Environ. Sci. Technol. 2011, 45, 958–963
Aging of Organochlorine Pesticides and Polychlorinated Biphenyls in Muck Soil: Volatilization, Bioaccessibility, and Degradation F I O N A W O N G †,‡ A N D T E R R Y F . B I D L E M A N * ,† Centre for Atmospheric Research Experiments, Science and Technology Branch, Environment Canada, 6248 Eighth Line, Egbert, Ontario L0L 1N0, Canada, and Department of Chemistry, University of Toronto Scarborough, 1265 Military Trail, Toronto, Ontario M1C 1A4, Canada
Received August 18, 2010. Revised manuscript received November 22, 2010. Accepted December 2, 2010.
An organic rich muck soil which is highly contaminated with native organochlorine pesticide (OCs) was spiked with known amounts of 13C-labeled OCs and nonlabeled polychlorinated biphenyls (PCBs). Spiked soils were aged under indoor, outdoor, and sterile conditions and the change in volatility, surrogate bioaccessibility, and degradation of chemicals was monitored periodically over 730 d. Volatility was measured using a fugacity meter to characterize the soil-air partition coefficient (KSA ) CSOIL/CAIR). The fraction of bioaccessible residues was estimated by comparing recoveries of chemical with a mild extractant, hydroxylpropyl-β-cyclodextrin (HPCD) vs a harsh extractant, DCM. KSA of the spiked OCs in the nonsterile (Indoor, Outdoor) soils were initially lower and approached the KSA of native OCs over time, showing reduction of volatility upon aging. HPCD extractability of spiked OCs and PCBs were negatively correlated with KSA, which suggests that volatility can be used as a surrogate for bioaccessibility. Degradation of endosulfans, PCB 8 and 28 was observed in the nonsterile soils, and 13C6-R-HCH showed selective degradation of the (+) enantiomer. Enantiomer fractions (EF) in air and HPCD extracts were lower than in nonsterile soils, suggesting greater sequestering of the (+) enantiomer in the soil during microbial degradation.
Introduction Soil is an important medium in controlling the global cycling of organic chemicals. After a chemical enters the soil, it can be leached, degraded, vaporized, or remain sorbed to soil particles for years. Models have shown that soils are acting as a sink, initially absorbing chemicals from the atmosphere during periods of increased emissions. As emissions decline, soils become a source and chemicals are slowly being released back to the atmosphere (1-4). This has been demonstrated by field studies in which volatilization of legacy and currentuse pesticides from contaminated soils was measured (5-10). The dimensionless soil-air partition coefficient (KSA ) CSOILS/CAIR) has been conventionally estimated by the modified Karickhoff model (11, 12) * Corresponding author phone: (705)458-3322; fax: (705)458-3301; e-mail:
[email protected]. † Environment Canada. ‡ University of Toronto Scarborough. 958 9 ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 45, NO. 3, 2011
KSA ) 0.411FsφocKOA
(1)
where φoc is the fraction of soil organic carbon, FS is soil density (kg L-1), KOA is the octanol-air partition coefficient, and 0.411 is a constant with units of L kg-1. Over the years, there have been many studies carried out to directly measure KSA under various environmental conditions (12-24). Hippelein and McLachlan (12) reported that the above model systematically underestimated the measured KSA by a factor of 2 for polychlorinated biphenyls (PCBs) and chlorobenzenes. Other studies have shown differences up to an orderof-magnitude between experimental and predicted KSA for organochlorine pesticides (OCs), polyaromatic hydrocarbons (PAHs), and PCBs (13, 15, 16, 18). Niederer et al. (21) found that the partition coefficient of a chemical between natural organic matter/air was highly variable and showed up to an order of magnitude difference depending on the source of the humic and fulvic acids (such as terrestrial vs aquatic). KSA is a key parameter used in soil emission models, which assume that the availability of a chemical for soil-air exchange does not vary over time. However, it has been well documented that as the contact time of a chemical in soil increases, its bioaccessability and extractability may decline as the chemical relocates within the soil matrix to sites where organisms cannot access, or forms strong irreversible bonds with the soil (25-32). Similarly, aging may reduce the volatilization of a chemical, a phenomenon which has largely been ignored. If only a fraction of the chemical is available for emission to the atmosphere, this could lead to overprediction in soil-air emission models. Cousins et al. (14) reported that after spiking a low organic carbon soil with PCBs, there was a short period of increased PCB volatilization for about 48 d, and no further changes in volatilization were observed for the subsequent 390 d. This study investigated the effect of aging on the volatilization and bioaccessibility of OCs and PCBs in a high organic matter soil. A contaminated muck soil was spiked with known amount of labeled chemicals and portions of the soil were aged in an indoor and outdoor environment. The volatility and bioaccessibility of the native and spiked chemicals in soil were monitored periodically up to 730 days of aging. The degradation kinetics of spiked OCs and PCBs, and the enantioselective degradation of 13C6-R-HCH, were also studied. Volatilization under equilibrium conditions was determined using a fugacity meter as described by Meijer et al. (20). Bioaccessibility was determined by a mild chemical extraction method using an aqueous solution of hydroxypropyl-β-cyclodextrin (HPCD) (32). HPCD extraction of PAHs has been shown to correlate well with microbial mineralization rates (33-36) and earthworm uptake of current-use pesticides from soils (37). Recently, an optimized HPCD extraction method was developed to estimate the bioaccessibility of OCs and PCBs from a high organic matter soil (38). It was shown that the extractability of the aged residues was lower than that of the freshly spiked chemicals, and the HPCD extractability of the spiked chemicals declined over time.
Experimental Method Soil Preparation. Muck soil with an organic carbon content of 42% (39) and pH of 5.5 (deionized water) was collected from a farm in Ontario, Canada. The soil was highly contaminated with OCs with last known application occurring about 25 to 40 years ago, except for endosulfan, which is a currently used pesticide. The soil contained 14400 ng g-1 10.1021/es102825w
Published 2011 by the American Chemical Society
Published on Web 01/04/2011
total DDTs (p,p′- and o,p′- isomers of DDT, DDE, and DDD), 600 ng g-1 dieldrin, 54 ng g-1 chlordanes (trans- + cischlordane + trans-nonachlor ) TC, CC, and TN), and 1120 ng g-1 endosulfans (endosulfan I, II and endosulfan sulfate ) ENDO I, ENDO II, ESUL). PCBs (
