Research Article

Received: 18 January 2008, Revised: 15 December 2008, Accepted: 16 December 2008, Published online 17 March 2009 in Wiley Interscience

(www.interscience.wiley.com) DOI 10.1002/bio.1120

Copyright © 2009 John Wiley & Sons, Ltd. Luminescence 2009; 24: 266–270

266

John Wiley & Sons, Ltd.A novel inhibitory kinetic fluorimetric method for the determination of trace methomyl in environmental samplesDetermination of trace methomyl in environmental samplesJing Fana, Xiaojing Shaoa, Haizhu Xub and Suling Fenga

ABSTRACT: A novel inhibitory kinetic fluorimetric method for the determination of trace methomyl was proposed. It wasshown that the Fenton reagent oxidized rhodamine B in acid medium which enabled the fluorescence quenching of the latter.The presence of trace methomyl clearly inhibited the reaction. Upon addition of EDTA, a good linear relationship betweenthe inhibitory effect and the concentration of methomyl was observed, together with improved stabilization and sensitivity.Factors affecting the determination of trace methomyl were investigated systematically. Under the optimum conditions,the linear range for the determination of methomyl was 0.04–2.2 mg/mL; the detection limit and the quantification limit formethomyl were 0.011 and 0.037 mg/mL, respectively. The proposed method was applied to the determination of methomyl infour environmental soil samples, six environmental water samples and one synthetic sample; the results were compared withthose determined by the HPLC method. The recoveries and the relative errors were 83.5–101.2 and 0.47–2.02%, respectively.The possible reaction mechanism has also been discussed. Copyright © 2009 John Wiley & Sons, Ltd.

Keywords: kinetic fluorimetric method; methomyl; rhodamine B; Fenton reagent; environmental samples

Introduction

Methomyl is one of the carbamate insecticides. Because of itsbroad biological activity, relatively rapid disappearance andhigh efficiency against insects, methomyl is widely used in manyagricultural countries for crop protection and soil or plant treat-ment (1). Considering the fact that methomyl has high solubilityin water, low affinity for sediment binding and strong inhibitoryeffect on acetylcholinesterase, it is expected that methomyl mayhave potential toxicological effects for groundwater (2) and forhuman beings. As a result, special attention had been paid tothe determination of methomyl in the environment (3). The USEnvironmental Protection Agency (EPA) drinking water regulationsrequires 45 unregulated substances to be monitoried, includingcarbamates, but the maximum contaminant levels have not yetbeen established. On the other hand, the European Union directiveon drinking water quality established a maximum permissibleconcentration for pesticides of 0.1 μg/L for individual pesticidesand 0.5 μg/L in total (4). Because of this legal limit, methods thatcan easily determine trace carbamates in water samples are needed.

Nowadays, the methods most frequently described in litera-ture for the determination of methomyl in water and soil samplesare based on chromatographic analysis, such as gas chromato-graphy/mass spectrometry (GC-MS) (5) and liquid chromato-graphy–mass spectrometry (LC-MS) (6–11), although other methods(12) have also been proposed. The detection of methomyl inenvironment using these techniques has been adequatelyreported at very low detection levels. However, these techniquesneed either an inconvenient derivatization approach or toxicorganic solvents. At the same time, they are time-consuming,expensive and require trained technicians, which make popular-ization difficult. Therefore, development of an inexpensive, fastand sensitive method for the determination of methomyl is ofprimary interest.

Hydroxyl radicals (•OH) generated by various approaches havevery high reaction activity. These radicals can join in series ofreactions as an oxidant or reducing agent. Of these, the Fentonreagent [Fe(II) + H2O2] can generate plenty of peroxyl radicals inacidic solutions, where H2O2 is used as the precursor of the •OH,and Fe(II) as the catalytic reagent (13, 14). So far the applicationof the oxidation of Fenton reagent has been most concentratedon the degradation of both environmental organic pollutants(15–17) and pesticide pollutants (18–20). However, to the bestof our knowledge, there is no report in the literature on theirapplication in the analytical determination of pesticides.

