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DIVISION S-2-SOIL CHEMISTRY

Effect of Dissolved Organic Matter in Treated Effluents on Sorption of Atrazine and Prometryn by Soils

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

lslee{at}purdue.edu

	ABSTRACT

TOP ABSTRACT INTRODUCTION Materials and methods Results and discussion Conclusions REFERENCES

 
The apparent enhanced transport of soil-applied atrazine following irrigation of treated effluents has been hypothesized to be from complexation of atrazine with effluent-borne dissolved organic matter (DOM). Under long-term effluent irrigation, even small DOM-induced decreases in pesticide sorption can result in significant enhanced pesticide movement due to cumulative effects. The effect of atrazine and prometryn association with DOM extracted from municipal wastewater (MW), swine-derived lagoon wastewater (SW), and dissolved Aldrich humic acid (HA) on sorption by two soils was measured in batch equilibration studies. Individual association of pesticides to DOM, sorption of DOM to soil, and pesticide sorption by soil were also quantified. Pesticide association to DOM normalized to organic carbon (OC) ranged from 30 to 1000 L/kg OC. DOM sorption by soil ranged from 1.5 to 10 L/kg with a silt loam having a higher affinity for the DOM than the sandy loam. DOM up to 150 mg OC/L did not significantly suppress sorption by soils of either atrazine or prometryne in agreement with predictions using the independently measured binary distribution coefficients in a model that assumed linear equilibrium behavior among pesticide, soil, and DOM. A sensitivity analysis was performed using the same model to identify what combination of soil, pesticide, and DOC variables may suppress sorption, resulting in facilitated transport. Results from the sensitivity analysis are presented and the potential for effluent properties other than DOM to facilitate pesticide transport is discussed.

Abbreviations: BF, Bloomfield • CEC, cation-exchange capacity • COD, chemical oxygen demand • DOC, dissolved organic carbon • DOM, dissolved organic matter • DR, Drummer • HOCs, hydrophobic organic compounds • HA, humic acid • HPLC, High Performance Liquid wastewater • SOC, soil organic carbon • SOM, soil organic matter • SW, swine effluent

	INTRODUCTION

TOP ABSTRACT INTRODUCTION Materials and methods Results and discussion Conclusions REFERENCES

 
EFFORTS TO ABATE pollution and supplement available water resources in many parts of the world have resulted in the increased use of effluents for irrigation. In semiarid areas, where fresh water is limited, the use of effluents for agricultural purpose has been common practice over the last few decades (Rebhun et al., 1987; Ronen and Magaritz, 1985). In the USA, treated effluent is one of the fastest growing sources of water for irrigating both agricultural and recreational facilities such as golf course, parks, school grounds and street medians (Miller, 1990). One potential side effect of this practice is the long-term deterioration of groundwater quality. Organic chemicals originally present in the wastewater and soil-borne organic pollutants have been identified in ground water at effluent-irrigated sites (Hutchins et al., 1985), or at greater depths in the soil profile than observed at sites irrigated with only high quality water (Muszkat et al., 1993; Graber et al., 1995). Effluent characteristics that may impact mobility of soil-borne chemicals include DOM, ionic strength, ionic composition, and pH.

Complexation or association of strongly hydrophobic substances (e.g., polychlorinated biphenyls, polyaromatic hydrocarbons, and organochlorine pesticides) with dissolved or colloidal organic matter resulting in the enhanced aqueous solubility, decreased sorption, and enhanced transport has been clearly documented in batch and column studies (Chiou et al., 1986; Dunnivant et al., 1992; Enfield et al., 1989; Hassett and Anderson, 1982; Hutchins et al., 1985; Kan and Tomson, 1990; McCarthy and Jimenez, 1985; Vinten and Nye, 1983; Chin et al., 1991). Based on studies with primarily aquatic humic substances, Chin et al. (1991) concluded that the influence of organic colloids on sorption of organic compounds where hydrophobic interactions dominated would be small for compounds with octanol-water partition coefficients (K_ow) <10⁵, especially given the low DOM concentrations typically in aquatic and subsurface water systems. However, in aqueous solutions containing dissolved organic matter from various sources including sludge, crop residues, and secondary wastewater, decreases in sorption or enhanced transport have also been observed for weakly basic or nonionic, moderately soluble pesticides (K_ow <10³) (Lee and Farmer, 1989; Barriuso et al., 1992; Graber et al., 1995; Celis et al., 1998; Nelson et al., 1998).

