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Adsorption of Organophosphate Pesticides with Humic FractionImmobilized Silica Gel in Hexane Yan-Shuo Lai and Shushi Chen* Department of Applied Chemistry, National Chiayi University, Chiayi 600, Taiwan R.O.C. ABSTRACT: Fractions collected from humic acid (HA) under acidic conditions and used as adsorbents for various agricultural organophosphate pesticides in hexane are immobilized on silica gel. For most organophosphate analytes examined in this study under the same conditions, the percentage of adsorption achieved nearly 100 % in 1 h and was found to be highly relevant to the structure of the analyte and the type of interaction that occurred between the functional groups attached to it and HA. The interaction leading to adsorption between the functional moieties of the analyte and HA (e.g., P−O or S bond of analyte vs carboxyl group of HA) is believed to be reversible and dipole−dipole oriented and is significantly enhanced in hexane. The enhancement of π−π interaction, even hydrogen bonding in some cases, was also observed in hexane and contributed to the percentage of adsorption to a certain degree. However, the interaction is subject to the steric hindrance effect caused by the bulky group or element surrounding the phosphorus element. Considering the nature of the analyte, the time required to reach the maximum percentage of adsorption is decreased as the amount of adsorbent is increased. Furthermore, the adsorption process is surface oriented because the longer the time that is elapsed, the higher the percent of the analyte that is adsorbed. Factors such as the type of liquid phase or the acidic or basic origin of the additive in the liquid phase of the matrix also affect the adsorption percentage of analyte.

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INTRODUCTION Organophosphate (OP) is the general name for esters of phosphoric acid, phosphorous acid, phosphinic acid, or thiophosphoric acid. Such compounds exhibit intense neurotoxic activity and are, therefore, highly poisonous to humans because they inhibit enzymes such as acetylcholinesterase, a critical material for nerve functioning.1,2 OP was originally developed as a nerve gas but is now widely used as the basis for many insecticides, herbicides, and plasticizers.3−10 Table 1 lists some typical pesticides and their target organisms. Many of the most crucial biochemicals are also OPs, including DNA, RNA, and many cofactors that are essential for life. Previous studies have suggested that the dietary intake of OP pesticides is a possible link to adverse effects in the neurobehavioral development of fetuses and children, even at extremely low levels of exposure.11,12 This is because small amounts of OP pesticides are still detected in food and drinking water, despite being degraded rapidly by hydrolysis when exposed to sunlight, air, and soil.13 Such a circumstance has promoted the European Union to regulate the maximum acceptable daily intake of various pesticide residues in processed and cereal-based foods for infants and children at a level lower than 10 μg/kg. It has also been reported that the health risks of farm workers exposed to pesticides in their work and home environments are rapidly increasing.14−16 Neurologic pathologies such as dementis, Parkinson’s disease, and childhood cancer have been diagnosed.17,18 Generally, OPs are nonpersistent; in this respect, they present an environ© 2013 American Chemical Society

Table 1. Pesticides and Their Targets pesticide type

target organism

acaricide adulticide algicide avicide bactericide culicide disinfectant fungicide grubicide herbicide insecticide larvicide miticide molluscicide nematicide piscicide rodenticide termiticides

mites insect adults algae birds bacteria mosquitoes microorganism fungi grubs plants insects insect larvae mites snails, slugs nematodes fish rodents termites

mental advantage over organochlorines.19 However, the degraded components from OP pesticides, such as phosphates, Received: April 24, 2013 Accepted: July 8, 2013 Published: July 17, 2013 2290

