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Degradation of 1-hydroxy-2,4-dinitrobenzene from aqueous solutions by electrochemical oxidation: Role of anodic material
Accepted Manuscript Title: Degradation of 1-hydroxy-2,4-dinitrobenzene from aqueous solutions by electrochemical oxidation: role of anodic material Author: Marco A. Quiroz Jos´e L. S´anchez-Salas Silvia Reyna Erick R. Bandala Juan M. Peralta-Hern´andez Carlos A. Mart´ınez-Huitle PII: DOI: Reference:
S0304-3894(13)00982-5 http://dx.doi.org/doi:10.1016/j.jhazmat.2013.12.050 HAZMAT 15643
To appear in:
Journal of Hazardous Materials
Received date: Revised date: Accepted date:
31-7-2013 11-12-2013 24-12-2013
Please cite this article as: M.A. Quiroz, J.L. S´anchez-Salas, S. Reyna, E.R. Bandala, J.M. Peralta-Hern´andez, C.A. Mart´inez-Huitle, Degradation of 1-hydroxy-2,4-dinitrobenzene from aqueous solutions by electrochemical oxidation: role of anodic material, Journal of Hazardous Materials (2014), http://dx.doi.org/10.1016/j.jhazmat.2013.12.050 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.
Degradation of 1-hydroxy-2,4-dinitrobenzene from aqueous solutions by electrochemical oxidation: role of anodic material
Marco A. Quiroz1, José L. Sánchez-Salas1, Silvia Reyna1, Erick R. Bandala1,
Universidad de las Américas Puebla. Grupo de Investigación en Energía y Ambiente. ExHda. Sta.
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Juan M. Peralta-Hernández2, Carlos A. Martínez-Huitle3,*
Catarina Martir s/n, Cholula 72820, Puebla, México. Centro de Innovación Aplicada en Tecnologías Competitivas, Departamento de Investigación
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2
Ambiental Omega-201, Fraccionamiento Industrial Delta. León, 37545, Guanajuato, México. Universidade Federal do Rio Grande do Norte, CCET – Institute of Chemistry, Lagoa Nova - CEP
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59.072-970 – Natal, RN; Brazil.
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E-mail address: [email protected]
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Highlights
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Degradation efficiency and pathways depend on the nature of electrode material.
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Denitration and substitution by •OH radicals on the aromatic rings is the first proposed step in reaction mechanism.
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ABSTRACT
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Diamond anodes generate a minor number of intermediates from 2,4-dinitrophenol oxidation.
Electrochemical oxidation (ECOx) of 1-hydroxy-2,4-dinitrobenzene (or 2,4-dinitrophenol: 2,4-DNP) in
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aqueous solutions by electrolysis under galvanostatic control was studied at Pb/PbO2, Ti/SnO2, Ti/IrxRuySnO2 and Si/BDD anodes as a function of current density applied. Oxidative degradation of
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2,4-DNP has clearly shown that electrode material as well as the current density applied were important
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parameters to optimize the oxidation process. It was observed that 2,4-DNP was oxidized at few
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substrates to CO2 with different results, obtaining good removal efficiencies at Pb/PbO2, Ti/SnO2 and Si/BDD anodes. Trends in degradation way depend on the production of hydroxyl radicals (! OH) on these anodic materials, as confirmed in this study. Furthermore, HPLC results suggested that two kinds of intermediates were generated, polyhydroxylated intermediates and carboxylic acids. The formation of these polyhydroxylated intermediates seems to be associated with the denitration step and substitution by ! OH radicals on aromatic rings, being this first proposed step in reaction mechanism. These compounds were successively oxidized, followed by the opening of aromatic rings and the formation of a series of carboxylic acids which were at the end oxidized into CO2 and H2O. On the basis of these information, a reaction scheme was proposed for each type of anode used for 2,4-D oxidation. 2 Page 2 of 29
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Keywords: hydroxyl radicals, electrode activity, 2,4-dinitrophenol, electro-oxidation.
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1. Introduction
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Persistent Organic Pollutants (POPs) are highly stable and toxic compounds with high levels of permanence in the environment, great atmospheric mobility and accumulative capacity in fatty tissue of
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most living organisms [1, 2]. These kinds of pollutants result from the use of chemicals in industry or
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as by-products of combustion. The presence of different types of POPs in water, constitutes one of the
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most important problems in environmental sciences today [3, 4], fundamentally as result of their widely known effects on human health and wildlife. Nitroaromatic compounds, in particular, can be formed by photochemical atmospheric reactions owing the presence of nitrogen oxides (NOX) in the industrial and automotive gases emission [5, 6]. They are also widely used in pesticide production, paints and explosive material and have been detected not only in industrial wastewaters but also in freshwater and marine environments [7, 8], and are considered by the United States Environmental Protection Agency (USEPA) as priority pollutants owing to their toxicity to humans [9, 10]. Detoxification of water contaminated with nitroaromatic compounds is usually a very difficult process since the presence of nitro group confers to the aromatic compound a strong chemical stability and resistance to microbial degradation [11]. Lypczynska-Kochany [7] has reported experimental 3 Page 3 of 29
conditions for the biodegradation of nitrophenol compounds (NPCs) where long range of incubation was required for the reduction of the nitro group. Bhatti et al., [12] reported a biological reactor where the use of polyester nonwovens for bacterial attachment significantly improved the biological
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degradation of NPCs. Nevertheless, long degradation time was required (11 h) as well as the use of a previous long biomass acclimation time (5-6 months) and the use of a supplementary substrate. More
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recently, Tomei et al. [13] has reported that the time and removal rate of NPCs biodegradation may be enhanced if the biodegradation process is carried out in a sequencing batch reactor. However, the
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mineralization process reported in this last work is not clear at all since the reported nitrite and nitrate formation and the absence of hydroquinone in the effluent are not sufficient evidence for a complete
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conversion of the NPCs to inorganic nitrogen species, CO2 and water. Indeed, the paper by Tomei et al.
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[13] offers a good alternative for the treatment of residual water containing NPCs even though the process continues being dependent on the bacterial culture and the acclimatization time.
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Nitroaromatic compounds in aqueous media has been also degraded by using irradiation sources
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with or without chemical mediators such as H2O2, Fe(II) and/or Fe(III) salts, ozone and TiO2 catalysts [14-17]. Thus, for nitrobenzene degradation in acidic media the best mineralization condition has been
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obtained by combination of solar light and iron salts (photo-Fenton process) [18]. Other photomediator combinations are also possible but the degradation efficiency is often affected due to competition effects between the chemical mediator and the chemical compound target of the degradation process.
A good alternative to biological, chemical and photochemical methods for the elimination of biorefractory organic pollutants in aqueous media (drinking water or wastewater) is the electrochemical oxidation (ECOx) [19-26] by using metallic oxides or boron-doped diamond (BDD), as anodes. These materials have shown high performance levels to the conversion and/or combustion of a wide variety of aromatic compounds. In the conversion process the aromatic compounds are only transformed to biocompatible compounds in order to they can be adequately eliminated in a subsequent biological 4 Page 4 of 29
treatment. The oxidation mechanism proceeds through an introduction of oxygen step into the oxide lattice which results in conversion of organic and a change of the oxidation number of metal (such as IrO2 and RuO2 for instance). On the other hand, the electrochemical combustion yields CO2 and H2O
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by direct surface oxidation of the undesired aromatic compounds and, therefore, the residual solution does not require further purification. In this last case, the accumulation of ! OH radicals on the electrode
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surface is the determinant factor for the electrochemical combustion. Anodic materials with these
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characteristics are SnO2 and PbO2 with or without doping species [20, 27-31].
Therefore, the aim of this work is to evaluate the electrocatalytic activity of PbO2, SnO2,
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IrxRuySnO2 and BDD as anodic materials to eliminate 2,4-DNP in aqueous acidic media at applied current density values in order to understand the role of the nature of electrode material, their ability to
2.1 Chemicals
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2. Materials and methods
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produce ! OH radicals and the more probable mechanism by which 2,4-DNP is degraded.
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Ultrapure water (18 M! ) was obtained by using a Simplicity water purification system. Chemicals were of the highest quality commercially available, and were used without further purification. 2,4DNP (puriss. p.a. ! 99.5%), N,N-dimethyl-p-nitrosoaniline (PNDA) (purum p.a. ! 98.0%) and H2SO4 (puriss. p.a. 95-97%) were purchased from Fluka. The PNDA concentration was 2×10-5 M and that of model organic compound 2,4-DNP was 50 mg L-1 (50 ppm), both of them in 0.5 M H2SO4 solution as supporting electrolyte.
2.2 Electrode materials The Pb/PbO2 electrode was prepared growing the anodic oxide at a current density of 50 mA cm-2 during an electrolysis time of 1.5 h in a 10% H2SO4 solution at 25 °C, in order to oxidize the lead 5 Page 5 of 29
surface into PbO2. More details concerning anode preparation and characterization are given elsewhere [32-34]. The SnO2-coated titanium (Ti/SnO2) electrodes [35, 36] were prepared by a sol-gel technique,
titanium base and drying at 50°C. The whole process was repeated 10 times.