In addition to intrinsic solute properties, the concentration, source, size, polarity, and molecular configuration of organic colloids will effect their impact on solubility, thus sorption of organic chemicals. 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. Complexation of s-triazines to DOM using well-characterized humic acids suggest that complexation occurs through either proton or electron transfer depending on the acidity of the humic acid and the basicity of the traizine rather than simple hydrophobic interactions (Senesi et al., 1987; Sposito et al., 1996). 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 impact of DOM on sorption and subsequent transport will be dependent on the intrinsic nature of the solute, soil, and DOM, and the competition among solute–soil, solute–DOM, and DOM–soil interactions.

In this study, the impact of effluent DOM extracted from swine waste lagoon effluent and municpal waste water effluent, as well as a well-characterized humic acid on triazine sorption, was measured. The association of atrazine and prometryne to DOM, sorption of DOM by soil, and pesticide sorption by soil were individually quantified to evaluate the impact of effluent–DOM on pesticide sorption, and subsequent mobility potential under effluent-irrigation. A sensitivity analysis using a model that assumed linear equilibrium behavior among solute, soil, and DOM was performed to identify what combination of soil, pesticide, and DOC variables may result in facilitated transport.

	Materials and methods

TOP ABSTRACT INTRODUCTION Materials and methods Results and discussion Conclusions REFERENCES

 
Pesticides
Two high purity s-triazine herbicides with different functional groups, atrazine (6-chloro-N²-ethyl-N⁴-isopropyl-1,3,5-triazine-2,4-diamine, 98.8%, Lot no. 153-122B), and prometryn [N,N'-bis(1-methylethyl)-6-(methylthio)-1,3,5-triazine-2,4-diamine, 99.5%, Lot no. 152-95B], were obtained from Chem Service (West Chester, PA). Selected properties of atrazine and prometryn are listed in Table 1 .

Dissolved Organic Matter Characterization
Freeze-dried DOM samples (HA, MW, and SW) were analyzed for C and N content with Fisons NA 1500 NC Analyzer (Fisons Scientific Equipment, Loughborough, UK). Dried materials were dissolved in 0.005 M CaCl₂ solution for characterization of pH, OC content (mg OC/L), inorganic C content (IC, mg C/L), and total acidity. For total acidity, 15 mL of NaOH (0.101 M) was mixed with 10 mL of DOM solution in 250-mL amber bottles and shaken for 24 h. The mixed solutions were titrated with HCl (0.0206 M) to obtain the total acidity of DOM. OC and IC contents in DOM solutions were measured with a Dohrmann TOC analyzer (Rosemount Analytical, Dohrmann Division, Santa Clara, CA). Reported DOM concentrations are OC-based concentrations (DOC).

