 UV-C/H2O2 process
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The effect of initial concentration of H2O2 on the degradation of 25 mg CAP/L by UV-
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C/H2O2 process can be observed in Fig.1. CAP degradation rate increased as H2O2 initial
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concentration was increased (5, 10, 20 and 50 mg/L), up to achieve an almost total removal of
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CAP after 120 min irradiation with 50 mg H2O2/L (t1/2= 19 min). The degradation of CAP was
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mainly due to the formation of ROS formed during UV-C/H2O2 process, as confirmed by the
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results from the control tests with H2O2 and UV-C as standalone processes, respectively. As
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matter of fact, H2O2 (50 mg/L) alone (dark control test) only removed approximately 4% of
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initial CAP concentration, even after 120 min treatment. UV-C light alone (UV-C control test)
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was more effective than H2O2 alone, and CAP removal after 120 min treatment was as high as
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45% (t1/2= 139 min). The absorption spectrum of CAP solution (Fig.S2 in supplementary
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material) is characterized by a curve in the range 240-370 nm, with an absorbance peak at 275
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nm. The UV-C light emits monochromatic radiation at 254 nm, which partially overlaps CAP
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curve, thus explaining its photodegradation (Rizzo et al., 2012).
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Fig.1
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The effect of initial concentration of CAP was also evaluated (Fig.2) and CAP degradation
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rate by UV-C/H2O2 (50 mg/L) decreased as initial CAP concentration was increased. While
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total removal was observed for 10 and 25 mg/L of CAP after 120 min treatment (t1/2= 15 and
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19 min, respectively), when initial CAP concentration was increased to 50 mg/L the
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maximum removal achieved after 120 min treatment was as high as 66% (t1/2= 81 min).
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The results achieved in our work are consistent with previously published results (Zuorro et
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al., 2014). The higher degradation rate (100 mg CAP/L removal in 60 min treatment),
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observed in the quoted work, may be due to the different experimental apparatus (a
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rectangular quartz cell with an internal volume of 4 mL and an optical path length of 1 cm
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was used as reactor). The decreased removal efficiency as initial CAP concentration was
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increased can be due to the absorption of UV radiation by the CAP molecule (Zuorro et al.,
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2014). The higher the initial CAP concentration, the higher the fraction of UV photons
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absorbed, which will finally result in a decreased concentration of HO• in the water solution.
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The effect of initial concentration of the oxidant on the degradation of 25 mg CAP/L was
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investigated in UV-C/PAA experiments too (Fig.3). CAP degradation rate increased as PAA
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initial concentration was increased (5, 10, 20 and 50 mg/L), up to achieve the total removal of
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CAP after 120 min irradiation with 50 mg PAA/L (t1/2= 20 min). Interestingly, when H2O2 was
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quenched by adding catalase to 50 mg PAA/L solution, process efficiency drastically
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decreased with a 42% CAP removal (t1/2= 99 min) compared to 100% observed at the same
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irradiation time (120 min) with the same PAA solution without catalase. Accordingly, PAA in
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combination with UV-C radiation can still produce ROS (the removal rate is higher than UV-
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C as standalone process) but at a significantly smaller rate compared to the PAA solution
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including H2O2.
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Fig.3
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Unfortunately, no data are available in scientific literature about the effect of UV-C/PAA
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process on the degradation of antibiotics in water. In a recently published paper, the effect of
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UV-C/PAA process was investigated on seven pharmaceuticals (bezafibrate, carbamazepine
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(CBZ), clofibric acid, diclofenac, ibuprofen (IBP), ketoprofen and naproxen (NAP)) and they
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were degraded by more than 93.5% under UV/PAA (1 mg/L) after 2 h treatment (Cai et al.,
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2017). The higher efficiency observed can be explained by the different experimental setup
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and photocatalytic reactor, but even by the higher PAA/contaminant ratio (0.8-5.0) in the
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quoted paper compared to our work (0.2-2.0). Interestingly, they observed that UV-C/PAA
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process played a greater role than UV-C/H2O2 in the degradation of IBP, NAP, and CBZ,
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PAA.
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Water matrix effect was also investigated by comparing CAP degradation (initial
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concentration 25 mg/L) in DW and WW (Fig.4). Due to the occurrence of radicals scavengers
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in wastewater (such as carbonates, bicarbonates and salts), AOPs efficiency in the removal of
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target contaminants is expected to decrease (Klamerth et al., 2010). Accordingly, the
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degradation rate of CAP by UV-C/H2O2 after 120 min irradiation decreased in WW (84%)
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compared to DW (94%) experiment (t1/2= 41 and 27 min, respectively). The decrease from
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DW (85%) to WW (79%) was less evident in UV-C/PAA experiments (t1/2= 45 and 48 min,
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respectively).
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Fig.4
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3.1.3 CAP degradation by AOPs: behaviour of residual oxidant
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The residual concentration of H2O2 in UV-C/H2O2 process and the residual concentration of
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PAA and H2O2 in UV-C/PAA process were respectively measured to evaluate their behaviour
269
with respect to CAP degradation. The experiments were performed by keeping the lamp
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switched off for the early 60 min, to evaluate possible consumption of the oxidants under dark
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conditions. In UV-C/H2O2 experiment, the initial concentration of H2O2 (18.3 mg/L) did not
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significantly change in the early 60 min under dark (Fig.5a). When the lamp was switched on
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the H2O2 concentration slightly decreased (2%) till to 30 min irradiation (90 min in the plot),
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while CAP degradation drastically increased (77%). After 30 min irradiation, H2O2 residual
275
concentration gradually decreased till to 12.8 mg/L (30% reduction) at 120 min irradiation
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time (180 min in the plot), when initial CAP was almost totally removed.
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Fig.5
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In UV-C/PAA experiment the residual concentration of both oxidants was measured because
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the commercial PAA solution also includes H2O2. Also in this experiment the initial
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concentrations of PAA (28.0 mg/L) and H2O2 (3.1 mg/L) did not significantly change in the
283
early 60 min under dark (Fig.5b). But when the lamp was switched on a fluctuation in H2O2
284
concentration was observed, possibly due to transformation of PAA in H2O2 and vice versa.
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The decay of PAA under UV radiation was due mainly to direct photolysis and partly to
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indirect photolysis with •OH (Cai et al., 2017). H2O2 fluctuation and PAA decay did not
287
significantly affect CAP degradation because both oxidants can result in the formation of HO•
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under UV-C radiation.
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3.1.4 CAP degradation by AOPs: effect of ROS
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To better understand the mechanisms of CAP degradation by UV-C/PAA process an attempt
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to characterize the effect of ROS was carried out. Accordingly, experiments aimed to quench
293
H2O2, superoxide radical anion (O2•_) and HO• by respectively adding catalase, benzoquinone
294
and butanol (Rodriguez-Mozaz et al., 2015; Cruz-Ortiz et al., 2017), in PAA solution exposed
295
to UV-C radiation were performed. Unfortunately, it was not possible to discriminate the
296
effect of the ROS neither in DW nor in WW experiments because CAP degradation was
297
strongly inhibited in the respective experiments with benzoquinone and butanol (Fig.S3 in
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supplementary material). The formation of ROS during PAA photolysis has been poorly
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investigated so far. Cai et al. (2017) evaluated just the effect of HO• (using tert-butyl alcohol
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ACCEPTED MANUSCRIPT (TBA) as HO• scavenger) in the degradation of seven pharmaceuticals by UV/PAA process
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and they made the conclusion that the target pharmaceuticals were degraded by a combination
302
of direct photolysis, oxidation by HO• and oxidation by other radicals. Each contribution was
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estimated based on the rate constants k (1/min) obtained for the processes UV+TBA,
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UV/PAA, and UV/PAA+TBA according to the following Eqs 4−7:
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kUV/PAA = kdirect photolysis + kHO• oxidation + kother radicals oxidation
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kdirect photolysis = kUV+TBA
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kother radicals oxidation = kUV/PAA+TBA - kUV+TBA
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kHO• oxidation = kUV/PAA - kUV/PAA+TBA
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3.1.5 CAP degradation by AOPs: effect of solar simulated radiation
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To reduce energy cost, sunlight has been investigated as alternative option to artificial light in
311
photo driven AOPs (Malato et al., 2009). Accordingly, AOPs experiments under simulated
312
solar radiation (SS) were carried out too.