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which consisted of the following steps: preparation of precursor solution, its brushing onto a pre-treated
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The Ti/IrxRuySnO2 electrodes were prepared as follows: a SnCl4/RuCl3 solution and a SnCl4/IrCl3 solution, both prepared with a tin mole ratio of 66%, were mixed in different proportions to render
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Ir:Ru ratios of 1:1, 2:1 and 3:1. A layer of each mixture was painted on a titanium support, dried for 10 minutes at 120°C and annealed for 25 minutes at 500°C; this procedure was repeated 11 times. Thus,
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solid solutions of the three metals were obtained by thermal decomposition. The electrode area was 10
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cm2. More details concerning anode preparation and characterization are given elsewhere [37]. BDD films were provided by CSEM (Neuchâtel, Switzerland) and synthetized on a conductive p-
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Si substrate (1 mm, Siltronix) via a hot filament using the chemical vapor deposition technique (HF-
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CVD) [38]. This procedure gave a columnar, randomly textured, polycrystalline diamond film, with a thickness of about 1 µm and a resistivity of 15 m! cm (! 30%) onto the conductive p-Si substrate.
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Boron doping level was about 5000 ppm.
2.3 Electrochemical measurements
Bulk oxidations were performed in a two-compartment electrochemical cell, the reaction compartment having a capacity of 250 mL. A schematic drawing of the experimental apparatus has been previously reported [34]. An ultra-high-purity graphite rod (EG&G PARC), enclosed in a 10 mL porous porcelain pot, was used as the counter electrode. Pb/PbO2, Ti/SnO2, Ti/IrxRuySnO2 and Si/BDD were used as the working electrode and a Hg/Hg2SO4/K2SO4(sat) was used, as the reference electrode. The anolyte consisted of 50 ppm of 2,4-DNP in 0.5 M H2SO4 solution, while the pure supporting electrolyte was chosen as the catholyte. A concentration of 2,4-DNP no higher than 50 ppm was chosen in order to 6 Page 6 of 29
cover the concentration level a little more than those permitted by government regulations. The temperature of the electrolyte was fixed at 25°C and maintained constant by using a water thermostat; also the stirring rate was kept almost constant (350 ! 50 rpm) by using a magnetic stirrer. The current
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density for the electrolysis (jappl) was kept at the desired level (30 and 50 mA·cm-2) with a Tacussel model PJT24-1 (24V–1A) potentiostat-galvanostat. During the runs, samples of anolyte (5 mL) were
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withdrawn and analyzed for the residual concentration of 2,4-DNP and that of oxidation products in the
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solution. The total time of electrolysis (~ 60 min) was established at the condition of the minimum detectable limit of 2,4-DNP concentration (![2,4-DNP] ! 0). All values of concentration were estimated
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considering the volume collected at each sample. After each run, both the cell and the anode were
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washed thoroughly with doubly (2 - 5 M! cm) and threefold distilled water (16 - 18 M! cm).
2.4 Chemical analysis
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The disappearance of 2,4-DNP during electrolysis was monitored by spectrophotometric measurements
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at wavelength of 261 nm by using a Cary model 50 UV-Vis spectrophotometer. High Performance
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Liquid Chromatography (HPLC) technique was also employed to analyze the presence of both 2,4DNP and oxidation products during the different stages of the electrolysis reaction. For HPLC, a Varian Model 9050/9012 equipped with a C18 Inertsil ODS3 150 × 4.6 mm column and a variable wavelength UV-Vis detector was used. The most suitable mobile phase was Acetonitrile/H2O (50/50) (pH 2.0 with H3PO4) at a flow rate of 1.5 mL min-1 and a column temperature of 35°C. In this case, the identification of the different chemical species in solution was performed by comparing the different UV spectra and chromatographic retention times with commercially available standards. Carboxylic acid and nitrate were identified by using a Bio-RAD Aminex HPX-87H 300x7.8 mm column at a flow rate of 0.05 M H2SO4 mobile phase of 0.8 mL min-1, using a column temperature of 40ºC and an wavelength of 210 nm. The Total Organic Carbon (TOC) data were obtained by using a Merck Model SQ118 spectrophotometer after digestion of samples in a Merck Model TR-300 thermoreactor. 7 Page 7 of 29
3. Results and discussion 3.1 UV spectroscopic characteristics of 2,4-DNP in aqueous media
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UV spectrum of 2,4-DNP exhibits three absorption bands whose wavelength values are shifted towards higher values due to the strong substitution of the benzene ring as well as to the influence of the
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aqueous media used as solvent. In 0.5M H2SO4 + x mg L-1 2,4-DNP solutions the observed wavelength values were 206, 261 and 300 nm, being these values associated to the 'B, 'La and benzenoid absorption
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bands respectively [39, 40]. Figure 1a shows UV spectra of 2,4-DNP as a function of 2,4-DNP
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concentration as well as the corresponding Beer's law applied to each absorption band observed (Figure 1b) and for which the following molar absorptivities (! ) were calculated: ! 300 = 7.1! 103 , ! 261 = 9.6! 103
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and ! 206 = 7.5! 103 L mol-1 cm-1. From these results, we have choose the wavelength at 261 nm as those
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at which quantitative analysis of 2,4-DNP would be performed.
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INSERT FIGURES 1a AND 1b HERE
3.2 Electrochemical oxidation of 2,4-dinitrophenol (2,4-DNP) The decay of 2,4-DNP concentration during the electrolysis at different current densities for the different anodes studied was followed by spectrophotometric and HPLC measurements. Thus, the variation in the 2,4-DNP concentration, as a function of the specific electrical charge (Q = AhL-1), at different current density values, is shown in Figures 2a (30 mA cm-2) and 2b (50 mA cm-2).
INSERT FIGURES 2a AND 2b HERE
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As expected, diverse behaviors were observed for the oxidation of 2,4-DNP at different anodic materials. The oxidation curves reveal a rapid removal of 2,4-DNP at Si/BDD (○), Pb/PbO2 () and Ti/SnO2 () anodes; contrarily for the three-metallic anodes, Ti/Ir3/8Ru1/8Sn1/2O2 () and
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Ti/Ir1/4Ru1/4Sn1/2O2 (×). The effect of the applied current was not significant for Pb/PbO2 and Ti/SnO2 when it increased from 30 to 50 mA cm-2, except for the Si/BDD anodes where a faster 2,4-DNP
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elimination was achieved when current density was increased. For Ti/Ir3/8Ru1/8Sn1/2O2 and
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Ti/Ir1/4Ru1/4Sn1/2O2, an increase in the applied current density produced a decrease in the removal efficiency (see Figure 2a and 2b).
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Since the specific electrical charge Q is proportional to the reaction time, Q = Ct, being C a constant whose value depends on the current density applied and the surface area, then the decay of
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2,4-DNP concentration could be fitted into a kinetic model of pseudo-first order, as already reported by
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d ! 2,4 - DNP! = k app ! 2,4 - DNP! dQ
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-
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Liu et al [41]. Kinetics equations used to estimate the pseudo-first order rate constants are:
(1)
which can be integrated to give
Ln
! 2,4 - DNP! t = Ln 2,4 - DNP = -k Q ! ! rel app ! 2,4 - DNP!o
(2)
where [2,4-DNP]rel is the relative concentration of 2,4-DNP at the time t with respect to the initial time and the kapp is the apparent observed pseudo-first order rate constant. Fitting experimental data with eq. (2) to estimate kapp for the different anodes tested is shown in Table 1.
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INSERT TABLE 1 HERE
According to the values reported in Table 1, the values clearly indicate that degradation rate
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depends on electrocatalytic material used. Based on the existing literature, for active electrodes, such as Ti/Ir3/8Ru1/8Sn1/2O2 and Ti/Ir1/4Ru1/4Sn1/2O2, the production of hydroxyl radicals as well as adsorption of
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organic pollutants during reaction, on the anode surface, plays an important role in the electrochemical oxidation process. In turn, the adsorption stages can limit the rate and efficiency of the total reaction.
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By contrast, the surface on a non-active anode exhibits weak interaction with •OH, enabling direct
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reaction of this radical with the organic pollutants. Thus, at Ti/SnO2; Pb/PbO2 and Si/BDD electrodes, •
OH radicals formed by water electrolysis (H2O ! ! OH + H+ + e-) can be electrochemically converted
+ ! OH ! intermediates ! ! CO2 + H2O).
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into molecular oxygen (! OH ! ½O2 H+ + e-), or contribute to the organic pollutant oxidation (2,4-DNP
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In this context, restricting now our analysis to the reaction rate for the 2,4-DNP oxidation, we
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can inferred two electrocatalytic activity order depending on the applied current density as well anode
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material. Thus, at 30 mAcm-2 the electrocatalytic activity order, for removing 2,4-DNP, was as follow:
Ti/SnO2 > Pb/PbO2 > Si/BDD >> Ti/Ir3/8Ru1/8Sn1/2O2 > Ti/Ir1/4Ru1/4Sn1/2O2
whereas at 50 mAcm-2 the order can be sorted as:
Si/BDD >> Ti/SnO2 ! Pb/PbO2 >> Ti/Ir3/8Ru1/8Sn1/2O2 > Ti/Ir1/4Ru1/4Sn1/2O2
Si/BDD, Pb/PbO2 and Ti/SnO2 are the anodic materials capable to degrade completely 2,4-DNP in acidic media. This outcome was already confirmed by Cañizares et al. [42] and Liu et al. [41] for 10 Page 10 of 29
Si/BDD and Ti/Bi-doped PbO2 anodes, respectively. This behavior can be explained taken into account that Si/BDD, Pb/PbO2 and Ti/SnO2 are considered “non-active anodes” with high oxygen evolution overpotential (2.2, 1.8 and 1.9 V/NHE respectively) [22, 40, 41], producing higher concentrations of
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! OH radicals and other oxidant species.
In the case of Si/BDD, in acid media containing sulphates, the formation of peroxodisulphate
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[21-23, 42, 43] can be attained (equation 3); which contribute in the oxidation of 2,4-DNP. At 30 and
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50 mAcm-2, the production of peroxodisulfates was favored, increasing the elimination rate of 2,4-DNP
(3)
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2SO 2-4 ! S2O82- + 2e - ,
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when an increase in the current density was applied (see Figure 2a and 2b), in both cases.