Interaction between Pesticide and Dissolved Organic Matter
The binding coefficients of pesticides to DOM from effluents were measured using a dialysis technique (Carter and Suffet, 1982). Spectral/Por 6 dialysis tubing (Spectrum Medical Industries, Houston, TX) with a molecular weight cut off of 1000 Da was washed in distilled water, 1 M Na₂CO₃, twice in 1 M NaHCO₃, and once again in distilled water. Freeze-dried DOM samples were dissolved in 0.005 M CaCl₂ solution at 140 mg OC/L for MW, 320 mg OC/L for SW, and 690 mg OC/L for HA. Aliquots of 5 or 10 mL were placed in dialysis tubing, and placed in 35-mL test tubes containing 5 or 10 mL of 0.005 M CaCl₂ solution ranging in pesticide concentrations from 0 to 20 mg/L. Tubes were wrapped with aluminum foil to minimize photolysis, and shaken on a platform shaker for 2 d at 21 ± 2°C. Each experiment included three tubes without DOM to verify that pesticide concentrations inside and outside of the bag were equal after 2 d (i.e., equilibrium achieved). Pesticide concentrations inside and outside the dialysis tubing were determined using a Shimadzu Automated High Performance Liquid Chromatography (HPLC) system (Shimadzu, Kyto, Japan) with a UV-vis detector ( = 254 nm), a Supelcosil ABZ⁺ reversed-phase column (Supelco, Supelco Park, Bellefonte, PA), and a mobile phase of 65/35 acetonitrile/water at a flow rate of 1.5 mL/min. Complete dissociation of the pesticide–DOM complex in the acetonitrile-water mobile phase during HPLC analysis was confirmed by comparing known pesticide concentrations in the presence and absence of DOM. The difference between pesticide concentrations inside and outside of the dialysis tubing was assumed to be the DOM-bound pesticide concentrations. A linear regression between bound pesticide (mg/kg OC) and free pesticide (mg/L) yields a slope equal to K^DOC_i.

Interaction between Dissolved Organic Matter and Soil
Varying amounts of soil (0–5 g) were shaken for 2 d with a fixed volume (5 mL) of DOM solution. For sorption by DR, initial DOM solution concentrations were 440, 110, and 300 mg OC/L for HA, MW, and SW, respectively. For BF, sorption of only HA was measured with initial concentrations of 84 mg OC/L. After shaking, samples were centrifuged (1000 g for 30 min) and OC content was measured in the supernatants using a Dohrmann TOC analyzer (Rosemount Analytical). Decrease in DOC concentration because of adsorption to the containers was not observed in blank samples without soil. Tubes with only soil and 0.005 M CaCl₂ solution were used to account for soil organic matter solubilized during equilibration. The OC-normalized distribution coefficient of DOM between the soil and aqueous phase, K^S_DOC = DOC_S/DOC_W, was estimated assuming DOM sorption obeys a linear isotherm and applying mass balance equations using the following relationship (Magee et al., 1991)

(1)

where m_S is soil mass (g), V_W is solution volume (mL), and DOC_O and DOC^*_W are OC-based DOM (kg OC/L) concentrations in the applied solution and remaining in the bulk water after equilibration, respectively, with DOC^*_W corrected for SOC released in control solutions containing no DOM. A linear regression between DOC_O/DOC_W and m_S/V_W yields a slope equal to K^S_DOC.

Pesticides, Soil, and Dissolved Organic Matter Interactions
Pesticide sorption coefficients (K^S_i and K^S_i,DOC) were measured for each pesticide–soil–DOM combination using batch isotherm experiments. Soil/water ratios were selected to achieve 50% sorption using reported K^SOC_i values and soil f_SOC. Pesticide solutions of varying concentrations in a 0.005 M CaCl₂ matrix, either with no DOM or at a single fixed DOM concentration, was added to air-dried soil in 17-mL glass test tubes with Teflon-lined screw caps and equilibrated in a end-over-end shaker for 2 d at 22 ± 1°C. The DOM concentration used was between 60 to 150 mg C/L depending on the DOM source. SW DOM and dissolved HA were used for Drummer while only dissolved HA was used in the Bloomfield soil experiments. Samples were centrifuged for 30 min at 1000 g and the supernatants were directly analyzed for the pesticides by HPLC/UV as previously described. K^S_i and K^S_i,DOC values were estimated from the slope of a linear regression between sorbed and apparent solution pesticide concentrations.

