Removal of 4-chlorophenol by visible-light photocatalysis using ammonium iron(II) sulfate-doped nano-titania

Accepted Manuscript Title: Removal of 4-chlorophenol by visible-light photocatalysis using ammonium iron(II) sulfate-doped nano-titania Authors: Finella Jianna A. Villaluz, Mark Daniel G. de Luna, James I. Colades, Sergi Garcia-Segura, Ming-Chun Lu PII: DOI: Reference:

S0957-5820(18)31335-1 https://doi.org/10.1016/j.psep.2019.03.001 PSEP 1683

To appear in:

Process Safety and Environment Protection

Received date: Revised date: Accepted date:

5 December 2018 25 February 2019 3 March 2019

Please cite this article as: Villaluz FJA, de Luna MDG, Colades JI, Garcia-Segura S, Lu M-Chun, Removal of 4-chlorophenol by visible-light photocatalysis using ammonium iron(II) sulfate-doped nano-titania, Process Safety and Environmental Protection (2019), https://doi.org/10.1016/j.psep.2019.03.001 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.

Removal of 4-chlorophenol by visible-light photocatalysis using ammonium iron(II) sulfatedoped nano-titania

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Finella Jianna A. Villaluza, Mark Daniel G. de Lunaa,b,*, James I. Coladesa,b, Sergi GarciaSegurac, Ming-Chun Lud,*

a

Environmental Engineering Program, National Graduate School of Engineering, University of

the Philippines, Diliman, Quezon City 1101, Philippines

Department of Chemical Engineering, University of the Philippines, Diliman, Quezon City

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b

Nanosystems Engineering Research Center for Nanotechnology-Enabled Water Treatment, School of

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c

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1101, Philippines

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Sustainable Engineering and the Built Environment, Arizona State University, Tempe, Arizona

Department of Environmental Resources Management, Chia Nan University of Pharmacy and

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d

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85287-3005, United States

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Science, Tainan 71710, Taiwan

Corresponding authors *Mark Daniel G. de Luna：[email protected] *Ming-Chun Lu: [email protected]

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Highlights Sol-gel synthesis method allows efficient doping of TiO2.



Doped Fe/N/S-TiO2 is active under visible light irradiation.



Degradation rates by Fe/N/S-TiO2 are one order of magnitude higher than TiO2

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

P25.

Complete degradation of 4-chlorophenol under visible light attained in 180 min.

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

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Abstract

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Halogenated aromatic compounds are toxic and carcinogenic. This is the case of 4-

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chlorophenol (4-CP), a priority pollutant found in large amounts in industrial wastewater

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effluents from pharmaceutical, dye, pulp and paper industries. Long term exposure to 4-CP even

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at low-concentration is associated to endocrine disruption. Photocatalysis is a promising advanced oxidation process that attains complete degradation of organic pollutants. The use of

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UV lamps undermines actual photocatalysis application due to the electrical energy requirements.

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In this frame, the development of visible light photoactive catalysts can overcome these challenges allowing the implementation using affordable light sources like light emitting diodes

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(LEDs) or natural sunlight. This work is to present the synthesis and use of an alternative photocatalyst that provided eight-fold increase on 4-CP degradation in comparison to commercial TiO2 Degussa P-25. Almost complete removal (99.20 %) was achieved with synthesized Fe/N/S-doped TiO2 at 1.0 g L-1 of photocatalyst dose and pH 7.0 for treating 10 ppm of 4-CP.The first-order rate constant, kLH, and Langmuir adsorption constant, KLH, were 2

calculated with values of 0.429 min-1 and 2.326 ppm-1, respectively. The Fe/N/S-TiO2. photocatalysts showed an excellent stability maintaining their performance during four cycles of

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recovery/reuse.

Keywords: persistent organic pollutants, wastewater treatment, visible light photocatalyst,

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nanotechnology, advanced oxidation process

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1. Introduction

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Phenolic compounds are co-carcinogens or promoters that increase genotoxicity of environmental carcinogens (Weisburger, 1992). Chlorinated derivatives of phenols are harmful

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substances that constitute a public health concern due to their increased toxicity and denoted

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carcinogenicity (Igbinosa et al., 2013). These hardly biodegradable compounds are released in

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the environment as a result of industrial activities, use of pesticides or as result of incomplete degradation of chlorinated organic compounds (Pozan and Kambur, 2013; Wang et al., 2016). In