In this paper, we describe a rapid, simple and sensitive kineticfluorimetric method for the determination of trace methomyl inenvironment water and soil samples based on the inhibitoryeffect of methomyl on the redox reaction between rhodamine Band Fenton reagent. It was found that hydroxyl radicals wereproduced by Fenton reagent oxidized rhodamine B (RhB), andcaused the fluorescence quenching of RhB. The presence oftrace methomyl has an inhibitory effect on the reaction, but nolinear relationship was observed between the inhibitory effect

* Correspondence to: Jing Fan, School of Chemistry and EnvironmentalScience, Key Laboratory for Yellow River and Huai River Water Environmentand Pollution Control, Henan Key Laboratory for Environmental PollutionControl, Henan Normal University, Xinxiang, Henan 453007, People’s Repub-lic of China. E-mail: fanjing@henannu.edu.cn

a School of Chemistry and Environmental Science, Key Laboratory for YellowRiver and Huai River Water Environment and Pollution Control, Henan KeyLaboratory for Environmental Pollution Control, Henan Normal University,Xinxiang, Henan 453007, People’s Republic of China

b Henan Yubei Administrative office of the National Natural ConservationArea for Birds and Wetland, Xinxiang, Henan 453000, People’s Republic ofChina

Determination of trace methomyl in environmental samples

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and the concentration of methomyl. Upon addition of EDTA, agood linear relationship was found and the stabilization and thesensitivity of the system were also improved. Compared withchromatographic methods, the proposed method is simple,inexpensive and does not need trained technicians. This methodhas been applied to determine methomyl in soil and environ-mental water samples; the results obtained were close to thosedetermined by the HPLC method.

Experimental

Reagents

Methomyl stock solution (1.0 g/L) was obtained by dissolving agiven amount of solid methomyl in a 100 mL volumetric flaskwith water and stored in the dark at 4°C. Working solutions(10 μg/mL) were prepared by diluting the stock solution beforeuse. A stock solution of 1.0 × 10−3 mol/L RhB was prepared bydissolving RhB in a given volume of water. EDTA–Fe(II) stocksolution (5.0 × 10−3 mol/L) was obtained by mixing the same volumeof 0.01 mol/L EDTA and 0.01 mol/L FeSO4·7 H2O. Solutions of0.036 mol/L H2SO4 and 0.15% hydrogen peroxide were preparedaccording to the standard methods. All the working solutionswere freshly prepared by appropriate dilution of the stocksolution. Methanol and acetonitrile (HPLC grade) were passedthrough a 0.45 μm filter before use. All the chemicals used wereof analytical reagent grade unless otherwise stated, and distilledwater was used throughout the work.

Apparatus

Fluorescence spectra were measured with an FP-6200 spectro-fluorometer (Jasco, Japan). Fluorescence intensity measurementswere carried out on a 930A spectrofluorimeter (Shanghai 3rdAnalytical Instrument Factory, China) using 1 cm quartz cells. Amodel 501 thermostat water bath (Chongqing ExperimentalInstrument Factory, China) was used to maintain the tempera-ture of the system. Professional software Origin, version 6.0,was used for data processing. The HPLC system used was 1100HPLC (Agilent, USA), which included an Ultra-Pure WaterSystem (SG Co., Germany) and a diode array detector set at232.0 nm.

Procedure

Into a 25 mL standard flask, a given amount of RhB, H2SO4 solu-tion, EDTA–Fe(II) solution, working solution of methomyl andan appropriate volume of hydrogen peroxide solution were added.The solution was mixed well, diluted to the mark with water andshaken until homogeneous, then kept at room temperature for10 min. The fluorescence intensity, F was determined at theexcitation wavelength of 556.0 nm and emission wavelength of576.0 nm. The fluorescence value for the blank sample withoutmethomyl, F0 was obtained under the same conditions, then thevalues of ΔF = F − F0 were calculated.

Results and discussion

Spectral characteristics (excitation and emission)

Excitation and emission spectra for different compositions ofsolutions were scanned using FP-6200 spectrofluorometer at the

same experimental conditions. It was found from Fig. 1 that,when RhB was oxidized by the hydroxyl radicals generated bythe Fenton reagent, the fluorescence of RhB disappeared sinceits molecular structure was destroyed (21) (Fig. 1, 5 and 5′). Uponaddition of trace methomyl, the rate of the redox reactiondecreased (Fig. 1, 4 and 4′). When EDTA–Fe(II) was absent, boththe blank and the sample reactions were very inconspicuous(Fig. 1, 1 and 1′, 2 and 2′ and 3 and 3′), indicating that the EDTA–Fe(II) influenced the production of hydroxyl radicals significantly.Furthermore, it was observed that there was a good linearrelationship between ΔF and the concentration of methomyl inthe presence of EDTA. The optimum excitation and emissionwavelengths are 556.0 and 576.0 nm, respectively.