dx.doi.org/10.1021/je400399x | J. Chem. Eng. Data 2013, 58, 2290−2301

Journal of Chemical & Engineering Data

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all measurements. The FTIR spectra were obtained with a Shimadzu model FTIR-8400 system. In FTIR measurements, the adsorbent was collected and then pelleted with KBr after being washed with fresh hexane and dried. FTIR spectra were obtained by scanning samples 10 times. Chemicals. All chemicals used in this study, including the organosilane reagent as a linker in chemical immobilization reactions, were purchased from Sigma (St. Louis, MO, USA) and Aldrich (Milwaukee, WI, USA) Chemical Companies, respectively. The pesticide compounds used as the analytes in the adsorption measurements were acquired from Chem Service, Inc. (West Chester, PA, USA). The silica gel (5 μm particle diameter, 100 Å porosity with a specific surface area of 400 m2·g−1) used as the supporting matrix of the solid phase in the adsorption evaluation at ambient temperatures was a product of Silicycle (Quebec City, QC, Canada) and was chemically modified before use according to the derivatization procedures reported previously.37,39,40 The solvents, such as toluene, acetonitrile, methanol, triethylamine, methylene chloride, hexane, and ethyl ether, were of HPLC grade and purchased from Fisher Scientific (Pittsburgh, PA, USA) and Merck Taiwan Ltd. (Taiwan, ROC). In all cases, filtered (0.2 μm) and distilled water were used. Immobilization of Humic Fraction on Silica Gel. As described previously, the fractions collected from HA under acidic conditions were dried under vacuum for 4 h before being added at a weight of approximately 0.15 g to 50 mL of dry dimethylformamide (DMF) in a three-neck reactor. The temperature for the reaction was raised to 93 °C, and then the approximately equivalent number of moles of the organosilane linker in 10 mL of dry DMF was added to the solution drop by drop over a time period of 2 h. Once this process was finished, the reaction was allowed to continue for 18 h. During the reaction, the reactor was circulated with desiccated nitrogen gas to maintain an inert (i.e., free from oxygen) and dry environment. Finally, the silica gel was added to the reactor at 3.22 g for another 24 h reaction. After the reaction, the functionalized silica gel, used as the solid phase, was collected and washed with DMF, methanol, toluene, acetonitrile, and distilled water several times before being dried again under vacuum and sent to the Instruments Center at National Chung Hsing University for elemental analysis. The elemental analysis data in wt % were then tabulated and compared with those from previous batches and discussed in the following section. Conditions for Adsorption Process. A weighted amount of solid-phase adsorbent (10 mg) was added to a 100 μL of a 2.57 × 10−3 M solution of analyte for a controlled period. In each measurement, the solution was sampled for HPLC analysis both before and after the adsorption process was complete during that specific period to calculate and compare the percentages of adsorption. To determine how the acidic or basic origin of the additive affected the adsorption of the analytes, an additive, such as glacial acetic acid or triethylamine in a volume ranging from 2.5 to 10 μL, was added to the mentioned matrix containing the adsorbent and the analyte. The analysis was then performed immediately following the 1 h adsorption period. The percentage of adsorption, an average of three measurements, was calculated based on the difference in peak area of the analyte before and after the 1 h adsorption.

play an essential role along with nitrates in causing eutrophication, an artificial or natural process that endangers the lifespan of fishes and decreases the value of rivers, lakes, estuaries for recreation, fishing, hunting, and aesthetic enjoyment.20−23 Consequently, carefully monitoring the OP pesticide residues present in our daily life through sensitive techniques seems to be a necessity to avoid lethal exposure.13 Chromatographic approaches including gas chromatography (GC) and high-performance liquid chromatography (HPLC) with UV and/or mass spectrometry (MS) detection are suitable for analyzing OP-based pesticide residues in a sample. However, for analyzing thermally labile and/or high boilingpoint pesticides, the HPLC approach represents the optimal choice.24−30 Despite the advancing development of techniques for quantitation analysis of OP pesticide residues, their recoveries prior to contaminating the food have rarely been investigated. Humic acid (HA), a complex mixture of many different acids, is derived by the microbial degradation of dead plant matter and can be found in soil nearly everywhere.31 Because it contains various components including quinone, phenol, catechol, and sugar moieties structurally, numerical benefits such as crop production and micronutrients transferring are available and have been proven experimentally and in the field.32 Applying HA to the removal of ions by forming the chelate complexes has been reported.33,34 Similarly, the separation and removal of humic acid, by using ion-exchange resins through an ultrafiltration system or the fractionation technique on the formation of complexes under the same mechanism, have also been documented.35,36 Because of its complex structure and having various functional components such as the carboxylate and phenolate groups, adsorptionoriented application also has been made to the carboxylcontaining pesticides and biogenic amines in acetonitrile.37 It is believed that adsorption may occur mainly because of complexation, π−π stacking between HA and analyte, and the acid−base type of interaction in the case of amine. For environmental protection, both the regulation and recovery of such pollutant compounds are necessary to prevent the contamination of underground water after their use.38 In this work, fractions of humic acid were collected under acidic conditions and then immobilized on silica gel and used as the adsorbents for various pesticidal compounds containing phosphorus elements in hexane. The variation in percentage adsorption was related to the analyte structure and the type of functional group on the analyte and was considered from a mechanistic perspective. In addition, factors that affected the percentage of adsorption, such as the type of liquid phase, the acidic or basic origin of the additive in the liquid phase, the time elapsed during the adsorption process, and the steric hindrance of the analyte, were examined and rationalized to help explore the adsorption mechanism involved, and thus, improve the adsorption efficiency.

