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TECHNICAL REPORT
Organic Compounds in the Environment

Effects of Dissolved Organic Matter from Animal Waste Effluent on Chlorpyrifos Sorption by Soils

Department of Agronomy, Purdue Univ., West Lafayette, IN 47907-1150

* Corresponding author (lslee{at}purdue.edu)

Received for publication June 8, 2001.

	ABSTRACT

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS AND DISCUSSION CONCLUSIONS REFERENCES

 
The increased use of animal waste–derived effluents for irrigation could result in the enhanced movement of pesticides through complexation with dissolved organic materials. Batch equilibrium studies were conducted to measure the interaction among soil, chlorpyrifos [O,O-diethyl O-(3,5,6-trichloro-2-pyridyl) phosphorothioate], and dissolved organic matter (DOM) from poultry, swine, and cow waste–derived lagoon effluents. All DOM was found to have a strong affinity for chlorpyrifos, resulting in reduced sorption of chlorpyrifos by soil, thus the potential for DOM-enhanced mobility. Effluent DOM was observed to sorb to soils. Thus, for increasingly higher soil mass to solution volume ratios, the effect of chlorpyrifos association with water-borne DOM on sorption decreases significantly. For high soil mass to solution volume ratios typical of soil profiles in the landscape, the potential for DOM-enhanced transport will be greatly attenuated. Dissolved organic matter concentration and the nonpolar nature of DOM in the lagoon effluent decreased with increasing residence time in the cells of the lagoon system, thus reducing the potential for DOM-enhanced transport.

Abbreviations: DOM, dissolved organic matter • HA, humic acid • K_DOM-soil, dissolved organic matter–soil distribution coefficient • K_i-DOM, pesticide–dissolved organic matter distribution coefficient • K_i-soil, pesticide–soil distribution coefficient • OC, organic carbon

	INTRODUCTION

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS AND DISCUSSION CONCLUSIONS REFERENCES

 
RECENT trends in agribusiness have led to large concentrated animal production facilities. These facilities produce huge quantities of animal manure. For example, the livestock produced in Pennsylvania generate 25 million tons of manure annually, or five times the waste of New York City's human population (Pelley, 1996). The use of lagoons as a biological treatment process to convert organic matter derived from manure as well as feed and bedding to more stable end products has become very popular. Lagoon supernatants may then be applied to field crops, grasslands, golf courses, and parks by sprinkler or surface irrigation (Miller, 1990) as part of a water conservation and/or waste optimization plan. Attention has been given to minimization of odor, surface runoff, and the accumulation of undesirable levels of nutrients and toxic materials in soils and plants irrigated with lagoon effluents. However, the potential of enhanced transport of pesticides previously applied to the soil via complexation with mobile effluent-borne organic matter has not been given much consideration in decisions regarding the proper use of lagoon effluents for agricultural irrigation.

Complexation of organic compounds with DOM or organic macromolecules parallels the well-known phenomenon of the sorption of dissolved organic compounds by the organic fraction in soils and sediments (Karickhoff et al., 1979; Chiou et al., 1983). Enhanced aqueous solubility, decreased sorption, and enhanced transport resulting from the complexation or association of strongly hydrophobic substances (e.g., polychlorinated biphenyls, polyaromatic hydrocarbons, and organochlorine pesticides) with dissolved or colloidal organic matter has been clearly documented in both batch and column studies (Hassett and Anderson, 1982; Vinten et al., 1983; Hutchins et al., 1985; McCarthy and Jimenez, 1985; Chiou et al., 1986; Enfield and Bengtsson, 1988; Enfield et al., 1989; McCarthy and Zachara, 1989; Kan and Tomson, 1990; Chin et al., 1991; Magee et al., 1991; Dunnivant et al., 1992). The relative effect of DOM on solubility and sorption will be greater for more hydrophobic chemicals (Chin et al., 1991) and will be attenuated by the concentration, source, size, polarity, and molecular configuration of the organic colloids in the water-borne organic matter. In solubility enhancement studies of hydrophobic organic compounds (HOCs) with humic and fulvic acids, Chiou et al. (1986) found the affinity of hydrophobic chemicals to associate with dissolved organic substances as follows: soil-derived humics > soil-derived fulvics > aquatic humics > aquatic fulvics. In addition to association of chemicals to DOM, DOM applied to soils can sorb to soil surfaces, resulting in an overall enhancement in sorption and retardation as demonstrated by Totsche et al. (1997) for HOCs with forest floor–derived DOM. Therefore, the effect of DOM on sorption and subsequent transport will be dependent on the nature of the solute, soil, and DOM, as well as the competition between solute–soil, solute–DOM, and DOM–soil interactions.