313
Due to the small portion of UV-C radiation emitted by the lamp, both SS/H2O2 (t1/2= 133-533
314
min, depending on H2O2 initial concentration) and SS/PAA (t1/2= 210-408 min, depending on
315
PAA initial concentration) were found to be poorly effective in the degradation of CAP
316
(Fig.6). Moreover, the low ratio oxidant/CAP further emphasized the low process efficiency.
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(Eq.4)
(Eq.6) (Eq.7).
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Fig.6
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Hydrogen peroxide is commonly used as a radical photoinitiator because in the presence of
321
UV-C radiation HO• are effectively produced. However, in the presence of simulated solar 14
ACCEPTED MANUSCRIPT radiation, the energy cannot produce a sufficient amount of HO• through H2O2 molecules
323
dissociation to effectively degrade CAP (Velo-Gala et al., 2017). Unlike of UV-C, SS as
324
stand-alone process did not result in a significant photodegradation of CAP (t1/2= 770 min),
325
because the emission spectrum of the lamp overlaps the absorbance spectrum of CAP only in
326
the final part of the curve (Fig.S2 in supplementary material) (Rizzo et al., 2012). Moreover,
327
because of the high power (250 W) of the lamp used in solar simulated experiments, the
328
temperature was found to increase up to 40-42°C in the reactor. Therefore the temperature
329
effect on CAP degradation during solar simulated AOPs experiments was also evaluated
330
through thermal experiments under dark and without oxidants, by storing 25 mg CAP/L DW
331
solutions at three different temperatures (20, 40 and 60°C), respectively. While 20°C
332
temperature did not affect CAP degradation, 40°C and 60°C values significantly decreased
333
CAP concentration up to 25% and 50% after 180 min exposure (15 and 25% after 120 min),
334
respectively (Fig.S4 in supplementary material). Accordingly, CAP removal observed in
335
SS/H2O2 (15-40% after 120 min irradiation, depending on H2O2 initial concentration) and
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SS/PAA (14-32% after 120 min irradiation, depending on PAA initial concentration)
337
experiments was remarkably affected by thermal degradation.
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3.1.6 CAP degradation by AOPs: effect of natural solar light
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Sunlight/H2O2 process decreased CAP concentration up to 67% in the early 120 min (254
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KJ/L) (Fig.7a). But, as the exposure time increased, CAP degradation did not significantly
342
change till the end of the experiment (71%, 1173 KJ/L), in spite of the addition of H2O2 after
343
150 min (356 KJ/L) to increase the oxidant dose to the initial value (20 mg/L). Possibly, and
344
according to the results at lab scale under simulated solar light, the small portion of UV-C
345
radiation emitted by the solar spectrum cannot produce a sufficient amount of HO• to
346
simultaneously degrade CAP molecules and their oxidation intermediates that act as radicals
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scavengers too. Moreover, the recombination between HO• molecules to form H2O2, as well
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as the reaction between H2O2 and HO• would further reduce their availability for CAP
349
degradation (Velo-Gala et al., 2017).
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Anyway, it is worthy to note that the presence of H2O2 only increased CAP degradation rate
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in the early 120 min compared to sunlight photodegradation because, as irradiation time
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increased, CAP degradation curve of sunlight process increasingly approached the
356
corresponding curve of sunlight/H2O2 process, up to achieve the same CAP removal after 240
357
min (793 KJ/L) (Fig.7a). This behaviour was more evident in sunlight/PAA experiment where
358
CAP removal was mainly due to a synergic effect of thermal degradation and sunlight
359
photodegradation (Fig.7b). Even the addition of PAA after 210 min (614 KJ/L) did not
360
improve the removal of CAP compared to sunlight process. The removal of pharmaceuticals,
361
and more specifically of antibiotics, from wastewater by solar driven AOPs has been mainly
362
investigated at initial concentrations of some tens of µg/L in the attempt to find a compromise
363
between experimental needs and real conditions in urban wastewater (Klamerth et al., 2010;
364
Ferro et al., 2015). Accordingly the high ratio H2O2/pollutant compared to that one
365
investigated in the present work resulted in a high removal of the target compounds (Ferro et
366
al., 2015).
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Transformation products
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The formation of photo-oxidation by-products is of great concern in wastewater treatment
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because they may be more toxic than their parent compounds (Rizzo 2011). Figs. 8 and 9
371
report the ESI-MS spectra in negative mode on CAP (25mg/l), dark and photo-oxidised
372
solutions with UV-C/H2O2 and UV-C/PAA (10 mg/L of H2O2 and PAA, respectively) over
373
the time (0, 15, 30 60 and 120 min). When H2O2 was added to CAP (25mg/L) solution, in dark
374
experiment, the CAP (α: 421 m/z, negative ion with z=1) was transformed in CAP-succinate
375
(β: 321 m/z, negative ion with z=1) (Fig.8a). Within the early 30 min of UV-C/H2O2 photo-
376
oxidation, the compounds identified as peaks 1 and 2 were formed (Fig.8b, c). After 120 min
377
of UV-C/H2O2 photo-oxidation the peak 1 disappeared and the peak 2 was significantly
378
reduced, whereas the formation of several compounds at low molecular weight (m.z < 290)
379
could be observed (Fig.8e).
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Fig.8
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The transformation products detected during photo-oxidation process by UV-C/PAA were
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different respect to those produced during UV-C/H2O2 process. Both CAP (α) and CAP-
385
succinate (β) were detected till 120 min, although they significantly decreased (Fig.9). After
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120 min most of the compounds detected presented a molecular weight > 300 m.z., meaning
387
that the photo-oxidation with UV-C/PAA needs more time to degrade the transformation
388
products.
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No previous study on the formation of CAP’s transformation products by UV/PAA are
393
available in literature, whereas CAP degradation in water solutions treated by UV/H2O2 has
394
been recently investigated (Jin et al., 2018). The oxidation of CAP yielded 4-(2-amino-1,3-
395
dihydroxy-propanyl)-nitrobenzene (m/z 212) with loss of dichloroacetic acid (m/z 129) by the
396
attack of HO• produced by UV/H2O2. Further oxidation of the amino group of 4-(2-amino-1,3-
397
dihydroxy-propyl)-nitrobenzene (m/z 212) to nitro group led to the production of 4-(2-nitro-
398
1,3-dihydroxy-propanyl)-nitrobenzene (m/z 242). The denitration of 4-(2-nitro-1,3-
399
dihydroxypropanyl)-nitrobenzene (m/z 242) induced the production of 4-nitro-(2R)-
400
hydroxy(phenyl) ethanoic acid (m/z 197). Subsequent oxidation of the lateral group of 4-
401
nitro-(2R)-hydroxy(phenyl) ethanoic acid (m/z 197) produced the aromatics 4-nitro-benzoic
402
acid (m/z 167). None of these compounds was detected in the present study. Anyway, a
403
comparison is still possible with other AOPs. For example, unlike of our results where 10
404
mg/L of H2O2 and PAA, respectively, were not sufficient to totally remove the transformation
405
products within 120 min irradiation, when UV/TiO2 photocatalytic process was investigated
406
in the degradation of CAP (25 mg/L) water solution, the transformation products were totally
407
removed after 120 min irradiation, at 1.6 g/L of TiO2 (Lofrano et al., 2016). It is also worthy
408
to note that the lower TiO2 loadings (in the range 0.1-0.8 g/L) investigated were not sufficient
409
to totally remove the transformation products. None of by products reported by Garcia-Segura
410
et al. (2014) as result of anodic oxidation electro-generated H2O2, electro-Fenton, photo-
411
electro Fenton and solar photo- electro-Fenton of 245 mg/L of CAP was found in our study.