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EXPERIMENTAL SECTION Apparatus. An elemental analyzer, Elementar model Vario EL III, was used to determine the carbon, hydrogen, sulfur, oxygen, and nitrogen contents in the weight percentage (wt %) of all solid phases examined in this study. The highperformance liquid chromatography (HPLC) system used in this study was a Hitachi model L-7100 coupled to a D-2500 Chromatopac data station and a UV detector. The detection wavelength in the adsorption evaluation was set at 260 nm for 2291



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Table 2. Organic Elemental Compositions for Two Separate Batches of Fractions Extracted from HA under Acidic Conditions organic elemental compositions (%) samplea HA batch batch batch batch

1b 2 1 on silica gelb 2 on silica gel

C

H

N

O

S

33.72 2.19 (3.69) 2.23 1.39 (1.59) 1.32

4.23 2.19 (1.23) 2.17 1.44 (1.28) 1.40

2.06 0.11 (0.05) 0.14 0.06 (0.04) 0.06

32.95 6.39 (5.97) 6.30 6.58 (6.24) 6.49

0 0 (0) 0 0 0

a

Humic acid is of sodium salt. Fractions (i.e., batches 1 and 2) are collected separately from HA under acidic conditions, respectively, and then immobilized on silica gel under the same conditions. bThe data in parentheses, for fraction collected under basic conditions, are cited from the ref 37 for comparison. The immobilization preparation is carried out under the same conditions.

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RESULTS AND DISCUSSION Two batches of fractions were collected from HA and immobilized chemically onto the surface of silica gel under the same conditions. The elemental analysis data for these materials are listed in Table 2 with the data from the humic fractions obtained under the basic conditions shown in parentheses for comparison. The variation of organic elemental compositions between batches of fractions collected, or fraction-modified silica gel, was slim. This indicated that the collection process for the humic fractions under acidic conditions was reproducible. In addition, the procedure for its chemical immobilization was efficient and reliable, which was essential for the adsorption evaluation with such a small amount of adsorbent (i.e., 10 mg) in the study. Regarding the variations among other elemental compositions between humic fractions collected under acidic and basic conditions, the possible explanation has been provided elsewhere.37,41 The percentage of oxygen for the humic fractions collected under acidic conditions was always higher (Table 2), compared to those obtained under basic conditions. Previous studies have indicated that the oxygen elements in HA exist mainly in the form of carboxyl groups determined according to the IR signal intensity.31,32 However, the absorption band for oxygen atoms to exist in a hydroxyl group, or the deformation of these functional groups (e.g., C−O, O−H of COOH or OH group, respectively), was not as intense. This may not seem to be the case for the humic fractions collected under acidic conditions in this study, after closely examining the FTIR results shown in Figure 1 (top). The figure shows the assignment of characteristic IR absorption bands for the humic fractions collected under acidic conditions after being immobilized onto the surface of silica gel. The most intense band corresponded to the frequency for the C−O stretching vibration at approximately 1125 cm−1. The singlet absorption band for CO stretching vibration near the frequency of 1680 cm−1, a doublet with the highest intensity in previous work, was perspectively minor in this study.32 In addition, several absorption bands below 1000 cm−1 were relatively distinguishable, which was not previously observed before for humic fractions or HAs. Note that the frequency in this region was contributed in part from N−H bending vibration of linker used in immobilization reaction. Therefore, the overall IR spectrum profile was expected to be relatively different, except for the frequency region centered at 3500 cm−1 for O−H vibration, indicating that the humic fractions collected under acidic conditions never resembled the reported HA molecules.31,32 Figure 1 also showed the FTIR spectrum (bottom) for the humic fraction modified silica gel with analyte phenamiphos adsorbed (compound 27 in Table 3). As can be seen, a red shift and