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this scenario, chloro-phenol is studied as model compound due to the recalcitrant character of phenolic compounds and the well-defined degradative pathway (Filipowicz et al., 2017;

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Shamaila et al., 2010).Conventional treatments have low efficiency removals of these persistent organic pollutants. Environmental management tools should be implemented to reduce human and environmental health associated to hazardous pollutants in water effluents. Advanced oxidation processes (AOPs) rely on the generation in situ of high oxidant species such as hydroxyl radical (●OH, Eº= 2.80 V vs SHE) that completely mineralize organic pollutants, 3

emerging as a promising solution (Garcia-Segura et al., 2018; D. Wang et al., 2018). Recently, additional attention has been drawn on novel persulfate based AOPs technologies ( Fernandes et al., 2018; Fernandes et al., 2019; Sayed et al., 2018). Several AOPs have been explored in

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literature including Fenton based processes (Garcia-Segura et al., 2016), ozone (Boczkaj and Fernandes, 2017), cavitation (Gągol et al., 2018a) and photocatalysis (Nakata and Fujishima, 2012).

Among AOPs photocatalysis has attracted special interest due to the promising capabilities of this water treatment technology (Nakata and Fujishima, 2012; Spasiano et al.,

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2015). Photocatalysis is a heterogeneous process based on the use of semiconductors that

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generate oxidant species on their surface under light irradiation. When photons of sufficient

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energy are delivered to the semiconductor, an electron from the filled valance band of the

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semiconductor catalyst may be promoted to the empty conduction band (ecb+) leaving a vacancy or hole behind (hvb+) as described by reaction (1) (Dávila-Jiménez et al., 2018). These charge

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carriers (ecb+ and hvb+) can be further exploited for water treatment. Photogenerated holes are

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highly oxidant species that can oxidize organics by direct charge transfer processes or can

2015).

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generate ●OH from water oxidation according to reaction (2) (Orha et al., 2017; Spasiano et al.,

(1)

hvb+ + H2O → ●OH + H+

(2)

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Semiconductor + hν → ecb− + hvb+

Even though different catalysts such as zinc oxides, niobium oxides or bismuth oxides

have also been studied (Batista et al., 2017; Deng et al., 2018; Dimapilis et al., 2018); titanium oxide is the most widely employed photocatalyst (Spasiano et al., 2015). The preferred use of TiO2 is associated to its high abundancy, stability, innocuous character and low cost (de Luna et

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al., 2016; Fujishima et al., 2008). The main advantage is that in photocatalysis oxidants can be continuously generated on TiO2 surface and from water, which reduces the need for continuous addition of chemicals (Nakata and Fujishima, 2012; Rokhmat et al., 2017). Unfortunately, the

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greatest challenge of TiO2 photocatalysis is the requirement if UV light sources to overcome the band gap energy required for reaction (1). The high energy consumption of these light sources dramatically increases electrical energy per order requirements to mineralize organic pollutants (Lee et al., 2018; Westerhoff et al., 2016). Moreover, the energy requirement diminishes chances of success in technology implementation in developing countries that require reliable off-grid

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treatments to provide clean water and sanitation for populations in need (United Nations, 2018).

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Introducing doping elements in TiO2 semiconductors can reduce energy band-gap and enable

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visible light photocatalysis to overcome this limitation (Fagan et al., 2016; Lin et al., 2018;

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Sanzone et al., 2018). Different approaches have been considered to mechanistically enhance visible-light driven photocatalysis such as photogenerated electrons transfer between

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semiconductor interfaces heterojunctions or selective doping (Li et al., 2019; K. Wang et al.,

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2018; Wang et al., 2019). Among them, doping with iron is a very affordable modification that

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may become a game-changer for visible light-driven photocatalysis (Isari et al., 2018; Velázquez-Martínez et al., 2018).

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In this manuscript we present a novel sol-gel synthesis method to obtaining Fe/N/S-TiO2 doped photocatalysts active under visible light irradiation. Synthesized catalyst is characterized.

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Comparative experiments with commercial TiO2 P25 demonstrate that Fe/N/S-TiO2 doped photocatalyst surpasses in more than one order of magnitude the removal of 4-chlorophenol. Operational variables that affect photocatalytic treatment performance are explored and optimized.

