Environ. Sci. Technol. 2004, 38, 6125-6131
Degradation of Methylparathion in Aqueous Solution by Electrochemical Oxidation APOSTOLOS VLYSSIDES,* ELLI MARIA BARAMPOUTI, AND SOFIA MAI School of Chemical Engineering, National Technical University of Athens, 157 70 Athens, Greece DIMITRIS ARAPOGLOU Institute of Technology of Agricultural Products, NAGREF, 141 23 Athens, Greece ANASTASIA KOTRONAROU Institute for Environmental Research and Sustainable Development, 152 36 Athens, Greece
The electrochemical degradation of methylparathion has been investigated by using Ti/Pt as anode, Stainless Steel 304 as cathode, and sodium chloride as electrolyte. The pesticide is rapidly degraded, but full mineralization is not observed. Degradation products have been monitored through gas chromatography and mass spectrometry, and the overall degradation process has been monitored through dissolved and particulate organic carbon, sulfur, and phosphorus measurements. Several intermediates have been identified, and oxalic, formic, and acetic acids as well as tetraphosphorus trisulfide have been recognized as final products of the degradation process. A proposed mechanism of the process is presented.
Introduction The disposal of pesticides can cause serious problems due to the chemical nature of the active ingredients in pesticide formulation and due to the large quantities of the unwanted products. These products undergo physical and chemical alterations either due to extended storage, beyond the recommended expiration date, or due to storage under improper conditions (high humidity and temperature). In many countries, large quantities of pesticides have accumulated since they have lost their desirable characteristics. Pesticides that have passed their self-life can be included in this category. Although, these products are not suitable for use, they still contain toxic compounds. In addition, many surplus pesticides, still within their expiration limits, may become useless, when their future use is prohibited due to toxicological or environmental concerns. FAO (Food and Agricultural Organization of the United Nations) estimated that more than 400 000 tonnes of obsolete pesticides are stocked worldwide (1). Methylparathion (O,O-methyl O-p-nitrophenyl thiophosphate) is a widely used pesticide. Organophosphate esters such as parathion have been used as alternatives to DDT and other chlorinated hydrocarbon pesticides. However, the organophosphate esters are not rapidly degraded in natural * Correspondingauthorphone: +302107723268;fax: +302107723269; e-mail:
[email protected]. 10.1021/es049726b CCC: $27.50 Published on Web 10/09/2004
2004 American Chemical Society
waters. At 20 °C and pH 7.4, parathion has a hydrolytic halflife of 108 days, and its toxic metabolite, paraoxon, has a similar half-life of 144 days (2). The biological degradation of pesticides is generally difficult due to their high content in toxic matter (3, 4). An ideal treatment method for pesticide surplus should be nonselective, should achieve rapid and complete mineralization, and should be suitable for small-scale wastes (5, 6). Today the main disposal method of obsolete pesticide stock is incineration, an impractical and expensive procedure. High-temperature incineration in dedicated hazardous waste incinerators is the currently recommended method for obsolete pesticide treatment. However, sophisticated incinerators do not exist in developing countries (1). However, safety on environmental and processing grounds may be questioned in many cases. For products for which a suitable inactivation method does not exist, long time storage in concrete tombs is recommended (3). However, oil-water emulsions and/or organic solvents from the solvent-based preparations may destroy the bituminous coatings on the concrete walls of the tomb. It is possible that the unprotected concrete walls might be corroded and that toxic chemicals may subsequently migrate to the surrounding area (1, 3). Various innovative technologies have been proposed for methylparathion treatment. These include the use of UV and hydrogen peroxide (7, 8), ultrasonic radiation (2), or mercurypromoted hydrolysis (9). The major disadvantage of these technologies is that they are designed for decontamination of aqueous solutions with a very low active ingredient content and are not suitable for the higher concentrations of unwanted pesticides. Recently, there has been increasing interest in the use of electrochemical methods for the treatment of recalcitrant toxic wastes. The organic and toxic pollutants present in such wastes, such as phenols which are present in many pesticides, are usually destroyed by anodic oxidation as a result of the production of oxidants such as hydroxyl radicals, ozone, etc. (10-13). These methods are environmentally friendly and do not produce new toxic wastes. Electrochemical methods have been successfully applied in the purification of domestic sewage (14, 15), landfill leachate (16), tannery wastes (17), olive oil wastewaters (18, 19), textile wastes (20), etc. In this paper a detailed investigation on the electrochemical degradation of high concentration of MeP in water solution in a laboratory scale plant using Ti/Pt as anode, Stainless Steel 304 as cathode, and sodium chloride as electrolyte is presented. Some new and characteristic features in the degradation patterns have been observed.
