Effect of Dissolved Organic Carbon on Sorption of Pyrethroids to

Oct 14, 2010 - Despite their strong hydrophobicity, recent studies showed widespread occurrence of pyrethroid in downstream surface waters bodies. In ...
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Environ. Sci. Technol. 2010, 44, 8473–8478

Effect of Dissolved Organic Carbon on Sorption of Pyrethroids to Sediments L. DELGADO-MORENO,* L. WU, AND J. GAN Department of Environmental Sciences, University of California, Riverside, California 92521

Received November 6, 2009. Revised manuscript received September 20, 2010. Accepted October 1, 2010.

Despite their strong hydrophobicity, recent studies showed widespread occurrence of pyrethroid in downstream surface waters bodies. In this work, the effect of dissolved organic carbon (DOC) on the sorption and desorption of pyrethroids in sediment was evaluated to understand the role of DOC in facilitating pyrethroid transport. Presence of DOC from three sources at 38 ( 2 mg L-1 in the aqueous phase decreased pesticide sorption to a sediment by 1.7 to 38.9 times and increased their desorption by 1.2 to 41.4 times. The effect on pyrethroid sorption to the sediment was linear. In addition, interactions between DOC and pyrethroids, when taking place prior to the contact with sediment, decreased sorption of some pyrethroids even further, implying that DOC-pyrethroid complexs were relatively stable in solution. DOC sources with higher contents of carboxylic and phenolic groups were found to have a higher potential to associate with pyrethroids. The DOC-water partition coefficients (KDOC) obtained by solidphase microextraction measurement were significantly correlated (P < 0.01) with Kd values measured for the sediment. These results provide evidence that DOC increases the distribution of pyrethroids from the sediment to the solution phase and plays an important role in mobilizing pyrethroids in runoff and surface streams.

Introduction Pyrethroids, despite their strong affinity for the soil phase, have been found to move from the original application or contamination site to downstream surface water bodies via surface runoff. In surface water bodies, some pyrethroids have the potential to bioaccumulate or exert toxic effects on aquatic wildlife (1, 2). A recent example of off-site transport of pyrethroids is their widespread occurrence in bed sediments in both urban and rural watersheds of California and other regions (1-5). Pyrethroids are extremely hydrophobic with log Koc from 5.1 to 6.0 (6). The strong sorption to the bed sediment should render these chemicals less mobile after application (7, 8). Previous studies have showed enhanced solubility of hydrophobic organic contaminants (HOCs) in the presence of dissolved organic carbon (DOC) from different sources and suspended materials (9-15). Thus, sorption to suspended materials and DOC has been assumed the mechanism for offsite HOCs movement via overland flow. Recent studies * Corresponding author phone: 951-827-3860; fax: 951-827-3993; e-mail: [email protected]. 10.1021/es102277h

 2010 American Chemical Society

Published on Web 10/14/2010

employing selective sampling methods such as solid-phase microextraction (SPME) have provided evidence on the strong association of pyrethroids with DOC (16-18). For instance, in sediment supernatant or porewater, 62.5-99% of bifenthrin, λ-cyhalothrin, permethrin and cyfluthrin were associated with DOC (19, 20), and the magnitude of sorption to DOC varied with the sources of DOC (21, 22). Concentrations of DOC in the environment range from 2-3 mg L-1 in precipitation to 10-50 mg L-1 in runoff water and in porewater of organic soil horizons (23). The level of DOC in runoff water is often elevated after a heavy rain event (24). Application of organic carbon amendments such as compost, mulches, and biosolids is a popular practice in agriculture and urban gardening operations. The addition of organic amendments increases the DOC content in runoff water (25-27). The strong sorption of pyrethroids to DOC may increase the mobility of pyrethroids during a runoff event by effectively decreasing their sorption to and increasing their desorption from the solid phase. However, so far no study has quantitatively examined the effect of DOC on sorption and desorption of pyrethroids and little is known about the influence of factors such as sources of DOC and preinteractions between DOC and pyrethroids. The objectives of this study were to evaluate the effect of DOC of different origins on the partition of pyrethroids between sediment and water and to understand the influences of preinteractions and properties of DOC solutions. The results from this study may be used to improve prediction of HOC transport via processes such as runoff by providing environmentally relevant Kd values.