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2. Experimental

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2.1 Chemicals and sol-gel synthesis procedure Synthetic solutions were prepared with 4-chlorophenol supplied by Alfa Aesar. Degussa P-25 titanium dioxide nanopowder was used as received. Titanium butoxide (Ti(OBu)4) (99% Acros Organics), ethanol (99.5% Shimakyu’s Pure Chemicals), nitric acid (65% Panreac Applichem), and ammonium iron(II) sulfate hexahydrate (NH4)2Fe(SO4)2•6H2O (99%, Panreac Applichem)

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were used without further treatment for the sol-gel synthesis of the modified photocatalyst.

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Ammonium iron(II) sulfate-doped nano-titania was prepared by following a facile sol-gel

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synthesis method. Ten mL of Ti(OBu)4 was mixed with 40 mL of ethanol in a beaker at 25 °C for

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5 min. The homogeneous solution was acidified dropwise with HNO3 and stirred for 10 min. TiO2 was doped through dropwise addition of 0.2 M (NH4)2Fe(SO4)2·6H2O. The resulting gel

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was aged for 24 h and dried completely at 105 °C in an oven for 16 h. Then, the dried gel was

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pulverized and calcined at 400 °C for 1 h.

2.2 Photocatalytic degradation experiments

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Photocatalysis was performed using a batch reactor equipped with 5 blue light emitting diodes (LED) as the light source with maximum wavelength (λmax) 450 nm and light intensity of

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16.85 mW cm-2 (HR16, 1W/110). LEDs were purchased from Sunlite (USA). Total volumes of 500 mL were treated under different photocatalyst dosage, 4-CP concentration and initial pH. Initial pH was adjusted using solutions of 0.5 M of NaOH and HNO3 and measured during experiment using a digital pH meter (PC-310, Suntex).Before photocatalytic experiments, adsorption-desorption equilibrium was established in the dark under continuous stirring at 300 6

rpm and 25°C of 4-CP solution in presence of photocatalyst. Photocatalytic experiments were conducted for 3 h and samples collected during treatment. Collected aliquots were filtered prior

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analysis using 25 mm Acrodiscs with 0.2 μm GHP membrane.

2.3 Analytical methods

Chlorophenol concentration during treatment was analyzed using a Thermo Scientific high-performance liquid chromatography (HPLC) equipped with an Asahipak ODP-506D column (5 μm, 150 mm x 6 mm) coupled to photodiode array detector selected at λ = 216nm.

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Aliquots of 20 μL were injected while using a mobile phase 60:40 (v/v) acetonitrile/water at 0.6

up to 15 mg L-1 were injected to define the calibration curve of the analysis method (including

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1

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mL min-1 at pH 3.5. Standard solutions containing 4-chlorophenol concentrations from 0.1 mg L-

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10 analytical points). Electrical energy per order (EEO, kW h m-3 order-1) requirements were calculated from the concentration abatement experimentally quantified by HPLC. Electrical

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energy per order is an engineering figure of merit defined by IUPAC to benchmark different

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AOPs technologies in terms of energy operational expenditures (Bolton et al., 2001). This figure

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of merit describes the electric energy required to reduce pollutant content by one order of magnitude in a unit volume. Electrical energy per order was calculated from the simplified

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expression (3) that uses the pseudo-first order kinetic constant determined from the pollutant

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abatement kinetics analysis (dos Santos et al., 2018). -4 EEO (kWh m-3 order-1) = 6.39 x 10 PLED Vs k1

(3)

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where 6.39 is a conversion factor (1 h / 3600 s / 0.4343), PLED is the rated power of the LEDs used as irradiation source (kW), Vs is the solution volume (m3); and k1 is the pseudo-first order rate constant (s-1).

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Catalysts morphology were observed through scanning electron microscopy (SEM) using a QUANTA 200 at 10 kV. Crystalline structure was determined by X-ray diffraction (XRD) with a Rigaku DX-2000 SSC X-ray Diffractometer scanning θ from 5º to 75º with increments of 0.06º. Diffuse reflectance was measured using a Hitachi U-3310 UV-vis spectrophotometer. Surface

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area was determined with BET method using a Micromeritics ASAP2010 surface analyzer.

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3. Results and discussion

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3.1 Photocatalyst characterization

Micrographs of commercial Degussa P25 TiO2 and synthesized Fe/N/S-TiO2 are shown

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in Fig. 1a and b, respectively. Both photocatalyst have a characteristic spheroidal shape, although

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a more irregular shape is observed for Fe/N/S-TiO2 catalyst due to the semiconductor TiO2

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lattice distortion due to the introduction of dopants, which also favors the disaggregation of particles. Electron dispersive X-ray spectroscopy confirmed the presence of dopants Fe, N, and S

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demonstrating the successful doping during the sol-gel synthesis of Fe/N/S-TiO2. b

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a

500 nm

10 μm 8

Fig. 1 - Scanning electron micrograph of (a) commercial Degussa P25 TiO2 and (b) sol-

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gel synthesized Fe/N/S-TiO2 photocatalyst powder.

