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Agriculture, Ecosystems and Environment, 18 (1987) 231-241Elsevier Science Publishers B.V., Am sterdam - Printed in The Netherlands231

Factors Affecting the Adsorption of 2,4-DMethyl Parathion in Soils and SedimentsKONDA S. REDD Y and ROBERT P. GAM BRELL

and

L aboratory for Wetland Soils and Sediments, Center for Wetland Resources, Louisiana StateUniversity, Baton Rouge, LA 70803-7511 (U.S.A.)(Accep ted for publication 26 August 1 986)

ABSTRACTReddy , K.S. and Gambrell, R.P., 1987 . Factors affecting the adsorption of 2,4-D and methyl par-athion in soils and sediments. Agric. Ecosystems Environ., 18: 231-241.

The adsorption of methyl parathion and 2,4-dichlorophenoxy acetic acid (2,4-D ) by 19 soil andsediment materials differing widely in their physical and chemical pro perties was investigatedusing a batch equilibration technique. Organic m atter was the most important single factor affect-ing adsorption of methyl parathion and 2,4-D. Soil pH and cation ex change capacity were alsoreasonably well associated with 2,4-D adsorption whe reas cation exch ange capacity con tributedsignificantly for methyl parathion. Data are presented which indicate th at in soil and sedimentmaterials whe re the organic matter content in less than l%, oxalate extractable Mn and Ca wereassociated with the adsorption of these synthetic organics. Additional work should be done in loworganic matter soil materials to better quantify the relationship between adsorption and soil geo-chemical properties.

INTRODUCTIONThe partitioning of pesticides between dissolved and adsorbed phases during

runoff events, and especially in sediment-water systems receiving runoff, is animportant process regulating their transport and potential availability to non-target organisms away from the point of application. Numerous studies andreviews during the past two decades have examined soil and sediment proper-ties affecting the distribution of various synthetic and energy related organicsbetween soluble and adsorbed forms (Karickhoff et al., 1979; Peck et al., 1980).

It is well established that organic matter content is the major factor influ-encing the immobilization of synthetic organics (Chiou et al., 1979; Karickhoffet al., 1979; Peck et al., 1980; Wilson et al., 1981). In recent years, it has beendemonstrated that expressing the distribution coefficients in terms of the soilPresent address: Texas Environmental Services, Nederland, TX.

0167-8809/87/$03.50 0 1987 Elsevier Science Publishers B.V.

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or sediment organic matter content substantially reduces the variability ofmeasured distribution coefficients from different soils and sediments. Thisimplies that the organic matter in different soils and sediments tends to besimilar in its affinity for synthetic organics. Knowledge of soil or sedimentorganic matter content and octanol-water partitioning coefficients have beenfound to be valuable parameters in predicting the immobilization of pesticidesby adsorption in soil and sediment-water systems (Karickhoff et al., 1979;Hassett et al., 1980; Brown and Flagg, 1981; Wilson et al., 1981). Other factorshave also been shown to influence the adsorption of pesticides; the importanceof parameters other than organic matter often depends on the chemical natureof the particular synthetic organic being considered. In particular, free ironoxides have been reported to affect pesticide adsorption (Pionke and Chesters,1973). This component can be highly variable in different local soils and sed-iments, and particularly between soils of different regions. This could beimportant in soils with a low organic matter content, and especially in subsoilsat waste disposal sites for pesticides and other synthetic organics.

The objectives of this study were to determine the role of selected physicaland chemical properties of soils and sediments on the distribution of methylparathion and 2,4-D between dissolved and solid phases using a batch equilib-rium adsorption procedure.EX PER I M EN T A L M ET HO D S

Soil.9

To study the factors affecting pesticide adsorption, 19 soil and sedimentmaterials were selected to include a broad range of physical and chemical prop-erties. Samples were air-dried and ground to pass through a 2-mm sieve forcharacterization and adsorption studies.