Materials and Methods Chemicals and Sediment. Standards of bifenthrin (BF) (99.0%), permethrin (PM) (38% cis-PM and 60% trans-PM) and cyfluthrin (CF) (95.2%) were purchased from Chem Service (West Chester, PA). These chemicals were dissolved in acetone separately as stock solutions. All other solvents and chemicals used were of gas chromatography (GC) or analytical grade. The sediment was taken from the Jordan Lake Reservoir (JL) in Chatham County, NC and was free of pyrethroid contamination. The sediment was wet-sieved through a 2-mm mesh, drained, air-dried, and mixed before use. The percentages of sand (78%), silt (17%), and clay (5%), pH (7.4), cation exchange capacity (2.8 meq 100 g-1), and OC content (0.24%) were measured, using the methods described in the Supporting Information (SI). The DOC content of the sediment porewater (extracted by centrifugation at 15 000 ×g for 20 min) was 3.8 ( 2.8 mg L-1 as determined by combustion of a porewater sample at 720 °C on an Apollo 9000 total OC analyzer (Tekmar-Dohrmann, Mason, OH). Preparation of DOC Solutions. Solutions of DOC were prepared from three sources: (a) the upper layer (0-25 cm) of a citrus orchard soil (DOCS) (Riverside, CA); (b) a commercial potting mix (DOCP) (Redi-Gro, Sacramento, CA) consisting of a mixture of composted forest products, composted organic matter, pumice, sphagnum peat moss, sand, and dolomite lime (pH stabilizer); and (c) a yard waste compost product (Riverside, CA) (DOCC). The DOC stock solutions were prepared by mixing the different organic materials with ultra pure water using a solution to solid ratio of 2:1 (v:w) on a shaker for 2 h. The slurries were centrifuged at 12 000 ×g for 20 min and filtered through polycarbonate membranes (0.45 µm pore diameter) to derive the DOC solutions. The DOC stock solutions were amended with NaN3 (0.01%) and kept at 4 °C in the dark. Immediately before use, VOL. 44, NO. 22, 2010 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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TABLE 1. Selected Properties of Dissolved Organic Carbon (DOC) and Characteristics of DOC Solutions Used in the Sorption and Desorption Measurements DOC Sourcea SUVA254 (L mgC-1 m-1)b HIXc C (%)d H (%) O (%) N (%) S (%) H/C O/C carboxilyc acid (molc kg-1) phenolic acid (molc kg-1) pH I Ca2+ (mg L-1) Mg2+ (mg L-1) Na+ (mg L-1) K+ (mg L-1)

DOCS

DOCP

DOCC

DOC properties 5.13 1.72 4.76 48.7 11.3 28.9 23.0 8.8 6.6 4.5 2.1 1.2 60.8 77.8 79.4 10.8 2.4 12.8 0.9 8.9 0.1 2.4 2.9 2.3 2.0 6.6 9.1 15.7 23.8 26.9 55.5 112.3 149.7 DOC solution charactertisticse 6.3 6.7 7.1 28.5 32.8 33.3 417 423 415 3.7 10.9 8.6 40.9 34.5 23.8 34.0 99.5 223.5

a DOCS, DOC extracted from soil; DOCP, DOC extracted from a potting mix; DOCC, DOC extracted from a compost product. b UV absorbance at 254 nm divided by the dissolved organic carbon concentration. c Humification index. d All elemental contents were corrected by the ash content. e Measured in the DOC solution containing CaCl2 0.01M.