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For Pb/PbO2 and Ti/SnO2 anodes, 2,4-DNP elimination depends principally on the production of
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! OH radicals and/or 2,4-DNP electrosorption on anode surface, as a pre-requisite for its oxidation. In fact, based on the literature relevant to 2,4-DNP [41, 42], ! OH radical has a strong electrophilic character, and attacks those groups or carbon atom of the aromatic ring with high electron density preferentially. The attack of electrophilic ! OH radical occurs at ring position activated by the presence of two substituents in nitrophenols, such as hydroxyl (-OH) and nitro (-NO2) group [41]. When both these substituents are present, the electrophilic attack will occur preferentially in ortho and para positions with respect to phenolic group. Taking into account the structure of 2,4-DNP, the ! OH radical would become tough owing to the steric effects of the second –NO2 group at the para and metha positions on aromatic ring, respectively. As a consequence, it could be inferred that the structure of 2,4DNP would affect its decomposition in some extent, justifying similar behaviors of Pb/PbO2 and 11 Page 11 of 29
Ti/SnO2 anodes. Therefore, we could also suppose that the adsorption steps, on electrode surface, are determining to complete oxidation for Pb/PbO2 and Ti/SnO2; contrary to the behavior observed at Si/BDD where oxidant species (! OH radicals and peroxodisulfates) contribute to 2,4-DNP degradation
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by release of the nitro group from the aromatic ring [42]. These assumptions can be also confirmed by TOC measurements, it is expected that a complete
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mineralization must be achieved at all anodic materials,
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2C6 H 4 N 2O5 +13O 2 ! 12CO 2 + 4NO-3 + 8H + + 8e - .
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(4)
Figure 3 shows the trend of the normalized values of TOC as a function of the specific electrical charge
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passed at 50 mAcm-2.
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INSERT FIGURES 3
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As it can be observed, for the Si/BDD anode a complete TOC decay was achieved, indicating complete 2,4-DNP mineralization. In the case of Pb/PbO2 and Ti/SnO2 anodes, TOC results show a partial mineralization (! 25%) after 6 AhL-1 of electrical charge passed at 50 mA cm-2. These results indicate that an important percentage of 2,4-DNP was oxidized by ! OH radicals and strong oxidant electrochemically formed on Si/BDD surface, while a partial oxidation was accomplished at Pb/PbO2 and Ti/SnO2 due to the production of several intermediates.
3.3 Degradation of 2,4-DNP by release of the nitro group As mentioned above, the degradation of 2,4-DNP can occur by different steps, including attack of strong oxidants (hydroxyl radicals/peroxodisulfates), substitution of -NO2 groups and electrosorption 12 Page 12 of 29
on anode surface. In this frame, the oxidation of organic compounds containing nitrogen can be monitored in order to determine the production of amonium (NH4+) and nitrate (NO3-) ions, as products of NO2- release [43]. The chemical analysis of nitrate in solution, produced by the mineralization reaction of chemical equation (4), allows to estimate the percentage of total nitrogen contained in the
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2,4-DNP mineralized. Since 1 mg L-1 of 2,4-DNP is equal to a [2,4-DNP] = 0.0054 mM and it
!in %N ! = !NO3 !/mM ! 100 !N!0 /mM -
(5)
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!NO !
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equivalent to a [N] = 0.011 mM, then
where [N]0 = 0.543 mM for a initial concentration of 2,4-DNP of 50 mg L-1. Equation (5) gives values
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of NO3! as percentage of the initial amount of nitrogen present, Fig. 4.
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INSERT FIGURE 4
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Figure 4 shows the NO3- ion production by applying 50 mAcm-2 of current density using different anodic materials. From these results, it can be inferred that, in a first instance, nitrogen is rapidly removed from 2,4-DNP at Si/BDD anode, producing more that 50% of NO3- after 1 AhL-1 of electrical charge passed. In the case of Pb/PbO2 and Ti/SnO2 anodes, under similar experimental conditions, only 20 and 12% of NO3- was produced. However, production of NO3- increased, as a function of time, at these both anodes. Conversely, at Ti/Ir3/8Ru1/8Sn1/2O2 and Ti/Ir1/4Ru1/4Sn1/2O2 anodes, lower concentrations of NO3- were generated, confirming their lower catalytic activity for degrading 2,4DNP.
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Taking into account the figures achieved, the behavior observed at Si/BDD confirms the assertions made above, where the oxidant species (! OH radicals and peroxodisulfates) contribute to release the nitro group from the aromatic ring, allowing its faster degradation, as showed by TOC
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decay. This assertion is also in agreement with the results reported by Cañizares et al. [42]. For Pb/PbO2 and Ti/SnO2 anodes, 2,4-DNP is attacked by ! OH radicals, produced on the anode
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surface, in the aromatic ring. In fact, the release of nitro group from aromatic ring has a moderate rate,
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achieving lower NO3- concentrations respect to Si/BDD anode. The attack of ! OH radicals to the aromatic ring promotes the generation of several intermediates, avoiding the complete elimination of
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organic matter in solution, as confirmed by TOC removal. These assumptions were also confirmed by Liu et al. [42]. In the case of Ti/Ir3/8Ru1/8Sn1/2O2 and Ti/Ir1/4Ru1/4Sn1/2O2 electrodes, the production of
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NO3- was lower, indicating a lower conversion of 2,4-DNP. Nevertheless, the production of different
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intermediates by the substitution of –NO2 group using Si/BDD anodes and by the ! OH radicals attack
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instrumental technique.
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to aromatic ring using Pb/PbO2 and Ti/SnO2 electrodes can be only confirmed by analytical
3.4 Organic by-products formed and proposed degradation mechanisms The formation of intermediate compounds, as a consequence of the 2,4-DNP oxidation, could be complex and dependent on several operational factors, such as anode material, current density, pH, temperature, etc. As a general rule, the oxidation of 2,4-DNP was verified by HPLC analysis of the electrolyzed solutions at 30 and 50 mA cm-2. Identification of different chemical species involved (2,4DNP and oxidation products) was performed by chromatographic comparison against commercially available standards. Two types of organic compounds were mainly identified:
(i) Aromatics: 2,4-dinitro-pyrocatechol, Catechol, 4-nitrocatechol, resorcinol and hydroquinone; 14 Page 14 of 29
(ii) Carboxylic acids: maleic, malic, oxalic, acetic and formic.
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Each type of anode has their own intermediate distribution, but it is important to point out that in the case of Si/BDD anodes, specific organic species were observed at both applied current densities,
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such as 4-nitrocatechol, and oxalic acid. The formation of these intermediates suggests a complete mineralization of 2,4-DNP and, as a result of the release of a nitro group, 4-nitrocatechol was
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generated. The identified by-products in this work are also in agreement with previously reported by Cañizares et al., [42]. These authors identified phenol, quinonic compounds and the release of the nitro
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groups from the aromatic ring in a first step. After that, the transformation to carboxylic acids, mainly
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maleic and oxalic acids, is attained and finally the generation of carbon dioxide was observed at Si/BDD, during electrochemical treatment of 2,4-DNP.
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In the case of Pb/PbO2 and Ti/SnO2 anodes, 2,4-dinitro-pyrocatechol, p-nitrophenol,
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benzoquinone and carboxylic acids were identified (maleic, malonic, oxalic and acetic acid), indicating that the degradation pathway of 2,4-DNP is via ! OH radicals attack in the aromatic ring. This outcome
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was in agreement with the results recently reported by several researchers that considered the addition of hydroxyl group to aromatic rings as a dominating reaction in the first part of the degradation [41, 44, 45]. However, in our work, we have demonstrated that whether denitration or addition of hydroxyl group happened in the first part of degradation depends on the nature of the anode used. In the case of Ti/Ir3/8Ru1/8Sn1/2O2 and Ti/Ir1/4Ru1/4Sn1/2O2 anodes, the formation of polyhydroxylated intermediates was established, but the concentrations were lower; predominating higher concentration of 2,4-DNP. Based on the intermediates identified, we suggest a degradation mechanism depending on anodic material used as shown in Fig. 5. The denitration, in the first stage, occurs at Si/BDD anode. Whereas, substitution by ! OH radicals on aromatic rings, in the first stage, take places when Pb/PbO2 and 15 Page 15 of 29
Ti/SnO2 anodes were employed. The polyhydroxylation would subsequently lead to the opening of aromatic rings to form variety of phenolic compounds and subsequently, carboxylic acids, in both cases. Although, in the case of Ti/Ir3/8Ru1/8Sn1/2O2 and Ti/Ir1/4Ru1/4Sn1/2O2 anodes, formation of polyhydroxylated intermediates, is the degradation pathway followed.
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Taking into account the above information, some questions are also addressed in this work:
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(1) Is it possible that the amount of hydroxyl radicals produced may affect the electrochemical pathway degradation?
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(2) The amount of hydroxyl radicals produced and reactivity of them are different at each
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electrode one studied?
To address these questions, in general, we investigated the production of ! OH radicals for each
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one of the anodes, by spectrophotometry technique by absorbance values, using as bleaching solution the N,N-dimethyl-p-nitrosoaniline (PNDA). The generation of adduct allows to estimate the rate
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anodes.
