Removal of the Pesticide Methamidophos from Aqueous Solutions by

Environ. Sci. Technol. 2008, 42, 6929–6935

Removal of the Pesticide Methamidophos from Aqueous Solutions by Electrooxidation using Pb/PbO2, Ti/SnO2, and Si/BDD Electrodes C A R L O S A . M A R T ´I N E Z - H U I T L E , † ACHILLE DE BATTISTI,‡ SERGIO FERRO,‡ ´ NICA CERRO-LO ´ PEZ,§ SILVIA REYNA,§ MO A N D M A R C O A . Q U I R O * ,§ Department of Analytical Chemistry, Laboratory of Electrochemistry, University of Milan, via Celoria 2-20133 Milan, Italy, Department of Chemistry, Laboratory of Electrochemistry, University of Ferrara, Via L. Borsari, 46 44100 Ferrara, Italy, and Department de Qu´imica y Biolog´ia, ´ Laboratorio de Electroqu´imica, Universidad de las Americas ´ ´ Puebla, Sta. Catarina Martir, Cholula 72820 Puebla, Mexico

Received March 25, 2008. Revised manuscript received May 28, 2008. Accepted June 12, 2008.

The anodic oxidation of methamidophos (MMD), a highly toxic pesticide used worldwide, was studied in a sodium sulfate aqueous solution on Pb/PbO2, Ti/SnO2, and Si/BDD (boron doped diamond) electrodes at 30 °C. Under galvanostatic conditions, it was observed that the performance of the electrode material is influenced by pH and current density as shown by HPLC and ATR-FTIR analysis of MMD and its oxidation products along the electrolysis. It was found that MMD degradation using Pb/PbO2 in acid media (pH 2.0 and 5.6) generates formaldehyde as the main product of the reaction giving evidence of an indirect mineralization mechanism. Under the same conditions, Ti/SnO2 showed poor formaldehyde production compared to the Pb/PbO2 electrode. On Si/BDD electrodes formaldehyde production was not observed, instead the ATRFTIR results showed the formation of phosphate as the reaction progressed suggesting a complete MMD mineralization on this electrode. In addition, HPLC results showed that the electrode efficiency is also dependent on the applied current density. This current density influence is remarkably clear on the Si/BDD electrodes where it was evident that the most efficient current density toward a complete MMD mineralization was reached with the application of 50 mA/cm2.

Introduction The use of pesticides essentially of organic nature has not only been limited to the protection of agriculture products but also at several other products related to the food chain as well as to the protection of natural resources (1). The use and production of several organic pesticides has continuously increased since introduction of DDT in the early 1940s, giving as a result a great family of organochlorine pesticides. However, owing to widespread use, misuse, and the fact that * Corresponding author e-mail: [email protected]. † University of Milan. ‡ University of Ferrara. § Universidad de las Ame´ricas Puebla. 10.1021/es8008419 CCC: $40.75