Theoretical Section There are two main processes in the electrochemical treatment of wastes (21): (i) the conversion of nonbiodegradable organics to biodegradable before a biological treatment and (ii) the combustion of organics to CO2 and water. Figure 1 shows a generalized scheme of the electrochemical conversion/combustion of organics on oxide anode (MOX). In the first step, H2O is discharged at the anode to produce absorbed hydroxyl radicals according to the reaction:
MOX + H2O f MOX(•OH) + H+ + e In a second step, the adsorbed hydroxyl radicals may interact with the oxygen already present in the oxide anode VOL. 38, NO. 22, 2004 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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FIGURE 1. Selective oxidation and combustion of organics in electrolochemical process. with possible transition of oxygen from the absorbed hydroxyl radical to the oxide anode forming the higher oxide MOX+1.
MOX(•OH) f MOX+1 + H+ + e At the anode surface the “active oxygen” can be present in two states: either as physico-sorbed (adsorbed hydroxyl radicals, •OH) or/and as chemico-sorbed (oxygen in the oxide lattice, MOX+1). In the absence of any oxidizable organics, the “active oxygen” produces dioxygen according to the following reactions:
MOX(•OH) f 1/2O2 + H+ + e + MOX MOX+1 f 1/2O2 + MOX In the presence of oxidizable organics the physico-sorbed “active oxygen” (•OH) should cause predominantly the complete combustion of organics, and chemico-sorbed (MOX+1) will participate in the formation of selective oxidation products (21). The physico-sorbed route of oxidation is the preferable way for waste treatment. It is probable that dioxygen participates also in the combustion of organics according to the following reaction schemes: (1) formation of organic radicals by a hydrogen abstraction mechanism: RH + •OH f R• + H2O; (2) reaction of organic radical with dioxygen formed at the anode: R• + O2 f ROO•; and (3) further abstraction of a hydrogen atom with formation of an organic hydroperoxide (ROOH) and another radical: ROO• + R′H f ROOH + R′•. Since the organic hydroperoxides formed are relatively unstable, decomposition of such intermediates leads to molecular breakdown and formation of subsequent intermediates with lower carbon numbers. These sequential reactions continue till the formation of carbon dioxide and water. In this case the diffusion rate of organics on the anode area controls the combustion rate (22, 23).
Experimental Section Reagents. The commercial formulation Folidol M (40% w/v methylparathion) was purchased from Bayer. Laboratory Scale Pilot Plant. The experimental plant is shown in Figure 2. The electrolytic cell was a cylindrical vessel (V), which contained 6 L of brine solution (H2O + NaCl). A Ti/Pt cylindrical electrode (14 cm long × 1.5 cm diameter) was used as anode. It was covered by platinum alloy foil approximately 0.22 mm thick. The electrode was located inside a perforated stainless steel 304 cylinder (14 cm long × 8 cm diameter) which served as cathode. This construction ensured homogeneous dynamic lines between anode and cathode and provided good contact of the waste with the 6126
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FIGURE 2. Experimental apparatus. electrode. The electrochemical system was operated at 3036 d.c. amperes. In all cases, to mix the brine solution and ensure the continuous presence of untreated organic matter close to the anode, an agitator at 300 rpm was used in the cell operated. The temperature was kept constant at 45 °C by using a water cooling system and a temperature controller. Experimental Procedure. During each batch experiment the oxidation reactor was filled up to 6 L with an 8% w/w aqueous suspension of methylparathion and 20 g/L of sodium chloride was added as electrolyte medium. During the electrolytic oxidation the agitation was constant at 300 rpm. The current density and temperature were kept constant at 0.56 A/cm2 and 45 °C, respectively. The total time of electrolysis was 2 h. The results as presented below are the mean values of 10 experimental runs. Analytical Determinations. The MeP detections carried out with a Perkin-Elmer HPLC Model P2000 pump with UV detection (Model LC 1200), equipped with a bonded-phase octadecylsilica column (Lichrospher C-18 Merck, 25-cm length, 46-mm i.d., 5-µm packing). The primary degradation of methylparathion was followed by direct injection of