Figure 1. FTIR spectra for the humic fraction collected under acidic conditions after being immobilized on the silica gel (top), adsorbed with the analyte phenamiphos (bottom, compound 27 in Table 3). Note that N−H bending vibration, in part, is contributed from linker molecules used in the chemical immobilization reaction.

additional peak were observed in the region for both C−O and CO stretching vibrations. The peak centered at about 3500 cm−1 for O−H and N−H vibrations became broader, and several new peaks were observed in this region. These all indicated the existence of the association between analyte and humic fraction molecules.42 Note that the C−H stretching in the region of 2930−2855 cm−1 became hardly to observe due to the peak broadening. The phosphorus-containing compounds examined in this study, known as OPs, are mainly esters of phosphoric acid and a few of phosphorous acid as described in the Introduction 2292



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Table 3. Percentage of Adsorption for Aromatic Organophosphate Pesticides with Humic Fraction Extracted under the Acidic Conditions after Its Immobilization on the Silica Gela

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Table 3. continued

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Table 3. continued

a

The mobile phases are mixtures of organic solvent and water acidified with glacial acetic acid at a ratio of 250:3 by volume, (v/v), and are A: 70(acetonitrile)/30, B: 60(methanol)/40, respectively. The mobile phase C is the acetonitrile solvent of HPLC grade. Most of the reported analytes are eluted with mobile phase C except for those numbered 15, 17, 25 (mobile phase B) and 5, 7, 11, 19, 22 (mobile phase A). bThe double bond formed between elements phosphorus and oxygen or sulfur in each analyte is tentatively assigned since the results in ref 43 suggest that the bonding should be a single, highly polarized σ bond where no π back-donation can clearly be found. cAll the reported percentage of adsorptions are measured over a controlled period of 1 h. The amount of humic fraction modified silica gel used as the solid phase in each adsorption measurement is 10 mg. The percentage of adsorption (%), an average of three measurements, is calculated based on the difference in peak area of the analyte before and after the adsorption. dThe LD50 value (mg/kg) in orally exposed male rats is shown based the information provided by manufacturers and suppliers.

strong, short, and polar. The sulfur analogues are similar, although, not as strong, or as polar.70,71 Sundberg et al. proposed that the P−O bond was a highly polarized σ bond (almost ionic) in phosphine oxide, and in its trimethyl derivative.43 The bonding became mainly covalent in nature, and thus, weaker in strength for sulfur analogues. The polarity of the P−O bond in the ester of phosphoric acid (i.e., OP) is believed to be further increased because of the extra surrounding oxygen atoms in ester bonds, which is beneficial to the interactions that are attractive in nature. Consequently, the dominated attractive force leading to the adsorptions of OP and organothiophosphate in this study is believed to be a dipole−dipole interaction. Replacing oxygen with sulfur atoms in the P−O bond is expected to generate more steric hindrance, thus affecting the attraction negatively to a certain degree. Other types of interactions that could lead to the possible adsorption, in part, include π−π stacking and hydrogen bonding, which were also enhanced in aprotic solvents such as hexane.72−74 Based on these considerations, the typical analyte methyl paraoxon (compound 7 in Table 3) appeared to have the maximum attraction with the collected humic fractions under acidic conditions. Hydrogen bonding was absent in this case. However, the π−π stacking was further enhanced to have a nearly 100 % adsorption because it possessed the electronwithdrawing nitro group derivatized at the para position of the aromatic moiety which would turn the π system into more acidic.75 Because it was thiolated to Compound 8, as shown in Table 3, the percentage of adsorption was not as impressive because of the bulky sulfur near the phosphorus atom. The other typical example also can be seen among Compounds 4−6 in Table 3. These analytes all have a double bond conjugated with the π system of the benzene ring, which thus may lower

