The effect of sorption on the oxidation of polychlorinated biphenyls (PCBs) by hydroxyl radical

Abstract

Sorption of hydrophobic organic compounds, such as polychlorinated biphenyls (PCBs), to particulate matter (diatomaceous earth) affects transformation rates by aqueous-phase hydroxyl radical. The reaction of three PCB congeners with hydroxyl radical (OH .), generated in a photo-Fenton system (i.e. iron, hydrogen peroxide and light at pH 3), was followed to evaluate predictive models of transformation kinetics in the presence of particulates. The OH . generated by this system only reacted with dissolved PCBs, and the overall rate of removal for each congener could be modeled using congener-specific data on desorption kinetics and solution phase reaction rates. Observed kinetics for the least hydrophobic congener evaluated, 2-monochlorobiphenyl, agreed with predictions based upon measured steady-state hydroxyl radical concentrations and second-order rate constants. 2,2′,5-Trichlorobiphenyl was also oxidized by OH ., but sorption to particulate matter resulted in slower transformation rates. A predictive model which utilized measured adsorption/desorption rate constants, steady-state hydroxyl radical concentrations and second-order rate constants adequately predicted 2,2′,5-trichlorobiphenyl transformation kinetics. Concentrations of 2,2′,4,5,5′-pentachlorobiphenyl, the most hydrophobic compound analyzed, did not change over the time scale of the experiments because dissolved concentrations were extremely low and desorption kinetics were very slow.