In the presence of DOM, apparent solution concentration will include solute-bound DOM with DOM normalized to organic carbon (C_DOM, kg kg^-1). Effluent DOM may also have some affinity for the soil. The following coefficients may be employed to account for soil–pesticide (K_i-soil, L kg^-1), DOM–pesticide (K_i-DOM, L kg^-1), and DOM–soil (K_DOM-soil, L kg^-1) interactions:

[1]

[2]

[3]

where C_s and C_w are the concentrations of the solute (i) associated with the soil (mg kg^-1) and free solute in solution (mg L^-1), respectively; C_i-DOM is the concentration of the solute (i) associated with DOM on a carbon basis (mg kg^-1); and DOM_w and DOM_s are the carbon-based mass of DOM remaining in the bulk water (kg L^-1) and associated with the soil (kg kg^-1), respectively. The overall soil–water distribution coefficient of an organic chemical in the presence of DOM can be defined in terms of Eq. [1]–[3]:

[4]

assuming linear distribution behavior between all constituents, no competition between DOM and pesticide for sorption sites on soil, and mass conservation of DOM (Enfield et al., 1989; Magee et al., 1991; Knabner et al., 1996; Brunk et al., 1997). Also, Eq. [4] does not include a unique sorption coefficient for the sorption to soil of a solute-associated DOM complex, but rather assumes that the affinity of a chemical for DOM (K_i-DOM) is the same for water-borne DOM and DOM that has been sorbed to soil. Chin et al. (1991) summarized that the relative effect of DOM on the sorption and transport behavior of organic molecules would not be significant unless DOM was present in concentrations greater than 40 mg L^-1 or the organic compound had an octanol–water partition coefficient (K_ow) greater than 10⁵.

At one of the animal science research facilities at Purdue University, lagoon systems are used for treating poultry, swine, and cow production wastes and effluents are often applied to nearby corn fields where chlorpyrifos, a widely used insecticide, is routinely applied. Chlorpyrifos is very hydrophobic with a K_ow of 10⁵ and low aqueous solubility (0.4 mg L^-1) (Verschueren, 1996), and thus is a likely candidate for facilitated transport. In this study, the affinity of chlorpyrifos to animal-derived effluent DOM, the affinity of the DOM to soil, and the net effect on sorption of chlorpyrifos in the presence of DOM was measured to evaluate the potential for enhanced transport of chlorpyrifos under animal-derived effluent irrigation.

	MATERIALS AND METHODS

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS AND DISCUSSION CONCLUSIONS REFERENCES

 
Chemicals
Chlorpyrifos was supplied by Chem Service, West Chester, PA. The compound was of greater than 98% purity. Acetonitrile, methyl alcohol (Mallinckrodt Baker, Paris, KY; Chrom AR HPLC), and chloroform (Lot no. 973220; Fisher Scientific, Fair Lawn, NJ) used in insecticide analysis were all greater than 99.9% pure.

Soils
Two silt loam soils typical of Indiana cultivated surface soils were collected from the Agronomy Research Center, Purdue University. The soils were a Toronto silt loam (fine-silty, mixed, superactive, mesic Udollic Epiaqualf) and a Raub silt loam (fine-silty, mixed, superactive, mesic Aquic Argiudoll). The collected soil was air-dried, gently crushed to pass a 2-mm sieve, and thoroughly mixed, and selected properties were measured (Table 1). Values for pH, cation exchange capacity (CEC), and organic carbon (OC) are averages of three replicates. Values for particle size analysis are from single measurements.