Published on Web 08/13/2008

 2008 American Chemical Society

many organochlorine pesticides are extremely persistent in the environment and in people’s bodies, in the early 1950s, new synthetic pesticides of the organophosphorus (OP) group were often used as supplements or substitutes. Although this second generation of synthetic pesticides is better accepted than those organochlorine compounds because they are generally of low persistence and do not tend to accumulate in the food chain, they are also more toxic due to their strong action as cholinesterase inhibitors. Therefore, all efforts to remove them from aquatic systems are welcome, especially if they can be transformed into simple compounds such as CO2 and H2O, among others. Recently, degradation of OP pesticides has been under study since they are extensively used and have shown to be environmentally persistent (2). Biological degradation methods to destroy pesticides are not effective enough due to their high toxicity (3). Nowadays, the main disposal method of pesticide stocks is incineration, which is expensive and not available in developing countries (3, 4). Therefore, some advanced types of treatment have been proposed such as the use of photolysis (5), ultrasonic radiation, mercury promoted hydrolysis, and electrochemical oxidation (6). Applying electrooxidation methods to decompose OP pesticides is recently attracting interest due to their capability of decomposing toxic organic materials without producing new toxic wastes (6, 7). The applications of electrochemistry for environmental pollution abatement have been thoroughly investigated (8). The feasibility of electrochemical conversion/destruction of organic substrates in wastewater, in particular, has attracted much attention since the first studies. During the last two decades, research work has focused on the efficiency for oxidizing various pollutants at different electrodes, the improvement of the electrocatalytic activity and electrochemical stability of the electrode materials, and the investigation of factors affecting the process performance and kinetics of pollutant degradation (8). Therefore, the application of electrooxidation processes to the aquatic systems represents an attractive alternative for the degradation of biorefractory or priority pollutants such as aromatics (8–11), phenol and its nitro and chlorinated derivatives (8, 12–15), pesticides (3, 4, 8), and others (8). On the other hand, identification of products plays an important role in determining the efficiency of an advanced wastewater treatment method. In electrochemical methods (8), determining these products helps to understand the mechanisms by which these organic compounds will decompose to less harmful compounds or better yet to their complete mineralization. It has been shown that the electrode material plays a key role in the evolution of the oxidation process (8, 15, 16) and consequently on the byproduct of oxidation. According to the mechanism involved in the pollutant oxidation (8), the electrode materials have been classified into two main groups: “active” and “non-active” electrode materials (8, 9). The proposed model assumes that the initial reaction in both kind of anodes (generically denoted as M) corresponds to the oxidation of water molecules leading to the formation of physisorbed hydroxyl radical (M(•OH)): M + H2O f M(•OH) + H++ e-

(1)

Both the electrochemical and chemical reactivity of heterogeneous M(•OH) are dependent on the nature of the electrode material. The surface of “active” anodes interacts strongly with •OH radicals and then (8, 9, 14, 17–19), a soVOL. 42, NO. 18, 2008 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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called higher oxide or superoxide (MO) may be formed following reaction 2. This may occur when higher oxidation states are available for a metal oxide anode, above the standard potential for oxygen evolution (E° ) 1.23 V vs SHE). M(•OH) f MO + H+ + e-

(2)

The redox couple MO/M acts as a mediator in the oxidation of organics by reaction 3, which competes with the side reaction of oxygen evolution via chemical decomposition of the higher oxide species from reaction 4. MO + R f M + RO 1

MO f M + ⁄2O2

(3) (4)

In contrast, the surface of a “non-active” anode interacts so weakly with •OH radicals that it allows the direct reaction of organics with M(•OH) to give fully oxidized reaction products such as CO2 as follows: M(•OH) + R f M + mCO2+nH2O + H++e-

(5)

where R is an organic compound with m carbon atoms and 2n hydrogen atoms, without any heteroatom, which needs (2m + n) oxygen atoms to be totally mineralized to CO2. The oxidative reaction 3 with the surface redox couple MO/M is much more selective than the mineralization reaction 5 with physisorbed heterogeneous hydroxyl radical. The latter reaction also competes with the side reactions of M(•OH) like direct oxidation to O2 from reaction 6 or indirect consumption through dimerization to hydrogen peroxide by reaction 7: M(•OH) f M + 1⁄2O2 + H+ + e-

(6)

2 M(•OH) f 2 M + H2O2

(7)