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Conclusions
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UV-C/PAA process was found to be effective in the degradation of CAP (t1/2= 20 min with 25
415
mg CAP/L), but the presence of H2O2 in the PAA solution significantly affected the removal
416
efficiency. When H2O2 was quenched by catalase addition, t1/2 increased to 99 min, meaning
417
that PAA can still produce HO• but at a lower rate compared to H2O2. Moreover, process
418
efficiency further decreased in the presence of both solar simulated and natural solar
419
irradiation, being CAP removal mainly due to a combination of thermal decomposition and
420
advanced oxidation. Possibly, the small portion of UV-C radiation emitted by the solar
421
spectrum cannot produce a sufficient amount of HO• to simultaneously degrade CAP
422
molecules and their oxidation intermediates that act as radicals scavengers too. Moreover, the
423
recombination between HO• molecules to form H2O2, as well as the reaction between H2O2
424
and HO• would further reduce their availability for CAP degradation.
425
The transformation products detected during CAP degradation by UV-C/PAA process were
426
different respect to those produced during UV-C/H2O2 process. Moreover, according to CAP
427
degradation kinetics, photo-oxidation with UV-C/PAA process needs more time to degrade
428
the transformation products than UV-C/H2O2 process.
429
In conclusion, taking into account the operating conditions (namely, initial oxidant
430
concentration and
431
degradation efficiency and transformation products removal) with UV/H2O2 and UV/PAA,
432
respectively, UV/H2O2 process is the best option for the removal of high CAP concentrations
433
from water/wastewater. Anyway, taking into account that sunlight/PAA process was found to
434
be more effective than sunlight/H2O2 process in the inactivation of bacteria in urban
435
wastewater (Formisano et al., 2016), different results can be expected in urban wastewater
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treatment. Accordingly, it would be worthy of investigation the use of UV/PAA (using
437
different light sources, namely sunlight and UV-C) as tertiary treatment of urban wastewater,
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treatment time/QUV dose) and the results achieved (namely, CAP
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to evaluate the removal of CECs (at realistic concentrations for urban wastewater, namely
439
ng/L – µg/L) and simultaneous bacteria inactivation.
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Acknowledgements
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The authors wish to thank the ERASMUS programme for supporting Carmen Gago’s visit at
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the University of Salerno as well as The Ministry of Education and Science of the Russian
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Federation for the grant to support Tatiana Bredneva’s visit at University of Salerno. Luigi
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Rizzo would also like to thank the University of Salerno for the financial support through
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FARB2015 (Trattamento fotocatalitico solare di acque reflue urbane destinate al riutilizzo).
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Figure 1: photodegradation kinetics of CAP (25 mg/L) by UV-C/H2O2 process: effect of
552
initial concentration of H2O2 and control tests.
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Figure 2: photodegradation kinetics of CAP by UV-C/H2O2 (50 mg/L) process: effect of
554
initial concentration of CAP (10, 25 and 50 mg/L).
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Figure 3: photodegradation kinetics of CAP (25 mg/L) by UV-C/PAA process: effect of both
556
initial concentration of PAA (5, 10, 20, and 50 mg/L) and quenching of H2O2 by catalase.
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Figure 4: photodegradation kinetics of CAP (25 mg/L) by UV-C/H2O2 (20 mg/L) and UV-
558
C/PAA (20 mg/L) processes: effect of aqueous matrix (distilled water (DW) and wastewater
559
(WW)).
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Figure 5: photodegradation of CAP (25 mg/L) by AOPs: behaviour of residual oxidant
561
concentration in (a) UV-C/H2O2 and (b) UV-C/PAA processes.
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Figure 6: photodegradation kinetics of CAP (25 mg/L) by solar simulated radiation (SS)
563
AOPs: effect of oxidant (PAA Vs. H2O2) and oxidant dose (10, 20 and 100 mg/L,
564
respectively).
565
Figure 7: photodegradation of CAP (25 mg/L) by AOPs under natural solar light: (a)
566
sunlight/H2O2 and (b) sunlight/PAA processes.
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Figures 8: ESI-MS spectra in negative mode on CAP (25mg/l) aqueous solutions, under dark
568
and no treatment (a) and during photo-oxidation by UV-C/H2O2 (10 mg/L) process (0 (b), 15
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(c), 30 (d), 60 (e) and 120 (f) min).
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ACCEPTED MANUSCRIPT Figures 9: ESI-MS spectra in negative mode on CAP (25mg/l) aqueous solutions, under dark
571
and no treatment (a) and during photo-oxidation by UV-C/PAA (10 mg/L) process (0 (b), 15
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(c), 30 (d), 60 (e) and 120 (f) min).
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ACCEPTED MANUSCRIPT Table 1: kinetic parameters of CAP degradation during AOPs experiments CAP initial k (1/min)×10-3 t1/2 (min) concentration (mg/L) DW 25 5.0 ± 0. 1 138 ± 3 UV-C DW 5 25 9.3 ± 0. 2 74± 1 UV-C/H2O2 DW 10 25 12.8 ± 0. 1 54.3± 0.5 UV-C/H2O2 DW 20 25 26.0 ± 1 27± 2 UV-C/H2O2 DW 50 25 36.4 ± 0. 3 19.0 ± 0.1 UV-C/H2O2 DW 50 10 46 ± 5 15 ± 2 UV-C/H2O2 DW 50 50 8.6 ± 0.3 81 ± 3 UV-C/H2O2 WW 20 25 17 ± 1 41 ± 3 UV-C/H2O2 DW 5 25 11. 5 ± 0.3 60 ± 2 UV-C/PAA DW 10 25 12.6 ± 0.9 55 ± 4 UV-C/PAA DW 20 25 15.5 ± 0.7 44.6 ± 0.2 UV-C/PAA DW 50 25 34.6 ± 0.7 20.0 ± 0.4 UV-C/PAA DW 50 25 7.0± 0.3 99 ± 5 UV-C/PAA* WW 20 25 14 ± 1 48 ± 4 UV-C/PAA DW 25 0.92 ± 0.06 750 ± 50 SS DW 10 25 3.2 ± 0.2 219 ± 10 SS/H2O2 DW 20 25 5.2 ± 0.5 134 ± 10 SS/H2O2 DW 100 25 1.3 ± 0.1 520 ± 40 SS/H2O2 DW 10 25 3.3 ± 0.2 210 ± 10 SS/PAA DW 20 25 1.7 ± 0.5 410 ± 120 SS/PAA DW 100 25 2.1 ± 0.2 340 ± 40 SS/PAA *Catalase was added to PAA solution to quench H2O2 and evaluate the effect of PAA alone. Oxidant dose (mg/L)
R2
0.9956 0.9974 0.9991 0.9695 0.9995 0.8669 0.9895 0.9592 0.9933 0.9909 0.9998 0.9965 0.9832 0.935 0.9594 0.9793 0.8921 0.9387 0.9773 0.437 0.8932
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ACCEPTED MANUSCRIPT Highlights First work on antibiotic degradation by UV/peracetic acid (PAA) process.
