Degradation of Methylparathion in Aqueous Solution by Electrochemical Oxidation

Degradation of Methylparathion in Aqueous Solution by Electrochemical OxidationA P O S T O L O S V L Y S S I D E S,*E L L I M A R I A B A R A M P O U T I,A N DS O F I A M A ISchool of Chemical Engineering,National Technical University of Athens,15770Athens,GreeceD I M I T R I S A R A P O G L O UInstitute of Technology of Agricultural Products,NAGREF,14123Athens,GreeceA N A S T A S I A K O T R O N A R O UInstitute for Environmental Research and Sustainable Development,15236Athens,GreeceThe electrochemical degradation of methylparathion has been investigated by using Ti/Pt as anode,Stainless Steel 304as 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.IntroductionThe 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 ac-cumulated 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 than400000tonnes of obsolete pesticides are stocked worldwide(1).Methylparathion(O,O-methyl O-p-nitrophenyl thiophos-phate)is a widely used anophosphate 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 waters.At20°C and pH7.4,parathion has a hydrolytic half-life of108days,and its toxic metabolite,paraoxon,has a similar half-life of144days(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 mineral-ization,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 incin-erators 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 mercury-promoted 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.Electrochemi-cal 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 electro-chemical degradation of high concentration of MeP in water solution in a laboratory scale plant using Ti/Pt as anode, Stainless Steel304as cathode,and sodium chloride as electrolyte is presented.Some new and characteristic features in the degradation patterns have been observed. Theoretical SectionThere are two main processes in the electrochemical treat-ment of wastes(21):(i)the conversion of nonbiodegradable organics to biodegradable before a biological treatment and (ii)the combustion of organics to CO2and water.Figure1 shows a generalized scheme of the electrochemical conver-sion/combustion of organics on oxide anode(MO X).In the first step,H2O is discharged at the anode to produce absorbed hydroxyl radicals according to the reaction:In a second step,the adsorbed hydroxyl radicals may interact with the oxygen already present in the oxide anode*Correspondingauthorphone:+302107723268;fax:+302107723269; e-mail:avlys@tee.gr.MO X+H2O f MO X(•OH)+H++eEnviron.Sci.Technol.2004,38,6125-613110.1021/es049726b CCC:$27.50©2004American Chemical Society VOL.38,NO.22,2004/ENVIRONMENTAL SCIENCE&TECHNOLOGY96125 Published on Web10/09/2004with possible transition of oxygen from the absorbed hydroxyl radical to the oxide anode forming the higher oxide MO X +1.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,MO X +1).In the absence of any oxidizable organics,the “active oxygen”produces dioxygen according to the following reactions:In the presence of oxidizable organics the physico-sorbed “active oxygen”(•OH)should cause predominantly the complete combustion of organics,and chemico-sorbed (MO X +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 •+H 2O;(2)reaction of organic radical with dioxygen formed at the anode:R •+O2f 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 SectionReagents.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 6L of brine solution (H 2O +NaCl).A Ti/Pt cylindrical electrode (14cm long ×1.5cm diameter)was used as anode.It was covered by platinum alloy foil approximately 0.22mm thick.The electrode was located inside a perforated stainless steel 304cylinder (14cm long ×8cm diameter)which served as cathode.This construction ensured homogeneous dynamic lines between anode and cathode and provided good contact of the waste with theelectrode.The electrochemical system was operated at 30-36d.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 300rpm 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 6L with an 8%w/w aqueous suspension of methylparathion and 20g/L of sodium chloride was added as electrolyte medium.During the electrolytic oxidation the agitation was constant at 300rpm.The current density and temperature were kept constant at 0.56A/cm 2and 45°C,respectively.The total time of electrolysis was 2h.The results as presented below are the mean values of 10experimental runs.Analytical Determinations.The MeP detections carried out with a Perkin-Elmer HPLC Model P2000pump with UV detection (Model LC 1200),equipped with a bonded-phase octadecylsilica column (Lichrospher C-18Merck,25-cm length,46-mm i.d.,5-µm packing).The primary degradation of methylparathion was followed by direct injection of filtered (0.45µm)electrolyzed sample;UV detection was performed at 276nm;the mobile phase was 60%acetonitrile/40%water at 1mL min -1;and retention time was 16.0min.Reaction intermediates such as paraoxon,p -nitrophenol (PNP),benzoquinone,and hydroquinone were identified by gas chromatography/mass spectrometry (GC/MS)and quan-tified 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 5890gas chromatograph and controlled by a SuperIncos data system.The spectra were recorded by setting the electron energy to 70eV and the accelerate to 3kV and