A “non-active” electrode does not participate in the direct anodic reaction of organics and does not provide any catalytic active site for their adsorption from the aqueous medium (8, 17). It only acts as an inert substrate and as a sink for the removal of electrons. In principle, only outer-sphere reactions and water oxidation are possible with this kind of anode. Hydroxyl radical produced from water discharge by reaction 1 is subsequently involved in the oxidation process of organics. The above model presupposes that the electrochemical activity (related to the overvoltage for O2 evolution) and chemical reactivity (related to the rate of organics oxidation) of physisorbed M(•OH) are strongly linked to the strength of the M-•OH interaction. As a general rule, the weaker the interaction, the lower the anode reactivity for organics oxidation with faster chemical reaction with M(•OH). The BDD anode is the best nonactive electrode verifying this behavior (8, 15, 17, 21), hence it’s being proposed as the preferable anode for treating organics by electrochemical oxidation. On the basis of this model, metal oxides such as IrO2 and RuO2 (8, 18) known as “active” electrodes, achieve an incomplete oxidation of organic pollutants; whereas “nonactive” oxides, such as Ti/SnO2 and Pb/PbO2 and their doped analogues, are capable of oxidizing organics to CO2 (8, 14, 20). Within this last group of electrode materials, boron doped diamond (Si/BDD) electrodes have received great attention due to the wide range of their electrochemical properties (17, 21). Thus, in order to compare the effect that the electrode material has on the oxidative removal of OP pesticides from aqueous solutions, the MMD (O,S-dimethyl phosphoramidothioate, C2H8NO2PS) degradation was carried out on three types of electrode material: Ti/SnO2, Pb/PbO2, and Si/BDD. 6930

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MMD is an OP compound classified by the Environmental Protection Agency (U.S. EPA) as a Class I compound with emphasis of Danger-Poison on commercial products and consequently being a restricted use pesticide (RUP). This OP pesticide is a highly active, systemic, residual OP insecticide/ acaricide/avicide with contact and stomach action which inhibits cholinesterase and is highly toxic to mammals, birds, and bees (22, 23). MMD is also a metabolite of acephate, another OP pesticide (24). The half-life of MMD in water is 309 days at pH 5.0, 27 days at pH 7.0, and 3 days at pH 9.0. Unfortunately, current surface waters as well as some groundwaters are usually acidic or slightly alkaline which made it understandable to look for an advanced method to remove MMD from aqueous media in shorter times than those obtained by typical biodegradation methods. Until now, the only reported study about the electrochemical removal of MMD from an aqueous medium was made on a Ti/Pt electrode (3). In this work, a decrease of both COD and BOD5 parameters as well as the pH value of electrolyzed brine solution was observed. Although the oxidation efficiency is low, an improvement of the biodegradability index (COD/BOD5) was achieved. However, no information about the oxidation pathway nor about the final surface characteristics of the working electrode was obtained. Important information about the oxidation pathway has been reported for analogous OP pesticides (6, 7). Vlyssides et al. (6), proposed that electrochemical oxidation of methylparathion takes place through a mechanism of five levels of degradation until the formation of final products within which CO2 and PO43- ion were detected. At the fourth level of degradation, carboxylic acids such as oxalic and formic acids were also detected. On the other hand, the photocatalytic degradation of OP insecticides (7) has shown that the PO43- ion is formed by the degradation and that its concentration is continuously increased event after the total amount of the insecticide has been removed. This result is explained by the formation of formaldehyde as one of the intermediates, which degraded as the illumination continued.

Experimental Section Reagents and Electrochemical Cell. Electrochemical MMD oxidation experiments were performed using a divided cell with a reaction compartment of 100 mL capacity and a 10 mL porous porcelain pot containing a zirconium plate as cathode. The anolyte consisted of 50 ppm MMD in acidic (pH 2.0 and 5.6) or alkaline (pH 9.0) aqueous solution. The supporting electrolyte consisted of a 5% Na2SO4 solution; MMD was added to this solution and the pH was then adjusted either using 1.0 M NaOH or 10% H2SO4 solutions. For every experiment, 100 mL of a fresh 50 ppm MMD solution was used to carry out oxidation. The catholyte was either acidic or alkaline pure supporting electrolyte. Reagent grade chemicals and 3-fold distilled water (16-18 MΩ) were used throughout the work. The temperature of the electrolyte was fixed at 30 °C and maintained constant by using a water thermostat. The stirring rate was kept almost constant (350 ( 10 rpm). In this condition, the estimated mass transfer coefficient in the cell, determined using the ferri/ferrocyanide couple, was 2 × 10-5 m s-1. Hg/Hg2SO4/K2SO4 (sat) was used as a reference electrode and one of the following materials was used as anode: Pb/PbO2, Ti/SnO2, or Si/BDD, which were prepared or obtained commercially. Electrodes Preparation. The initial geometric surface area of the sheet of pure lead used to prepare the Pb/PbO2 anode was approximately 10 cm2. After a deep cleaning of the exposed lead surface, it was anodized at a current density of 50 mA · cm-2 during an electrolysis time of 1.5 h in a 10% H2SO4 solution at 298 K to oxidize the lead surface to lead dioxide (14, 15). The SnO2-coated titanium (Ti/SnO2) anodes (25–28) were prepared by a sol-gel technique, which