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UV-C/H2O2 was more effective than UV-C/PAA in chloramphenicol (CAP) degradation.
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The presence of H2O2 in the commercial PAA solution significantly affected CAP removal.
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Sunlight/PAA was not effective in the removal of CAP.
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Different transformation products were detected after UV-C/PAA and UV-C/H2O2.
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S0959-6526(18)32821-X
DOI:
10.1016/j.jclepro.2018.09.101
Reference:
JCLP 14236
To appear in:
Journal of Cleaner Production
Received Date: 26 March 2018 Revised Date:
11 September 2018
Accepted Date: 12 September 2018
Please cite this article as: Rizzo L, Lofrano G, Gago C, Bredneva T, Iannece P, Pazos M, Krasnogorskaya N, Carotenuto M, Antibiotic contaminated water treated by photo driven advanced oxidation processes: Ultraviolet/H2O2 vs ultraviolet/peracetic acid, Journal of Cleaner Production (2018), doi: https://doi.org/10.1016/j.jclepro.2018.09.101. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
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Antibiotic contaminated water treated by photo driven advanced oxidation processes:
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ultraviolet/H2O2 Vs ultraviolet/peracetic acid
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Luigi Rizzoa*, Giusy Lofranob, Carmen Gagoc, Tatiana Brednevad, Patrizia Ianneceb, Marta
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Pazosc, Nataliya Krasnogorskayad, Maurizio Carotenutob
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Fisciano (SA), Italy.
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b
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Department of Civil Engineering, University of Salerno, Via Giovanni Paolo II 132, 84084
Department of Chemistry and Biology “Adolfo Zambelli”, University of Salerno, Via
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Giovanni Paolo II 132, 84084 Fisciano (SA), Italy.
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c
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University, Ufa, Republic of Bashkortostan, Russian Federation.
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Department Production Safety and Industrial Ecology, Ufa State Aviation Technical
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Department of Chemical Engineering, University of Vigo, 36310 Vigo, Spain.
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*Corresponding author: Tel.: +39089969334; fax: +39089969620
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E-mail address: [email protected]
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ACCEPTED MANUSCRIPT Abstract
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The release of antibiotics in aquatic ecosystems from the effluents of wastewater treatment
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plants (WTPs) is of great concern due to possible chronic toxic effects as well as contribution
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to antibiotic resistance spread. In the present work, the degradation of the antibiotic
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chloramphenicol (CAP) by ultraviolet (UV)/peracetic acid (PAA) (an advanced oxidation
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process that has been poorly investigated so far) and UV/H2O2 processes as well as its
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transformation products were studied under different light sources. UV-C/PAA process was
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found to be effective in the degradation of CAP (half life time (t1/2) = 20 min, initial CAP
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concentration 25 mg/L), but not effective when solar radiation was used as light source. It is
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worthy to note that the presence of H2O2 in the commercial PAA solution significantly
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affected the removal efficiency of CAP by UV-C/PAA process. When H2O2 was quenched by
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catalase addition, t1/2 increased to 99 min, meaning that PAA can still produce hydroxyl
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radicals but at a lower rate compared to H2O2. Moreover, process efficiency further decreased
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in the presence of both solar simulated and natural solar irradiation, being CAP removal also
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the result of thermal decomposition. The transformation products detected during CAP
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degradation by UV-C/PAA were different respect to those produced during UV-C/H2O2
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process. In particular, after 120 min most of the compounds detected presented a molecular
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weight > 300 m/z., meaning that UV-C/PAA process needs more time to degrade the
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transformation products.
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Keywords: chloramphenicol, oxidation intermediates, pharmaceutical wastewater,
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photodegradation, solar-driven processes.
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Introduction
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The intensive use of antibiotics for human (domestic and hospital use), veterinary and
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agriculture purposes, results in a continuous release into the environment. Wastewater
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treatment plants (WTPs) are among the main anthropogenic sources of antibiotic
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environmental pollution (Michael et al., 2013; Krzeminski et al., 2019) and they are
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considered as one of the main ‘hotspots’ for antibiotic resistance spread into the environment
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(Rizzo et al., 2013; Fiorentino et al., 2018). Although antibiotics are typically detected at low
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concentrations (from a few to some hundreds ng/L) in the effluents of urban WTPs (Michael
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et al., 2013), significantly higher concentrations (up to some tens of mg/L) were detected in
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the effluent of pharmaceutical WTPs (Larsson et al., 2007; Hou et al., 2015; Guo et al., 2018).
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Conventional biological processes are not or poor effective in the removal of a so high
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concentration of antibiotics, therefore a pre-treatment (to improve the biodegradability of the
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wastewater) or a post-treatment step should be used to effectively minimize the release of
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antibiotics into the environment.
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Advanced Oxidation Processes (AOPs), among which ultraviolet (UV)/H2O2, photo-Fenton
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and TiO2 photocatalysis, have been widely investigated for the removal of recalcitrant
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contaminants (RCs) from wastewater (Rizzo, 2011). The capacity of AOPs to effectively
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degrade refractory pollutants is due to their ability to produce reactive oxygen species (ROS),
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such as hydroxyl radicals (HO•). Moreover, in photo driven AOPs, energy costs can be saved
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by using solar light (Sacco et al., 2018; Foteinis et al., 2018). Among AOPs, UV/peracetic
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acid (PAA) process has been poorly explored so far, and basically focused on bacteria
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inactivation (Koivunen and Heinonen-Tanski, 2005; de Souza et al. 2015); only recently its
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effect on pharmaceuticals has been investigated (Cai et al., 2017). PAA is used as alternative
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option to chlorination in wastewater disinfection (Antonelli et al., 2013; Formisano et al.,
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2016). However, disinfection efficiency can be improved by coupling PAA with UV radiation
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(Formisano et al., 2016) due to the formation of HO• according to the following reaction
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(Caretti and Lubello, 2003):
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CH3CO3H + hν → CH3CO2• + HO•
(Eq.1)
Commercially available PAA aqueous solutions include hydrogen peroxide which contributes
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not only to the formation of new PAA as soon as it is consumed, but also to the formation of
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new HO• under UV radiation.
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To our knowledge, no data are available in scientific literature on the effect of UV/PAA
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process on antibiotics and on the formation of their oxidation intermediates. Therefore, in the
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present work, the effect of UV/PAA process on a model antibiotic, namely chloramphenicol
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(CAP), was investigated and compared to UV/H2O2 process. This is an AOP widely used for
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the removal of RCs from water and wastewater (Andreozzi et al., 2003; Baumgarten et al.,
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2007; Ferro et al., 2015) and it has an higher sustainability potential compared to other solar
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driven AOPs (Foteinis et al., 2018). CAP is an antibiotic applied in the treatment of various
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infectious diseases such as meningitis, plague, cholera, and typhoid fever. Despite CAP has
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been banned from several countries due to its carcinogenic effects and other serious adverse
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reactions (i.e., bone marrow depression, aplastic anemia and severe blood disorders), it has
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been detected in surface waters as well as in WTPs effluents (Peng et al. 2006; Choi et al.,
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2008; Liu et al., 2009).
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In this work the degradation of CAP by UV/PAA and UV/H2O2 processes was investigated by
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evaluating the effect of (i) initial CAP concentration, (ii) initial oxidant dose (PAA and H2O2,
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respectively), (iii) different light sources (UV-C and (simulated) solar radiation), (iv) radicals
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effect, (v) water matrix effect (distilled water (DW) Vs wastewater (WW)) as well as (v) the
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formation of oxidation intermediates. Moreover, the effect of natural sunlight on photo-
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oxidation of CAP by H2O2 and PAA, respectively, was also evaluated in outdoor experiments
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through a solar compound triangular collector (CTC) based reactor.