the DOC stock solutions were diluted to the desired concentrations with ultra pure water. Selected physical and chemical properties of the DOC solutions are given in Table 1. Details on the DOC characterization methods are described in SI. Sorption Experiments. Sorption of pyrethroids to sediment was determined using two methods. In the first method, 2.0 g of sediment was placed in a 250-mL glass centrifuge bottle and spiked with 0.5 mL of a mixture of BF, PM isomers and CF in acetone to give a final concentration of 1 to 5 µg g-1. After the carrier solvent was evaporated, 100 mL of 0.01 M CaCl2 solution or a DOC solution containing 0.01 M CaCl2 and DOC at 38 ( 2 mg L-1 was added and the samples were mechanically shaken end-overend at 15 rpm for 16 h. Preliminary kinetic experiments showed that sorption of BF, PM, and CF to the sediment was very rapid, and an equilibrium was reached within 3 h of mixing. In the second method, 0.01 M CaCl2 or DOC (containing 38 ( 2 mg L-1 DOC and 0.01 M CaCl2) solutions were spiked with BF, PM isomers, and CF at 20, 40, 60, 80, and 100 µg L-1 and mixed at 15 rpm for 24 h, time sufficient to ensure equilibrium based on preliminary experiments. The pesticide-spiked solutions (100 mL) were then mixed with 2.0 g of sediment in 250-mL glass centrifuge bottles for 16 h. In this experiment, pesticides were allowed to interact with DOC prior to their contact with the primary sorbent phase (i.e., sediment). In both experiments, the use of a relatively large solution-tosediment ratio was necessary because the aqueous-phase concentration at equilibrium was very low due to strong sorption. Triplicate samples were used for each treatment. Upon equilibrium, the sediment and aqueous phases were separated by centrifugation at 500 ×g for 15 min. To determine the apparent pyrethroid concentration in the aqueous phase, 50 mL of the supernatant was transferred to a 250-mL glass separatory funnel and was extracted with methylene chloride. The recovery of the extraction method was 74-87% for BF, 76-95% for cis-PM, 80-93% for transPM, and 89-102% for CF. Analysis of the final extract on a GC equipped with an electron capture detector gave the whole aqueous-phase concentration (Cw). Pyrethroid residues in 8474

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the sediment phase were extracted by ultrasound-assisted extraction with methylene chloride: acetone (1:1, v/v). The average recovery of the sediment extraction method was 85% for BF, 90% for cis-PM, 88% for trans-PM and 89% for CF. An aliquot of the final extract was analyzed on GC-ECD to derive the sorbed concentration (Cs). Detailed sample preparation and analysis information is given in the SI. The sorption isotherms were initially fitted to both the Freundlich equation and a linear relationship, and a better fit was found with the linear model, from which the distribution coefficient Kd (L kg-1) was estimated from the slope of the linear regression line: Kd ) Cs /Cw

(1)

Sorption of pyrethroids to the sediment was further measured by adding DOCC at incremental concentrations (0, 5, 15, 38, 55 mg L-1). This experiment was intended to understand the dependence of Kd on the DOC level in the aqueous phase. Sorption of pyrethroids to DOC was also determined without the sediment phase using conditions similar to those described above. Solutions of DOC (containing DOC at 38 ( 2 mg L-1 and 0.01 M CaCl2) were spiked with BF, PM isomers or CF at 0.5, 2, 5, and 10 µg L-1 and the pesticide solutions were agitated end-overend at 15 rpm for 24 h. An aliquot (18 mL) of the equilibrated solution was analyzed using a previously developed SPME method (20) to determine Cw-free (µg L-1). Another aliquot (50 mL) was subjected to liquid-liquid extraction with methylene chloride to determine the whole concentration Cw. The DOC-sorbed concentration of pyrethroids (CDOC, in µg kg-1) was calculated as follows: CDOC )

Cw - Cw-free [DOC]

(2)

where [DOC] is the DOC concentration. The DOC-water partition coefficient KDOC (L kg-1) was estimated using: KDOC )

CDOC Cw-free

(3)

Desorption Experiment. Desorption of pyrethroids from the sediment was measured using samples of the highest concentration following the sorption experiment. Desorption was achieved by replacing 90 mL of the supernatant with 90 mL of pesticide-free 0.01 M CaCl2 solution or DOC solution (containing DOC at 38 ( 2 mg L-1 and 0.01 M CaCl2). At each desorption step, the tubes were mechanically shaken endoverend for 24 h at 15 rpm. The same desorption step was repeated for four successive times for the same samples. Pyrethroid concentrations in the aqueous and solid phases were determined as described above. Total solid content (TS) was also measured in the supernatant after one desorption step, as described in SI. Statistical Analysis. The regression lines of the sorption isotherms were compared using the STATGRAPHICS Plus 5.1. statistical software (Statistical Graphics, Princeton, NJ). SPSS version 13.0.1 statistical software (SPSS, Chicago, Illinois) was used for the one-way ANOVA analysis.

