Environmental Toxicology and Chemrstry, zyxwvutsrqponml Vol 9, zyxwvutsrqpo pp 963-973, 1990 Printed in the USA Pergamon Press plc 0730 7268/90 $3 00 zyxw + 00 zyxw Environmental Chemistry HYDROLYSIS OF CHLOROSTILBENE OXIDE 11. MODELING OF HYDROLYSIS IN AQUIFER SAMPLES AND IN SEDIMENT-WATER SYSTEMS MOHAMMED EL-SAYED METWALLY and N. LEE WOLFE* Environmental Research Laboratory, U.S. Environmental Protection Agency, Athens, Georgia 30613 (Received 27 July 1989; Accepted 27 October 1989) Abstract -The disappearance kinetics of 4-chlorostilbene oxide (CSO) were determined in aquifer samples and in sediment-water systems to derive kinetic expressions that describe heterogeneous ef- fects. Disappearance rate constants were determined in heterolytic systems and compared with those obtained in distilled water to delineate the effect of solids on the kinetics of hydrolysis. In both neu- tral and acid hydrolysis studies, the sorption-desorption rates of CSO were faster than the hydro- lysis rates in either the aqueous or sediment-sorbed phases. Both dissolved and sorbed CSO hydrolyzed at either neutral or acidic pHs. Above pH 5, where neutral hydrolysis dominates, the hydrolysis rate constant of CSO in sterile sediment-water systems was the same as in distilled wa- ter. This suggests that sorption neither retards nor promotes the neutral hydrolysis pathway. At pHs below 5, where acid hydrolysis dominates, the pseudo-first-order hydrolysis rate constant was lower for the sorbed fraction than the rate constant in distilled water at the same pH. This indicates that hydrolysis rate constant in the sorbed phase is slower than that in the bulk aqueous phase. Kinetics of hydrolysis were studied in raw sediment samples and samples sterilized by heat or with formalin to distinguish between abiotic and biotic hydrolysis processes. Above pH 5, the hy- drolysis rate constants were larger in nonsterile systems when compared with the rate constants in the sterile systems, suggesting that both biotic and abiotic pathways contribute to the disappearance of CSO. Below pH 5, the hydrolysis rate constants were, within experimental error, the same in the sterile and nonsterile systems. In aquifer samples, the average disappearance rate constant of CSO was 0.85 (kO.11) x min-', which is about the same as the rate constant in distilled water. These results suggest that there is no heterolytic enhancement of the neutral hydrolysis rate constant by the aquifer materials. Keywords - Chlorostilbene oxide Hydrolysis Sorption Sediments Aquifer INTRODUCTION Because many anthropogenic organic pollutants have low water solubilities, a large fraction of these compounds can be associated with suspended or bottom sediments in aquatic systems. It has been reported for selected organic compounds that al- kaline hydrolysis does not occur in the sediment- sorbed phase. Hydrolysis occurs only in the aqueous phase with equilibrium between the sediment and the aqueous phases [l]. Macalady and Wolfe [2-41 suggest that acid-catalyzed rate constants are en- hanced in the sorbed phase and while those for neutral hydrolysis are the same as the correspond- ing rate constants in the dissolved aqueous phase. Additional study is needed to challenge this work- *To whom correspondence may be addressed. The permanent address of M.E, Metwally is Depart- ment of Analytical Chemistry, Faculty of Pharmacy, Mansoura University, EbMansoura, Egypt. ing model for acid-catalyzed hydrolysis. The quan- titative effect of sorption to sediments and aquifer materials on the kinetics of acidic-catalyzed and neutral hydrolysis of chlorostilbene oxide (CSO) has been researched. Epoxides are a class of compounds that are formed as intermediates in the biological transfor- mation of polycyclic aromatic compounds. Consid- erable attention has been devoted to the chemistry of these epoxide intermediates because of their role in mutagenesis and carcinogenesis [5]. In addition to the importance of studying the fate of these compounds in the environment, they are excellent model compounds for studying acid-catalyzed hy- drolysis in heterolytic systems. There are at least three kinetically distinguishable pathways for the hydrolysis of epoxides in aqueous solutions [6-131. At low pH values, acid-catalyzed hydrolysis dom- inates, whereas at higher pH values base-catalyzed hydrolysis dominates. At intermediate pHs, hydro- 963