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2.
Material and methods
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2.1
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H2O2 (30% w/w) was purchased from Sigma-Aldrich (Switzerland) while PAA aqueous
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solution (15% w/w and including H2O2, PAA:H2O2 ≅ 2:1) was purchased from Lenntech
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(Italy). Sodium thiosulfate (95% w/w), ammonium molybdate, KI and N,N-diethyl-p-
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phenylenediamine were used for the measurement of residual PAA and H2O2. Catalase from
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bovine liver (2000-5000 units/mg protein) was purchased as hydrolyzed powder from Sigma-
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Aldrich and used as obtained. Chloramphenicol sodium succinate was purchased by Sigma
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Aldrich (CAS Number 982-57-0). Figure S1 (in supplementary material) shows the HPLC
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chromatogram of CAP (molecular weight of 445.18 g/mol) and its chemical structure.
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Chemicals
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2.2
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Wastewater samples were taken from a large WTP (700,000 population equivalent) placed in
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Salerno (Italy), from the effluent of the biological process (activated sludge) and upstream of
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disinfection unit. Average values for some parameters are: pH 7.6, 20.0 mg BOD5/L, 48.0 mg
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COD/L,18.0 mg TSS/L, 7.9 mg Total N/L, 1.8 mg Total P/L. As possible (micro)biological
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effect on CAP removal is of concern, no significant biodegradation contribution is expected in
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parallel to AOPs experiments, taking into account the high initial CAP concentration, the slow
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reaction time for biological process (Krzeminski et al., 2019) and, most important, the
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disinfection effect of the investigated AOPs on the indigenous bacteria occurring in the
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wastewater samples (Caretti and Lubello, 2003; Formisano et al., 2016).
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2.3
AOPs and control experiments
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2.3.1 Lab scale experiments
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The experimental set-up for lab scale AOPs tests includes a light source and an open glass
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cylindrical vessel (total volume of 0.5 L), with aqueous solutions (DW and WW)
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continuously mixed by a magnetic stirrer. UV-C driven AOPs experiments were carried out
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with a 16 W low-pressure mercury vapour lamp (Novus, Italy). The lamp was fixed at 15 cm
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from the upper water level in the reactor. The incident radiation intensity (0.7×10-8 Einstein/s
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at 253.7 nm, 0.77 W/m2) was measured by actinometry. Simulated solar radiation AOPs
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experiments were carried out with a wide spectrum 250 W lamp equipped with a filter
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(Procomat, Italy) (main radiation emission in the range 320–450 nm). The lamp was fixed at
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40 cm from the upper water level in the reactor and aqueous solutions were exposed to a
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range of UV doses (0-12.5 W/m2, spectrometer model HR-2000,Ocean Optics, Florida, USA)
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by varying the exposure time from 0 to 120 min. The effect of oxidant dose, initial
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concentration of CAP, radicals and water matrix (DW Vs. WW) were also investigated.
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Moreover, taking into account that commercial PAA solution also includes H2O2, the
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contribution of PAA to the formation of radicals was evaluated by adding catalase to remove
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H2O2 from the aqueous solution. Control tests were also carried out by investigating the
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contribution of the light source and the oxidant as standalone processes, respectively. Finally,
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taking into account that high temperature (>40°C) can be reached by aqueous matrix during
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outdoor experiments under sunlight, temperature effect on the degradation of CAP was also
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evaluated through the incubation at different temperature of the CAP aqueous solution, under
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dark conditions and in absence of the oxidant.
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2.3.2 Outdoor sunlight experiments
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Laboratory of Sanitary and Environmental Engineering (latitude 40°N, longitude 14°E), in a
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CTC solar reactor. The CTC reactor (10.24 L irradiated volume) consists of 8 acrylic glass
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tubes with an external diameter of 3.3 cm and a length of 150 cm each (Sacco et al., 2018).
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The tubes are housed in the mid of triangular shaped aluminum collectors. The reactor is
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mounted on a mobile and inclinable platform, which was inclined of 40° during the
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experiments, according to the latitude of the Laboratory. A pyrex vessel, filled with the CAP
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DW solution, was connected to a peristaltic pump, which allowed to operate the system in a
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recirculation mode. Only one tube of the reactor was used for the solar driven AOPs
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experiments, so that the total water volume (tube + vessel + connections) was 2.15 L. The
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solution was recirculated at a flow rate of 625 mL/min and the experiments were operated for
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300 min from the morning (typical start time 11.00) to the afternoon (16.00), in June 2017. In
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solar driven AOPs experiments, after the reactor was filled in with the CAP water solution,
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the oxidant (H2O2 and PAA, respectively) was added and the solution was circulated in the
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system under dark for 5 min to allow the oxidant to mix with the water solution. Then the
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cover was removed from the tube and t0 sample was collected to measure initial CAP
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concentration and oxidant dose. To make experiments performed in different days
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comparable, CAP degradation rate was plotted as a function of the cumulative energy per unit
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of volume (QUV) received in the photoreactor, which was calculated according to the
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following Eq.2:
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QUV,n= QUV,n−1 +∆tn·UVG,n · Ar/(1000·Vt)
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QUV,n and QUV,n-1 are the UV energy accumulated per litre (kJ/L) at times n and n-1,
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respectively; UVG,n is the average incident radiation on the irradiated area (W/m2), ∆tn is the
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experimental time of the sample, Ar is the illuminated area of the reactor (m2) and Vt is the
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(Eq.2)
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total volume of the water treated (L). The light intensity was measured by a radiometer (Black
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Comet, StellarNet Inc.) which probe was fixed close to the irradiated tube.
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2.4
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PAA and H2O2 concentrations were measured according to the method from HACH (2014).
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CAP concentration was measured by HPLC-UV (Finnigan Surveyer) equipped with a
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reversed phase C18 analytical column (Vydac, 5µm, 150 mm ×3.0 mm). CAP was separated
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using as mobile phase a mixture of methanol/ultrapure water (30%/70%) at flow rate of 1
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mL/min. The injection volume was 30µL and the wavelength was set at 275 nm according to
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the maximum light absorption of the CAP. The limit of quantification (LOQ) of this method
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was 0.5µg/mL.
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Mass spectra identification of oxidation intermediates and by-products was achieved through
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a Bruker solarix XR Fourier transform mass spectrometer (Bruker Daltonik GmbH, Bremen,
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Germany) equipped with a 7T refrigerated actively shielded superconducting magnet (Bruker
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Biospin, Wissembourg, France). The samples were ionized in negative ion mode using ESI
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(Bruker Daltonik GmbH, Bremen, Germany). Sample solutions were continuously supplied
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using a syringe pump at a flow rate of 120 µL/h. The detection mass range was set at 150-
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3000 m/z. The mass spectra were calibrated externally with NaTFA clusters in negative ion
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mode using a linear calibration. The source voltage was set to -3.9 kV, the dry gas (nitrogen)
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flow rate to 4L/min at 200 °C.
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Analytical measurements
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3.
Results and discussion
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3.1
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The results of CAP degradation fit quite well the pseudo-first order kinetic (Eq.3):
Degradation of CAP by AOPs
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(Eq.3)
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where [CAP] and [CAP0] are the concentrations of CAP (mg/L) at time t and time zero,
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respectively, and k is the pseudo-first-order rate constant for the degradation (1/min). In the
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next subparagraphs the effect of (i) oxidant initial concentration, (ii) CAP initial
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concentration, (iii) water matrix, (iv) residual oxidant concentration, (v) ROS species and (vi)
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solar (simulated) radiation on CAP degradation is explained and discussed according to the
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scientific literature. Kinetic parameters (namely k, half life time (t1/2) and R2) are given in
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table 1.