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constant, for ! OH radicals formation, respect to time (k’) for each one of the electrodes studied as
3.5 Hydroxyl radicals (! OH) production and their effect on 2,4-DNP oxidation The production of free ! OH radicals on Pb/PbO2, Ti/SnO2, Ti/IrxRuySnO2 and Si/BDD electrodes was confirmed by the bleaching of N,N-dimethyl-p-nitrosoaniline (PNDA) solutions. PNDA was spectrophotometrically traced at 440 nm in 0.1 M (pH = 9.5) borax buffer. From calibration by using the Beer’s Law (Absorbance vs [PNDA] with r2 = 0.9975) a molar absorptivity of ! 440 = 33580 M-1cm-1 was obtained. Based on the results obtained from the PNDA-OH radicals trapping, the value of relative ! OH radicals’ production rate (k’), respect to time, was estimated; for each one of the anodes studied (assuming a pseudo first-order reaction, as already proposed by Liu et al. [42]) (Table 2). For Si/BDD anodes, the production of peroxodissulfates was also quantified and confirmed at 30 and 50 mAcm-2. 16 Page 16 of 29
INSERT TABLE 2 HERE
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From these PNDA-OH radicals trapping formation data it is possible to point out that a variation from 30 to 50 mA cm-2 causes a remarkable improvement on the ! OH radicals’ production rate. For
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Pb/PbO2, this production rate is more effective than the other materials; while for Ti/SnO2 and Si/BDD anodes these rates are very similar when the applied current density was increased. However, in the
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case of Si/BDD anode the production of peroxodisulfates was confirmed, when an increase on applied
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current density was attained; and consequently, this reaction competes with the production of ! OH radicals. With these results, we cannot be certain whether the nitro group release took place by ! OH
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radicals substitution or alternatively favored by peroxodisulfate oxidation of 2,4-DNP to a radical cation and after that, nucleophilic attack by ! OH radical. However, as already confirmed by HPLC
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measurements, the radical cation mechanism could be an alternative elimination of –NO2 group from
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2,4-DNP at Si/BDD anode where on its surface is Si/BDD(! OH/S2O82-).
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The electrochemical values of k’ obtained at Ti/SnO2 and Pb/PbO2 anodes, is significantly higher than that achieved at Si/BDD. These figures suggest either that the electrochemical species produced are stable (less reactive) compared with Si/BDD(! OH) or more likely, that the electrochemical oxidation of 2,4-DNP occurs by the proximity of it towards anode surface containing with higher concentrations of ! OH radicals, favoring hydroxyl radical attack in ortho position (as already confirmed by HPLC measurements). From a thermodynamic point of view, the substitution of –NO2 group requires more energy; then, the SnO2(! OH) and PbO2(! OH) species should be less reactive and more abundant on the anode surface than the analogous BDD(! OH) species. However, the BDD(! OH) could be considered more reactive
17 Page 17 of 29
and stable, close to anode surface and in the reaction cage [47, 48]. This hypothesis is consistent with our conclusions on the degradation of 2,4-DNP.
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4. CONCLUSIONS Electrochemical oxidation of 2,4-DNP was achieved at Si/BDD, Ti/SnO2 and Pb/PbO2 anodes;
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obtaining higher removal efficiencies at Si/BDD anode. The high chemical reactivity of these anodes for organics oxidation is the result of a weak electrode-hydroxyl species interaction and, consequently,
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a low electrochemical activity for the oxygen evolution reaction. By the contrary, for Ti/IrxRuySnO2
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anodes, the degradation of 2,4-DNP was lower due, fundamentally, to a strong electrode-hydroxyl species interaction resulting in high electrochemical activity for the oxygen evolution reaction and low
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chemical reactivity for organics oxidation (low current efficiency for organics oxidation). This behavior could be due to the preparation method and Ir, Sn and Ru proportion within metallic lattice.
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Based on the results obtained, the production of free hydroxyl radicals and strong oxidants
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promotes for a complete degradation of 2,4-DNP at Si/BDD anode; while for Ti/SnO2 and Pb/PbO2, production of hydroxyl radicals and the adsorption oxidation steps are determining factors for complete
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degradation (as recently indicated by Comninellis [46]). Finally, depending on the nature of the material and experimental conditions, the denitration or hydroxyl radicals attack of aromatic rings seem to be the first stage on 2,4-DNP oxidation.
Acknowledgments This work was supported jointly by the Consejo Nacional de Ciencia y Tecnología (CONACYT) México (Project No. 156200 CB-2010-01) and Dirección de Investigación y Posgrado from VAUDLAP, México (Convocatoria 2013).
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Angew. Chem. Int. Ed., 47 (2008) 1998-2005. [27] S. Stucki, R. Kotz, B. J. Carcer, W. Suter, Electrochemical waste water treatment using high overvoltage anodes Part II: Anode performance and applications, J. Appl. Electrochem., 21 (1991) 99-104.
[28] A. M. Polcaro, S. Palmas, S. Dernini, Electrochemical removal of organic pollutants from wastes water, Current Topics in Electrochemistry, 4 (1997) 137-146. [29] J. Iniesta, J. González-García, E. Expósito, V. Montiel, A. Aldaz, Influence of chloride ion on electrochemical degradation of phenol in alkaline medium using bismuth doped and pure PbO2 anodes, Water Res., 35 (2001) 3291-3300.
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simultaneous oxygen evolution, J. Chim. Phys., 93 (1996) 673-679. [36] B. Correa-Lozano, Ch. Comninellis, A. De Battisti, Electrochemical properties of Ti/SnO2-Sb2O5 electrodes prepared by the spray pyrolysis technique, J. Appl. Electrochem., 26 (1996) 683-688. [37] A. Morozov, A. De Battisti, S. Ferro, G. N. Martelli, European Patent WO2005/014885 A1 (2005).
[38] S. Ferro, Synthesis of diamond, J. Mat. Chem., 12 (2002) 2843-2855. [39] E. F. H. Brittain, W.O. George, C. N. J. Well, Introduction to molecular spectroscopy theory and experiment, Academic Press, New York, 1970. [40] A. E. Gillam, E. S. Stern, Electronic absorption spectroscopy in organic chemistry, Edward APNDAld, London, 1970. 22 Page 22 of 29
[41] Y. Liu, H. Liu, J. Ma, X. Wang, Comparison of degradation mechanism of electrochemical oxidation of di- and tri-nitrophenols on Bi-doped lead dioxide electrode: Effect of the molecular structure, Appl. Catal. B: Environ., 91 (2009) 284-299.
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[43] A. M. Sales Solano, C. K. C. de Araújo, J. Vieira de Melo, J. M. Peralta-Hernandez, D. Ribeiro da Silva a, C. A. Martínez-Huitle, Decontamination of real textile industrial effluent by strong
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oxidant species electrogenerated on diamond electrode: Viability and disadvantages of this electrochemical technology, Appl. Catal. B: Environ. 130–131 (2013) 112-120.
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[44] K. Tanaka, W. Luesaiwong, T. Hisanaga, Photocatalytic degradation of mono-, di- and
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trinitrophenol in aqueous TiO2 suspension, J. Mol. Catal. A: Chem., 122 (1999) 67-74. [45] B. Sangchakr, T. Hisanaga, K. Tanaka, Photocatalytic degradation of sulfonated aromatics in
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[46] Ch. Comninellis, G. Chen, (Eds.) Electrochemistry for the Environment, 2010, Springer, NY,
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[47] M. Panizza, C.A. Martinez-Huitle, Role of electrode materials for the anodic oxidation of a real landfill leachate - Comparison between Ti-Ru-Sn ternary oxide, PbO2 and boron-doped diamond anode, Chemosphere 90 (2013) 1455-1460. [48] J.H.B. Rocha, A.M.S. Solano, N.S. Fernandes, D.R. da Silva, J.M. Peralta-Hernandez, C.A. Martínez-Huitle, Electrochemical degradation of Remazol Red BR and Novacron Blue C-D dyes using diamond electrode, Electrocatalysis 3 (2012) 1-12.
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TABLES
Table 1. Rate constant for the 2,4-DNP oxidation in acidic media at 298 K.
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Anode Pb/PbO2 Ti/SnO2 Ti/Ir3/8Ru1/8Sn1/2O2 Ti/Ir1/4Ru1/4Sn1/2O2 Si/BDD
kapp (A-1Lh-1) 30 mAcm-2 50 mAcm-2 0.772 0.798 0.904 0.818 0.085 0.059 0.071 0.048 0.481 2.345
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Respect to reaction time (k’ x 102 (min-1)) Production of S2O82- (mmol L-1) 30 mAcm-2 50 mAcm-2 30 mAcm-2 50 mAcm-2 1.89 5.37 3.61 3.11 1.49 1.08 0.99 0.30 1.22 1.44 0.89 3.95
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Anode Pb/PbO2 Ti/SnO2 Ti/Ir3/8Ru1/8Sn1/2O2 Ti/Ir1/4Ru1/4Sn1/2O2 Si/BDD
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Table 2. Rate constant for the PNDA bleaching reaction in pH = 9.5 borax buffer at 298 K.
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FIGURE CAPTIONS
Figure 1. (a) UV absorption spectra of 0.5 M H2SO4 + x mg L-1 2,4-DNP solutions as a function of 2,4-
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DNP concentration, 0 < x ! 50 mg L-1 . (b) Beer's law applied to each absorption bands observed: , 300 nm; , 261 nm; , 206 nm.
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Figure 2. Plot of % [2,4-DNP]rel vs. specific electrical charge passed (AhL-1) in 0.5 M H2SO4 solution
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at: (a) 30 mAcm-2 and (b) 50 mAcm-2. % [2,4-DNP]rel = ([2,4-DNP]t/[2,4-DNP]0)! 100. Anode material: ○ BDD, PbO2, SnO2, Ir3/8Ru1/8Sn1/2O2, × Ir1/4Ru1/4Sn1/2O2. The variation of Ln [2,4-
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DNP]rel with specific electrical charge passed (AhL-1) on different anodes is depicted in the inset plot.