consisted of the following steps: preparation of precursor solution, brushing it onto a pretreated titanium base, and drying at 50 °C. The whole process was repeated 10 times. More details concerning anode preparation and characterization are given elsewhere (29). Si/BDD thin-film electrodes were supplied by Adamant Technologies (Neuchatel, Switzerland). They were synthesized by a hot filament chemical vapor deposition technique (HF CVD) on single crystal p-type Si 〈1 0 0〉 wafers (1-3 mΩcm, Siltronix). The filament temperature ranged from 2440 to 2560 °C, while the substrate temperature was 830 °C. The reactive gas was methane in excess dihydrogen (1% CH4 in H2). The doping gas was trimethylboron with 3 mg L-1 concentration. The gas mixture was supplied to the reaction chamber at a flow rate of 5 L min-1, with a diamond layer growth rate of 0.24 µmh-1. The obtained diamond film had 1 µm thickness, with 10-30 m Ω cm resistivity. Instrumentation and Experimental Conditions. The current density for the electrolysis (jappl) was kept constant at the desired level (10, 20, 30, and 50 mA · cm-2) from a Tacussel model PJT24-1 (24V - 1A) potentiostat-galvanostat. During the runs samples of anolyte were withdrawn and analyzed for the concentration of residual MMD and oxidation products in the solution. The total time of electrolysis (∼ 70 min) was established at the condition of the minimum detectable limit of MMD concentration (∆[MMD] ≈ 0). After each run the cell and the anode were washed thoroughly with doubly (2-5 MΩ) and 3-fold distilled water (16-18 MΩ). A Waters model 1515 HPLC equipped with an Inertsil ODS3, 5 µm, 4.6 mm I.D. × 150 mm reversed-phase column and a dual wavelength UV-vis Waters detector model 2487 was used to determine MMD and oxidation products concentrations during various stages of the electrolysis. The most suitable mobile phase was 5:95 CH3CN/H2O at a flow rate of 1.0 mL/min, a column temperature of 30 °C, and an analytical wavelength of 220 nm. In addition, reaction products were also analyzed with a Varian 800 Fourier transform infrared spectrometer based on attenuated total reflectance (ATRFTIR). For these studies, a drop of sample solution taken at different reaction times was placed directly on a zinc selenide (ZnSe) crystal for its IR analysis. The COD content of the solution was followed in order to determine the general current efficiency for the anodic oxidation of MMD; for this type of analysis, a HACH DR/2010 portable datalogging spectrophotometer was used. Efficiency Parameter. Current efficiency (CE) for anodic oxidation of MMD was calculated from COD values (8), using the following relationship:

(

CE(%) ) FV

)

[(COD)0 - (COD)t] × 100 8It

(8)

Where COD0 and CODt are chemical oxygen demands at times t ) 0 (initial) and t (in gO2 L-1), respectively, I is the current (A), F is the Faraday constant (96,487 Cmol-1), V is the electrolyte volume (L), and 8 is the oxygen equivalent mass (g equiv-1).