Selection of the optimum conditions

In order to obtain an optimized system, various experimentalparameters have been investigated. The reagent concentrationsand reaction conditions were optimized by setting all para-meters to be constant and optimizing one each time. Eachexperiment was replicated at least three times and the concen-tration of methomyl was kept at 1.0 μg/mL.

In order to choose a best acid media, several aqueous acidswith the same concentration were tested in the present work. Itwas found that the indicator reaction may take place in aqueoussolution of sulfuric acid, phosphoric acid, perchloric acid or nitricacid. The reaction rate followed the order: nitric acid > sulfuricacid ≈ phosphoric acid > perchloric acid. However, we foundthat the reaction was striking and the system was stable onlyin aqueous sulfuric acid. Therefore, sulfuric acid was selectedas the reaction medium. The rate of reaction is known todepend on the pH of the system. Thus the effect of sulfuric acidconcentration was studied and an optimum concentration of11.52 × 10−4 mol/L was selected for the further studies.

In the present work, the Fenton reagent [Fe(II) + H2O2] wasused to produce hydroxyl radicals. However, it was found that,although the fluorescence of RhB can be quenched significantly,there is no linear relationship between the concentration of

Figure 1. Excitation (a) and emission (b) spectra of RhB in the presence of differ-ent reagents: (1, 1′), RhB + H2SO4; (2, 2′), H2O2 + RhB + H2SO4; (3, 3′), methomyl +H2O2 + RhB + H2SO4; (4, 4′), EDTA–Fe(II) + methomyl +H2O2 + RhB + H2SO4; (5, 5′),EDTA–Fe(II) + H2O2 + RhB + H2SO4. RhB, 3.2 × 10−6 mol/L; H2O2, 0.006%; EDTA–Fe(II),3.4 × 10−5 mol/L; H2SO4, 11.52 × 10−4 mol/L; methomyl, 1.0 μg/mL; room tempera-ture; time, 10.0 min.

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methomyl and the ΔF. Once EDTA was added, a good linear rela-tionship was observed, and the stabilization of the system aswell as the sensitivity of the determination were improved. Theeffect of the EDTA–Fe(II) concentration on both the sample andthe blank reactions was studied. The concentration of hydrogenperoxide, as the precursor of the •OH, may greatly influence the•OH concentration. As a result, the effect of hydrogen peroxideconcentration was also studied. At the same time, the concen-tration of the fluorescent indicator of the reaction, RhB, was alsoinvestigated. All the concentration range investigated and theoptimal conditions are listed in Table 1.

The effects of reaction temperature and time were investig-ated in the range indicated in Table 1. It was observed that anincrease in temperature caused a decrease in the ΔF, but ΔFvalues were constant in the range of reaction time studied. How-ever, when the reaction time was less than 9.0 min, the ΔF valuewas not stable. Therefore, we selected room temperature as thereaction temperature and 10.0 min as the reaction time.

Analytical parameters

Under the optimum conditions, the calibration graph formethomyl was obtained by a fixed-time method. The graph waslinear in the concentration range of 0.04–2.2 μg/mL. The regres-sion equation was found to be ΔF = 15.78 + 40.48C (μg/mL) (n =8), with a correlation coefficient of 0.9994. The detection limit[LOD = 3Sb/k, where Sb is the standard deviation of the regent

blank (n = 11) and k is the slope of the calibration graph] and thequantification limit (LOQ = 10 Sb/k) of the method were 0.011and 0.037 μg/mL, respectively.

To evaluate the precision of the method, two independentstandard samples were used in the measurements. The RSD(n = 11) was found to be 3.65 and 1.53% for the determination ofmethomyl at concentrations of 0.2 and 1.0 μg/mL, respectively.

Selectivity and stability

To study the selectivity of the proposed method, the effect ofcommon ions and compounds on the determination of 0.8 μg/mLmethomyl was tested under the optimum conditions. Theresults are summarized in Table 2. It was shown that, exceptfor Hg(II), As(III), PO4

3−, Br− and NO3−, the studied common ions

did not interfere with the determination. The interfering effectof the other ions in the analysis of the real samples can becompletely removed by using a cation exchange resin and ananion exchange resin, and the interference of the phenolic com-pounds can be removed by distilling. However, because of thestrong oxidation ability of hydroxyl radicals, other coexistingorganics can also interfere with the determination. Therefore,appropriate separation method should be selected on the basisof the compositions of the real samples.

It was found that F and F0 decreased with the reaction time atroom temperature, but ΔF can remain constant for about 1 h.This indicated that the system is stable enough.