filtered (0.45 µm) electrolyzed sample; UV detection was performed at 276 nm; the mobile phase was 60% acetonitrile/40% water at 1 mL min-1; and retention time was 16.0 min. Reaction intermediates such as paraoxon, p-nitrophenol (PNP), benzoquinone, and hydroquinone were identified by gas chromatography/mass spectrometry (GC/MS) and quantified by either GC/flame ionization detection or GC/MS. Electron impact (EI) mass spectra were obtained with a Hewlett-Packard Model 5989A mass spectrometer, interfaced to a Hewlett-Packard Model 5890 gas chromatograph and controlled by a SuperIncos data system. The spectra were recorded by setting the electron energy to 70 eV and the accelerate to 3 kV and applying a source temperature of 200 °C. The magnet was scanned from 29 to 800 Da in 0.8 s (1.0-s cycle time). Samples were injected splitless into SE-52 30 m × 0.32 mm capillary column inserted directly into ion source. The auxiliary carrier gas was nitrogen at 30 mL/min velocity. The oven temperature was programmed as follows: isothermal at 60 °C for 1 min, from 60 to 300 °C at 10 °C min-1, isothermal at 300 °C for 10 min. Ten milliliters of the reaction mixture was extracted with 10 mL of ethyl acetate, dried over Na2SO4, and evaporated at 40 °C. The residue was dissolved in 0.2 mL of chloroform for injection (24).
FIGURE 3. Electrolytic degradation of methylparathion. Experimental conditions are reported in the Experimental Section. Inset: logarithmic plot and deviation from first-order kinetics are shown.
FIGURE 4. Total organic carbon (TOC) variation as a function of the electrolytic oxidation time. The ordinate values are scaled to the number of carbon atoms of MeP. Experimental conditions are reported in the Experimental Section. Inset: logarithmic plot and deviation from first-order kinetics are shown. Nitrate, sulfate, and dimethyl phosphate were determined by ion chromatography on a Dionex IONPAC ASA4 column with postcolumn micromembrane suppression and conductivity detection under conditions that were previously reported (25). Phosphate was determined by the phosphomolybdate method with isobutanol modification (26, 27), which is suitable for inorganic phosphate. Particulate sulfur and dissolved organic carbon determinations were carried out by filtering samples on a 0.45µm cellulose acetate membrane. The solids were dried over silica gel and analyzed with Carlo Erba elemental analyzer, Model 1106. Determinations on liquids were carried out by using a Dohrman total carbon analyzer, Model DC-190. The content of N2, H2, and CO2 in the electrolysis gas was determined with a dual-column gas chromatograph with a thermoconductivity detector. The columns were 2.6-m, 3.5mm in diameter copper packed with Poropack Q, and helium was used as a carrier gas (29). Total O2, CO2, NOx, and SO2 production was measured separately by gas scrubbing through bubbler traps containing potassium pyrogallate, barium hydroxide, ferrous sulfate, and sodium tetrachloromercurate solutions, respectively, under conditions reported by Scott (28). Organic acids (oxalic, acetic, formic) were measured with gas chromatograph using a flame ionization detector and a
FIGURE 5. Moles of paraoxon, O-methyl-O-(p-nitrophenyl)monothiophosphoric acid (HMeP) and methanol formed per mole of MeP as a function of electrolytic oxidation time. Experimental conditions are reported in the Experimental Section.
FIGURE 6. Moles of p-nitrophenol, benzoquinone, and hydroquinone formed per mole of MeP as a function of electrolytic oxidation time. Experimental conditions are reported in the Experimental Section.
FIGURE 7. Moles of dimethylphosphoric acid (DMP) and methylmonothiophosphoric (MSP) acid formed per mole of MeP as a function of electrolytic oxidation time. Experimental conditions are reported in the Experimental Section.
1.8-m, 6-mm diameter glass column packed with Chromosorb 101 and helium was used as a carrier gas (29). The formation of peroxodisulfuric acid was detected using the test with Ni(OH)2, in the presence of silver nitrate to avoid interference from other oxidants (30, 31) and was estimated from the sulfur balance. VOL. 38, NO. 22, 2004 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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FIGURE 8. Moles of oxalates, acetates, and formates formed per mole of MeP as a function of electrolytic oxidation time. Experimental conditions are reported in the Experimental Section.