Results and Discussion Electrodes Characterization. In any case, the electrochemical characteristics of each one of the anodes for the oxidation process were tested: cell potential as stability criteria for Pb/ PbO2 and Ti/SnO2 (Figure 1a) and potential window to Si/ BDD (Figure 1b). In fact, at any given electrode material, the j/E function decides not only the faradaic efficiency of the oxidation of the organic substrate, but also of the electrode itself. Accordingly, it is important to establish the current density range for the oxidation reaction at which the anode and the electrochemical cell are stable during the entire electrolysis

FIGURE 1. (a) Plot of the cell potential, at (open star) Pb/PbO2 and (•)Ti/SnO2 anodes, as a function of the electrolysis time for the electrochemical oxidation of MMD in 0.5 M H2SO4, at 25 °C and 30 mA cm-2. (b) CV curves for the Si/BDD and Pt electrodes, in acidic media (0.5 M H2SO4). Si/BDD area of the electrode was 0.78 cm2 and a small Pt wire was used, with the same real surface area. Scan rate: 100 mV s-1.

FIGURE 2. ATR-FTIR spectrum of MMD in (a) crystalline form and (b) aqueous solution. time (14). Cell potential measurements are shown in Figure 1a. As it can be observed, the cell potential remains almost constant over the entire electrolysis time (∼250 min); such behavior indicates a reasonable stability of the electrochemical cell and, therefore, allows assumption that the electrocatalytic activity of the Pb/PbO2 and Ti/SnO2 anodes for the MMD oxidation is not modified by the experimental conditions during the electrolysis time. Finally, a comparison between the potentiodynamic profiles of Pt and Si/BDD electrode is shown in Figure 1b, where the Si/BDD electrode exhibits its most important characteristic: a potential window 4V wide (17). The cyclic voltammetric curves for the Si/BDD electrode obtained after purging with nitrogen gas is featureless, and the background current is very low (less than 1 µA) in both cases. In addition, in acidic media, the potentiodynamic profile of Si/BDD electrode shows that the oxygen evolution peak appears about +2.0V vs SCE which is an essential characteristic of an electrode material for applications in wastewater treatment. ATR-FTIR Standards Analysis. Before investigating the MMD removal by electrochemical oxidation, the IR characteristics of MMD in a crystalline form and in aqueous solution were established. The corresponding ATR-FTIR spectra of MMD are shown in Figure 2a and b. The more important characteristics in these IR spectra of MMD are the phosphate bands observed at 1199 cm-1 (PdO stretch) and 1028 cm-1 (PsOsC stretch) in the solid state and at 1185 and 1055 cm-1 for the aqueous solution, respectively. The band observed at 2344 cm-1 in Figure 2b corresponds to the CO2 gas dissolved in the aqueous solution. Anodic Oxidation. Once MMD was spectroscopically characterized, its oxidation process was then investigated as a function of (a) the electrode material (Pb/PbO2, Ti/SnO2, and Si/BDD) used as anode, (b) the current density applied to the electrolytic solution (10, 20, 30, and 50 mA cm-2), and (c) the pH of the aqueous solution (2.0, 5.6, and 9.0). The aim of these experiments was to find the best electrochemical conditions for an adequate removal of MMD from aquatic systems. VOL. 42, NO. 18, 2008 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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FIGURE 3. Plot of normalized MMD concentration (C/C0) as a function of the specific electrical charge (Ah L-1) of the electrode material and at (a) pH 2.0 and (b) pH 5.6. The importance of pH on the electrolytic reaction relies on the influence that the medium has on the molecular dissociation process and also on some steps of the mechanism (2, 3). In this respect, it is important to point out that MMD undergoes hydrolysis in either alkaline or acidic media but the hydrolysis degree varies with respect to the media as it is shown in chemical eqs 9 and 10.