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Table 1
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3.1.1 UV-C/H2O2 process
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The effect of initial concentration of H2O2 on the degradation of 25 mg CAP/L by UV-
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C/H2O2 process can be observed in Fig.1. CAP degradation rate increased as H2O2 initial
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concentration was increased (5, 10, 20 and 50 mg/L), up to achieve an almost total removal of
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CAP after 120 min irradiation with 50 mg H2O2/L (t1/2= 19 min). The degradation of CAP was
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mainly due to the formation of ROS formed during UV-C/H2O2 process, as confirmed by the
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results from the control tests with H2O2 and UV-C as standalone processes, respectively. As
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matter of fact, H2O2 (50 mg/L) alone (dark control test) only removed approximately 4% of
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initial CAP concentration, even after 120 min treatment. UV-C light alone (UV-C control test)
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was more effective than H2O2 alone, and CAP removal after 120 min treatment was as high as
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45% (t1/2= 139 min). The absorption spectrum of CAP solution (Fig.S2 in supplementary
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material) is characterized by a curve in the range 240-370 nm, with an absorbance peak at 275
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nm. The UV-C light emits monochromatic radiation at 254 nm, which partially overlaps CAP
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curve, thus explaining its photodegradation (Rizzo et al., 2012).
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The effect of initial concentration of CAP was also evaluated (Fig.2) and CAP degradation
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rate by UV-C/H2O2 (50 mg/L) decreased as initial CAP concentration was increased. While
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total removal was observed for 10 and 25 mg/L of CAP after 120 min treatment (t1/2= 15 and
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19 min, respectively), when initial CAP concentration was increased to 50 mg/L the
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maximum removal achieved after 120 min treatment was as high as 66% (t1/2= 81 min).
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The results achieved in our work are consistent with previously published results (Zuorro et
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al., 2014). The higher degradation rate (100 mg CAP/L removal in 60 min treatment),
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observed in the quoted work, may be due to the different experimental apparatus (a
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rectangular quartz cell with an internal volume of 4 mL and an optical path length of 1 cm
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was used as reactor). The decreased removal efficiency as initial CAP concentration was
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increased can be due to the absorption of UV radiation by the CAP molecule (Zuorro et al.,
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2014). The higher the initial CAP concentration, the higher the fraction of UV photons
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absorbed, which will finally result in a decreased concentration of HO• in the water solution.
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The effect of initial concentration of the oxidant on the degradation of 25 mg CAP/L was
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investigated in UV-C/PAA experiments too (Fig.3). CAP degradation rate increased as PAA
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initial concentration was increased (5, 10, 20 and 50 mg/L), up to achieve the total removal of
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CAP after 120 min irradiation with 50 mg PAA/L (t1/2= 20 min). Interestingly, when H2O2 was
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quenched by adding catalase to 50 mg PAA/L solution, process efficiency drastically
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decreased with a 42% CAP removal (t1/2= 99 min) compared to 100% observed at the same
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irradiation time (120 min) with the same PAA solution without catalase. Accordingly, PAA in
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combination with UV-C radiation can still produce ROS (the removal rate is higher than UV-
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C as standalone process) but at a significantly smaller rate compared to the PAA solution
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including H2O2.
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Fig.3
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Unfortunately, no data are available in scientific literature about the effect of UV-C/PAA
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process on the degradation of antibiotics in water. In a recently published paper, the effect of
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UV-C/PAA process was investigated on seven pharmaceuticals (bezafibrate, carbamazepine
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(CBZ), clofibric acid, diclofenac, ibuprofen (IBP), ketoprofen and naproxen (NAP)) and they
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were degraded by more than 93.5% under UV/PAA (1 mg/L) after 2 h treatment (Cai et al.,
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2017). The higher efficiency observed can be explained by the different experimental setup
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and photocatalytic reactor, but even by the higher PAA/contaminant ratio (0.8-5.0) in the
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quoted paper compared to our work (0.2-2.0). Interestingly, they observed that UV-C/PAA
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process played a greater role than UV-C/H2O2 in the degradation of IBP, NAP, and CBZ,
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PAA.
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Water matrix effect was also investigated by comparing CAP degradation (initial
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concentration 25 mg/L) in DW and WW (Fig.4). Due to the occurrence of radicals scavengers
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in wastewater (such as carbonates, bicarbonates and salts), AOPs efficiency in the removal of
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target contaminants is expected to decrease (Klamerth et al., 2010). Accordingly, the
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degradation rate of CAP by UV-C/H2O2 after 120 min irradiation decreased in WW (84%)
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compared to DW (94%) experiment (t1/2= 41 and 27 min, respectively). The decrease from
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DW (85%) to WW (79%) was less evident in UV-C/PAA experiments (t1/2= 45 and 48 min,
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respectively).
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Fig.4
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3.1.3 CAP degradation by AOPs: behaviour of residual oxidant
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The residual concentration of H2O2 in UV-C/H2O2 process and the residual concentration of
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PAA and H2O2 in UV-C/PAA process were respectively measured to evaluate their behaviour
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with respect to CAP degradation. The experiments were performed by keeping the lamp
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switched off for the early 60 min, to evaluate possible consumption of the oxidants under dark
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conditions. In UV-C/H2O2 experiment, the initial concentration of H2O2 (18.3 mg/L) did not
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significantly change in the early 60 min under dark (Fig.5a). When the lamp was switched on
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the H2O2 concentration slightly decreased (2%) till to 30 min irradiation (90 min in the plot),
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while CAP degradation drastically increased (77%). After 30 min irradiation, H2O2 residual
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concentration gradually decreased till to 12.8 mg/L (30% reduction) at 120 min irradiation
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time (180 min in the plot), when initial CAP was almost totally removed.
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Fig.5
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In UV-C/PAA experiment the residual concentration of both oxidants was measured because
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the commercial PAA solution also includes H2O2. Also in this experiment the initial
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concentrations of PAA (28.0 mg/L) and H2O2 (3.1 mg/L) did not significantly change in the
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early 60 min under dark (Fig.5b). But when the lamp was switched on a fluctuation in H2O2
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concentration was observed, possibly due to transformation of PAA in H2O2 and vice versa.
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The decay of PAA under UV radiation was due mainly to direct photolysis and partly to
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indirect photolysis with •OH (Cai et al., 2017). H2O2 fluctuation and PAA decay did not
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significantly affect CAP degradation because both oxidants can result in the formation of HO•
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under UV-C radiation.
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3.1.4 CAP degradation by AOPs: effect of ROS
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To better understand the mechanisms of CAP degradation by UV-C/PAA process an attempt
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to characterize the effect of ROS was carried out. Accordingly, experiments aimed to quench
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H2O2, superoxide radical anion (O2•_) and HO• by respectively adding catalase, benzoquinone
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and butanol (Rodriguez-Mozaz et al., 2015; Cruz-Ortiz et al., 2017), in PAA solution exposed
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to UV-C radiation were performed. Unfortunately, it was not possible to discriminate the
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effect of the ROS neither in DW nor in WW experiments because CAP degradation was
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strongly inhibited in the respective experiments with benzoquinone and butanol (Fig.S3 in
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supplementary material). The formation of ROS during PAA photolysis has been poorly
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investigated so far. Cai et al. (2017) evaluated just the effect of HO• (using tert-butyl alcohol
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and they made the conclusion that the target pharmaceuticals were degraded by a combination
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of direct photolysis, oxidation by HO• and oxidation by other radicals. Each contribution was
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estimated based on the rate constants k (1/min) obtained for the processes UV+TBA,
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UV/PAA, and UV/PAA+TBA according to the following Eqs 4−7:
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kUV/PAA = kdirect photolysis + kHO• oxidation + kother radicals oxidation
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kdirect photolysis = kUV+TBA
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kother radicals oxidation = kUV/PAA+TBA - kUV+TBA
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kHO• oxidation = kUV/PAA - kUV/PAA+TBA
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3.1.5 CAP degradation by AOPs: effect of solar simulated radiation
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To reduce energy cost, sunlight has been investigated as alternative option to artificial light in
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photo driven AOPs (Malato et al., 2009). Accordingly, AOPs experiments under simulated
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solar radiation (SS) were carried out too.