FIGURE 9. Moles of peroxydisulfuric acid and tetraphosphate sulfide formed per mole of MeP as a function of electrolytic oxidation time. Experimental conditions are reported in the Experimental Section.
FIGURE 10. Moles of sulfates, phosphates, and nitrates formed per mole of MeP as a function of electrolytic oxidation time. Experimental conditions are reported in the Experimental Section
FIGURE 11. Moles of nitrogen oxides, oxygen, and sulfur dioxide produced per mole of MeP as a function of electrolytic oxidation time. Experimental conditions are reported in the Experimental Section.
FIGURE 12. Moles of carbon dioxide and total gas produced per mole of MeP as a function of electrolytic oxidation time. Experimental conditions are reported in the Experimental Section.
Results and Discussion
FIGURE 13. pH and redox changes during the electrolytic oxidation of MeP. Experimental conditions are reported in the Experimental Section.
Stoichiometry of the Overall Process. O,O-Dimethyl 4-nitrophenyl phosphate, O-methyl-O-(p-nitrophenyl)monothiophosphoric acid, dimethylphosphoric acid, methylmonothiophosphoric acid, benzoquinone, hydroquinone methanol, oxalic acid, acetic acid, formic acid, peroxydisulfuric acid, sulfates, phosphates, and nitrates were identified in liquid phase as products of methylparathion electrolytic oxidation, and carbon dioxide, sulfur dioxide, nitrogen
dioxide, hydrogen, and traces of oxygen were detected in gas phase. Also in the solid phase tetraphosphate sulfide was identified. Figures 3-12 illustrate the concentrations of the major products and intermediates during the batch electrolytic degradation of a 0.152 M parathion suspension. The results presented on all figures are the mean values from five independent experiments.
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FIGURE 14. Proposed degradation pathways for methylparathion electrolytic degradation.
SCHEME 1
As shown in Figure 13, the pH of the solution decreased during electrolytic oxidation. After 60 min of electrolysis, the pH dropped from 6.1 to 2.8 and remained close to that value thereafter. The observed decrease in pH is consistent with the formation of sulfuric and nitric acid (Figure 10) and peroxydisulfuric acid (Figure 9) as well as the oxalic, acetic, and formic acids (Figure 11). Over a broad pH range, p-nitrophenol is found to be the major degradation product of parathion (32). The degradation of methylparathion to p-nitrophenol can be proceeded via two pathways (2, 33, 34) (see Scheme 1). In alkaline or neutral solution probably route (1) is proceeded (2, 32), while our experimental results suggest that in acidic solution, route (2)
is more favorable. The results presented in Figures 5 and 7 in relation to Figure 13 confirm the observations above where route (2) seems to be the main pathway of the electrolytic oxidation. During electrolysis p-nitrophenol has been degraded mainly to benzoquinone and secondary to hydroquinone (Figure 6), while the nitrogen content is oxidized to NOx (Figures 10 and 11) where [NOx] ) [NO2-] + [NO3-] +[NOy (gas)]. Therefore [NOx] and [p-NP] can be considered as the total amount of PNP that was formed from decomposition of parathion (35). It was previously proved (36) that methylmonothiophosphoric acid (MSP) is degraded electrochemically to yield VOL. 38, NO. 22, 2004 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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methanol tetraphospahate sulfide (TPS) and sulfates, while the dimethyl phosphoric acid (DMP) is degraded under the same experimental conditions to phosphates and methanol. As shown, in Figure 10, the concentration of phosphates during the electrolysis was almost zero, while all the residual phosphorus is accumulated as TPS (Figure 9). This is another evidence that the methylparathion electrochemical degradation procedure follows mainly route (2). Finally benzoquinone and hydroquinone, as shown in Figures 6 and 8, are degraded to organic acids (formate, acetate, and