Soil samples were characterized for pH, cation exchange capacity, organiccarbon, particle size distribution, free iron oxides, and Ca, Al and Mn extractedwith the free iron oxides. Sample pH was determined on a 1:l soil-water sus-pension (Peech, 1965). Cation exchange capacity was determined by theammonium acetate method (Chapman, 1965). Organic carbon was deter-mined by the Walkley and Black method (Jackson, 1967)) and organic mattercontent was calculated by multiplying organic carbon by a factor of 1.724. Theparticle size distribution was determined by the hydrometer method (Day,1965)) and free iron oxide was extracted by an ammonium oxalate extractionprocedure ( McKeague and Day, 1966). Iron, Al, Ca and Mn were measured inthe ammonium oxalate extract by an ICP emission spectrometer.Pesticides

The pesticides selected for this study were the insectide methyl parathion( 0-0-dimethyl-O-p-nitrophenyl phosphorothioate) and the herbicide, 2,4-D

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(2,4dichlorophenoxyacetic acid). The partitioning of these pesticides betweendissolved and adsorbed forms would be expected to be affected differently bychanges in soil properties. The solubility of methyl parathion in water at 25 Cis 50 mg 1-l (Wauchope, 1978)) and the solubility of 2,4-D in water is approx-imately 620 mg 1-l (Loos, 1969).

Pesticide standard compounds from Chem-Service, Inc. were amended with14C-labeled materials obtained from Pathfinder Laboratories, Inc. for this study.Stock solutions containing labeled methyl parathion and 2,4-D (specific activ-ity 1 PCi ml-) were prepared in acetone and stored in the refrigerator. Thestock solutions were diluted with water such that on a soil solids basis, theamended levels of compound and label were 1 ,ug compound and 0.1 PCi specificactivity per g soil, respectively. A batch equilibrium technique described belowwas used for this study (Leenheer and Ahlrichs, 1971).

Twenty milliliters of a given pesticide solution were added to 2 g of air-driedsoil in a 50-ml stainless steel centrifuge tube. The tubes were capped with teflon-lined, stainless steel lids and equilibrated for 2 h on a mechanical box shaker.Each soil treatment combination was replicated three times. Following equi-libration, the tubes were centrifuged for 15 min at 12000 rpm in a DuPontSorvall SA-600 rotor. Aliquots of the clear supernatant solution (1 ml) weretransferred to counting vials and 14C-labeled materials were measured on aBeckman Model LSlOOC liquid scintillation counter. To prepare a sample forcounting, 1 ml of an aqueous sample was mixed with 15 ml of a commercialliquid scintillation counting reagent. Decreases in pesticide solution concen-tration were attributed to adsorption by the soil. After the l-ml sample aliquotwas removed at 2 h, the caps were replaced on the centrifuge tubes and theequilibration continued for another 22 h, when another l-ml aliquot was sam-pled. Thus data were obtained for 2-h and 24-h equilibrations. Each samplewas counted for 10 min. Standards and blanks were also counted and includedin calculations to obtain adsorption coefficients.

The amount of pesticides adsorbed on soils has been expressed as thesoil-water distribution coefficient (K) calculated by the formula given below(Wahid and Sethunathan, 1978; Luchini et al., 1981). *

K= pg adsorbed/g soilM dissolved/g solutionWhen the sorption of compounds is expressed as a function of the organic

carbon content of a soil or sediment, a coefficient (K,,) is generated that isdependent upon the organic component (Hamaker and Thompson, 1972;Briggs, 1973; Karickhoff et al., 1979; Khan et al., 1979).

Km=K

% OC (decimal equivalent)

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K,,, is equal to the distribution coefficient (K) divided by percent organic car-bon (OC) in the respective soil or sediment, and is a measure of the partition-ing of the compound between an aqueous solution and a stationary organicphase ( humus ) .RESULTS

The physical and chemical properties of the soil and sediment materialsstudied are given in Table I. Adsorption/desorption coefficients for methylparathion equilibrated with the 19 soils and sediments are given in Table II.Distribution coefficients were calculated for 2 h and 24 h adsorption. Soils 1,3,4,7, 11,14 and 15 adsorbed methyl parathion relatively strongly after a 2-hequilibration and soils 2, 5,9,16, 18 and 19 did not strongly adsorb this com-pound. Soils that adsorbed relatively strongly were generally those with thehighest organic matter content and those in the group that weakly adsorbedtended to have the lowest organic matter levels. The adsorption coefficientswere generally about the same or slightly higher after 24 h equilibration as at2-h equilibration, except for the muck which adsorbed substantially more at24 h.

Distribution coefficients of 2,4-D are presented in Table III. The values inthe table show about the same inter-soil trend as methyl parathion, except thatthe coefficients were less, indicating that 2,4-D is less tightly bound than methylparathion. The data show that with the exception of the Shubuta series,adsorption coefficients for 2,4-D were greater as the pH of the soil decreased.The very low CEC in the Shubuta material compared to the others may havebeen an interacting factor contributing to very low adsorption.

