The sorption of pentachlorophenol (PCP, pKa’ = 4.75) onto natural solids from aqueous phase was investigated by batch sorption experiments. The experimental aqueous phase was prepared for set values of pH (2 to 12) and ionic strength (0.1 M). Experimental results indicated that the sorption decreased with increasing pH over the entire pH range tested. A simple mathematical model, based on the hypotheses that the sorption coefficients of non-ionized and ionized species are different and the pH has only negligible effect on the natural solid characteristics, was applied to the pH range between 6 and 8, and the sorption coefficients (Kd) of both species were estimated. The Kd of ionized species (phenolate anion) was smaller than that of non-ionized species. The Kd of both species had poor correlation to the organic carbon content of natural solids. The Kd, however, correlated well with the swelling clay content and cation exchange capacity of natural solids. These results indicated that the sorption of PCP was not controlled by the organic carbon referenced hydrophobic sorption. For broader pH range (i.e., below 6 or above 8), the dependence of the natural solid characteristics on pH must be additionally included in the model.

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