applying a source temperature of 200°C.The magnet was scanned from 29to 800Da in 0.8s (1.0-s cycle time).Samples were injected splitless into SE-5230m ×0.32mm capillary column inserted directly into ion source.The auxiliary carrier gas was nitrogen at 30mL/min velocity.The oven temperature was programmed as follows:iso-thermal at 60°C for 1min,from 60to 300°C at 10°C min -1,isothermal at 300°C for 10min.Ten milliliters of the reaction mixture was extracted with 10mL of ethyl acetate,dried over Na 2SO 4,and evaporated at 40°C.The residue was dissolved in 0.2mL of chloroform for injection (24).FIGURE 1.Selective oxidation and combustion of organics in electrolochemical process.MO X (•OH)f MO X +1+H ++eMO X (•OH)f 1/2O 2+H ++e +MO XMO X +1f 1/2O 2+MOXFIGURE 2.Experimental apparatus.61269ENVIRONMENTAL SCIENCE &TECHNOLOGY /VOL.38,NO.22,2004Nitrate,sulfate,and dimethyl phosphate were determined by ion chromatography on a Dionex IONPAC ASA4column with postcolumn micromembrane suppression and con-ductivity 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 deter-minations were carried out by filtering samples on a0.45-µm cellulose acetate membrane.The solids were dried over silica gel and analyzed with Carlo Erba elemental analyzer, Model1106.Determinations on liquids were carried out by using a Dohrman total carbon analyzer,Model DC-190.The content of N2,H2,and CO2in the electrolysis gas was determined with a dual-column gas chromatograph with a thermoconductivity detector.The columns were2.6-m,3.5-mm in diameter copper packed with Poropack Q,and helium was used as a carrier gas(29).Total O2,CO2,NO x,and SO2production was measuredseparately 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 1.8-m,6-mm diameter glass column packed with Chro-mosorb101and 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.FIGURE3.Electrolytic degradation of methylparathion.Experimental conditions are reported in the Experimental Section.Inset:loga-rithmic plot and deviation from first-order kinetics are shown. FIGURE4.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.FIGURE5.Moles of paraoxon,O-methyl-O-(p-nitrophenyl)mono-thiophosphoric acid(HMeP)and methanol formed per mole of MeP as a function of electrolytic oxidation time.Experimental conditions are reported in the Experimental Section.FIGURE6.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. FIGURE7.Moles of dimethylphosphoric acid(DMP)and methyl-monothiophosphoric(MSP)acid formed per mole of MeP as a function of electrolytic oxidation time.Experimental conditions are reported in the Experimental Section.VOL.38,NO.22,2004/ENVIRONMENTAL SCIENCE&TECHNOLOGY96127Results and DiscussionStoichiometry of the Overall Process .O ,O -Dimethyl 4-ni-trophenyl phosphate,O -methyl-O -(p -nitrophenyl)mono-thiophosphoric acid,dimethylphosphoric acid,methyl-monothiophosphoric acid,benzoquinone,hydroquinone methanol,oxalic acid,acetic acid,formic acid,peroxydis-ulfuric acid,sulfates,phosphates,and nitrates were identified in liquid phase as products of methylparathion electrolytic oxidation,and carbon dioxide,sulfur dioxide,nitrogendioxide,hydrogen,and traces of oxygen were detected in gas phase.Also in the solid phase tetraphosphate sulfide was identified.Figures 3-12illustrate the concentrations of the major products and intermediates during the batch elec-trolytic degradation of a 0.152M parathion suspension.The results presented on all figures are the mean values from five independent experiments.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 ExperimentalSection.FIGURE10.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 SectionFIGURE 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.FIGURE 13.pH and redox changes during the electrolytic oxidation of MeP.Experimental conditions are reported in the Experimental Section.61289ENVIRONMENTAL SCIENCE &TECHNOLOGY /VOL.38,NO.22,2004As shown in Figure 13,the pH of the solution decreased during electrolytic oxidation.After 60min of electrolysis,the pH dropped from 6.1to 2.8and 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 5and 7in relation to Figure 13confirm 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 NO x (Figures 10and 11)where [NO x ])[NO 2-]+[NO 3-]+[NO y (gas)].Therefore [NO x ]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 methylmonothiophos-phoric acid (MSP)is degraded electrochemically to yieldFIGURE 14.Proposed degradation pathways for methylparathion electrolytic degradation.SCHEME 1VOL.38,NO.22,2004/ENVIRONMENTAL SCIENCE &TECHNOLOGY96129methanol 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 Figure10,the concentration of phosphates during the electrolysis was almost zero,while all the residual phosphorus is accumulated as TPS(Figure9).This is another evidence that the methylparathion electrochemical degrada-tion procedure follows mainly route(2).Finally benzoquinone and hydroquinone,as shown in Figures6and8,are degraded to organic acids(formate, acetate,and oxalate)and eventually to CO2(Figure12).Another important intermediate byproduct is the per-oxydisulfuric acid that is increased rapidly during the first 20min of the experimental procedure followed by a slower degradation(from0.08to0.02mol 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(Figure9) follows the