oxalate) and eventually to CO2 (Figure 12). Another important intermediate byproduct is the peroxydisulfuric acid that is increased rapidly during the first 20 min of the experimental procedure followed by a slower degradation (from 0.08 to 0.02 mol per MeP mol) and remained constant thereafter. The peroxydisulfuric acid is a strong oxidative agent but less effective than hydroxyl radicals. The peroxydisulfuric acid concentration curve (Figure 9) follows the DMP concentration curve (Figure 7), so this is an indication that the produced peroxydisulfuric acid plays an important role, by an unknown mechanism, to the catalysis of route (2). Although the experimental oxidation conditions were very strong (i ) 0.56 A/cm2), the redox of the solution was gradually decreased from 700 mV to -150 mV in 120 min (Figure 13). This result was in accordance with the almost zero oxygen (gas) production (Figure 11) proving that the degradation of methylparathion using the electrochemical oxidation procedure described above is very effective for the applied energy (10). The overall proposed degradation pathways for methylparathion electrolytic oxidation are shown schematically in Figure 14. The electrochemical degradation mechanism for methylparathion has been investigated in detail. Due to very low pH and redox conditions that rapidly occurred during the experimental procedure, the m-parathion is degraded to p-nitrophenol via the O-methyl-O-(p-nitrophenyl)monothiophosphoric acid route. Depending on the operational parameters (temperature, current intensity, resident time in the electrolytic cell, etc.), the final products of the degradation can be the following: residues from methylparathion, residues from O-methyl-O-(p-nitrophenyl)monothiophosphoric acid, residues from p-nitrophenol, residues from methylmonothiophosphoric acid, residues from benzoquinone, residues from hydroquinone, residues from methyl paraoxon, residues from dimethylphosphoric acid, residues from methanol, residues from oxalic acid, residues from formic acid, residues from acetic acid, tetraphosphate sulfide, sulfates, nitrates, carbon dioxide, nitrogen oxides, sulfur dioxide, oxygen, and hydrogen. The efficiency during a batch electrochemical degradation of a 0.152 M methylparathion water suspension in an electrolytic cell of 6 L volume with a resident time of 120 min and current intensity 0.56 A/cm2 at 45 °C temperature was about 82% based on parathion degradation (Figure 3) or 66% based on total organic carbon degradation (Figure 4).
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(5) Krueger, F. N.; Seiber, J. N. Treatment and Disposal of Pesticide Wastes; ACS Symposium Series 259; American Chemistry Society: Washington, DC, 1984. (6) Pesticide Waste Management Technology and Regulation; Bourke, J. B., Felson, A. S., Gilding, T. J., Jensen, J. K., Seiber, J. N., Eds.; ACS Symposium Series 510; American Chemistry Society: Washington, DC, 1991. (7) Pignatello, J. J.; Sun, Y. Complete oxidation of metolachlor and methyl parathion in water by the photoassisted Fenton reaction. Water Res. 1995, 29(8), 1837-1843. (8) Chen, T.; Doong, R.; Lei, W. Photocatalytic degradation of parathion in aqueous TiO2 dispersion: the effect of hydrogen peroxide and light intensity. Water Res. 1998, 37(8), 187-192. (9) Zeinali, M.; Torrents, A. Mercury-Promoted Hydrolysis of Parathion-methyl: Effect of Chloride and Hydrated Species. Environ. Sci. Technol. 1998, 32, 2338-2342. (10) Comninellis, C. Electrochemical treatment of wastewater. GWA 1992, 11, 792-797. (11) Comninellis, C. Electrocatalysis in the electrochemical conversion/combustion of organic pollutants for wastewater treatment. Electrochim. Acta 1994, 39(11/12), 1857-1862. (12) Comninellis, C.; Pulgarin, C. Anoxic oxidation of phenol for wastewater treatment. J. Appl. Electrochem. 1991, 21, 703-708. (13) Comninellis, C.; Nerini, A. Anodic oxidation of phenol in the presence of NaCl for wastewater treatment. J. Appl. Electrochem. 