Hydrolysis of chlorostilbene oxide II. Modeling of hydrolysis in aquifer samples and in sediment-water systems

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 effects. 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 neutral and acid hydrolysis studies, the sorption-desorption rates of CSO were faster than the hydrolysis 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 water. 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 hydrolysis 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 (±0.11) × 10−4 min−1, 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.