section. In addition, many other organothiophosphates were studied for comparison for the adsorption mechanism exploration. These OPs and organothiophosphates were further structurally divided into two groups that contained aromatic versus heterocyclic moiety for clarity and easy discussion, as summarized in Tables 3 and 4, respectively. Figure 2 shows the chromatogram of the adsorption evaluation of the aromatic moiety containing edifenphos, the 20th compound in Table 3, in hexane. The percentage of adsorption was calculated to be 99.1 % in 1 h. Many other such OPs and organothiophosphates are listed in Table 3, with the information on the type of use and LD50. The percent adsorption reached well above 90 % for most of the analytes that were examined. The longer the elapsed time was, the higher the percentage of analyte adsorbed as shown in Figure 3. As expected, the time required to reach the maximum percentage of adsorption was shortened as more adsorbent was used in the process because the adsorption process was more than 80 % complete in 50 min. Therefore, the adsorption process was surface-oriented.37 None of the mentioned results could be reproduced using other organic solvents such as acetonitrile or ethyl ether, or in an aqueous environment, due to the competition for binding sites from solvent molecules. Such a competition also revealed the nature or type of interaction leading to the adsorption in hexane. The nature of the bonding between phosphorus and oxygen, or sulfur elements in phosphine oxides43−55 and thiophosphenes,55−57 has been of great interest, and thus, extensively reviewed over the years; however, it remains controversial because no virtual d orbitals are involved in the bond resonance between structures R3P+O−, or S− ↔ R3PO, or S based on several theoretical calculations.55,58−69 In addition, the overall experimental results indicate that the phosphoryl bond is 2295



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Table 4. Percentage of Adsorption for Organophosphate Pesticides Containing Heterocyclic Moiety with Humic Fraction Extracted under the Acidic Conditions after Its Immobilization on the Silica Gel

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Table 4. continued

a

The mobile phase is the acetonitrile solvent of HPLC grade for all reported analytes. Compounds numbered 19 and 20 are containing neither aromatic nor heterocyclic moiety. bThe double bond formed between elements phosphorus and oxygen or sulfur in each analyte is tentatively assigned since the results in ref 43 suggest that the bonding should be a single, highly polarized σ bond where no π back-donation can clearly be found. cAll of the reported percentage of adsorptions are measured over a time period of 1 h. The amount of humic fraction modified silica gel used as the solid phase in each adsorption measurement is 10 mg. The percentage of adsorption (%), an average of three measurements, is calculated based on the difference in peak area of the analyte before and after the adsorption. dThe LD50 value (mg/kg) in orally exposed male rats is shown based the information provided by manufacturers and suppliers.

Furthermore, bulky derivatives are also around the aromatic moiety. Likewise for organothiophosphate having thioester bond, as in Compounds 17 to 19, the percentage of adsorption is unsatisfactory, as expected. Based on these results, the major interaction that occurred around the phosphorus atom was deduced to be dipole−dipole in nature. The π−π stacking, enhanced by the presence of the electron-withdrawing group on the ring, is considered minor because adsorptions for OP and organothiophosphate containing heterocyclic moieties (summarized in Table 4) were also observed. Except for these two types of interaction forces, hydrogen bonding

the overall energy of the molecule and increase stability in general. As a result, the π−π complexation reaction was believed not in favor to form between the analyte and the humic fraction molecules. The adsorption observed, nearly 100 % for Compounds 5 and 6, was obviously due to the dipole− dipole interaction around the phosphorus atom. The relatively low adsorption for Compound 4 was because it was thiolated. Conversely, the typical analytes that received the minimum interaction because of significant steric hindrance are Compounds 1 to 4 and 17 in Table 3. These analytes are either organothiophosphates or OPs having thioester bonds. 2297



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Figure 3. Effect of time elapsed during the adsorption process on the percentage of adsorption for edifenphos in hexane (compound 20 in Table 3). As can be seen, the adsorption process is more than 80 % complete in 50 min.

