Amphoteric effect of humic acids in surfactant-aided photolysis of polychlorobiphenyls

Polychlorobiphenyls (PCBs) are one of the most environmentally problematic organic contaminants present in soils and sediments due to their long-life in direct disposal or spills in soil. It is known that surfactant-aided soil washing followed by a photodechlorination process has the ability to separate PCBs from the soil and decompose them all in one step. However, the influence of humic substances on the removal of PCBs in surfactant-aided ultraviolet systems is of special interest because of the involvement of humic materials in the soil-washing process. A PCB congener, 4,4′-dichlorobiphenyl (DCB) was used as a probe in this study. At low humic acid (HA) concentrations, humic substances promote the photochemical reaction, but a high dose of HA leads to retardation of the DCB decay. A mechanism for the amphoteric effect of HA on the photodecay of DCB is proposed in this study, and two mathematical models are developed to successfully rationalize the amphoteric effect of HA in the photolysis process.