The XRD analysis of Degussa P-25 shown in Fig. 2 depicts peaks with characteristic distribution of crystalline phase composition of 75% anatase and 25% rutile in agreement with previous reports (Han et al., 2018). Meanwhile, the difractogram of Fe/N/S-TiO2 catalyst of Fig. 2 shows solely anatase representative peaks associated to crystalline planes (101), (004), (200),

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(105), (204) and (220) (Khan and Bashir, 2011). It is important to note the widening of the

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diffraction peaks is indicative of a more amorphous structure. This is explained by the anatase

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crystalline lattice distortion due to dopants incorporation such as observed previously for sulfur

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species into TiO2 (Ohno et al., 2003), although the main crystalline structure is retained. The distortion of TiO2 lattice deduced from XRD analysis is in agreement with the irregular

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spheroidal shape structure observed in SEM micrographs (see Fig. 1b). A

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Intensity (a.u.)

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Degussa P-25

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A A

A A

(101)

Fe/N/S-TiO2 (004)

20

30

40

(200)

(105)

50

(204)

60

(220)

70

2 (degrees)

Fig. 2 - X-ray diffractograms of (a) commercial Degussa P25 TiO2 and (b) sol-gel synthesized Fe/N/S-TiO2 photocatalyst powder identifying characteristic peaks of anatase.

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The isotherm of nitrogen adsorption-desorption allowed determining the specific surface area of both catalysts. Commercial P25 shows the usually reported SBET of 50 m2 g-1 usually

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reported (Han et al., 2018). The synthesized Fe/N/S-TiO2 catalyst is in the range of SBET obtained during sol-gel synthesis procedures of 227.35 m2 g-1.

Optical properties of the photocatalyst were analyzed by the UV-vis diffuse reflectance spectra. Fig. 3 . shows higher photo-absorption capabilities in the visible region by Fe/N/S-TiO2. Note that the absorption edge of 406.6 nm corresponds to a lower band gap energy of 3.05 eV,

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while commercial P-25 TiO2 presents its characteristic 3.2 eV band gap (Das et al., 2016; Kumar

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et al., 2016). The red shift towards higher wavelength, lower energy and lower band gap is

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advantageous for visible photocatalysts enabling efficient use of natural sunlight (visible

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radiation) for photogeneration of charge carriers on the photocatalyst surface for environmental protection applications. The visible excitation is enabled due to the insertion of dopant species in

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the semiconductor lattice (Fagan et al., 2016; Ohno et al., 2004), which has been also

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corroborated by the characterization discussed in this section.

Absorbance (a.u.)

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Degussa P-25 Fe/N/S-TiO2

300

400

500

600

700

Wavelength (nm)

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Fig. 3 – UV-vis diffuse reflectance spectra of X (a) commercial Degussa P25 TiO2 and (b) sol-gel synthesized Fe/N/S-TiO2 photocatalyst powder.

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XPS analysis allowed identifying Ti, O, C, and N signals corresponding to the successful doping (see Fig. 4). The low doping of Fe that remains diluted in the TiO2 lattice is below the detection limit of XPS in agreement with results reported in literature that identified weak

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interaction of Fe and Ti (Crisan et al., 2014; Khan et al., 2008).

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Fig. 4 – XPS spectra of the synthesized Fe/N/S-TiO2 photocatalyst powder.

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3.2 Comparative visible-light photocatalytic performance with commercial photocatalysts Photocatalysis is one of the most studied AOPs that have shown great capabilities to treat

polluted waters with persistent organic pollutants (Spasiano et al., 2015; D. Wang et al., 2018). Conventional TiO2 has excellent performance under UV irradiation light (Chang et al., 2005;

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Nakata and Fujishima, 2012). However, in order to enhance photocatalytic treatment competitiveness for their actual implementation in water treatment trains alternative light sources must be exploited (Fagan et al., 2016; Tugaoen et al., 2017). Developing countries would benefit

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from the use of visible-light active photocatalysts as green and low-cost treatment approach. Fig. 5 compares the performance on 4-CP removal of commercial P25 TiO2 with synthesized Fe/N/STiO2 photocatalyst under visible light photoexcitation. Control experiments were conducted to exclude contribution of other removal mechanisms associated to pure adsorption and photolysis. As can be seen in Fig. 5, both catalysts showed discrete removals of ~9.0 % after 180 min of

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treatment in absence of light irradiation due to pollutant adsorption on the catalyst surface.