Table 1. The optimized experimental conditions for the determination of 1.0 μg/mL methomyl

Parameters of study Range of study Optimum condition

Sulfuric acid (mol/L) 2.88 × 10−4 to 4.32 × 10−3 11.52 × 10−4

EDTA-Fe(II) (mol/L) 1.0 × 10−5 to 6.0 × 10−5 3.4 × 10−5

RhB (mol/L) 0.8 × 10−6 to 4.0 × 10−6 3.2 × 10−6

H2O2 (%) 0.002–0.010 0.006Temperature (°C) 15–55 Room temperatureTime (min) 2.0–50.0 10.0

Table 2. Effect of foreign substances on the determination of 0.8 μg/mL methomyl

Foreignsubstance

Ratioa Change in fluorescenceintensity (%)

Foreignsubstance

Ratioa Change in fluorescenceintensity (%)

Na+ 3.1 × 103 +4.5 Cl− 1.0 × 102 +5.0K+ 1.6 × 103 +3.3 CH3COO− 1.0 × 10 −5.4NH4

+ 1.2 × 104 +4.4 Mn2+ 2.2 × 10 −5.2Hg (II) 2.4 −5.4 Cd2+ 6.5 × 10 −6.0Mg2+ 1.4 × 103 +4.6 Cu2+ 1.2 × 10 +5.4Al3+ 5.3 × 102 −5.4 As(III) 2.0 +4.2CO3

2− 3.1 × 102 −5.2 ClO4− 8.2 × 102 −5.5

BrO3− 2.4 × 103 −4.8 Formic acid 2.2 × 10 +4.8

Zn2+ 2.9 × 103 −3.8 Sucrose 9.3 × 10 +4.1Br− 0.5 +4.9 Amylum 1.1 × 102 +5.1PO4

3− 2.0 +3.6 p-Nitrophenol 0.2 +4.8NO3

− 0.1 −4.2 m-Nitrophenol 1.0 +4.6HPO4

2− 1.6 × 10 +5.4 Imidacloprid 1.0 +5.9H2PO4

− 6.4 × 10 +2.1 Acetamiprid 1.25 +4.8aThe ratio of concentration between the interfering substance and methomyl, i.e. [ion]/[methomyl].

Determination of trace methomyl in environmental samples

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Processing and analysis of real and synthetic samples

Several environmental water samples such as tap water, surfacewater, river water and lake water were collected and filteredthree times. River water sediment, lake water sediment, kaleyardsoil and generic soil were collected, air-dried and ground intoparticles with a diameter of 80 μm; an appropriate amount ofthe particles was put into conical flasks. Then, an appropriatevolume of distilled water was added and mixed at room temper-ature for 5 h, kept for some time and filtered. All the filtrateswere passed through a 0.45 μm filter in order to remove the smallparticles and bacteria, then they were passed through an anionexchange resin (strong-base OH− form, Xi’an, China) and a cationexchange resin (strong-acid H+ form, Xi’an, China), respectively, toremove the interfering anions and cations.

A synthetic sample was prepared with the following composi-tion: methomyl, 1.0 μg/mL; Na+, 1000 μg/mL; NH4

+, 1000 μg/mL;Hg (II), 10 μg/mL; As(III), 10 μg/mL; PO4

3−, 10 μg/mL; NO3−, 10 μg/

mL; Br−, 10 μg/mL; phenol, 2.0 μg/mL; and m-nitrophenol,2.0 μg/mL. The synthetic water sample was adjusted to pH 11, inwhich the phenolic compounds existed as anions, and thenpassed through the anion exchange resin. Then, the watersample was adjusted to pH 5, and passed through the cationexchange resin. In this way, the above-mentioned foreign inter-fering substances can be removed.

Under the optimum conditions, these samples were analyzedby the proposed method, and recovery experiments werealso carried out for the real samples. The results are shown inTables 3 and 4. To evaluate the reliability of the proposed method,a comparison between the proposed method and the HPLCmethod was made for tap water, lake water sediment andsynthetic samples. An Agilent 1100 HPLC system was employedfor the determination at 232.0 nm. Water–methanol (40:60, v/v)was used as mobile phase, and the flow rate was fixed at 1.0 mL/min.The volume of sample injected by an autosampler was 50 μL,and temperature of the column was maintained at 30°C. Theexperimental results are given in Table 5. Good agreement wasobserved for the results determined by the two methods.