Distribution Studies
Distribution coefficients K_i-DOM, K_i-soil, K_DOM-soil, and K^*_i-soil were determined for various combinations of chlorpyrifos, DOM, and soils using batch isotherm techniques. All samples were done in duplicate for each treatment and equilibrated at 23 ± 1°C for 24 h on an end-over-end shaker. In preliminary experiments conducted for 24 h, no changes in solution concentration of chlorpyrifos or DOM were observed between 12 and 24 h; therefore, an apparent equilibrium was assumed at 24 h.

Chlorpyrifos to Dissolved Organic Matter (K_i-DOM)
Binding of chlorpyrifos to DOM was estimated using a solubility enhancement technique (Chiou et al., 1986). A series of concentrations ranging from 0 to 100 mg OC L^-1 were prepared for each type of DOM. Chlorpyrifos was added to 35-mL Corex glass centrifuge tubes (Kimble/Kontes, Vineland, NJ) with Teflon-lined screw caps in amounts more than required to saturate DOM solutions using a plating method followed by addition of DOM solutions. The plating method involves application of a chemical dissolved in an organic solvent followed by volatilization of the solvent prior to any other addition (Karickhoff and Brown, 1979). After equilibration for 24 h, samples were centrifuged at 3500 x g for 1 h at 23°C to separate the excess solute. Preliminary studies using equilibration times up to 48 h showed that 24 h was sufficient to achieve equilibrium. Undissolved solute particles adhering to the meniscus were aspirated from the surface, and glass walls with undissolved solute particles were carefully cleaned with tissue paper. The centrifugation and cleaning process was repeated. A 5-mL aliquot of the centrifuged solution was carefully withdrawn with a volumetric pipette and extracted with 2 mL of chloroform. A 1-mL aliquot of the chloroform was transferred to a 5-mL conical evaporator vial and subsequently concentrated to dryness using an N₂ evaporator system. The concentrate was dissolved in 1 mL methanol and analyzed for chlorpyrifos using a Shimadzu automated high performance liquid chromatography (HPLC) system with a UV-Vis detector ( = 230 nm), a Supelcosil ABZ⁺ reversed-phase column (Supelco, Bellefonte, PA), and a mobile phase of 65:35 acetonitrile and water at a flow rate of 1.5 mL min^-1. Chlorpyrifos concentrations were estimated using a linear external calibration curve that encompassed the concentration range of sample extracts.

Apparent chlorpyrifos solubility (S, mg L^-1) includes free and DOM-bound solute and can be expressed in terms of K_i-DOM:

[5]

where S_w is the pure aqueous solubility (Chiou et al., 1986). A linear regression of S versus DOM_w yields a slope equal to S_w K_i-DOM and an intercept equal to S_w.

Dissolved Organic Matter to Soils (K_DOM-soil)
The partitioning of DOM to soil was determined by combining 10 mL of DOM solution (100 mg OC L^-1) and varying amounts of soil (1, 2, 3, 4, and 5 g) in 18-mL Corex centrifuge tubes with Teflon-lined screw caps. Samples were equilibrated and centrifuged as described previously for K_i-DOM and carbon-based DOM concentrations (mg L^-1) were measured. The DOM released from the soil during the experiment was accounted for by analysis of the soil–0.005 M CaCl₂ controls. Dissolved organic matter concentrations extracted by 0.005 M CaCl₂ solutions increased linearly with increasing soil mass (m_s) to solution volume (V_w) ratios from 0.1 to 0.6 g mL^-1 for both soils (Toronto: slope = 78, r² = 0.81; Raub: slope = 61, r² = 0.90). The highest DOM release concentrations observed at the highest m_s to V_w ratio were 30 and 37 mg L^-1 for Toronto and Raub soils, respectively. Calcium chloride solution has been shown to have a relatively small effect on DOM release from soils compared with other salt solutions, with ionic strength effects from CaCl₂ also being small (Reemtsma et al., 1999). Dissolved organic matter sorption to containers was not observed in solution controls. The K_DOM-soil values were estimated using the following equation, which assumes sorption is linear within the concentration range investigated:

[6]

where DOM₀ is the initial DOM concentration (mg L^-1), m_s is the soil mass (kg), and V_w is solution volume (L). The slope of a linear regression between DOM₀ DOM^-1_w versus m_s V^-1_w is K_DOM-soil.

