The extent of reversibility of polychlorinated biphenyl adsorption

Abstract

The extent of reversibility of PCB bonding to sediments has been characterized in studies on the partitioning behavior of a hexachlorobiphenyl isomer. Linear non-singular isotherms have been observed for the adsorption and desorption of 2.4.5.2′,4′,5′ hexachlorobiphenyl (HCBP) to 1100 ppm sediment suspensions. Partition coefficients, π(1 kg−1), for desorption from lake sediments (Saginaw Bay. Lake Huron. Michigan) are substantially greater ( π d ∼ 20.000–35.000) than those obtained for adsorption ( π 4 ∼ 9000–14,000). HCBP was found to be more weakly adsorbed to montmorillonite ( π a − 3000, π d ∼ 9000) and kaolinite ( π a ∼ 1000, π d ∼ 3000) clay samples than to the natural sediment samples. Desorption results ( π d) for Saginaw Bay sediments were quite similar to π values (15,000–35.000) calculated from field measurements of aqueous and particulate PCB concentrations. For Saginaw Bay sediments and clay minerals partitioning appeared to be correlated both to sediment surface area and to sediment organic content. A regression analysis using both of these variables explained ∼ 90% of the observed variations. HCBP adsorption at 40 C ( π a ∼ 14,000) was significantly greater than at 1° C ( π a ∼ 6500) resulting in a calculated enthalpy of adsorption of +3.3 kcal mol −1. Non-singular isotherm behavior was not found to be readily attributable to microbiological, kinetic or experimental effects. Evidence from consecutive desorption studies suggests that while HCBP adsorption may ultimately be reversible. release from sediments appeared to involve desorption along two distinct isotherms. These results have been interpreted in terms of possible similarities between the sorption properties observed in the distilled water systems of the present study and PCB bonding processes in natural water systems.