The relationship between sorption and the soil organic carbon content canbe expressed by dividing the individual distribution coefficients of the samplesby their respective organic carbon contents to produce Kc, values (Hamakerand Thompson, 1972; Karickhoff et al., 1979). Calculation of K,, values hasthe effect of putting the sorption on a uniform organic carbon basis, assumingthat all the organic carbon is equally effective (Lambert, 1968) in sorbingmethyl parathion and 2,4-D. The methyl parathion K,,,values obtained for allsamples investigated in this research ranged from 466 to 2815 and 554 to 2831for 2 h and 24 h adsorption, respectively, with average values of 1374 and 1516,respectively ( Table IV ) .

The K,, values for 2,4-D were also lower than for methyl parathion, rangingfrom 9 to 330 and 40 to 415 for 2 h and 24 h adsorption, respectively, with anaverage value of 93 and 160, respectively. The substantially decreased eoeffi-cients of variation [ (S/f) x 1001 of K,,,values compared to coefficients of var-iation of adsorption coefficients for both compounds indicate the reducedvariability between soils when adsorption is considered on an organic carbonbasis (Table IV). This supports what others have reported in the literature.

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236TABLE IIDistribution coefficients for methyl parathion equilibrated with 19 soil and sediment materialsNo. Soil material K*

2-h 24-h1 Airplane Lake2 Atchafaiaya3 Bayou Chevreuil4 CaIcasieu River5 Cecil subsoil6 Cecil topsoil7 Chastain8 Crowley9 Gallion10 Hartwell Lake

11 Lafitte muck12 Lake Pontchartrain13 Lake Providence14 Leeville15 Loring16 Mhoon17 Mississippi River18 Norwood19 Shubuta

70.63 rf:0.30b2.03 + 0.08

60.85 + 0.5656.35 Y? .40

2.58 rt 0.1216.17 + 0.0939.58 f 0.9821.78 f0.26

3.46kO.1418.16f0.1079.93 + 3.6914.08 + 0.5918.77 + 0.9350.48 + 3.92

162.45 t 3.255.80 rt 0.23

23.32 f 0.542.69kO.122.05rto.12

64.21+ 0.5Sb2.52 + 0.07

64.15 + 0.4049.28+ 1.17

4.62 + 0.5223.36 +0.6548.94 rL_.3 919.53 & 0.11

3.85 +0.1320.32f0.11251.41 It8.93

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*Adsorption coefficient.Sediment.bMean of triplicate samples and standard deviation.

Table V presents results of a statistical analysis examining the relationshipbetween a number of soil properties and the adsorption coefficients of 24-Dand methyl parathion after 2-h and 24-h equilibrations. The SAS RSQUAREprocedure used employs linear regression to identify subsets of multiple inde-pendent variables, measured soil properties in this case, that best predict adependent variable, or adsorption coefficients for this work ( SAS, 1985). Thusan R2 value was calculated for each adsorption coefficient for soil propertiesconsidered individually, and for every combination of pairs of soil properties.In a regression analysis, R2 represents the proportion of the total sum of squaresof the dependent variable that is attributable to the independent varable ( s)(Steel and Torrie, 1960). Where all 19 soil and sediment materials wereincluded, organic matter content gave the highest R2 values (except for methylparathion with a 2-h adsorption). Cation exchange capacity was also reason-ably well associated with 2,4-D adsorption where all 19 soils were included, andsoil pH contributed significantly, though not with high R2 values. CEC con-tributed significantly for methyl parathion. The soil property most often cor-

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TABLE IIIDistribution coefficients for 2,4-D equilibrated with 19 soil and sediment materials

No. Soil material K*2-h 24-h

1 Airplane Lake 0.85 f 0.05b2 Atchafalaya River 0.16f0.073 Bayou Chevreuil 0.69 f 0.074 CaIcasieu Rivei 1.42f0.115 Cecil subsoil 0.69 f 0.046 Cecil topsoil 0.83 f 0.047 Chastain 8.45f0.138 Crowley 0.50 f 0.079 GaIlion 0.33 + 0.07

10 Hartwell Lake 3.7550.1011 Lafitte muck 20.16 f 0.2412 Lake Pontchartrain 1.82f0.0813 Lake Providence 0.19 f 0.0914 Leeville 1.29f0.0215 Loring 8.96f0.3116 M hoon 0.05 f 0.0817 Mississippi River 0.51+ 0.0318 Norwood 0.31+ 0.0719 Shubuta 0.09 f 0.02*Adsorption coefficient.Sediment.bMean of triplicate subsamples and standard deviation.