Figure 3. Plot of %TOC (as a percentage of the normalized value: (TOCt/TOC0)! 100) as a function of
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the specific electrical charge passed (AhL-1) in 0.5 M H2SO4 solution at 50 mAcm-2 . Anode material:
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BDD, PbO2, SnO2, Ir3/8Ru1/8Sn1/2O2, × Ir1/4Ru1/4Sn1/2O2.
Figure 4. Trend in specific electrical charge (AhL-1) of nitrate ion concentration as percentage of the initial amount of nitrogen present. Anode material: BDD, PbO2, SnO2, Ir3/8Ru1/8Sn1/2O2, × Ir1/4Ru1/4Sn1/2O2.
Figure 5. Proposed scheme of 2,4-DNP degradation depending on electrocatalytic material used.
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FIGURES
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Figure 1 (b)
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Figure 1 (a)
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Figure 2 (a)
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Figure 2 (b)
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S0304-3894(13)00982-5 http://dx.doi.org/doi:10.1016/j.jhazmat.2013.12.050 HAZMAT 15643
To appear in:
Journal of Hazardous Materials
Received date: Revised date: Accepted date:
31-7-2013 11-12-2013 24-12-2013
Please cite this article as: M.A. Quiroz, J.L. S´anchez-Salas, S. Reyna, E.R. Bandala, J.M. Peralta-Hern´andez, C.A. Mart´inez-Huitle, Degradation of 1-hydroxy-2,4-dinitrobenzene from aqueous solutions by electrochemical oxidation: role of anodic material, Journal of Hazardous Materials (2014), http://dx.doi.org/10.1016/j.jhazmat.2013.12.050 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.
Degradation of 1-hydroxy-2,4-dinitrobenzene from aqueous solutions by electrochemical oxidation: role of anodic material
Marco A. Quiroz1, José L. Sánchez-Salas1, Silvia Reyna1, Erick R. Bandala1,
Universidad de las Américas Puebla. Grupo de Investigación en Energía y Ambiente. ExHda. Sta.
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1
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Juan M. Peralta-Hernández2, Carlos A. Martínez-Huitle3,*
Catarina Martir s/n, Cholula 72820, Puebla, México. Centro de Innovación Aplicada en Tecnologías Competitivas, Departamento de Investigación
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2
Ambiental Omega-201, Fraccionamiento Industrial Delta. León, 37545, Guanajuato, México. Universidade Federal do Rio Grande do Norte, CCET – Institute of Chemistry, Lagoa Nova - CEP
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3
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59.072-970 – Natal, RN; Brazil.
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E-mail address: [email protected]
1 Page 1 of 29
Highlights
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Degradation efficiency and pathways depend on the nature of electrode material.
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Denitration and substitution by •OH radicals on the aromatic rings is the first proposed step in reaction mechanism.
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ABSTRACT
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Diamond anodes generate a minor number of intermediates from 2,4-dinitrophenol oxidation.
Electrochemical oxidation (ECOx) of 1-hydroxy-2,4-dinitrobenzene (or 2,4-dinitrophenol: 2,4-DNP) in
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aqueous solutions by electrolysis under galvanostatic control was studied at Pb/PbO2, Ti/SnO2, Ti/IrxRuySnO2 and Si/BDD anodes as a function of current density applied. Oxidative degradation of
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2,4-DNP has clearly shown that electrode material as well as the current density applied were important
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parameters to optimize the oxidation process. It was observed that 2,4-DNP was oxidized at few
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substrates to CO2 with different results, obtaining good removal efficiencies at Pb/PbO2, Ti/SnO2 and Si/BDD anodes. Trends in degradation way depend on the production of hydroxyl radicals (! OH) on these anodic materials, as confirmed in this study. Furthermore, HPLC results suggested that two kinds of intermediates were generated, polyhydroxylated intermediates and carboxylic acids. The formation of these polyhydroxylated intermediates seems to be associated with the denitration step and substitution by ! OH radicals on aromatic rings, being this first proposed step in reaction mechanism. These compounds were successively oxidized, followed by the opening of aromatic rings and the formation of a series of carboxylic acids which were at the end oxidized into CO2 and H2O. On the basis of these information, a reaction scheme was proposed for each type of anode used for 2,4-D oxidation. 2 Page 2 of 29
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Keywords: hydroxyl radicals, electrode activity, 2,4-dinitrophenol, electro-oxidation.
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1. Introduction
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Persistent Organic Pollutants (POPs) are highly stable and toxic compounds with high levels of permanence in the environment, great atmospheric mobility and accumulative capacity in fatty tissue of
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most living organisms [1, 2]. These kinds of pollutants result from the use of chemicals in industry or
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as by-products of combustion. The presence of different types of POPs in water, constitutes one of the
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most important problems in environmental sciences today [3, 4], fundamentally as result of their widely known effects on human health and wildlife. Nitroaromatic compounds, in particular, can be formed by photochemical atmospheric reactions owing the presence of nitrogen oxides (NOX) in the industrial and automotive gases emission [5, 6]. They are also widely used in pesticide production, paints and explosive material and have been detected not only in industrial wastewaters but also in freshwater and marine environments [7, 8], and are considered by the United States Environmental Protection Agency (USEPA) as priority pollutants owing to their toxicity to humans [9, 10]. Detoxification of water contaminated with nitroaromatic compounds is usually a very difficult process since the presence of nitro group confers to the aromatic compound a strong chemical stability and resistance to microbial degradation [11]. Lypczynska-Kochany [7] has reported experimental 3 Page 3 of 29
conditions for the biodegradation of nitrophenol compounds (NPCs) where long range of incubation was required for the reduction of the nitro group. Bhatti et al., [12] reported a biological reactor where the use of polyester nonwovens for bacterial attachment significantly improved the biological
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degradation of NPCs. Nevertheless, long degradation time was required (11 h) as well as the use of a previous long biomass acclimation time (5-6 months) and the use of a supplementary substrate. More
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recently, Tomei et al. [13] has reported that the time and removal rate of NPCs biodegradation may be enhanced if the biodegradation process is carried out in a sequencing batch reactor. However, the
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mineralization process reported in this last work is not clear at all since the reported nitrite and nitrate formation and the absence of hydroquinone in the effluent are not sufficient evidence for a complete
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conversion of the NPCs to inorganic nitrogen species, CO2 and water. Indeed, the paper by Tomei et al.
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[13] offers a good alternative for the treatment of residual water containing NPCs even though the process continues being dependent on the bacterial culture and the acclimatization time.
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Nitroaromatic compounds in aqueous media has been also degraded by using irradiation sources
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with or without chemical mediators such as H2O2, Fe(II) and/or Fe(III) salts, ozone and TiO2 catalysts [14-17]. Thus, for nitrobenzene degradation in acidic media the best mineralization condition has been
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obtained by combination of solar light and iron salts (photo-Fenton process) [18]. Other photomediator combinations are also possible but the degradation efficiency is often affected due to competition effects between the chemical mediator and the chemical compound target of the degradation process.
A good alternative to biological, chemical and photochemical methods for the elimination of biorefractory organic pollutants in aqueous media (drinking water or wastewater) is the electrochemical oxidation (ECOx) [19-26] by using metallic oxides or boron-doped diamond (BDD), as anodes. These materials have shown high performance levels to the conversion and/or combustion of a wide variety of aromatic compounds. In the conversion process the aromatic compounds are only transformed to biocompatible compounds in order to they can be adequately eliminated in a subsequent biological 4 Page 4 of 29
treatment. The oxidation mechanism proceeds through an introduction of oxygen step into the oxide lattice which results in conversion of organic and a change of the oxidation number of metal (such as IrO2 and RuO2 for instance). On the other hand, the electrochemical combustion yields CO2 and H2O
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by direct surface oxidation of the undesired aromatic compounds and, therefore, the residual solution does not require further purification. In this last case, the accumulation of ! OH radicals on the electrode
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surface is the determinant factor for the electrochemical combustion. Anodic materials with these
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characteristics are SnO2 and PbO2 with or without doping species [20, 27-31].
Therefore, the aim of this work is to evaluate the electrocatalytic activity of PbO2, SnO2,
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IrxRuySnO2 and BDD as anodic materials to eliminate 2,4-DNP in aqueous acidic media at applied current density values in order to understand the role of the nature of electrode material, their ability to
2.1 Chemicals
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2. Materials and methods
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produce ! OH radicals and the more probable mechanism by which 2,4-DNP is degraded.
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Ultrapure water (18 M! ) was obtained by using a Simplicity water purification system. Chemicals were of the highest quality commercially available, and were used without further purification. 2,4DNP (puriss. p.a. ! 99.5%), N,N-dimethyl-p-nitrosoaniline (PNDA) (purum p.a. ! 98.0%) and H2SO4 (puriss. p.a. 95-97%) were purchased from Fluka. The PNDA concentration was 2×10-5 M and that of model organic compound 2,4-DNP was 50 mg L-1 (50 ppm), both of them in 0.5 M H2SO4 solution as supporting electrolyte.
2.2 Electrode materials The Pb/PbO2 electrode was prepared growing the anodic oxide at a current density of 50 mA cm-2 during an electrolysis time of 1.5 h in a 10% H2SO4 solution at 25 °C, in order to oxidize the lead 5 Page 5 of 29
surface into PbO2. More details concerning anode preparation and characterization are given elsewhere [32-34]. The SnO2-coated titanium (Ti/SnO2) electrodes [35, 36] were prepared by a sol-gel technique,
titanium base and drying at 50°C. The whole process was repeated 10 times.
