Effects of heavy metal species, concentrations, and speciation on pentachlorophenol sorption by river biofilms

The sorption of trace organic pollutants at solid/liquid interfaces is one of the most important processes that influence their fate and behaviours in the aquatic environment. Sorption is affected by coexisting contaminants. The process and extent to which coexisting heavy metals affect the sorption of organochlorine pesticides (OCPs), especially acid radical anion heavy metals, are still unclear. Here, the effects of the species, concentrations, and speciation of the heavy metals Cu, Pb, and Cr, and the metalloid As on the sorption of pentachlorophenol (PCP), as a model OCP, by river biofilms were investigated through batch experiments. The results show that the presence of Cu, Pb, Cr, and As decreased the maximum sorption quantity of PCP onto the biofilms by 67.7, 9.2, 58.4, and 14.4%, respectively. The inhibitory effect of heavy metals on sorption decreased as the initial concentration ratios of heavy metals to PCP increased. In addition, the impact of heavy metals on PCP sorption was attributed to differences in heavy metal speciation. Cu and Pb commonly existed as divalent cations, but Cr and As existed as anionic acid radicals under the experimental conditions. The inhibitory effects of heavy metals on PCP sorption by biofilms were enhanced as the cation valence state increased, while the effects were weakened as the anionic acid radical valence state increased. Although all four heavy metals had inhibitory effects on PCP sorption by biofilms, there were distinct differences in the mechanisms causing these effects.