Figure 2. Chromatograms showing the adsorption of edifenphos (compound 20 in Table 3) before (top) and after (bottom) the process was completed in hexane. The percentage of adsorption was estimated to be 99.1 % for a time period of 1 h. Refer to Table 3 for chromatographic conditions.

appeared to be beneficial to the adsorption of Compounds 24, 26, and 27, as listed in Table 3. The percentage of adsorption reached nearly 100 % for these analytes. One of the chromatograms leading to the calculation of the 1 h percent adsorption for phenamiphos (compound 27 in Table 3) is shown in Figure 4. Table 4 lists the adsorption data for OP and organothiophosphate containing heterocyclic moieties, along with the information on the type of use and LD50. Adsorption was more effective compared to that for the analytes listed in Table 3 under the same conditions. Compounds 19 and 20 do not have the heterocyclic moiety; however, they exhibit nearly 100 % of adsorption in 1 h. Clearly, the interaction force from π−π stacking leading to adsorption is auxiliary. In other words, the dipole−dipole interaction around the phosphorus atom was responsible for the observed adsorption. The adsorption

Figure 4. Chromatograms showing the adsorption of phenamiphos (compound 27 in Table 3) before (top) and after (bottom) the process was completed in hexane. The percentage of adsorption was estimated to be nearly 100 % for a time period of 1 h. Refer to Table 3 for chromatographic conditions.

mechanism could be further studied by introducing acidic or basic additives such as acetic acid or triethylamine, respectively, to the liquid phase. The additive was expected to compete with the analyte molecules for the binding site on the humic fraction-modified silica gel. Consequently, any interaction related to the nature of polarity was affected, thus influencing 2298



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moiety can still interact with adsorbent molecules through a type of π−π stacking force.

the percentage of adsorption negatively. Figure 5 shows the effect of the acidic or basic origin of the additive in the liquid

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Funding

Support of this work by the National Science Council of Taiwan (100-2113-M-415-001-MY2) is gratefully acknowledged. Notes

The authors declare no competing financial interest.