DMP concentration curve(Figure7),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)0.56A/cm2),the redox of the solution was gradually decreased from700mV to-150mV in120min(Figure13). This result was in accordance with the almost zero oxygen (gas)production(Figure11)proving that the degradation of methylparathion using the electrochemical oxidation pro-cedure described above is very effective for the applied energy (10).The overall proposed degradation pathways for meth-ylparathion electrolytic oxidation are shown schematically in Figure14.The electrochemical degradation mechanism for meth-ylparathion 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)mono-thiophosphoric 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)monothiophos-phoric acid,residues from p-nitrophenol,residues from methylmonothiophosphoric acid,residues from benzo-quinone,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 a0.152M methylparathion water suspension in an electrolytic cell of6L volume with a resident time of120min and current intensity0.56A/cm2at45°C temperature was about82%based on parathion degradation(Figure3)or66% based on total organic carbon degradation(Figure4). Literature Cited(1)FAO Pesticide Disposal Series.Baseline study on the problem ofobsolete pesticide stocks;Rome,2000.(2)Kotronarou,A.;Mills,G.;Hoffmann,R.M.Decomposition ofparathion in aqueous solution by ultrasonic irradiation.Environ.Sci.Technol.1992,26(7),1460-1462.(3)Zaleska,A.;Hupka,J.Problem of disposal of unwanted pesticidesdeposited in concrete tombs.Waste Manage.Res.1999,17,220-226.(4)Felsot,A.S.Options for cleanup and disposal of pesticide wastesgenerated on a small scale.J.Environ.Sci.Health1996,B31(3), 365-381.(5)Krueger,F.N.;Seiber,J.N.Treatment and Disposal of PesticideWastes;ACS Symposium Series259;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 Series510;American Chemistry Society: Washington,DC,1991.(7)Pignatello,J.J.;Sun,plete oxidation of metolachlor andmethyl parathion in water by the photoassisted Fenton reaction.Water Res.1995,29(8),1837-1843.(8)Chen,T.;Doong,R.;Lei,W.Photocatalytic degradation ofparathion in aqueous TiO2dispersion:the effect of hydrogen peroxide and light intensity.Water Res.1998,37(8),187-192.(9)Zeinali,M.;Torrents, A.Mercury-Promoted Hydrolysis ofParathion-methyl:Effect of Chloride and Hydrated Species.Environ.Sci.Technol.1998,32,2338-2342.(10)Comninellis,C.Electrochemical treatment of wastewater.GWA1992,11,792-797.(11)Comninellis,C.Electrocatalysis in the electrochemical conver-sion/combustion of organic pollutants for wastewater treatment.Electrochim.Acta1994,39(11/12),1857-1862.(12)Comninellis,C.;Pulgarin,C.Anoxic oxidation of phenol forwastewater treatment.J.Appl.Electrochem.1991,21,703-708.(13)Comninellis,C.;Nerini,A.Anodic oxidation of phenol in thepresence of NaCl for wastewater treatment.J.Appl.Electrochem.1995,25(1),23-28.(14)Della Monica,M.;Agostino,A.;Ceglie,A.An electrochemicalsewage 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 SanitaryLandfill by an Electrolysis System.Environ.Technol.2001,22, 1467-1476.(16)Chang,L.C.;Wen,T.C.Indirect oxidation effect in electro-chemical oxidation treatment of landfill leachate.Water Res.1995,29(2),671-678.(17)Vlyssides,G.A.;Israilides,J.C.Detoxification of tannery wasteliquors 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 the2nd Specialised Confer-ence on Pretreatment of Industrial Wastewaters,IAWQ,Athens, Greece,8401996.(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 usinga Pt/Ti electrode.J.Hazard.Mater.1999,70(1-2),41-52.(21)Comninellis,Ch.Electrocatalysis in the electrochemical conver-sion/combustion of organic pollutants for wastewater treatment.Electrochim.Acta1994,39(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.mun.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,plete oxidation of metolachlor andmethyl 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.;MarcelDekker:New York,1987;p257.(26)APHA-AWWA-WPCF,In Standard Methods for the examinationof 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;p2348.(29)Therkelsen,H.H.;Carlson, A. D.Thermophilic anaerobicdigestion of a strong complex substrate.J.WPCF1979,51(7), 1949-1964.(30)Serrano,K.;Michaud,A.P.;Comninellis,C.;Savall,A.Elec-trochemical preparation of peroxodisulfuric acid using boron doped diamond thin film electrodes.Electrochim.Acta2002, 48(4),431-436.61309ENVIRONMENTAL SCIENCE&TECHNOLOGY/VOL.38,NO.22,2004(31)Feigl,F.;Anger,V.Spot Tests in Inorganic Analysis;Elsevier:Amsterdam,1972;p458.(32)Faust,S.D.;Gomaa,H.M.Chemical hydrolysis of some organicphosphorus 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 tin-selective detector.Environ.Sci.Technol.1982,16(2),110-119.(34)Bricker,J.L.Atomic emission spectrometric determination ofmercury in natural waters at the part-per-trillion level.Anal.Chem.198052(3),492-496.(35)Kotronarou, A.;Mills,G.;Hoffmann,M.R.J.Ultrasonicirradiation of p-nitrophenol in aqueous solution.Phys.Chem.1991,95,3630-3638.(36)Arapoglou, D.Degradation of organophosphoric obsoletepesticides stocks by electrochemical oxidation,Ph.D.Thesis, Athens,NTUA,2003.Received for review February20,2004.Revised manuscript received August12,2004.Accepted August27,2004.ES049726BVOL.38,NO.22,2004/ENVIRONMENTAL SCIENCE&TECHNOLOGY96131。