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However, no significant degradation was observed during direct photolysis in absence of

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photocatalyst due to the photostability of 4-CP. Noteworthy is the similar removal attained by

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P25 TiO2 in both, photocatalytic and in dark experiments. It may be inferred that removal mechanism of 4-CP is then solely associated to adsorption processes. This result is explained by

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the inability of visible light irradiation to provide enough energy to overcome TiO 2 bangap,

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therefore charge carriers (ecb- and hvb+) are not photogenerated (Tugaoen et al., 2017). In contrast,

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complete 4-CP abatement was attained in 180 min using doped Fe/N/S-TiO2 photocatalyst. The narrower band gap due to the inclusion of dopant species explains the efficient photoexcitation

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under visible light irradiation and consequently the photogeneration of oxidants hvb+ and OH following reactions (1) and (2). Use of selective scavengers as trapping species is used in

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literature to identify the role of different oxidants (Li et al., 2018; K. Wang et al., 2018). It is well-understood that the main oxidant species involved in TiO2 photocatalytic oxidation of organic compounds is mainly conducted by OH and photogenerated hvb+, whereas doping does

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not modify modify noticeably the active species involved in the degradation of recalcitrant organics (Deng et al., 2017; Zhang et al., 2011). The kinetic analysis demonstrated good fittings to pseudo-first order kinetics for 4-CP

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photocatalytic abatement with R2 values over 0.998. A quick comparison of the estimated rate constants (k1) (Fig. 2b) highlights the notorious difference higher than one order of magnitude between commercial P25 TiO2 with a k1 = 1.28 x 10-5 s-1 and visible light photoactive Fe/N/STiO2 with a k1 =4.42 x 10-4 s-1. The trend can be explained by the higher ability of the doped photocatalyst to generate charge carriers (ecb- and hvb+) according to reaction (1) using visible

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irradiation light sources, which has a direct impact on the oxidation capabilities of the

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photocatalyst (Laciste et al., 2017; Lin et al., 2018). A further analysis in terms of electrical

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energy per order as engineering figure of merit (Bolton et al., 2001; Lanzarini-Lopes et al., 2017)

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shows a reduction of energy requirement from 499.2 kWh m-3 order-1 for commercial P25 TiO2 down to 14.5 kWh m-3 order-1 which represents a decrease over one order of magnitude (ca. 35-

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fold). On top of that, this high efficiency under visible light suggests that further diminishing

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operation expenditures may be possible by using natural sunlight irradiation.

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0.6 0.4

99.2 % 40 k = 44.2

60

30

40

20

20

0.2

2.0 % k = 0.19

40

80

120

time / min

Adsorption p25 Adsorption Doped PC P25 Doped

160

200

-1

80

-5

% Removal

0.8

0 0

50

b

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0

1.0

100

k/ x10 s

a [4-CP]/[4-CP]

PEC

13.1 % k = 1.28

7.8 % k = 0.76

9.7 % k = 0.99

P25 TiO

Fe/N/S-TiO

10

0

0 Photolysis

2

In the dark

2

P25 TiO

2

Fe/N/S-TiO

2

Visible light

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Fig. 5 – (a) Degradation kinetics of 500 mL solutions of 10 mg L-1 of 4-chlorophenol during different treatments using catalyst doses of 1 g L-1 at pH 7.0: () Photolysis in absence of catalyst, () P25 TiO2 in the dark, () Fe/N/S-TiO2 in the dark, () P25 TiO2 under visible

treatment and pseudo-first order rate constants determined.

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light, () Fe/N/S-TiO2 under visible light. (b) Percentage of removal attained after 180 min of

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3.3 Impact of pH on photocatalytic degradation of 4-chlorophenol

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Treated solution pH affects the electrostatic surface charge of the catalyst at the interface

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semiconductor/solution by acid-base equilibria. The point of zero charge (pHpzc) of the doped

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catalyst Fe/N/S-TiO2 presented the isoelectric point at pHpzc = 6.2, similarly reported to other TiO2 based photocatalysts (Anotai et al., 2012; Zawawi et al., 2017). TiO2 photocatalyst surface

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is positively charged for pH < 6.2, whereas remains negatively charged for pH > 6.2. On the

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other hand, 4-CP speciation is defined also by an acid-base equilibria with a pKa = 9.4.