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which consisted of the following steps: preparation of precursor solution, its brushing onto a pre-treated
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The Ti/IrxRuySnO2 electrodes were prepared as follows: a SnCl4/RuCl3 solution and a SnCl4/IrCl3 solution, both prepared with a tin mole ratio of 66%, were mixed in different proportions to render
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Ir:Ru ratios of 1:1, 2:1 and 3:1. A layer of each mixture was painted on a titanium support, dried for 10 minutes at 120°C and annealed for 25 minutes at 500°C; this procedure was repeated 11 times. Thus,
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solid solutions of the three metals were obtained by thermal decomposition. The electrode area was 10
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cm2. More details concerning anode preparation and characterization are given elsewhere [37]. BDD films were provided by CSEM (Neuchâtel, Switzerland) and synthetized on a conductive p-
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Si substrate (1 mm, Siltronix) via a hot filament using the chemical vapor deposition technique (HF-
te
CVD) [38]. This procedure gave a columnar, randomly textured, polycrystalline diamond film, with a thickness of about 1 µm and a resistivity of 15 m! cm (! 30%) onto the conductive p-Si substrate.
Ac ce p
Boron doping level was about 5000 ppm.
2.3 Electrochemical measurements
Bulk oxidations were performed in a two-compartment electrochemical cell, the reaction compartment having a capacity of 250 mL. A schematic drawing of the experimental apparatus has been previously reported [34]. An ultra-high-purity graphite rod (EG&G PARC), enclosed in a 10 mL porous porcelain pot, was used as the counter electrode. Pb/PbO2, Ti/SnO2, Ti/IrxRuySnO2 and Si/BDD were used as the working electrode and a Hg/Hg2SO4/K2SO4(sat) was used, as the reference electrode. The anolyte consisted of 50 ppm of 2,4-DNP in 0.5 M H2SO4 solution, while the pure supporting electrolyte was chosen as the catholyte. A concentration of 2,4-DNP no higher than 50 ppm was chosen in order to 6 Page 6 of 29
cover the concentration level a little more than those permitted by government regulations. The temperature of the electrolyte was fixed at 25°C and maintained constant by using a water thermostat; also the stirring rate was kept almost constant (350 ! 50 rpm) by using a magnetic stirrer. The current
ip t
density for the electrolysis (jappl) was kept at the desired level (30 and 50 mA·cm-2) with a Tacussel model PJT24-1 (24V–1A) potentiostat-galvanostat. During the runs, samples of anolyte (5 mL) were
cr
withdrawn and analyzed for the residual concentration of 2,4-DNP and that of oxidation products in the
us
solution. The total time of electrolysis (~ 60 min) was established at the condition of the minimum detectable limit of 2,4-DNP concentration (![2,4-DNP] ! 0). All values of concentration were estimated
an
considering the volume collected at each sample. After each run, both the cell and the anode were
M
washed thoroughly with doubly (2 - 5 M! cm) and threefold distilled water (16 - 18 M! cm).
2.4 Chemical analysis
d
The disappearance of 2,4-DNP during electrolysis was monitored by spectrophotometric measurements
te
at wavelength of 261 nm by using a Cary model 50 UV-Vis spectrophotometer. High Performance
Ac ce p
Liquid Chromatography (HPLC) technique was also employed to analyze the presence of both 2,4DNP and oxidation products during the different stages of the electrolysis reaction. For HPLC, a Varian Model 9050/9012 equipped with a C18 Inertsil ODS3 150 × 4.6 mm column and a variable wavelength UV-Vis detector was used. The most suitable mobile phase was Acetonitrile/H2O (50/50) (pH 2.0 with H3PO4) at a flow rate of 1.5 mL min-1 and a column temperature of 35°C. In this case, the identification of the different chemical species in solution was performed by comparing the different UV spectra and chromatographic retention times with commercially available standards. Carboxylic acid and nitrate were identified by using a Bio-RAD Aminex HPX-87H 300x7.8 mm column at a flow rate of 0.05 M H2SO4 mobile phase of 0.8 mL min-1, using a column temperature of 40ºC and an wavelength of 210 nm. The Total Organic Carbon (TOC) data were obtained by using a Merck Model SQ118 spectrophotometer after digestion of samples in a Merck Model TR-300 thermoreactor. 7 Page 7 of 29
3. Results and discussion 3.1 UV spectroscopic characteristics of 2,4-DNP in aqueous media
ip t
UV spectrum of 2,4-DNP exhibits three absorption bands whose wavelength values are shifted towards higher values due to the strong substitution of the benzene ring as well as to the influence of the
cr
aqueous media used as solvent. In 0.5M H2SO4 + x mg L-1 2,4-DNP solutions the observed wavelength values were 206, 261 and 300 nm, being these values associated to the 'B, 'La and benzenoid absorption
us
bands respectively [39, 40]. Figure 1a shows UV spectra of 2,4-DNP as a function of 2,4-DNP
an
concentration as well as the corresponding Beer's law applied to each absorption band observed (Figure 1b) and for which the following molar absorptivities (! ) were calculated: ! 300 = 7.1! 103 , ! 261 = 9.6! 103
M
and ! 206 = 7.5! 103 L mol-1 cm-1. From these results, we have choose the wavelength at 261 nm as those
d
at which quantitative analysis of 2,4-DNP would be performed.
Ac ce p
te
INSERT FIGURES 1a AND 1b HERE
3.2 Electrochemical oxidation of 2,4-dinitrophenol (2,4-DNP) The decay of 2,4-DNP concentration during the electrolysis at different current densities for the different anodes studied was followed by spectrophotometric and HPLC measurements. Thus, the variation in the 2,4-DNP concentration, as a function of the specific electrical charge (Q = AhL-1), at different current density values, is shown in Figures 2a (30 mA cm-2) and 2b (50 mA cm-2).
INSERT FIGURES 2a AND 2b HERE
8 Page 8 of 29
As expected, diverse behaviors were observed for the oxidation of 2,4-DNP at different anodic materials. The oxidation curves reveal a rapid removal of 2,4-DNP at Si/BDD (○), Pb/PbO2 () and Ti/SnO2 () anodes; contrarily for the three-metallic anodes, Ti/Ir3/8Ru1/8Sn1/2O2 () and
ip t
Ti/Ir1/4Ru1/4Sn1/2O2 (×). The effect of the applied current was not significant for Pb/PbO2 and Ti/SnO2 when it increased from 30 to 50 mA cm-2, except for the Si/BDD anodes where a faster 2,4-DNP
cr
elimination was achieved when current density was increased. For Ti/Ir3/8Ru1/8Sn1/2O2 and
us
Ti/Ir1/4Ru1/4Sn1/2O2, an increase in the applied current density produced a decrease in the removal efficiency (see Figure 2a and 2b).
an
Since the specific electrical charge Q is proportional to the reaction time, Q = Ct, being C a constant whose value depends on the current density applied and the surface area, then the decay of
M
2,4-DNP concentration could be fitted into a kinetic model of pseudo-first order, as already reported by
te
d ! 2,4 - DNP! = k app ! 2,4 - DNP! dQ
Ac ce p
-
d
Liu et al [41]. Kinetics equations used to estimate the pseudo-first order rate constants are:
(1)
which can be integrated to give
Ln
! 2,4 - DNP! t = Ln 2,4 - DNP = -k Q ! ! rel app ! 2,4 - DNP!o
(2)
where [2,4-DNP]rel is the relative concentration of 2,4-DNP at the time t with respect to the initial time and the kapp is the apparent observed pseudo-first order rate constant. Fitting experimental data with eq. (2) to estimate kapp for the different anodes tested is shown in Table 1.
9 Page 9 of 29
INSERT TABLE 1 HERE
According to the values reported in Table 1, the values clearly indicate that degradation rate
ip t
depends on electrocatalytic material used. Based on the existing literature, for active electrodes, such as Ti/Ir3/8Ru1/8Sn1/2O2 and Ti/Ir1/4Ru1/4Sn1/2O2, the production of hydroxyl radicals as well as adsorption of
cr
organic pollutants during reaction, on the anode surface, plays an important role in the electrochemical oxidation process. In turn, the adsorption stages can limit the rate and efficiency of the total reaction.
us
By contrast, the surface on a non-active anode exhibits weak interaction with •OH, enabling direct
an
reaction of this radical with the organic pollutants. Thus, at Ti/SnO2; Pb/PbO2 and Si/BDD electrodes, •
OH radicals formed by water electrolysis (H2O ! ! OH + H+ + e-) can be electrochemically converted
+ ! OH ! intermediates ! ! CO2 + H2O).
M
into molecular oxygen (! OH ! ½O2 H+ + e-), or contribute to the organic pollutant oxidation (2,4-DNP
d
In this context, restricting now our analysis to the reaction rate for the 2,4-DNP oxidation, we
te
can inferred two electrocatalytic activity order depending on the applied current density as well anode
Ac ce p
material. Thus, at 30 mAcm-2 the electrocatalytic activity order, for removing 2,4-DNP, was as follow:
Ti/SnO2 > Pb/PbO2 > Si/BDD >> Ti/Ir3/8Ru1/8Sn1/2O2 > Ti/Ir1/4Ru1/4Sn1/2O2
whereas at 50 mAcm-2 the order can be sorted as:
Si/BDD >> Ti/SnO2 ! Pb/PbO2 >> Ti/Ir3/8Ru1/8Sn1/2O2 > Ti/Ir1/4Ru1/4Sn1/2O2
Si/BDD, Pb/PbO2 and Ti/SnO2 are the anodic materials capable to degrade completely 2,4-DNP in acidic media. This outcome was already confirmed by Cañizares et al. [42] and Liu et al. [41] for 10 Page 10 of 29
Si/BDD and Ti/Bi-doped PbO2 anodes, respectively. This behavior can be explained taken into account that Si/BDD, Pb/PbO2 and Ti/SnO2 are considered “non-active anodes” with high oxygen evolution overpotential (2.2, 1.8 and 1.9 V/NHE respectively) [22, 40, 41], producing higher concentrations of
cr
ip t
! OH radicals and other oxidant species.
