Effect of aerobic and anaerobic conditions on chlorophenol sorption in wetland soils

Sorption of four chlorophenols (CPs) was studied in ten wetland soils with organic C contents between 1 and 44%, which were incubated under aerobic or anaerobic conditions to simulate wetland conditions. The objectives of the study were to (i) determine the influence of aerobic and anaerobic processes on sorption, and (ii) develop sorption models to predict the distribution coefficient based on chemical characteristics of soils and compounds. Aerobic soils consistently had lower pH than anaerobic treatments, which was a function of the amount of oxidizable constituents present in the sample. Depending on the pK_a of the compound relative to the pH shift, a greater fraction of the CP was in the neutral form in the aerobic treatments, which was sorbed to a much greater extent than the ionic form (by about 25 times). The organic C normalized distribution coefficient (K_oc) was strongly related to the octanol-water distribution coefficient (K_ow) and soil pH. Sorption models accurately predicted distribution coefficients within a factor of 2 from the K_ow and pK_a of the compounds and the pH and organic C content of the sorbent. The role of sorption on CP retention was partially negated by the formation of the nonseparable phase, which composed up to 8.6% of the total solid mass (depending on the soil redox status) and had similar distribution coefficients as the separable phase. This study demonstrated that microbial redox processes significantly influenced the soil properties and CP retention characteristics, and should be considered when designing a bioremediation plan for these compounds.