Results and Discussion Decrease of Sorption to Sediment by DOC. Sorption was first determined using the conventional batch method where the pesticides and DOC source were added simultaneously. To eliminate the potential effect of pyrethroid sorption to glass surfaces, concentrations from both the aqueous and sediment phases were determined. Mass balance for the different treatments ranged from 73 to 104%. In preliminary experiments, after equilibration for 24 h, no significant

TABLE 2. Partition Cefficient (Kd, L kg-1) of Bifenthrin, cis-Permethrin, trans-Permethrin, and Cyfluthrin in Jordan Lake Sediment Equilibrated with Water and Dissolved Organic Carbon Solutions from Different Sources at 38 ± 2 mg L-1 (Mean ± Sandard Dviation) compound bifenthrin cis-permethrin trans-permethrin cyfluthrin bifenthrin cis-permethrin trans-permethrin cyfluthrin

water

DOCSa

DOCP

TABLE 3. Concentrations (µg L-1) of Bifenthrin and Cyfluthrin in the Aqueous Phase at Equilibrium (Cw) (Mean ± Standard Deviation, n = 3) spiked conc. (µg/g)

water

1.0 2.0 3.0 4.0 5.0

0.11 ( 0.01 0.24 ( 0.01 0.34 ( 0.02 0.46 ( 0.02 0.58 ( 0.01

1.0 2.0 3.0 4.0 5.0

0.27 ( 0.03 0.56 ( 0.11 0.88 ( 0.02 1.13 ( 0.09 1.21 ( 0.11

DOCC

without preinteraction (×102) 83.7 ( 0.0 38.9 ( 1.0 3.8 ( 0.1 2.1 ( 0.1 49.7 ( 1.4 23.2 ( 0.5 4.3 ( 0.2 1.8 ( 0.1 37.3 ( 1.3 19.9 ( 0.5 4.3 ( 0.2 2.0 ( 0.1 36.4 ( 1.1 21.1 ( 0.4 5.8 ( 0.1 2.7 ( 0.1 with preinteraction (×102) 81.0 ( 1.7 29.4 ( 1.1 2.5 ( 0.1 1.0 ( 0.1 43.6 ( 1.5 21.9 ( 0.8 2.8 ( 0.1 1.2 ( 0.1 33.0 ( 1.2 18.8 ( 0.6 3.2 ( 0.1 1.3 ( 0.1 34.0 ( 1.1 21.0 ( 0.6 4.6 ( 0.1 1.4 ( 0.1

a DOCS, DOC extracted from soil; DOCP, DOC extracted from a potting mix; DOCC, DOC extracted from a compost product.

differences (P ) 0.55-0.84) were found between the initial and final DOC concentrations in the DOC amended treatments (data not shown), suggesting that DOC was not sorbed to the sediment under the conditions used. Sorption isotherms were well described by the linear relationship (Table 2) (r2 g 0.90), suggesting that sorption of pyrethroids to the sediment did not reach saturation within the used concentration ranges. The linear relationship allowed the calculation of Kd using eq 1. Pyrethroids were strongly sorbed to the sediment when water was the aqueous phase, and the average Kd values were 3.64-8.37 × 103 L kg-1. However, addition of DOC at 38 mg L-1 to the system consistently decreased Kd for the same compound. For BF, the Kd values were 2.2-39.9 fold smaller in the DOC-amended systems than in the sediment-water system. The decreases were 2.1-27.6 and 1.8-18.7 fold for cis and trans-PM, respectively, and 1.7-13.48 fold for CF. Statistically significant differences were observed (P < 0.05) among the different DOC sources, with Kd increasing in the order of water > DOCS > DOCP > DOCC for all the test pyrethroids (Table 2). As the DOC level was kept the same for the different DOC treatments, the observed differences suggested that different characteristics of the DOC sources contributed to their different magnitudes of effect on the sorption of pyrethroids on the sediment. The concentrations sorbed on the sediment phase (CS) were similar with and without DOC addition (data not shown) because the fraction of pyrethroids in the solution phase was very small ( DOCP > DOCS > water. In particular, mixing with DOCC resulted in desorption of 45.5-55.5% of pyrethroids that were initially sorbed on the sediment (Figure 3). Since DOC concentrations were the same for the different DOC sources, the differences may be attributed to the different physicochemical properties of these DOC sources. DOC Properties Affecting Pyrethroid Sorption. The aromaticity of DOC increased in the order DOCS > DOCC > DOCP as indicated by the SUVA254 (UV absorbance at 254 nm divided by DOC concentration) and the humification index (HIX) (Table 1). A high H/C ratio would indicate a high content of aliphatic functional groups, while the O/C ratio is related to the oxidation degree of DOC sources. The O/C ratio increases as the content of acids increases. In this work, the