In the case of Si/BDD, in acid media containing sulphates, the formation of peroxodisulphate
us
[21-23, 42, 43] can be attained (equation 3); which contribute in the oxidation of 2,4-DNP. At 30 and
an
50 mAcm-2, the production of peroxodisulfates was favored, increasing the elimination rate of 2,4-DNP
(3)
d
2SO 2-4 ! S2O82- + 2e - ,
M
when an increase in the current density was applied (see Figure 2a and 2b), in both cases.
te
For Pb/PbO2 and Ti/SnO2 anodes, 2,4-DNP elimination depends principally on the production of
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! OH radicals and/or 2,4-DNP electrosorption on anode surface, as a pre-requisite for its oxidation. In fact, based on the literature relevant to 2,4-DNP [41, 42], ! OH radical has a strong electrophilic character, and attacks those groups or carbon atom of the aromatic ring with high electron density preferentially. The attack of electrophilic ! OH radical occurs at ring position activated by the presence of two substituents in nitrophenols, such as hydroxyl (-OH) and nitro (-NO2) group [41]. When both these substituents are present, the electrophilic attack will occur preferentially in ortho and para positions with respect to phenolic group. Taking into account the structure of 2,4-DNP, the ! OH radical would become tough owing to the steric effects of the second –NO2 group at the para and metha positions on aromatic ring, respectively. As a consequence, it could be inferred that the structure of 2,4DNP would affect its decomposition in some extent, justifying similar behaviors of Pb/PbO2 and 11 Page 11 of 29
Ti/SnO2 anodes. Therefore, we could also suppose that the adsorption steps, on electrode surface, are determining to complete oxidation for Pb/PbO2 and Ti/SnO2; contrary to the behavior observed at Si/BDD where oxidant species (! OH radicals and peroxodisulfates) contribute to 2,4-DNP degradation
ip t
by release of the nitro group from the aromatic ring [42]. These assumptions can be also confirmed by TOC measurements, it is expected that a complete
cr
mineralization must be achieved at all anodic materials,
us
2C6 H 4 N 2O5 +13O 2 ! 12CO 2 + 4NO-3 + 8H + + 8e - .
an
(4)
Figure 3 shows the trend of the normalized values of TOC as a function of the specific electrical charge
M
passed at 50 mAcm-2.
te
d
INSERT FIGURES 3
Ac ce p
As it can be observed, for the Si/BDD anode a complete TOC decay was achieved, indicating complete 2,4-DNP mineralization. In the case of Pb/PbO2 and Ti/SnO2 anodes, TOC results show a partial mineralization (! 25%) after 6 AhL-1 of electrical charge passed at 50 mA cm-2. These results indicate that an important percentage of 2,4-DNP was oxidized by ! OH radicals and strong oxidant electrochemically formed on Si/BDD surface, while a partial oxidation was accomplished at Pb/PbO2 and Ti/SnO2 due to the production of several intermediates.
3.3 Degradation of 2,4-DNP by release of the nitro group As mentioned above, the degradation of 2,4-DNP can occur by different steps, including attack of strong oxidants (hydroxyl radicals/peroxodisulfates), substitution of -NO2 groups and electrosorption 12 Page 12 of 29
on anode surface. In this frame, the oxidation of organic compounds containing nitrogen can be monitored in order to determine the production of amonium (NH4+) and nitrate (NO3-) ions, as products of NO2- release [43]. The chemical analysis of nitrate in solution, produced by the mineralization reaction of chemical equation (4), allows to estimate the percentage of total nitrogen contained in the
ip t
2,4-DNP mineralized. Since 1 mg L-1 of 2,4-DNP is equal to a [2,4-DNP] = 0.0054 mM and it
!in %N ! = !NO3 !/mM ! 100 !N!0 /mM -
(5)
an
3
us
!NO !
cr
equivalent to a [N] = 0.011 mM, then
where [N]0 = 0.543 mM for a initial concentration of 2,4-DNP of 50 mg L-1. Equation (5) gives values
d
M
of NO3! as percentage of the initial amount of nitrogen present, Fig. 4.
te
INSERT FIGURE 4
Ac ce p
Figure 4 shows the NO3- ion production by applying 50 mAcm-2 of current density using different anodic materials. From these results, it can be inferred that, in a first instance, nitrogen is rapidly removed from 2,4-DNP at Si/BDD anode, producing more that 50% of NO3- after 1 AhL-1 of electrical charge passed. In the case of Pb/PbO2 and Ti/SnO2 anodes, under similar experimental conditions, only 20 and 12% of NO3- was produced. However, production of NO3- increased, as a function of time, at these both anodes. Conversely, at Ti/Ir3/8Ru1/8Sn1/2O2 and Ti/Ir1/4Ru1/4Sn1/2O2 anodes, lower concentrations of NO3- were generated, confirming their lower catalytic activity for degrading 2,4DNP.
13 Page 13 of 29
Taking into account the figures achieved, the behavior observed at Si/BDD confirms the assertions made above, where the oxidant species (! OH radicals and peroxodisulfates) contribute to release the nitro group from the aromatic ring, allowing its faster degradation, as showed by TOC
ip t
decay. This assertion is also in agreement with the results reported by Cañizares et al. [42]. For Pb/PbO2 and Ti/SnO2 anodes, 2,4-DNP is attacked by ! OH radicals, produced on the anode
cr
surface, in the aromatic ring. In fact, the release of nitro group from aromatic ring has a moderate rate,
us
achieving lower NO3- concentrations respect to Si/BDD anode. The attack of ! OH radicals to the aromatic ring promotes the generation of several intermediates, avoiding the complete elimination of
an
organic matter in solution, as confirmed by TOC removal. These assumptions were also confirmed by Liu et al. [42]. In the case of Ti/Ir3/8Ru1/8Sn1/2O2 and Ti/Ir1/4Ru1/4Sn1/2O2 electrodes, the production of
M
NO3- was lower, indicating a lower conversion of 2,4-DNP. Nevertheless, the production of different
d
intermediates by the substitution of –NO2 group using Si/BDD anodes and by the ! OH radicals attack
Ac ce p
instrumental technique.
te
to aromatic ring using Pb/PbO2 and Ti/SnO2 electrodes can be only confirmed by analytical
3.4 Organic by-products formed and proposed degradation mechanisms The formation of intermediate compounds, as a consequence of the 2,4-DNP oxidation, could be complex and dependent on several operational factors, such as anode material, current density, pH, temperature, etc. As a general rule, the oxidation of 2,4-DNP was verified by HPLC analysis of the electrolyzed solutions at 30 and 50 mA cm-2. Identification of different chemical species involved (2,4DNP and oxidation products) was performed by chromatographic comparison against commercially available standards. Two types of organic compounds were mainly identified:
(i) Aromatics: 2,4-dinitro-pyrocatechol, Catechol, 4-nitrocatechol, resorcinol and hydroquinone; 14 Page 14 of 29
(ii) Carboxylic acids: maleic, malic, oxalic, acetic and formic.
ip t
Each type of anode has their own intermediate distribution, but it is important to point out that in the case of Si/BDD anodes, specific organic species were observed at both applied current densities,
cr
such as 4-nitrocatechol, and oxalic acid. The formation of these intermediates suggests a complete mineralization of 2,4-DNP and, as a result of the release of a nitro group, 4-nitrocatechol was
us
generated. The identified by-products in this work are also in agreement with previously reported by Cañizares et al., [42]. These authors identified phenol, quinonic compounds and the release of the nitro
an
groups from the aromatic ring in a first step. After that, the transformation to carboxylic acids, mainly
M
maleic and oxalic acids, is attained and finally the generation of carbon dioxide was observed at Si/BDD, during electrochemical treatment of 2,4-DNP.
d
In the case of Pb/PbO2 and Ti/SnO2 anodes, 2,4-dinitro-pyrocatechol, p-nitrophenol,
te
benzoquinone and carboxylic acids were identified (maleic, malonic, oxalic and acetic acid), indicating that the degradation pathway of 2,4-DNP is via ! OH radicals attack in the aromatic ring. This outcome
Ac ce p
was in agreement with the results recently reported by several researchers that considered the addition of hydroxyl group to aromatic rings as a dominating reaction in the first part of the degradation [41, 44, 45]. However, in our work, we have demonstrated that whether denitration or addition of hydroxyl group happened in the first part of degradation depends on the nature of the anode used. In the case of Ti/Ir3/8Ru1/8Sn1/2O2 and Ti/Ir1/4Ru1/4Sn1/2O2 anodes, the formation of polyhydroxylated intermediates was established, but the concentrations were lower; predominating higher concentration of 2,4-DNP. Based on the intermediates identified, we suggest a degradation mechanism depending on anodic material used as shown in Fig. 5. The denitration, in the first stage, occurs at Si/BDD anode. Whereas, substitution by ! OH radicals on aromatic rings, in the first stage, take places when Pb/PbO2 and 15 Page 15 of 29
Ti/SnO2 anodes were employed. The polyhydroxylation would subsequently lead to the opening of aromatic rings to form variety of phenolic compounds and subsequently, carboxylic acids, in both cases. Although, in the case of Ti/Ir3/8Ru1/8Sn1/2O2 and Ti/Ir1/4Ru1/4Sn1/2O2 anodes, formation of polyhydroxylated intermediates, is the degradation pathway followed.
ip t
Taking into account the above information, some questions are also addressed in this work:
cr
(1) Is it possible that the amount of hydroxyl radicals produced may affect the electrochemical pathway degradation?
us
(2) The amount of hydroxyl radicals produced and reactivity of them are different at each
an
electrode one studied?
