Reversible and resistant components of PCB adsorption-desorption: isotherms

Abstract

Results from laboratory equilibration studies indicate that sediment-adsorbed 2,4,5,2',4',5'-hexachlorobiphenyl (HCBP) fractions may be comprised of both reversibly and strongly bound or resistant components. This evidence suggests that for many environmental modeling efforts it may be inappropriate to treat this and other PCB isomer adsorption reactions as either completely reversible or completely irreversible. The initial refinement of such models requires a means of estimating the relative magnitudes of the respective sorbed PCB fractions. To this end, a computational method has been derived to allow prediction of the magnitude of the reversible and more strongly adsorbed HCBP fractions from conventional isotherm data. The derivation, which is based upon experimentally observed linear HCBP isotherms, has also been applied to other organic molecules of similar isotherm character. This methodology provides an initial quantitative approximation of the strongly bound, resistant, sediment fractions while utilizing relatively simple experimental adsorption-desorption data. Results suggest that for montmorillonite clay, kaolinite clay, and natural lake sediment (Saginaw Bay, MI) linear isotherms effectively describe the relationships between both adsorbed HCBP fractions and the aqueous HCBP concentration. The implications of these and other model predictions for the cycling of PCB's in natural waters are discussed.