According to chemical eq 9, the formation of species C is clearly favored under alkaline conditions. Owing to this chemical aspect, monitoring MMD degradation at pH higher than 5.6 was not performed by HPLC technique since light absorption properties of the chemical species involved significant changes. Therefore, the degradation process of MMD was studied in this first experimental step on Pb/PbO2, Ti/SnO2, and for Si/BDD electrodes at pH 2.0 (Figure 3a), pH 5.6 (Figure 3b), and at 20 mA cm-2 of applied current density. From Figure 3, it is evident that MMD degradation not only depends on pH of electrolyzed solution but also of the electrode material. In both cases, pH 2.0 and 5.6, the Si/BDD electrode showed the best removal degree of MMD whereas on both Ti/SnO2 and the Pb/PbO2 electrodes the MMD removal was scarce and with a low removal rate. Even though at pH 5.6 the Si/BDD electrode showed the best MMD removal degree, the condition of a more acidic solution seems to be more favorable for all material electrode studied. Now, under this pH 2.0 condition, the MMD removal was investigated as a function of the current density applied to the same electrodes, Figure 4. 6932

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FIGURE 4. Plot of normalized MMD concentration (C/C0) as a function of the specific electrical charge (Ah/L), of the applied current density and on (a) Pb/PbO2, (b) Ti/SnO2, and (c) Si/BDD electrodes. As shown in Figure 4a, interesting results were obtained at the Pb/PbO2 electrodes; in this case, a nearly complete elimination of MMD was observed at current densities of 10 and 20 mA cm-2, although the faradaic efficiency of the initial stages of oxidation process is moderately low. For these applied current density values, the oxidative attack is faster in its first-intermediate part, allowing a 85% of mineralization of organics with essentially the same charge consumption. However, at higher j values (j ) 30 mA cm-2) similar improvements in the efficiency of the MMD mineralization were achieved, in its initial stage. Whereas, in the final part of the process the MMD oxidation was limited, this could be due to mass transport limitations. At the Ti/SnO2 electrode, a slower MMD electrooxidation at all applied current densities was observed, as indicated by normalized concentration values (Figure 4b). This behavior can be explained according to the mechanism proposed by Comninellis (9), who suggested that metal cations in the oxide lattice may reach higher oxidation states under anodic polarization and, consequently, a stabilization of adsorbed · OH radicals takes place, which favors the oxygen evolution at the expense of the electrochemical incineration reaction. As can be seen, Figure 4 shows the MMD electrooxidation at 20 and 30 mA cm-2, but lower j value (10 mA cm-2) was not considered in these experiments because this process could require longer times. The electrooxidation experiments of MMD at Si/BDD anodes were also carried out exploring the whole range of current densities (20 and 30 mA cm-2). It is worth mentioning that semimetallic Si/BDD, as well as Pb/PbO2, is an anodic material with a very low exchange current density for the oxygen evolution reaction. The oxidation trend of MMD at Si/BDD as a function of the specific electric charge passed is shown in Figure 4c for the different j values. As can be observed (Figure 4c), a complete elimination of MMD from the electrolyzed solution requires about 0.3-0.5 Ah L-1 at 20 and 30 mA cm-2, quite lower with respect to the values required at the Pb/PbO2 and Ti/SnO2 (more than 1.4 Ah L-1),