To address these questions, in general, we investigated the production of ! OH radicals for each
M
one of the anodes, by spectrophotometry technique by absorbance values, using as bleaching solution the N,N-dimethyl-p-nitrosoaniline (PNDA). The generation of adduct allows to estimate the rate
te Ac ce p
anodes.
d
constant, for ! OH radicals formation, respect to time (k’) for each one of the electrodes studied as
3.5 Hydroxyl radicals (! OH) production and their effect on 2,4-DNP oxidation The production of free ! OH radicals on Pb/PbO2, Ti/SnO2, Ti/IrxRuySnO2 and Si/BDD electrodes was confirmed by the bleaching of N,N-dimethyl-p-nitrosoaniline (PNDA) solutions. PNDA was spectrophotometrically traced at 440 nm in 0.1 M (pH = 9.5) borax buffer. From calibration by using the Beer’s Law (Absorbance vs [PNDA] with r2 = 0.9975) a molar absorptivity of ! 440 = 33580 M-1cm-1 was obtained. Based on the results obtained from the PNDA-OH radicals trapping, the value of relative ! OH radicals’ production rate (k’), respect to time, was estimated; for each one of the anodes studied (assuming a pseudo first-order reaction, as already proposed by Liu et al. [42]) (Table 2). For Si/BDD anodes, the production of peroxodissulfates was also quantified and confirmed at 30 and 50 mAcm-2. 16 Page 16 of 29
INSERT TABLE 2 HERE
ip t
From these PNDA-OH radicals trapping formation data it is possible to point out that a variation from 30 to 50 mA cm-2 causes a remarkable improvement on the ! OH radicals’ production rate. For
cr
Pb/PbO2, this production rate is more effective than the other materials; while for Ti/SnO2 and Si/BDD anodes these rates are very similar when the applied current density was increased. However, in the
us
case of Si/BDD anode the production of peroxodisulfates was confirmed, when an increase on applied
an
current density was attained; and consequently, this reaction competes with the production of ! OH radicals. With these results, we cannot be certain whether the nitro group release took place by ! OH
M
radicals substitution or alternatively favored by peroxodisulfate oxidation of 2,4-DNP to a radical cation and after that, nucleophilic attack by ! OH radical. However, as already confirmed by HPLC
d
measurements, the radical cation mechanism could be an alternative elimination of –NO2 group from
te
2,4-DNP at Si/BDD anode where on its surface is Si/BDD(! OH/S2O82-).
Ac ce p
The electrochemical values of k’ obtained at Ti/SnO2 and Pb/PbO2 anodes, is significantly higher than that achieved at Si/BDD. These figures suggest either that the electrochemical species produced are stable (less reactive) compared with Si/BDD(! OH) or more likely, that the electrochemical oxidation of 2,4-DNP occurs by the proximity of it towards anode surface containing with higher concentrations of ! OH radicals, favoring hydroxyl radical attack in ortho position (as already confirmed by HPLC measurements). From a thermodynamic point of view, the substitution of –NO2 group requires more energy; then, the SnO2(! OH) and PbO2(! OH) species should be less reactive and more abundant on the anode surface than the analogous BDD(! OH) species. However, the BDD(! OH) could be considered more reactive
17 Page 17 of 29
and stable, close to anode surface and in the reaction cage [47, 48]. This hypothesis is consistent with our conclusions on the degradation of 2,4-DNP.
ip t
4. CONCLUSIONS Electrochemical oxidation of 2,4-DNP was achieved at Si/BDD, Ti/SnO2 and Pb/PbO2 anodes;
cr
obtaining higher removal efficiencies at Si/BDD anode. The high chemical reactivity of these anodes for organics oxidation is the result of a weak electrode-hydroxyl species interaction and, consequently,
us
a low electrochemical activity for the oxygen evolution reaction. By the contrary, for Ti/IrxRuySnO2
an
anodes, the degradation of 2,4-DNP was lower due, fundamentally, to a strong electrode-hydroxyl species interaction resulting in high electrochemical activity for the oxygen evolution reaction and low
M
chemical reactivity for organics oxidation (low current efficiency for organics oxidation). This behavior could be due to the preparation method and Ir, Sn and Ru proportion within metallic lattice.
d
Based on the results obtained, the production of free hydroxyl radicals and strong oxidants
te
promotes for a complete degradation of 2,4-DNP at Si/BDD anode; while for Ti/SnO2 and Pb/PbO2, production of hydroxyl radicals and the adsorption oxidation steps are determining factors for complete
Ac ce p
degradation (as recently indicated by Comninellis [46]). Finally, depending on the nature of the material and experimental conditions, the denitration or hydroxyl radicals attack of aromatic rings seem to be the first stage on 2,4-DNP oxidation.
Acknowledgments This work was supported jointly by the Consejo Nacional de Ciencia y Tecnología (CONACYT) México (Project No. 156200 CB-2010-01) and Dirección de Investigación y Posgrado from VAUDLAP, México (Convocatoria 2013).
18 Page 18 of 29
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based on Fenton's reaction chemistry, Chem. Rev., 109 (2009) 6570-6631. [26] C.A. Martínez-Huitle, E. Brillas, Electrochemical alternatives for drinking water disinfection,
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Angew. Chem. Int. Ed., 47 (2008) 1998-2005. [27] S. Stucki, R. Kotz, B. J. Carcer, W. Suter, Electrochemical waste water treatment using high overvoltage anodes Part II: Anode performance and applications, J. Appl. Electrochem., 21 (1991) 99-104.
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simultaneous oxygen evolution, J. Chim. Phys., 93 (1996) 673-679. [36] B. Correa-Lozano, Ch. Comninellis, A. De Battisti, Electrochemical properties of Ti/SnO2-Sb2O5 electrodes prepared by the spray pyrolysis technique, J. Appl. Electrochem., 26 (1996) 683-688. [37] A. Morozov, A. De Battisti, S. Ferro, G. N. Martelli, European Patent WO2005/014885 A1 (2005).
[38] S. Ferro, Synthesis of diamond, J. Mat. Chem., 12 (2002) 2843-2855. [39] E. F. H. Brittain, W.O. George, C. N. J. Well, Introduction to molecular spectroscopy theory and experiment, Academic Press, New York, 1970. [40] A. E. Gillam, E. S. Stern, Electronic absorption spectroscopy in organic chemistry, Edward APNDAld, London, 1970. 22 Page 22 of 29
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TABLES
Table 1. Rate constant for the 2,4-DNP oxidation in acidic media at 298 K.
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Anode Pb/PbO2 Ti/SnO2 Ti/Ir3/8Ru1/8Sn1/2O2 Ti/Ir1/4Ru1/4Sn1/2O2 Si/BDD
kapp (A-1Lh-1) 30 mAcm-2 50 mAcm-2 0.772 0.798 0.904 0.818 0.085 0.059 0.071 0.048 0.481 2.345
d
M
Respect to reaction time (k’ x 102 (min-1)) Production of S2O82- (mmol L-1) 30 mAcm-2 50 mAcm-2 30 mAcm-2 50 mAcm-2 1.89 5.37 3.61 3.11 1.49 1.08 0.99 0.30 1.22 1.44 0.89 3.95
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Anode Pb/PbO2 Ti/SnO2 Ti/Ir3/8Ru1/8Sn1/2O2 Ti/Ir1/4Ru1/4Sn1/2O2 Si/BDD
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Table 2. Rate constant for the PNDA bleaching reaction in pH = 9.5 borax buffer at 298 K.
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FIGURE CAPTIONS
Figure 1. (a) UV absorption spectra of 0.5 M H2SO4 + x mg L-1 2,4-DNP solutions as a function of 2,4-
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DNP concentration, 0 < x ! 50 mg L-1 . (b) Beer's law applied to each absorption bands observed: , 300 nm; , 261 nm; , 206 nm.
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Figure 2. Plot of % [2,4-DNP]rel vs. specific electrical charge passed (AhL-1) in 0.5 M H2SO4 solution
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at: (a) 30 mAcm-2 and (b) 50 mAcm-2. % [2,4-DNP]rel = ([2,4-DNP]t/[2,4-DNP]0)! 100. Anode material: ○ BDD, PbO2, SnO2, Ir3/8Ru1/8Sn1/2O2, × Ir1/4Ru1/4Sn1/2O2. The variation of Ln [2,4-
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DNP]rel with specific electrical charge passed (AhL-1) on different anodes is depicted in the inset plot.
Figure 3. Plot of %TOC (as a percentage of the normalized value: (TOCt/TOC0)! 100) as a function of
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the specific electrical charge passed (AhL-1) in 0.5 M H2SO4 solution at 50 mAcm-2 . Anode material:
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BDD, PbO2, SnO2, Ir3/8Ru1/8Sn1/2O2, × Ir1/4Ru1/4Sn1/2O2.
Figure 4. Trend in specific electrical charge (AhL-1) of nitrate ion concentration as percentage of the initial amount of nitrogen present. Anode material: BDD, PbO2, SnO2, Ir3/8Ru1/8Sn1/2O2, × Ir1/4Ru1/4Sn1/2O2.
Figure 5. Proposed scheme of 2,4-DNP degradation depending on electrocatalytic material used.
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FIGURES
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Figure 1 (b)
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Figure 1 (a)
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Figure 2 (a)
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Figure 2 (b)
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Figure 3
Figure 4 29 Page 28 of 29
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Figure 5
[49] 30 Page 29 of 29