1995, 25(1), 23-28. (14) Della Monica, M.; Agostino, A.; Ceglie, A. An electrochemical sewage treatment process. J. Appl. Electrochem. 1980, 10(4), 527-533. (15) Vlyssides, A.; Karlis, P.; Loizidou, M.; Zorpas, A.; Arapoglou, D.Treatment of Leachate from a Domestic Solid Waste Sanitary Landfill by an Electrolysis System. Environ. Technol. 2001, 22, 1467-1476. (16) Chang, L. C.; Wen, T. C. Indirect oxidation effect in electrochemical oxidation treatment of landfill leachate. Water Res. 1995, 29(2), 671-678. (17) Vlyssides, G. A.; Israilides, J. C. Detoxification of tannery waste liquors with an electrolysis system. Environ. Pollut. 1997, 97(1-2), 147-152. (18) Israilides, C. J.; Vlyssides, A. G.; Loizidou, M.; Mourafeti, V. N.; Karvouni, G. Olive oil waste treatment with the use of an electrolysis system. Proceedings of the 2nd Specialised Conference on Pretreatment of Industrial Wastewaters, IAWQ, Athens, Greece, 840 1996. (19) Israilides, C. J.; Vlyssides, A. G.; Mourafeti, V. N.; Karvouni, G. Olive oil wastewater treatment with the use of an electrolysis system. Bioresour. Technol. 1997, 61(2), 163-170. (20) Vlyssides, A. G.; Loizidou, M.; Karlis, P.; Zorpas, A.; Papaioannou, D. Electrochemical oxidation of a textile dye wastewater using a Pt/Ti electrode. J. Hazard. Mater. 1999, 70(1-2), 41-52. (21) Comninellis, Ch. Electrocatalysis in the electrochemical conversion/combustion of organic pollutants for wastewater treatment. Electrochim. Acta 1994, 39(11/12), 1857-1962. (22) Panizza, M.; Michaud, A. P.; Cerisola, G.; Comninellis, Ch. Electrochemical treatment of wastewaters containing organic pollutants on boron-doped diamond electrodes: Prediction of specific energy consumption and required electrode area. Electrochem. Commun. 2001, 3(7), 336-339. (23) Ouattara, L.; Duo, I.; Diaco, Th.; Invandini, A.; Honda, K.; Rao, T.; Fujishima, A.; Comninellis, Ch. New Diamond Front. Carbon Technol. 2003, 13(2), 97-108. (24) Pignatello, J. J.; Sun, Y. Complete oxidation of metolachlor and methyl parathion in water by the photoassisted Fenton reaction. Water Res. 1995, 29(8), 1837-1844. (25) Dasgupta, P. K. In Ion Chromatography; Tarter, J. G., Ed.; Marcel Dekker: New York, 1987; p 257. (26) APHA-AWWA-WPCF, In Standard Methods for the examination of water and wastewater, 17th ed.; American Public Health Association: Washington, DC, 1989. (27) Martin, J. B.; Doty, D. M. Determination of Inorganic Phosphate. Anal. Chem. 1949, 21, 965-967. (28) Scott, W. W. In Scott’s Standard Methods of Chemical Analysis, 5th ed.; D. Van Nostram Co.: 1939; p 2348. (29) Therkelsen, H. H.; Carlson, A. D. Thermophilic anaerobic digestion of a strong complex substrate. J. WPCF 1979, 51(7), 1949-1964. (30) Serrano, K.; Michaud, A. P.; Comninellis, C.; Savall, A. Electrochemical preparation of peroxodisulfuric acid using boron doped diamond thin film electrodes. Electrochim. Acta 2002, 48(4), 431-436.
(31) Feigl, F.; Anger, V. Spot Tests in Inorganic Analysis; Elsevier: Amsterdam, 1972; p 458. (32) Faust, S. D.; Gomaa, H. M. Chemical hydrolysis of some organic phosphorus and carbamate pesticides in aquatic environments. Environ. Lett. 1972, 3, 171-201. (33) Jackson, J. A.; Blair, W. R.; Brinckman, F. E.; Iverson, W. P. E. Gas-chromatographic speciation of methylstannanes in the Chesapeake Bay using purge and trap sampling with a tinselective detector. Environ. Sci. Technol. 1982, 16(2), 110-119. (34) Bricker, J. L. Atomic emission spectrometric determination of mercury in natural waters at the part-per-trillion level. Anal. Chem. 1980 52(3), 492-496.
(35) Kotronarou, A.; Mills, G.; Hoffmann, M. R. J. Ultrasonic irradiation of p-nitrophenol in aqueous solution. Phys. Chem. 1991, 95, 3630-3638. (36) Arapoglou, D. Degradation of organophosphoric obsolete pesticides stocks by electrochemical oxidation, Ph.D. Thesis, Athens, NTUA, 2003.
Received for review February 20, 2004. Revised manuscript received August 12, 2004. Accepted August 27, 2004. ES049726B
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