FIGURE 5. ART-FTIR spectrum in 5% Na2SO4 aqueous solution of (a) formic acid and (b) formaldehyde. under similar conditions (Figure 4a and b). These results clearly show the effectiveness of the Si/BDD electrodes for the anodic mineralization of organic pollutants. These results show in general that the reaction pathway for each electrode material is not dependent on the applied current density and that the reaction extent depends only on the electrolysis time. In any case, the removal efficiency of MMD follows the order Si/BDD . Pb/PbO2 > Ti/SnO2. Moreover, the influence of the anode material on the elimination of MMD seems to be very important, with Si/ BDD being the electrode at which the best incineration efficiency has been attained. It is known that the surface stability of an electrode is often affected by the level of the applied current density; therefore the only electrode analyzed at 50 mA cm-2 was Si/BDD, for which HPLC studies showed a complete disappearance of MMD during the first 20 min of reaction (eq 0 >.20 Ah L-1). As shown, the best efficiency for MMD elimination is achieved at Si/BDD electrode. For this reason, CE values were calculated in order to analyze the faradaic efficiency of the oxidation process. MMD elimination is reached at 50 mA cm-2 with a CE value of 14%, which decreases to 8 and 6% for current densities of 30 and 20 mA cm-2, respectively. This behavior is characteristic of discontinuous electrochemical oxidation of wastewaters with conductive-diamond anodes. It is usually explained in terms of mass transfer limitations, assuming that the main mechanism involved in electrochemical oxidation on conductive diamond anodes is a direct or a hydroxyl-radical mediated electrochemical oxidation process (17). Degradation Products. As previously reported (7), the degradation of OP pesticides by photolysis processes gave formaldehyde and PO43- among some oxidation products. These oxidation products were analyzed by colorimetric methods, the inherent limitation of which is their susceptibility to have strong chemical interferences. Therefore, in order to compare electrode performance toward MMD removal, the formation of degradation products was followed by ATR-FTIR assuming in first instance that formaldehyde is formed through chemical eq 11: C2H8NO2PS + 8 •OH f 2CH2O + NH4+ + SO42- + PO43- + 8H+ + 4e-(11) However, the MMD oxidation could also form formic acid instead of formaldehyde according to chemical eq 12: C2H8NO2PS + 10 •OH f 2CH2O + NH4+ + SO42- + PO43- + 12H+ + 8e-(12) Newly, the ATR-FTIR technique allow us to discern between these two possibilities since the spectroscopic profiles in the 800-1600 cm-1 range are clearly distinctive as shown in the corresponding standard spectra of Figure 5a and b. It is clear that IR band at 1368.1 cm-1 (CH2 scissor) is the key band to do the spectroscopic difference. The differentiation between these two compounds by other chemical and/

FIGURE 6. ATR-FTIR spectra for the formaldehyde formation during MMD electrooxidation on a Pb/PbO2 electrode at pH 2.0 and 20 mA/cm2. Insert shows the formaldehyde formation (as % transmittance) during MMD electrooxidation on Pb/PbO2 and Ti/ SnO2 electrodes at 20 mA/cm2 as a function of pH of solution and also with the electrolysis time.

FIGURE 7. ATR-FTIR spectrum for the MMD electrooxidation on the Si/BDD electrode at pH 2.0 and 50 mA/cm2. Insert: ART-FTIR spectrum of K3PO4 in 5% Na2SO4 solution. or instrumental techniques is difficult in extreme mainly if the compound is in solution and in the presence of other chemical species as it is the case of reaction media. Once formaldehyde was confirmed as the organic byproduct of MMD by electrochemical oxidation, Figure 6, this compound was tracked through ATR-FTIR as the reaction proceeded. For this, the IR band at 1740 cm-1 (CdO stretching) was chosen to monitor its developing on the different electrodes. Figure 6 shows how the formaldehyde bands grow as the MMD electrooxidation proceeds on a Pb/PbO2 electrode. This behavior is clearly observed only on Pb/PbO2 and in a less appreciable amount on Ti/SnO2; the insert in Figure 6 gives us evidence that MMD removal on these electrode materials is achieved through the formation of a single organic intermediate, according to eq 11. Moreover, it is observed that on Pb/PbO2, formaldehyde production is favored at low pH values. It is important to remark that in some cases, the concentration of the formaldehyde decreases after 60 min of electrolysis at Pb/PbO2 and Ti/SnO2 under specific conditions of pH (inset Figure 6). According to these data, the formaldehyde oxidation could be achieved in further steps, forming formic acid (fast step) and subsequently to CO2 according to eq 13: CH2O + • OH f HCOOH + • OH f CO2+H2O + 2 H++2e(13) Si/BDD: Direct Mineralization. In the case of the Si/ BDD electrode, no bands of formaldehyde formation appear on the corresponding ATR-FTIR spectrum of the electrolyzed solution, Figure 7. However, the plot of normalized MMD concentration as a function of the specific electrical charge VOL. 42, NO. 18, 2008 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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FIGURE 8. Methamidophos degradation on Si/BDD at pH 2.0 as a function of the electrolysis time and the applied current density. (a) Phosphate ion formation through different electrolysis times. PO43- was tracked through FTIR band at 1015 cm-1 and expressed in instrumental units (au). (b) CO2 formation through different electrolysis times. CO2 was tracked through FTIR band at 2361 cm-1 and expressed in instrumental units (au). (Ah L-1), Figure 3c, shows a high MMD removal rate which leads to a nondetectable MMD concentration after 40 min of electrolysis time. The absence of formaldehyde and the complete consumption of MMD can only be explained assuming a direct mineralization process of MMD to form CO2 and PO43- among other byproducts of oxidation according to the chemical eq 14. +