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In this frame, Fe/N/S-TiO2 and 4-CP relative charges play a key role on the degradation since may ensure or hinder the interaction between the photogenerated oxidants (hvb+ and OH) on the

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catalyst surface. Fig. 6a depicts the notorious differences observed in the photocatalytic degradation kinetics in function of the initial solution pH. Complete abatement is attained at

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circumneutral pH where Fe/N/S-TiO2 surface is slightly negatively charged and 4-CP remains neutral. However, 4-CP removal percentage decreases radically down to 45.0 % at pH 9.0. Kinetics deceleration may be explained by the electrostatic repulsion between the Fe/N/S-TiO2 surface and 4-CP molecules, both negatively charged. This result is in agreement with previous

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observations on degradation efficiency associated to the impeded adsorption of pollutants on photocatalyst surface due to electrostatic repulsion (D. Wang et al., 2018; Yamazaki et al., 2012). Adsorption or approaching of the pollutant to the catalyst surface is the initial step for

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heterogeneous catalytic degradation of organics. Mass transport of 4-CP from solution to the catalyst surface is controlled by diffusion for pH 5.0 and 7.0, where electrostatic forces neither repulsive nor attractive contribute in any extent (Anotai et al., 2012; Spasiano et al., 2015). In this frame, the differences on pollutant degradation performance were related to the oxidation mechanism. Note that even though oxidizing holes are photogenerated in similar extent allowing

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direct charge transfer, degradation events of 4-CP are mostly occurring through mediated

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oxidation by OH through reaction (Lin et al., 2008). Generation of OH is favored at higher pH

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due to the higher coverage of hydroxyl ions (OH-) on the catalyst surface (Barakat et al., 2005).

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Faster degradation under natural water pH is highly favorable to reduce operational costs associated to pH adjustments prior and after treatment. Thus, synthesized Fe/N/S-TiO2 catalyst

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can attain complete degradation of highly recalcitrant 4-CP PECunder visible light and neutral pH 7.0.

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0,6 0,4

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0 0

40

60

3

40

2

20

0,2

80

120

time / min

160

200

4

99.2 % k = 4.42

-1

% Removal

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0,8

5

b

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[4-CP]/[4-CP]

0

1

100

57.5 % k = 0.83

-4

a

k/ x10 s

1,2

1 42.5 % k = 0.68

0

0 5

7 pH

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Fig. 6– (a) Photocatalytic degradation kinetics of 500 mL solutions of 10 mg L-1 of 4Adsorption p25

chlorophenol withAdsorption 1 g L-1 Fe/N/S-TiO Doped 2 at different operational pH: () 5.0, () 7.0, () 9.0. (b) PC P25 Doped

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Percentage of removal attained after 180 min of treatment and pseudo-first order rate constants determined.

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3.4 Defining optimum dosage requirements for optimum performance Catalyst dose requirements not only define costs associated to material requirements, but it is known to have impact also on light penetration and photocatalytic performance. Fig. 7explores the effect the influence of catalyst dosage on degradation kinetics. One may infer that increasing dosage should result in a proportional increase of degradation kinetics due to the

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larger availability of catalytic sites per treated volume (higher semiconductor/solution interface).

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However, the trends described by Fig. 7b evidence that such trend is only correct up to certain

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point. Catalyst doses higher than 1.0 g L-1 have a clear detrimental effect on the removal

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capabilities using a slurry system. The dramatic decay on k1 from 4.42 x 10-3 s-1 at 1.0 g L-1 to 1.12 x 10-3 s-1 at 1.6 g L-1 may be explained by two main effects. First, the aggregation of

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suspended catalyst particles forming larger colloids diminishes the specific interfacial surface

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area and consequently the number of catalytic sites (i) accessible to react with the pollutant and

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(ii) accessible to the delivered photons (Tugaoen et al., 2017; D. Wang et al., 2018). Last, the higher turbidity due to the Fe/N/S-TiO2 particles in slurry may enhance light scattering effects

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and definitely diminish light penetration due to solution opacity. Both effects diminish considerably the efficient light transport and consequently the efficient photoexcitation according

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to reaction (1) (Bhatia et al., 2017). The reduced quantum yield of the system has a direct repercussion on the generation of oxidants hvb+ and OH, decreasing the overall degradation performance of the photocatalytic treatment. The optimum photocatalyst dosage was defined as 1.0 g L-1 of Fe/N/S-TiO2.