The possible reaction mechanism

From the excitation and emission spectra shown in Fig. 1, we cansee that methomyl has little inhibitory effect on the redox reac-tion between H2O2 and RhB, but significant inhibitory effect wasobserved for this pesticide on the redox reaction between RhBand Fenton reagent. In this process, methomyl is oxidized bystrong oxidant. In the present work, •OH produced by Fentonreagent oxidized RhB in the acid medium, which caused thestructural destruction and the fluorescence quenching of RhB. Itis possible that methomyl was involved in the reaction and com-peted with RhB, which decreased the reaction rate of •OH with

RhB. Combined with the data reported in the literature (22), thepossible reaction mechanism is suggested as follows:

EDTA–Fe(II) + H2O2 → EDTA–Fe(III) + ·OH + OH− (1)

EDTA–Fe(III) + H2O2 → EDTA–Fe(II) + HO2· + H+ (2)

·OH + RhB → P1 (3)

·OH + methomyl → P2 (4)

Reaction (1) shows the redox reaction of Fenton reagent, andthe peroxyl radicals are produced in reaction (2). P1 is the productof the oxidized RhB. Reaction (4) occurs when methomyl isadded, where P2 stands for the products of the methomyl oxidizedby •OH.

Combined with the analytical result for the reaction productsreported in the literature (23, 24), reaction (4) can be describedby

CH3SC(CH3) = NOOCNHCH3 + ·OH → CH3CN+ CH3NH2 + CH3SH + CO2 (5)

Table 5. Comparison in the results of the proposed method and the HPLC method (n = 3)

Sample Proposed method RSD(%)

HPLC method RSD(%)

Methomyl added(μg/mL)

Methomyl found(μg/mL)

Methomyl added(μg/mL)

Methomyl found(μg/mL)

Tap water 0.400 0.393 1.02 0.400 0.397 0.67Lake sediment 0.400 0.411 2.01 0.400 0.394 2.83Synthetic sample 1.000 0.978 1.41 1.000 0.989 1.12

Table 3. Analytical results for methomyl in water samples(n = 3)

Sample Methomyladded

(μg/mL)

Methomylfound

(μg/mL)

Recovery(%)

RSD(%)

Tap water 1.000 0.993 99.3 1.04Surface water 0.400 0.400 100.0 1.02River water 1 1.000 0.858 85.8 0.68Lake water 1 1.000 0.889 88.9 0.90River water 2 1.000 0.969 96.9 0.98Lake water 2 1.000 0.936 93.6 1.78

Table 4. Analytical results for methomyl in soil samples(n = 3)

Samples Methomyladded

(μg/mL)

Methomyl found

(μg/mL)

Recovery(%)

RSD(%)

Generic soil 0.400 0.396 99.0 2.02Kaleyard soil 0.400 0.334 83.5 1.25River sediment 0.400 0.360 90.0 0.47Lake sediment 1.000 1.012 101.2 1.96

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When methomyl was oxidized by •OH, the structure of methomylwas destroyed and some small molecules were produced, suchas CH3CN, CH3NH2, CH3SH and CO2. These small molecules werefurther oxidized into carbon dioxide, water and so on.

ConclusionsCombined with the spectrofluorimetry which offers excellentdetection limits in the determination of trace amounts of organiccompounds, the conventional Fenton reagent was applied tothe determination of methomyl in environmental samples. Themethod presents a competitive sensitivity with an LOD of 11 ng/mL. The samples were prepared with pre-separation usinganionic and cationic exchange resins, then methomyl can bedetermined directly at ng/mL levels. In all the cases, recoveriesof 83.5–101.2% were obtained. Compared with the HPLC method,the proposed method is rapid, inexpensive, simple to operateand easy to popularize.

Acknowledgements

This work was financially supported by the Science and TechnologyDepartment of Henan Province (No. 072102320007).

References1. WHO, Methomyl Environmental Health Criteria 178. WHO: Geneva,

1996; 74.2. Strathmann TJ, Stone AT. Reduction of the carbamate pesticides

oxamyl and methomyl by dissolved FeII and CuI. Environ. Sci. Technol.2001; 35: 2461–2469.

3. Fernandez M, Pico Y and Manes J. Determination of carbamateresidues in fruits and vegetables by matrix solid-phase dispersionand liquid chromatography–mass spectrometry. J. Chromatogr. A 2000;871: 43–56.

4. The Council of the European Union, EU’s Drinking Water Standards.Council Directive 98/83/EC on the quality of water intended forhuman consumption, adopted by the Council on 3 November 1998.