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Due to the small portion of UV-C radiation emitted by the lamp, both SS/H2O2 (t1/2= 133-533
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min, depending on H2O2 initial concentration) and SS/PAA (t1/2= 210-408 min, depending on
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PAA initial concentration) were found to be poorly effective in the degradation of CAP
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(Fig.6). Moreover, the low ratio oxidant/CAP further emphasized the low process efficiency.
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(Eq.4)
(Eq.6) (Eq.7).
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Fig.6
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Hydrogen peroxide is commonly used as a radical photoinitiator because in the presence of
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UV-C radiation HO• are effectively produced. However, in the presence of simulated solar 14
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dissociation to effectively degrade CAP (Velo-Gala et al., 2017). Unlike of UV-C, SS as
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stand-alone process did not result in a significant photodegradation of CAP (t1/2= 770 min),
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because the emission spectrum of the lamp overlaps the absorbance spectrum of CAP only in
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the final part of the curve (Fig.S2 in supplementary material) (Rizzo et al., 2012). Moreover,
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because of the high power (250 W) of the lamp used in solar simulated experiments, the
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temperature was found to increase up to 40-42°C in the reactor. Therefore the temperature
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effect on CAP degradation during solar simulated AOPs experiments was also evaluated
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through thermal experiments under dark and without oxidants, by storing 25 mg CAP/L DW
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solutions at three different temperatures (20, 40 and 60°C), respectively. While 20°C
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temperature did not affect CAP degradation, 40°C and 60°C values significantly decreased
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CAP concentration up to 25% and 50% after 180 min exposure (15 and 25% after 120 min),
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respectively (Fig.S4 in supplementary material). Accordingly, CAP removal observed in
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SS/H2O2 (15-40% after 120 min irradiation, depending on H2O2 initial concentration) and
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SS/PAA (14-32% after 120 min irradiation, depending on PAA initial concentration)
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experiments was remarkably affected by thermal degradation.
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3.1.6 CAP degradation by AOPs: effect of natural solar light
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Sunlight/H2O2 process decreased CAP concentration up to 67% in the early 120 min (254
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KJ/L) (Fig.7a). But, as the exposure time increased, CAP degradation did not significantly
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change till the end of the experiment (71%, 1173 KJ/L), in spite of the addition of H2O2 after
343
150 min (356 KJ/L) to increase the oxidant dose to the initial value (20 mg/L). Possibly, and
344
according to the results at lab scale under simulated solar light, the small portion of UV-C
345
radiation emitted by the solar spectrum cannot produce a sufficient amount of HO• to
346
simultaneously degrade CAP molecules and their oxidation intermediates that act as radicals
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scavengers too. Moreover, the recombination between HO• molecules to form H2O2, as well
348
as the reaction between H2O2 and HO• would further reduce their availability for CAP
349
degradation (Velo-Gala et al., 2017).
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Anyway, it is worthy to note that the presence of H2O2 only increased CAP degradation rate
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in the early 120 min compared to sunlight photodegradation because, as irradiation time
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increased, CAP degradation curve of sunlight process increasingly approached the
356
corresponding curve of sunlight/H2O2 process, up to achieve the same CAP removal after 240
357
min (793 KJ/L) (Fig.7a). This behaviour was more evident in sunlight/PAA experiment where
358
CAP removal was mainly due to a synergic effect of thermal degradation and sunlight
359
photodegradation (Fig.7b). Even the addition of PAA after 210 min (614 KJ/L) did not
360
improve the removal of CAP compared to sunlight process. The removal of pharmaceuticals,
361
and more specifically of antibiotics, from wastewater by solar driven AOPs has been mainly
362
investigated at initial concentrations of some tens of µg/L in the attempt to find a compromise
363
between experimental needs and real conditions in urban wastewater (Klamerth et al., 2010;
364
Ferro et al., 2015). Accordingly the high ratio H2O2/pollutant compared to that one
365
investigated in the present work resulted in a high removal of the target compounds (Ferro et
366
al., 2015).
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Transformation products
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The formation of photo-oxidation by-products is of great concern in wastewater treatment
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because they may be more toxic than their parent compounds (Rizzo 2011). Figs. 8 and 9
371
report the ESI-MS spectra in negative mode on CAP (25mg/l), dark and photo-oxidised
372
solutions with UV-C/H2O2 and UV-C/PAA (10 mg/L of H2O2 and PAA, respectively) over
373
the time (0, 15, 30 60 and 120 min). When H2O2 was added to CAP (25mg/L) solution, in dark
374
experiment, the CAP (α: 421 m/z, negative ion with z=1) was transformed in CAP-succinate
375
(β: 321 m/z, negative ion with z=1) (Fig.8a). Within the early 30 min of UV-C/H2O2 photo-
376
oxidation, the compounds identified as peaks 1 and 2 were formed (Fig.8b, c). After 120 min
377
of UV-C/H2O2 photo-oxidation the peak 1 disappeared and the peak 2 was significantly
378
reduced, whereas the formation of several compounds at low molecular weight (m.z < 290)
379
could be observed (Fig.8e).
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The transformation products detected during photo-oxidation process by UV-C/PAA were
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different respect to those produced during UV-C/H2O2 process. Both CAP (α) and CAP-
385
succinate (β) were detected till 120 min, although they significantly decreased (Fig.9). After
386
120 min most of the compounds detected presented a molecular weight > 300 m.z., meaning
387
that the photo-oxidation with UV-C/PAA needs more time to degrade the transformation
388
products.
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No previous study on the formation of CAP’s transformation products by UV/PAA are
393
available in literature, whereas CAP degradation in water solutions treated by UV/H2O2 has
394
been recently investigated (Jin et al., 2018). The oxidation of CAP yielded 4-(2-amino-1,3-
395
dihydroxy-propanyl)-nitrobenzene (m/z 212) with loss of dichloroacetic acid (m/z 129) by the
396
attack of HO• produced by UV/H2O2. Further oxidation of the amino group of 4-(2-amino-1,3-
397
dihydroxy-propyl)-nitrobenzene (m/z 212) to nitro group led to the production of 4-(2-nitro-
398
1,3-dihydroxy-propanyl)-nitrobenzene (m/z 242). The denitration of 4-(2-nitro-1,3-
399
dihydroxypropanyl)-nitrobenzene (m/z 242) induced the production of 4-nitro-(2R)-
400
hydroxy(phenyl) ethanoic acid (m/z 197). Subsequent oxidation of the lateral group of 4-
401
nitro-(2R)-hydroxy(phenyl) ethanoic acid (m/z 197) produced the aromatics 4-nitro-benzoic
402
acid (m/z 167). None of these compounds was detected in the present study. Anyway, a
403
comparison is still possible with other AOPs. For example, unlike of our results where 10
404
mg/L of H2O2 and PAA, respectively, were not sufficient to totally remove the transformation
405
products within 120 min irradiation, when UV/TiO2 photocatalytic process was investigated
406
in the degradation of CAP (25 mg/L) water solution, the transformation products were totally
407
removed after 120 min irradiation, at 1.6 g/L of TiO2 (Lofrano et al., 2016). It is also worthy
408
to note that the lower TiO2 loadings (in the range 0.1-0.8 g/L) investigated were not sufficient
409
to totally remove the transformation products. None of by products reported by Garcia-Segura
410
et al. (2014) as result of anodic oxidation electro-generated H2O2, electro-Fenton, photo-
411
electro Fenton and solar photo- electro-Fenton of 245 mg/L of CAP was found in our study.