	Results and discussion

TOP ABSTRACT INTRODUCTION Materials and methods Results and discussion Conclusions REFERENCES

 
Effluent Dissolved Organic Matter Characterization
Chemical analyses of final DOM solutions used in the different interaction studies are summarized in Table 3 . Also shown for comparison are selected characteristics of the raw effluent after filtration or centrifugation in Table 3. Final DOM solutions were prepared at elevated DOC concentration relative to the initial effluent to expand the DOM range investigated. Because of the ultra-filtration step, IC contents were significantly reduced. The pH values of DOM solutions were also 0.5 to 0.7 pH units lower than the initial effluents. Changes in solution pH may impact the behavior of weak bases such as the triazines. However, for the effluent and soil solution pH range investigated in these studies, both atrazine and prometryn were 99.99% neutral.

View larger version (19K): [in this window] [in a new window]   Fig. 1 Association of pesticides to DOM from municipal waste effluent (MW), swine effluent (SW), and Aldrich humic acid (HA). DOM concentrations were as follows: 136 mg OC/L for MW, 320 mg/L for SW, and 690 mg OC/L for HA. Lines represent linear regressions with zero intercepts

View larger version (16K): [in this window] [in a new window]   Fig. 2 Sorption of DOM from municipal waste effluent (MW), swine effluent (SW), and Aldrich humic acid (HA) by Drummer (DR) and Bloomfield (BF) soils. Initial DOM concentrations applied to DR were 110 mg OC/L for MW, 300 mg OC/L for SW, and 440 mg OC/L for HA, and 84 mg OC/L of HA was applied to BF

Dissolved organic matter (DOM) sorption to soil (K^S_DOC) was greatest for HA and smallest for SW, similar to what was observed for the association of a given pesticide to DOM: HA > MW > SW. Humic acid (HA) sorption was significantly lower on the Bloomfield soil compared with the Drummer soil. Bloomfield is a much sandier soil with lower OC, clay content, CEC, and surface area, all of which would impact DOM sorption.

The ratios of K^DOC_i/ K^SOC_i summarized in Table 4 were calculated to assess the relative affinity of pesticides to DOC vs. SOC. The K^DOC_i/K^SOC_i ratios of both pesticides were greater than 1.0 for HA and MW DOM, and prometryn showed greater ratios than atrazine. Similar trends can be cited for nonpolar organic chemicals. Phenanthrene binding to organic matter water extracted from a clayey silty loam (5.9% OM), and dissolved HA resulted in K^DOC_i values of 43 800 L/kg (Magee et al., 1991) and 50 000 L/kg (Gauthier et al., 1986), respectively, which is much higher than the commonly reported K^SOC_i value for phenanthrene of 16000 L/kg. However, Chiou et al. (1986) found K^DOC_i values measured for the association of several chlorinated organic chemicals to alkaline-extracted organic matter to be consistently less than reported K^SOC_i values.

Values of K^DOC_i/K^SOC_i greater than unity will result in a greater potential for DOM to facilitate pesticide transport; however, the latter may be counteracted by a concomitant increase in sorption of DOM by soils. For the DOM in this study, sorption of DOM by soils followed a similar trend as the association of a given pesticide with the same DOM sources (i.e., HA > MW > SW). DOM that sorbs to the soil will in turn serve as an additional sorption domain for pesticides. The net effect can not only minimize enhanced transport potential, but result in an overall increase in sorption and subsequent decrease in mobility.

Pesticide–Soil–Dissolved Organic Matter Systems
The net impact of DOM on pesticide sorption from the combined interactions among pesticide, soil, and DOM was measured using batch isotherm experiments and estimated using a simple model that assumes linear equilibrium distribution behavior among soil, solute, and DOM. The overall distribution coefficient of a pesticide in a soil–water system containing DOM is, by definition

(2)

From Eq. [2], K^S_i,DOC can be defined in terms of the individual linear distribution coefficients by applying a series of mass-balance equations, assuming linear distribution behavior among all constituents, mass conservation of DOM, and no competition between DOM and pesticide for adsorption sites on soil

(3)

Various forms of Eq. [3] and the concepts incorporated therein have been applied by others to assess the effect of DOM on solubility, sorption, and transport (Gschwend and Wu, 1985; Chiou et al., 1986; Enfield et al., 1989; Magee et al., 1991; Knabner et al., 1996).