5. Reeves RG, Woodham DW. Gas chromatographic analysis ofmethomyl residues in soil, sediment, water, and tobacco utilizing theflame photometric detector. J. Agric. Food Chem. 1974; 22: 76–78.

6. Fontanals N, Galia M, Marce RM, Borrull F. Solid-phase extraction ofpolar compounds with a hydrophilic copolymeric sorbent. J. Chro-matogr. A. 2004; 1030: 63–68.

7. Li HP, Li JH, Li GC, Jen JF. Simultaneous determination of airbornecarbamates in workplace by high performance liquid chromatographywith fluorescence detection. Talanta 2004; 63: 547–553.

8. Sanchez-Brunete C, Rodriguez A, Tadeo JL. Multiresidue analysis ofcarbamate pesticides in soil by sonicationq assisted extraction in

small columns and liquid chromatography. J. Chromatogr. A 2003;1007: 85–91.

9. Bogialli S, Curini R, Corcia AD, Nazzari M, Tamburro D. A simple andrapid assay for analyzing residues of carbamate insecticides invegetables and fruits: hot water extraction followed by liquidchromatography–mass spectrometry. J. Agric. Food Chem. 2004; 52:665–671.

10. Nogueira JMF, Sandra T, Sandra P. Multiresidue screening of neutralpesticides in water samples by high performance liquid chromato-graphy–electrospray mass spectrometry. Anal. Chim. Acta. 2004;505: 209–215.

11. Nogueira JMF, Sandra T, Sandra P. Considerations on ultra traceanalysis of carbamates in water samples. J. Chromatogr. A 2003;996: 133–140.

12. Lin ZY, Chen GN. Determination of carbamates in nature water basedon the enhancement of electrochemiluminescent of Ru(bpy)3

2+ atthe multi-wall carbon nanotube-modified electrode. Talanta 2006;70: 111–115.

13. Fenton HJH. Oxidation of tartaric acid in presence of iron. J. Chem.Soc. 1894; 65: 899–910.

14. Luzzatto E, Cohen H, Stockheim C, Wieghardt K, Meyerstein D.Reactions of low valent transition metal complexes with hydrogenperoxide: are they ‘Fenton-like’ or not? 4. The case of Fe (II) L,L = EDTA, HEDTA and TCMA. Free Radic. Res. 1995; 23: 453–463.

15. Saltmiras DA, Lemley AT. Degradation of ethylene thiourea (ETU)with three fenton treatment processes. J. Agric. Food Chem. 2000;48: 6149–6157.

16. Selvam K, Murugan M, Swaminathan M. Enhanced heterogeneousferrioxalate photo—Fenton degradation of reactive orange 4 bysolar light. Sol. Energy Mater. Sol. Cells 2005; 89: 61–74.

17. Neppolian B, Park JS, Choi H. Effect of fenton-like oxidation onenhanced oxidative degradation of para-chlorobenzoic acid byultrasonic irradiation. Ultrason. Sonochem. 2004; 11: 273–279.

18. Shemer H, Sharpless CM, Elovitz MS, Linden KG. Relative rateconstants of contaminant candidate list pesticides with hydroxylradicals. Environ. Sci. Technol. 2006; 40(14): 4460–4466.

19. Evgenidou E, Konstantinou I, Fytianos K, Poulios I. Oxidation of twoorganophosphorous insecticides by the photo-assisted Fentonreaction. Water Research 2007; 41(9): 2015–2027.

20. Wang QQ, Lemley AT. Oxidation of carbaryl in aqueous solution bymembrane anodic fenton treatment. J. Agric. Food Chem. 2002; 50:2331–2337.

21. Feng S, Chen X, Fan J, Zhang G, Jiang J, Wei X. Kinetic determinationof ultratrace amounts of ascorbic acid with spectrofluorimetricdetection. Anal. Lett. 1998; 31: 463–474.

22. Lu MC, Chen JN, Chang CP. Oxidation of dichlorvos with hydrogenperoxide using ferrous ion as catalyst. J. Hazard. Mater. 1999; 65:277–288.

24. Strathmann TJ, Stone AT. Reduction of the pesticides oxamyl andmethomyl by FeII: effect of pH and inorganic ligands. Environ. Sci.Technol. 2002; 36: 653–661.

25. Strathmann TJ, Stone AT. Reduction of oxamyl and related pesticidesby FeII: influence of organic ligands and natural organic matter.Environ. Sci. Technol. 2002; 36: 5172–5183.