1.52 f 0.06b0.67 + 0.051.37+0.011.89?0.071.31+0.081.67kO.12

10.85 f 0.090.93 ? 0.060.87? 0.084.46 f 0.07

39.09 + 0.672.28 + 0.070.85 & 0.051.93 + 0.099.99zbo.110.38 f 0.051.17+0.080.70 f 0.040.44 f 0.03

TABLE IVMeans, standard deviations and coefficients of variation for K an d K, values of methyl parathionand 2,4-D adsorption in 19 soil and sediment materialsCompo und Equilibration Parameter f s cv

time (h)Methyl parathion 2Methyl parathion 2Methyl parathion 24Methyl parathion 242,4-D 22,4-D 22,4-D 242,4-D 24

34.3 39.9 1141374 782 57

43.2 61.8 1431516 642 42

2.7 5.0 18593 93 100

4.3 8.9 206160 115 72

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related with 2,4-D adsorption is organic matter (Bailey and White, 1964). Ourdata on 19 soil and sediment materials indicate that low pH also promotes 2,4-D adsorption (Kuo, 1973).DISCUSSION

In soil and sediment materials where the organic matter content is very low,such as typical subsurface soil horizons, other soil properties may then becomesubstantially more important in regulating the mobility of synthetic organics.One object of this work was to examine the importance of properties other thanorganic matter content in adsorbing these pesticides in soil materials with loworganic matter content. In particular, we were interested in examining the roleof reducible iron levels as well as the Al extracted by an ammonium oxalateextractant. Therefore, R2 values describing the relationship between adsorp-tion and soil properties were also determined on those soil and sediment mate-rials with less than 1% organic carbon, an arbitrary cutoff on our part.

In these low organic matter materials, pH gave low R2 values of 0.31-0.36for both 2- and 24-h adsorption coefficients for both compounds, and all of theother properties we originally planned to examine gave much lower values.Oxalate extractable Fe and Al levels gave especially low R2 values. In these loworganic matter materials, however, we observed that the relationship betweenadsorption and oxalate extractable Mn and Ca increased, especially for 2,4-D.Table V presents R2 values where the regression includes two independentvariables. For each combination of compound, adsorption equilibration time,and level of soil organic matter, the pair of soil parameters giving the first,second, and third highest R2 values are presented. In the low organic mattersoil and sediment materials, the R 2values were greatly improved by includingpairs of soil properties in the regression analyses. Oxalate extractable Mn orCa appeared in all of the independent variable pairs giving the highest threeR2 values.

The reason for the frequent inclusion of oxalate extractable Ca and Mn asan independent variable contributing to some of the higher R2 values was notinvestigated further. However, a previous study has shown different saturatingcations have an effect on the adsorption of Dasanit, another organosphateinsecticide, (O,O-diethyl O- p- (methyl sulfinyl) phenyl] phosphorothioate)in montmorillonite suspensions (Bowman, 1973). This effect was attributedto the nature of the cation-pesticide interaction, the limited water solubility ofthe compound, and the effect of the saturating cation on the aggregation of theclay in the suspension used.

Thus in soil and sediment materials with low organic matter content, Ca andMn, or factors associated with weak chelate extractable levels of these ele-ments, may be important for regulating the mobility of these synthetic organics.

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CONCLUSIONS

Organic matter content of the soil materials was generally indicated to bethe most important property affecting adsorption. As expected, 2,4-D was lessstrongly associated with the solid phase than methyl parathion, and pH wasmore closely associated with the adsorption of 2,4-D than with methylparathion.

Data were presented indicating that in low organic matter content soils (1 %organic matter was arbitrarily selected as the cutoff), the association betweenadsorption and oxalate extractable Mn and Ca may be an important relation-ship. Additional work should be done in low organic matter soil material tobetter quantify the relationship between adsorption and soil geochemical prop-erties. As our work has demonstrated, those properties closely associated withthe adsorption of some synthetic organics in typical surface soils may not beuseful in predicting the immobilization of synthetic organics in subsoilsUnderstanding the relationship between geochemical properties and themobility of synthetic organics is especially important in typical, low-organicmatter subsoil materials that have been contaminated with hazardous organicssuch as has occurred at many hazardous waste disposal sites.ACKNOWLEDGEMENTS

This research was supported by the Environmental Research Laboratory,U.S. Environmental Protection Agency, Athens, GA. This support does notsignify that the contents necessarily reflect the views and policy of the Agency;nor does mention of trade names of commercial products constitute endorse-ment or recommendation for use.

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