Dehydrochlorination of activated carbon-bound 1,1,2,2-tetrachloroethane: Implications for carbonaceous material-based soil/sediment remediation

Carbonaceous materials such as activated carbon (AC) are effective adsorbents for a number of organic contaminants, and are often used in soil and sediment remediation to sequester organic contaminants. However, little is known about the abiotic reactivity (and thus, the long-term fate) of the carbonaceous material-bound contaminants. We studied the base-promoted dehydrochlorination kinetics of low-level (∼1.25 mg/g) 1,1,2,2-tetrachloroethane (TeCA) adsorbed to 17 different ACs varying in origin and physicochemical properties. Activated carbon-bound TeCA exhibited significant reactivity—at a given pH the apparent reaction kinetic constants of AC-bound TeCA are 13–70% of the respective kinetic constants of dissolved TeCA. Binding to ACs likely have affected the reactivity of TeCA via two important mechanisms. First, the deprotonated surface O-functional groups (e.g., carboxyl and phenolic groups) catalyzed the base-promoted dehydrochlorination reaction by serving as the conjugate bases. Second, at low contaminant loadings TeCA molecules likely resided primarily in the micropores of ACs (particularly, micropores with diameters of 0.67–1.6 nm), wherein mass transfer of OH− (the nucleophile that reacts with TeCA) was inhibited. An important environmental implication is that carbonaceous materials not only sequester contaminants, but also allow contaminants to be slowly degraded in situ.