FIGURE 2. Dependence of measured sediment Kd of pyrethroids on KDOC after equilibration of a lake sediment with different dissolved organic carbon (DOC) solutions containing DOC at 38 ( 2 mg L-1. 8476

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FIGURE 3. Fractions of (A) bifenthrin, (B) cis-permethrin, (C) trans-permethrin, and (D) cyfluthrin desorbed from a lake sediment by water and different dissolved organic carbon (DOC) solutions containing DOC at 38 ( 2 mg L-1. The x-axis denotes incremental desorption steps. O/C ratio increased in the order DOCS > DOCP > DOCC, which coincided with the trend of carboxylic acid and phenolic acid contents (Table 1). Potential relationships between DOC properties and KDOC values were considered. No significant correlation (P > 0.1) was found between KDOC and SUVA254 or HIX for any of the compounds considered, indicating that aromaticity was not the main DOC property influencing pyrethroid sorption. This finding was contrary to other studies considering other HOCs (11-14, 28). However, correlation coefficients between carboxylic and phenolic acid contents (Table 1) of the DOC solutions and KDOC values were between 0.81 and 0.99, indicating a moderate to relatively strong relationship. Carboxylic and phenolic groups may form complexes with inorganic ionic species (29) facilitating intra- and intermolecular interactions and leading to the formation of more organized molecular aggregates. Aggregation expels a portion of the hydration water that surrounds the molecule, leaving it less hydrated. Thus, upon interactions with cations, humic molecules may change from a hydrophilic to a hydrophobic colloid (30), increasing their binding potential for HOCs such as pyrethroids (14, 31, 32). Although 0.01 M CaCl2 was used to normalize the ionic strength in all treatments, as shown in Table 1, DOCC generally contained higher levels of other cations and higher carboxylic and phenolic acid content, which may have contributed to its relatively stronger effect on pyrethroid sorption. The enhanced desorption observed with DOCC and DOCP may be also partly attributed to the dispersion effect of monovalent cations (Na+ and K+) that could lead to disaggregation of sediment aggregates and increased dispersion of colloids into the aqueous phase (28). Measurement of total solids after one desorption step during the desorption experiment suggested that DOCC and DOCP caused a significant increase in the level of total solids in the aqueous phase after centrifugation. For instance, mixing with DOCC and DOCP resulted in 21.1 ( 1.2 and 17.7 ( 2.0 mg L-1 total solids in the supernatant, as compared to only 4.8 ( 0.6 mg L-1 in the DOCS treatment. Implications for Contaminant Transport. The role of DOC in decreasing sorption and enhancing desorption of pyrethroids to sediment has several implications for the

transport of pyrethroids. First, surface erosion and runoff is the principal process governing the off-site movement of pyrethroids, where Kd is one of the most important parameters imbedded in transport models for describing such transport (33, 34). Therefore, knowing the effective Kd by considering the effect of DOC may improve the prediction of contaminant transport to downstream water bodies. DOC is ubiquitous in surface water, and may increase in runoff after a storm event (23, 24). The level of DOC may further increase in runoff originating from areas rich in plant residues or receiving applications of organic materials such as compost, mulches, animal wastes, and biosolids (26, 27). The inhibition of DOC on sorption and the opposite effect on desorption suggests that considerably more pyrethroids than predicted through literature Kd (or KOC) may be partitioned into the aqueous phase and subjected to mass transport over distance. Results from this study serve as experimental evidence providing explanation of the reasons for the widespread occurrence of HOCs such as pyrethroids in the surface water systems (1, 2, 5). The knowledge that interactions lead to the formation of stable DOC-pyrethroids complexes is also important for mitigation management. For instance, practices aiming at preventing erosion of organic materials from soil surfaces should be generally effective at reducing offsite runoff of pyrethroids.

Acknowledgments L.D.M. thanks the Spanish Ministry of Education and Science for the postdoctoral fellowship received.

Supporting Information Available Additional details on sample preparation and chemical analysis are available free of charge via the Internet at http:// pubs.acs.org.

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