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Conclusions
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UV-C/PAA process was found to be effective in the degradation of CAP (t1/2= 20 min with 25
415
mg CAP/L), but the presence of H2O2 in the PAA solution significantly affected the removal
416
efficiency. When H2O2 was quenched by catalase addition, t1/2 increased to 99 min, meaning
417
that PAA can still produce HO• but at a lower rate compared to H2O2. Moreover, process
418
efficiency further decreased in the presence of both solar simulated and natural solar
419
irradiation, being CAP removal mainly due to a combination of thermal decomposition and
420
advanced oxidation. Possibly, the small portion of UV-C radiation emitted by the solar
421
spectrum cannot produce a sufficient amount of HO• to simultaneously degrade CAP
422
molecules and their oxidation intermediates that act as radicals scavengers too. Moreover, the
423
recombination between HO• molecules to form H2O2, as well as the reaction between H2O2
424
and HO• would further reduce their availability for CAP degradation.
425
The transformation products detected during CAP degradation by UV-C/PAA process were
426
different respect to those produced during UV-C/H2O2 process. Moreover, according to CAP
427
degradation kinetics, photo-oxidation with UV-C/PAA process needs more time to degrade
428
the transformation products than UV-C/H2O2 process.
429
In conclusion, taking into account the operating conditions (namely, initial oxidant
430
concentration and
431
degradation efficiency and transformation products removal) with UV/H2O2 and UV/PAA,
432
respectively, UV/H2O2 process is the best option for the removal of high CAP concentrations
433
from water/wastewater. Anyway, taking into account that sunlight/PAA process was found to
434
be more effective than sunlight/H2O2 process in the inactivation of bacteria in urban
435
wastewater (Formisano et al., 2016), different results can be expected in urban wastewater
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treatment. Accordingly, it would be worthy of investigation the use of UV/PAA (using
437
different light sources, namely sunlight and UV-C) as tertiary treatment of urban wastewater,
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treatment time/QUV dose) and the results achieved (namely, CAP
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to evaluate the removal of CECs (at realistic concentrations for urban wastewater, namely
439
ng/L – µg/L) and simultaneous bacteria inactivation.
440
Acknowledgements
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The authors wish to thank the ERASMUS programme for supporting Carmen Gago’s visit at
443
the University of Salerno as well as The Ministry of Education and Science of the Russian
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Federation for the grant to support Tatiana Bredneva’s visit at University of Salerno. Luigi
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Rizzo would also like to thank the University of Salerno for the financial support through
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FARB2015 (Trattamento fotocatalitico solare di acque reflue urbane destinate al riutilizzo).
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Figure 1: photodegradation kinetics of CAP (25 mg/L) by UV-C/H2O2 process: effect of
552
initial concentration of H2O2 and control tests.
553
Figure 2: photodegradation kinetics of CAP by UV-C/H2O2 (50 mg/L) process: effect of
554
initial concentration of CAP (10, 25 and 50 mg/L).
555
Figure 3: photodegradation kinetics of CAP (25 mg/L) by UV-C/PAA process: effect of both
556
initial concentration of PAA (5, 10, 20, and 50 mg/L) and quenching of H2O2 by catalase.
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Figure 4: photodegradation kinetics of CAP (25 mg/L) by UV-C/H2O2 (20 mg/L) and UV-
558
C/PAA (20 mg/L) processes: effect of aqueous matrix (distilled water (DW) and wastewater
559
(WW)).
560
Figure 5: photodegradation of CAP (25 mg/L) by AOPs: behaviour of residual oxidant
561
concentration in (a) UV-C/H2O2 and (b) UV-C/PAA processes.
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Figure 6: photodegradation kinetics of CAP (25 mg/L) by solar simulated radiation (SS)
563
AOPs: effect of oxidant (PAA Vs. H2O2) and oxidant dose (10, 20 and 100 mg/L,
564
respectively).
565
Figure 7: photodegradation of CAP (25 mg/L) by AOPs under natural solar light: (a)
566
sunlight/H2O2 and (b) sunlight/PAA processes.
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Figures 8: ESI-MS spectra in negative mode on CAP (25mg/l) aqueous solutions, under dark
568
and no treatment (a) and during photo-oxidation by UV-C/H2O2 (10 mg/L) process (0 (b), 15
569
(c), 30 (d), 60 (e) and 120 (f) min).
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ACCEPTED MANUSCRIPT Figures 9: ESI-MS spectra in negative mode on CAP (25mg/l) aqueous solutions, under dark
571
and no treatment (a) and during photo-oxidation by UV-C/PAA (10 mg/L) process (0 (b), 15
572
(c), 30 (d), 60 (e) and 120 (f) min).
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ACCEPTED MANUSCRIPT Table 1: kinetic parameters of CAP degradation during AOPs experiments CAP initial k (1/min)×10-3 t1/2 (min) concentration (mg/L) DW 25 5.0 ± 0. 1 138 ± 3 UV-C DW 5 25 9.3 ± 0. 2 74± 1 UV-C/H2O2 DW 10 25 12.8 ± 0. 1 54.3± 0.5 UV-C/H2O2 DW 20 25 26.0 ± 1 27± 2 UV-C/H2O2 DW 50 25 36.4 ± 0. 3 19.0 ± 0.1 UV-C/H2O2 DW 50 10 46 ± 5 15 ± 2 UV-C/H2O2 DW 50 50 8.6 ± 0.3 81 ± 3 UV-C/H2O2 WW 20 25 17 ± 1 41 ± 3 UV-C/H2O2 DW 5 25 11. 5 ± 0.3 60 ± 2 UV-C/PAA DW 10 25 12.6 ± 0.9 55 ± 4 UV-C/PAA DW 20 25 15.5 ± 0.7 44.6 ± 0.2 UV-C/PAA DW 50 25 34.6 ± 0.7 20.0 ± 0.4 UV-C/PAA DW 50 25 7.0± 0.3 99 ± 5 UV-C/PAA* WW 20 25 14 ± 1 48 ± 4 UV-C/PAA DW 25 0.92 ± 0.06 750 ± 50 SS DW 10 25 3.2 ± 0.2 219 ± 10 SS/H2O2 DW 20 25 5.2 ± 0.5 134 ± 10 SS/H2O2 DW 100 25 1.3 ± 0.1 520 ± 40 SS/H2O2 DW 10 25 3.3 ± 0.2 210 ± 10 SS/PAA DW 20 25 1.7 ± 0.5 410 ± 120 SS/PAA DW 100 25 2.1 ± 0.2 340 ± 40 SS/PAA *Catalase was added to PAA solution to quench H2O2 and evaluate the effect of PAA alone. Oxidant dose (mg/L)
R2
0.9956 0.9974 0.9991 0.9695 0.9995 0.8669 0.9895 0.9592 0.9933 0.9909 0.9998 0.9965 0.9832 0.935 0.9594 0.9793 0.8921 0.9387 0.9773 0.437 0.8932
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ACCEPTED MANUSCRIPT Highlights First work on antibiotic degradation by UV/peracetic acid (PAA) process.
•
UV-C/H2O2 was more effective than UV-C/PAA in chloramphenicol (CAP) degradation.
•
The presence of H2O2 in the commercial PAA solution significantly affected CAP removal.
•
Sunlight/PAA was not effective in the removal of CAP.
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Different transformation products were detected after UV-C/PAA and UV-C/H2O2.
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