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PEC

100

a

5

b 80

0.4

40

72.9 % k = 1.12

37.5 % k = 0.41

40

80

120

160

200

0

-1

0.4 g L

time / min

2

-4

3

20

0.2 0 0

60

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% Removal

0.6

-1

0

[4-CP]/[4-CP]

0.8

4

99.2 % k = 4.42

k/ x10 s

1.0

1

-1

1.0 g L Catalyst dosage

1.6 g L

-1

0

Fig. 7 – (a) Photocatalytic degradation kinetics of 500 mL solutions of 10 mg L-1 of 4Adsorption p25

after 180 min of treatment and pseudo-first order rate constants

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Percentage of

PC P25 removal Doped attained

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chlorophenol at pH 7.0 with Adsorption Dopeddifferent Fe/N/S-TiO2 dosage: () 0.4, () 1.0, () 1.6. (b)

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determined.

3.5 Identifying pollutant concentration range treatable

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The main objective of photocatalytic treatment is to attain complete removal of a target

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pollutant in a timely manner. Initial concentration of pollutant may define the contact time

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needed to ensure the achievement of this goal; therefore it would define reactor design to meet hydraulic retention time requirements. Fig. 8 describes the photocatalytic degradation kinetics

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under different initial concentration of 4-CP. The kinetic analysis shows the decrease of the apparent k1 2-fold from 4.42 x 10-3 s-1 at 10 mg L-1 down to 2.85 x 10-3 s-1 at 20 mg L-1, and a

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more dramatic decay of 3-fold to 1.55 x 10-3 s-1 at 40 mg L-1. The slower removal observed for increasing pollutant concentrations is related to light

attenuation due photon absorption by 4-CP, which reduces photon transport efficiency towards photocatalyst surface and thus the extent of photogeneration of charge carriers by reaction

17

(Jiménez-Tototzintle et al., 2015; Tugaoen et al., 2018). However, one of the driving effects is the competition of higher concentration of organics (4-CP and yielded by-products) for Fe/N/STiO2 catalytic sites and generated oxidants (Fagan et al., 2016; Lin et al., 2018). The competitive

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reaction of oxidant with yielded by-products decreases the ratio of events concerning 4-CP oxidation by hvb+ and OH in agreement with the experimental results observed.

Even under high pollutant concentration conditions (40 mg L-1) high removals ~80 % were attained in 180 min. These promising results suggest potential applicability of Fe/N/S-TiO2

M

A

N

U

solar photocatalytic treatment under a wide range of concentrations.

PEC

100

0.6 0.4 0.2

CC

0 0

A

40

80

120

60

3 95.1 % k = 2.85

40

200

2 80.0 % k = 1.55

20

160

-1

4

99.2 % k = 4.42

-4

% Removal

TE

0.8

80

EP

[4-CP]/[4-CP]

0

D

1.0

5

b

k/ x10 s

a

0

-1

10 mg L

time / min

-1

20 mg L [4-Chlorophenol]

40 mg L

-1

1 0

Fig. 8 – (a) Photocatalytic degradation kinetics of 500 mL solutions at pH 7.0 with 1.0 g Adsorption p25

L-1 of Fe/N/S-TiOAdsorption treating different initial concentrations of 4-chlorophenol: () 10 Doped 2 catalyst -1

mg L , () 20

PC P25 -1 mg L , Doped

() 40 mg L-1. (b) Percentage of removal attained after 180 min of

treatment and pseudo-first order rate constants determined.

18

3.6 Understanding catalyst reuse capabilities Development of green treatment processes should aim for catalyst recycling and reuse during

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several treatment cycles. Reutilization without a significant lost on photocatalytic performance is therefore the utmost desired characteristic for actual application along with low material cost.

The consecutive degradation of 4-CP solutions using recovered photocatalyst is plotted in Fig. 9. The removal percentage attained at 180 min of treatment decreases after several cycles showing values of 99.2 % during first cycle, 94.2 % during second cycle, 83.7 % during third cycle, and

U

72.5 % during fourth cycle. Similar trend is observed for the estimated kinetic constants that

N

decrease from 4.42 x 10-3 s-1 in the first cycle down to 1.16 x 10-3 s-1 in the fourth cycle. The

A

decrease on treatment efficiency can be explained by the discrete leaching of dopants, which

M

would reduce the photoexcitation events taking place under visible light irradiation. However, solution samples analyzed contained dopant species below of the limit of detection and

D

differences were not observed during the characterization of used catalyst respect to fresh

TE

catalysts. In this frame, a possible explanation to the reduced performance may be the catalytic

EP

sites poisoning by strongly adsorbed by-products on Fe/N/S-TiO2 surface. Even though, the

A

CC

slight decrease of catalyst performance suggest possible reuse for several cycles.