Batch isotherms for pesticide sorption by BF and DR soils in the presence and absence of DOM are shown in Fig. 3 , and the slopes (K^S_i,DOC) from the linear isotherm fits are summarized in Table 5 . Also shown in Table 5 is the percent change in pesticide sorption upon addition of DOM, with a negative sign representing a decrease in sorption. The lines shown in Fig. 3 are predictions using Eq. [3] with the individual distribution coefficients summarized in Table 4. In almost all cases pesticide sorption increased in the presence of DOM, with the exception of prometryne sorption in the presence of HA (97 mg C/L). DOM-pesticide interactions were counteracted by the concomitant sorption of DOM by soils resulting in decreases in available DOM and additional sorptive domains on the soil. The K^S_i,DOC values estimated from Eq. [3] were generally in very good agreement with measured values (Table 5) with one exception: Eq. [3] predicted a substantial increase in sorption of prometryn in the presence of HA whereas a decrease was observed.

View larger version (21K): [in this window] [in a new window]   Fig. 3 Sorption of pesticide by Drummer (DR) and Bloomfield (BF) soils in the presence and absence of DOM from different sources. Numbers in parentheses are DOM concentrations in mg OC/L, and lines are Eq. [3] predictions

View this table: [in this window] [in a new window]   Table 5 Measured and predicted distribution coefficients of pesticides to soils in the presence and absence of DOM.

View larger version (48K): [in this window] [in a new window]   Fig. 4 Equation [3] estimates for the minimium DOCo concentrations needed to produce a 2% decrease in pesticide sorption, i.e., KSi,DOC 0.98 fSOCKSOCi for values of KD (fSOC* KSOCi) < 20 L/kg (upper graph); and for a case where KD (fSOC* KSOCi) is near unity, KSDOC = 0 and KDOCi = 100 and 200 (lower graph)

	Conclusions

TOP ABSTRACT INTRODUCTION Materials and methods Results and discussion Conclusions REFERENCES

 
The net effect of pesticide association with DOM on pesticide sorption is dependent on the nature of, and interactions among, the pesticide, soil, and DOM. For moderately polar s-triazines, sorption data and model simulations indicate that the association with effluent DOM will have a small to negligible impact on pesticide sorption in most cases. However, under long-term effluent irrigation, even a small suppression in sorption may lead to enhanced pesticide movement especially when coupled to the potential influence of other effluent properties as well as transient field-scale processes.

	ACKNOWLEDGMENTS

 
This research was funded in part by the Purdue University Research Program and USDA-BARD under contract IS-2384-94. Approved for publication as Purdue Agricultural Research Programs Journal Series no. 15961. Special thanks are extended to Judy Santini for her help in the sensitivity analysis.

	REFERENCES

TOP ABSTRACT INTRODUCTION Materials and methods Results and discussion Conclusions REFERENCES

 

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This Article Abstract Figures Only Full Text (PDF) Free Alert me when this article is cited Alert me if a correction is posted Services Similar articles in this journal Similar articles in ISI Web of Science Alert me to new issues of the journal Download to citation manager Citing Articles Citing Articles via HighWire Citing Articles via ISI Web of Science (16) Citing Articles via Google Scholar Google Scholar Articles by Seol, Y. Articles by Lee, L. S. Search for Related Content PubMed Articles by Seol, Y. Articles by Lee, L. S. GeoRef GeoRef Citation Agricola Articles by Seol, Y. Articles by Lee, L. S.