Sorption of the neutral and charged forms of pentachlorophenol on soil: evidence for different mechanisms

Laboratory soil sorption experiments have been conducted on pentachlorophenol (PCP) at different pH values in an attempt to elucidate differences in sorption mechanisms between the charged and neutral species. Sorption of PCP on soil was investigated by maintaining pH 4 or 8 in batch sorption experiments. Pre-equilibration of the soil was necessary to maintain a constant pH over the course of the experiments. Additionally, a CaCO3-CO2/N2 buffered solution was necessary to maintain a pH of 8. Sorption of the neutral PCP species conformed to a linear isotherm model, while a Langmuir model provided the best fit for the charged species. Desorption of the neutral form was completely reversible over the sorption times studied but the charged species exhibited some resistance to desorption. Temperature effects on the distribution coefficients (Kd) were investigated and thermodynamic parameters were calculated. The ionized species showed a clear decrease in Kd with increasing temperature while the protonated species showed no apparent trend. Enthalpies (deltaHo), entropies (deltaSo), and free energies (deltaGo), support the conclusions that the neutral form of PCP partitions by hydrophobically binding to the soil while the charged form sorbs by a more specific exothermic adsorption reaction.