2-

C2H8NO2PS + 12 •OH f 2CO2 + 2H2O + NH4 + SO4 3-

PO4

+

+ -

+ 12H + 8e (14)

To verify this last assumption, the IR characteristics of PO43- ion in 5% Na2SO4 were established, as can be observed in the insert of Figure 7. The IR band at 1006 cm-1 is the typical IR band of free PO43- ion in aqueous medium which differs from that observed in Figure 2b at 1185 cm-1 (PdO stretch) and 1055 cm-1 (PsOsC stretch) of MMD in aqueous solution, nevertheless all of them are related to PO43- groups. The set of IR band in the 1015-1055 cm-1 range from Figure 7, and whose growth as the electrolysis time increases, can be adequately associated with the release of PO43- ion from the MMD fragmentation during mineralization process of chemical eq 14. Therefore, the increase of the height of IR band at 1015 cm-1 with the electrolysis time at pH 2.0 and at different current densities is shown in Figure 8a. Moreover, the IR bands around 3000 cm-1 (not shown) of Figure 7 seem to be consistent with the NH4+ formation. In addition, the IR band of CO2 formation at 2361 cm-1 is also observed for the MMD oxidation at 20 and 30 mA cm-1 (both at pH 2.0) as electrolysis time increases, as shown in Figure 8b. From these results, the formation of CO2 is an indisputable fact although its magnitude is lower than that expected for the direct mineralization of MMD. However, it is only an appreciation since not all CO2 formed remains in solution owing to its low solubility in the acid medium and to the condition of continuous stirring. When the pH condition of the solution is changed toward more alkaline values, in the region of 9.6-12 for instance, the amount of CO2 monitored is significantly greater as can be observed in the plot from Figure 9a. On the other hand, the formation of PO43- ion at this last condition of pH is similar to that observed at pH 2.0, as it is shown in 9b for comparison. 6934

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FIGURE 9. Methamidophos degradation on Si/BDD at pH 12.0 and 20 mA/cm2. (a) CO2 formation through different electrolysis times. CO2 was tracked through FTIR band at 2361 cm-1 and expressed in instrumental units (au). (b) Phosphate ion formation through different electrolysis times at (0) pH 2.0 and (I) pH 12.0. PO43- ion was tracked through FTIR band at 1015 cm-1 and expressed in instrumental units (au). Thus, the efficiency of the Si/BDD electrode is related to complete mineralization according to ATR-FTIR studies which show that PO43-, NH4+, and CO2 are the main byproducts of MMD oxidation at all conditions of applied current densities with the reaction extent being the only parameter dependent on the current density. Consequently, the best current efficiency of the Si/BDD electrode is achieved at 50 mA/cm2.

Acknowledgments This research was supported in part by a grant-in-aid from University of the Americas Puebla (UDLAP/VIPE) and by a grant from the National Council of Science and Technology of Me´xico (CONACYT reg. no. 53024).

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