Chlorpyrifos to Soils (K_i-soil and K^*_i-soil)
The adsorption of chlorpyrifos onto Toronto and Raub soil was determined by combining 0.1 g soil with 20 mL of a 0.005 M CaCl₂ solution containing chlorpyrifos ranging from 0 to 0.35 mg L^-1 in 35-mL Corex centrifuge tubes with Teflon-lined screw caps. For chlorpyrifos sorption in the presence of added DOM , DOM was added to chlorpyrifos solutions to achieve initial carbon-based DOM concentrations of approximately 70 mg L^-1 and mixed for 8 h prior mixing with the soil in the Corex tubes. After equilibration, samples were centrifuged for 30 min at 1350 x g. Included as controls in all cases were samples with water and soil only, and samples with the initial chlorpyrifos solution without soil. Both chlorpyrifos sorbed by soil and chlorpyrifos in solution at equilibrium were determined directly. A 10-mL aliquot of the supernatant phases was extracted with 2 mL chloroform; chlorpyrifos in soil was extracted for 48 h with 1:1 (v/v) of methanol and chloroform. Chlorpyrifos concentrations were determined by high performance liquid chromatography as previously described. The slope of a linear regression between concentration of chlorpyrifos sorbed and that in solution at equilibrium is equal to the soil–water distribution coefficients (K_i-soil or K^*_i-soil).

	RESULTS AND DISCUSSION

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS AND DISCUSSION CONCLUSIONS REFERENCES

 
Binding of Chlorpyrifos to Dissolved Organic Matter
The apparent water solubility of chlorpyrifos increased linearly with DOM concentration for all DOM sources (Fig. 1 and Table 3). Correlation coefficients (r²) ranged from 0.78 to 0.94 (Table 3) with lower r² values usually resulting from a scatter in the data at the lower DOM concentrations. The largest solubility enhanced was observed with HA, thus resulting in the greatest K_i-DOM value. The log K_i-DOM value of 4.28 for chlorpyrifos binding to HA is in excellent agreement with the prediction of 4.24 from the log K_ow–log K_i-DOM correlation reported by Chin et al. (1991) for binding of a series of hydrophobic chemicals by Fluka (Buchs, Switzerland) and Aldrich HAs. Values for K_i-DOM varied with DOM source, but K_i-DOM values for all lagoon systems generally decreased from Cells 1 to 3.

View larger version (13K): [in this window] [in a new window]   Fig. 1. Representative plots of apparent water solubility (S) of chlorpyrifos as a function of dissolved organic matter (DOM) source and concentration. Error bars represent standard deviations.

View this table: [in this window] [in a new window]   Table 4. Elemental analysis of freeze-dried organic matter extracted from animal waste–derived lagoon effluents on a dry-weight basis.

View larger version (14K): [in this window] [in a new window]   Fig. 2. Correlation between pesticide–dissolved organic matter distribution coefficient (Ki-DOM) and C/(O + N) ratios. Error bars represent standard deviations.

View larger version (14K): [in this window] [in a new window]   Fig. 3. Representative isotherms for sorption of dissolved organic matter (DOM) by Toronto soil. Error bars represent standard deviations.

[7]

Various forms of Eq. [7] and the concepts incorporated therein have been applied by others to assess the effect of DOM on solubility, sorption, and/or transport (Gschwend and Wu, 1985; Chiou et al., 1986; Enfield et al., 1989; Magee et al., 1991; Knabner et al., 1996). A summary of measured versus predicted K^*_i-soil values is shown in Fig. 4 along with a line representing a 1:1 correlation. Reduced sorption in the presence of DOM was predicted; however, Eq. [7] generally predicted greater reductions in sorption than what was measured. Effluent DOM is not composed of a single molecule but a mixture of molecules that will range in size and polarity. As illustrated previously, both K_i-DOM and K_DOM-soil coefficients tend to increase with decreasing DOM polarity; therefore, the more nonpolar DOM molecules will preferentially sorb to soil leaving the less hydrophobic DOM molecules in solution for chlorpyrifos–DOM interactions. The net result will be a reduced effect of DOM on chlorpyrifos sorption than would be predicted by the individual binary distribution coefficients, in agreement with our observations. Although predicted K^*_i-soil values were consistently lower than measured values, Eq. [7] is still useful in screening for the potential of DOM-facilitated transport of an organic chemical.