19

PEC

100

a

0.6 0.4

3

94.2 % k = 2.69

40

0 0

40

80

120

160

200

2

83.7 % k = 1.68

20

0.2

0

-1

60

4

-4

% Removal

0.8

99.2 % k = 4.42

72.5 % k = 1.16

k/ x10 s

0

80

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1.0

[4-CP]/[4-CP]

5

b

1 0

1

4

U

time / min

2 3 Reuse cycle

N

Fig. 9 – (a) Reuse of Fe/N/S-TiO2 catalyst in consecutive treatments of 500 mL solutions Adsorption p25

nd

() 4th cycle. (b) Percentage of removal attained after 180 min of

M

() 2 cycle,

PC P25 rd () 3 cycle, Doped

A

-1 st Of 10 mg L-1 of 4-chlorophenol Adsorption Doped with 1.0 g L of Fe/N/S-TiO2 catalyst at pH 7.0: () 1 cycle,

EP

4. Conclusions

TE

D

treatment and pseudo-first order rate constants determined.

Photocatalysis is a promising advanced oxidation process for organic pollution

CC

remediation. Conventional TiO2 photocatalysts rely on the use of UV light to efficiently photogenerate oxidants, which arises as a major challenge due to the high energy requirements

A

associated to UV irradiation sources (i.e. Hg lamps). The sol-gel synthesis method developed herein allows the efficient doping of TiO2 that leads to the photocatalytic activation in the visible light range. The Fe/N/S-TiO2 catalyst achieved complete abatement of 10 mg L-1 of 4-CP in 180 min of photocatalytic treatment. Evaluation of the effect of initial pH demonstrates the direct correlation between 4-CP speciation and Fe/N/S-TiO2 surface charge on the photocatalytic 20

degradation kinetics, which is mostly explained by electrostatic repulsions that inhibit the heterogeneous reaction on the catalyst surface. Experimental results show great capabilities to treat polluted hydric resources at natural pH conditions without requiring pH readjustments

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prior- and/or post-treatment. The use of slurry systems enhances mass transfer and degradation kinetics but limits the catalyst dosing that is efficiently excited by the light source. Excessive dosing enhances scattering events and undermines light penetration. Moreover, high doses of catalyst may result in particles agglomeration diminishing the catalytic surface area in contact with the solution. Experiments conducted at different initial 4-CP concentrations allow

U

concluding that a wide range of contaminated sources may be treated under solar light irradiation

N

using Fe/N/S-TiO2 catalyst. Even though, faster kinetics is observed at lower pollutant

A

concentrations where the kinetics is defined by the mass transfer from solution to the catalyst

M

surface and not limited by available catalytic sites. Moreover, increasing organics concentration may result in 4-CP kinetics degradation deceleration due to competitive reactions with by-

D

products yielded. Under optimal treatment conditions of 10 mg L-1 of 4-chlorophenol with 1.0 g

TE

L-1 of Fe/N/S-TiO2 catalyst at pH 7.0 total abatement was observed at 180 min of photocatalytic

EP

treatment under visible light irradiation. Further studies must consider total organic carbon abatement, evolution of organochlorinated and inorganic chlorine (e.g. Cl-, ClO-) and the

CC

biotoxicity of treated effluents (Garcia-Segura et al., 2016; Gągol et al., 2018b; Shah et al., 2018). Catalyst can be reused for several cycles although ore research efforts should be conducted to

A

enhance stability. Operational expenditures benchmark against commercial Degussa P25 TiO2 demonstrates reduction of electrical energy per order requirements from 499.2 kWh m -3 order-1 down to 14.5 kWh m-3 order-1 for the synthesized Fe/N/S-TiO2 catalyst.

21

Declarations of interest

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The authors declare no conflicting financial interest.

Acknowledgments

The authors would like to thank the National Science Council, Taiwan (NSC 101-2923-E-

U

041-001-MY2) and the Department of Science and Technology, Philippines for funding this

N

research.

M

A

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