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REFERENCES

(1) Pandit, V.; Seshadri, S.; Rao, S. N.; Samarasinghe, C.; Kumar, A.; Valsalan, R. A case of organophosphate poisoning presenting with seizure and unavailable history of parenteral suicide attempt. J. Emerg. Trauma Shock 2011, 4 (1), 132−134. (2) Yurumez, Y.; Durukan, P.; Yavuz, Y.; Ikizceli, I.; Avsarogullari, L.; Ozkan, S.; Akdur, O.; Ozdemir, C. Acute organophosphate poisoning in university hospital emergency room patients. Intern. Med. 2007, 46 (13), 965−969. (3) Meister, R. T.; Sine, C. Farm Chemicals Handbook ’99; Meister Publishing Co.: Willoughby, OH, 1999. (4) Worthing, C. R. The Pesticide Manual: A World Compendium, 8th ed.; The British Crop Protection Council: Hampshire, U.K., 1987. (5) Paxman, J.; Harris, R. A Higher Form of Killing: The Secret History of Chemical and Biological Warfare, Rando ed.; Random House Press: New York, 2002. (6) Van Emden, H. F.; Pealall, D. B. Beyond Silent Spring; Chapman & Hall: London, 1996. (7) Morrison, H. I.; Wilkins, K.; Semenciw, R.; Mao, Y.; Wigle, D. Herbicides and cancer. J. Natl. Cancer Inst. 1992, 84 (24), 1866−1874. (8) Doolittle, A. K. Mechanism of Plasticization. In Plasticizer Technology; Reinhold: New York, 1965. (9) O’Brien, J. L. Plasticizers. In Modern Plastics Encyclopedia; McGraw Hill: New York, 1988. (10) Doolittle, A. K. The Technology of Solvents and Plasticizers; Wiley: New York, 1954. (11) Jurewicz, J.; Hanke, W. Prenatal and childhood exposure to pesticides and neurobehavioral development: review of epidemiological studies. Int. J. Occup. Environ. Med. 2008, 21 (2), 121−132. (12) Eskenazi, B.; Bradman, A.; Castorina, R. Exposures of children to organophosphate pesticides and their potential adverse health effects. J. Environ. Health Perspect. 1999, 107, 409−419. (13) González-Rodríguez, R. M.; Rial-Otero, R.; Cancho-Grande, B.; Gonzalez-Barreiro, C.; Simal-Gándara, J. A review on the fate of pesticides during the processes within the food production chain. Crit. Rev. Food Sci. Nutr. 2011, 51 (2), 99−114. (14) Bonner, M. R.; Coble, J.; Blair, A. Malathion exposure and the incidence of cancer in the agricultural health study. Am. J. Epidemiol. 2007, 166 (9), 1023−34. (15) Rothlein, J.; Rohlman, D.; Lasarev, M.; Phillips, J.; Muniz, J.; McCauley, L. Organophosphate pesticide exposure and neurobehavioral performance in agricultural and nonagricultural Hispanic workers. Environ. Health Perspect. 2006, 114 (5), 691−696. (16) Mills, P. K.; Zahm, S. H. Organophosphate pesticide residues in urine of farm workers and their children in Fresno County, California. Am. J. Ind. Med. 2001, 40 (5), 571−577. (17) Baldi, I.; Filleul, L.; Mohammed-Brahim, B.; Fabrigoule, C.; Dartigues, J. F.; Schwall, S.; Drevet, J. P.; Salamon, R.; Brochard, P. Neuropsychologic effects of long-term exposure to pesticides: results from the French Phytoner study. Am. J. Ind. Med. 2001, 109 (8), 839− 844. (18) Patel, S.; Singh, V.; Kumar, A.; Gupta, Y. K.; Singh, M. P. Status of antioxidant defense system and expression of toxicant responsive genes in striatum of maneb- and paraquat-induced Parkinson’s disease

Figure 5. Effect of the acidic or basic origin of the additive in the liquid phase of the matrix on the percentage of the analytes, edifenphos (top, compound 20 in Table 3) and chlorpyrifos (bottom, compound 16 in Table 4), adsorbed on the solid phase in hexane. As can be seen, the effect of the acidic or basic origin of the additive on the adsorption was relatively analyte-dependent. However, the adsorption deteriorated dramatically in the presence of an additive in both cases.

phase on the percentage of adsorption for two analytes different structurally. The percentage of adsorption of the analyte decreased in the presence of an additive of acidic or basic origin and decreased to nearly zero as the amount of additive in the liquid phase increased in the case of chlorpyrifos (compound 16 in Table 4). However, because the amount of additive introduced was 10 μL, the percentage of adsorption for edifenphos (compound 20 in Table 3) was still observed. The π−π stacking interaction, which was not negatively influenced in the presence of an additive of acidic or basic origin but was enhanced in hexane, is likely responsible.

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CONCLUSION The humic fraction-modified silica gel as the adsorbent for various phosphorus-containing compounds in the aprotic solvent of hexane was successfully demonstrated with the percentage of adsorption reached well above 90 % for most analytes examined. Based on the FTIR data, the collected humic fractions were not the same as those for the reported HAs, when considering the type of functional groups. The adsorption process was found to be surface-oriented because it was dependent on both time and the amount of adsorbent. The highly polarized σ bond (nearly ionic) in phosphine oxide seems to be mainly responsible for the observed adsorption and is believed to be dipole−dipole in nature and greatly enhanced in hexane. In addition, adsorption was also observed for the analyte with heterocyclic moiety. The other auxiliary forces leading to the adsorption includes π−π stacking and hydrogen bonding for some of the analytes examined. These types of forces were also enhanced in hexane. In the presence of an additive of acidic or basic origin in the liquid phase, the attractive type of interaction is severely suppressed to have zero adsorption because of the competition for interaction from additive molecule. Therefore, only those having an aromatic 2299



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