View larger version (21K): [in this window] [in a new window]   Fig. 4. Relationship between the measured and predicted soil–water distribution coefficient of an organic chemical in the presence of dissolved organic matter . Error bars for predicted values were estimated by propagation of errors.

[8]

where and are average bulk density (g cm^-3) and water content (cm³ cm^-3), respectively. K^*_i-soil values were predicted with Eq. [7] using effluent DOM concentrations reported for the last cell in each lagoon system (Table 2); K_i-soil, K_i-DOM, and K_DOM-soil values reported in Tables 6, 3, and 5, respectively; and assuming a value of 4 for /. Estimates of retardation factors for clean water irrigation are 363 and 353 for Toronto and Raub soils, respectively. Retardation in the presence of effluent DOM was reduced no more than 6% for any of the waste-derived effluents with either soil. If the sorption of effluent DOM to the soil had been assumed to be zero, significant reductions of approximately 60, 40, and 20% for cow, swine, and poultry waste–derived effluents would have been estimated. In the latter case, R values were still large, ranging from 140 to almost 300. However, the latter case does clearly exemplify the need to consider sorption of applied effluent DOM when assessing the potential effect of DOM-facilitated pesticide transport under effluent irrigation. For high soil mass to solution volume ratios typical of soil profiles in the landscape, the potential for DOM-enhanced transport will be greatly attenuated by sorption of DOM to soil. For soil-applied chlorpyrifos under animal-derived effluent irrigation, even if DOM is not sorbed by soils, retardation factors (R) are likely to remain sufficiently large such that movement past the upper few centimeters of the soil profile is unlikely.

This study focused on the effect of sorption on DOM-facilitated transport, which while important is not sufficient in assessing the appropriate use of treated effluent for irrigation. There are other factors that need to be considered. Effluent pH and ionic composition, especially monovalent salts (e.g., sodium salts), can enhance the release of soil organic matter, thereby mobilizing sorbed contaminants otherwise retained by the soil surface (Reemtsma et al., 1999). Also under natural field conditions, soil undergoes wetting and drying cycles that can result in DOM accumulation, enhanced soil organic matter solubilization, and an increase in the strength of the DOM–pesticide complex (Williams et al., 1999; Seol, 1998). Furthermore, if preferential flow processes are significant, the pesticide mass that can be transported down preferential flow paths includes both the free pesticide and the pesticide associated with water-borne DOM (Nelson et al., 2000; Williams et al., 2000).

	CONCLUSIONS

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS AND DISCUSSION CONCLUSIONS REFERENCES

 
Dissolved organic matter applied to the soil through effluent irrigation will undergo sorption reactions with the soil, and in both the sorbed and water-borne states will serve as a partitioning domain for hydrophobic chemicals. Chlorpyrifos is a very hydrophobic compound with a small water solubility, thus having a high affinity to soils as well as to DOM. Sorption of DOM by soil and DOM–chlorpyrifos complexation was measured for several sources of DOM on two silt loams. The affinity of DOM for soil and to associate with chlorpyrifos followed the same order, with the less polar DOM having a higher affinity. Both predicted and measured sorption coefficients showed that the presence of DOM can reduce sorption by soils (K_i-soil). When the DOM concentration was fixed, the effect of DOM on the K_i-soil increased with decreasing DOM polarity. The effect of decreased chlorpyrifos sorption to soil in the presence of DOM on retardation was estimated using DOM concentrations measured in the last cells of each lagoon system. Changes in retardation were small (less than 6%) unless sorption of effluent DOM to soils was ignored. In the latter case, significant reductions of 20 to 60% in retardation were estimated. In the case of chlorpyrifos under field conditions, chlorpyrifos is likely to remain in the upper few centimeters of the soil profile. Lagoon treatment of animal wastes is a good method for decomposing organic matter. In this process, not only DOM concentration, suspended particles, and electrical conductivity were reduced, but also the polarity of DOM increased with each subsequent cell. The longer the residence time of animal wastes in the lagoon system, the less chance of binding between DOM and a nonpolar pesticide like chlorpyrifos.

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TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS AND DISCUSSION CONCLUSIONS REFERENCES

 

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