Time-dependent sorption-desorption behavior of 2,4-dichlorophenol and its polymerization products in surface soils

Contact time-dependent sorption-desorption of 2,4-dichlorophenol (DCP) and DCP polymerization products (DPP) was investigated in the context of agricultural and woodland soils. DPP was generated in soil slurry reactors by the addition of H2O2 to solutions containing horseradish peroxidase (HRP) and DCP. Size-exclusion chromatography confirmed the formation of oligomeric products including dimers, trimers, and tetramers. DCP removal from HRP-amended, soil-free solutions occurred as a result of DPP formation and sorption of DCP to the oligomeric precipitate. In reactors containing soil particles, additional removal occurred due to sorption of residual DCP and DPP to the soil. Sorption of DCP and DPP to the surface soils was rapid and appeared to be complete within 1 day. DPP sorbed to a greater extent than DCP, especially at higher solute concentrations. Water-extraction data indicated that while sorbed DCP was readily released into solution, sorbed DPP was more resistant to desorption. Both DCP and DPP were more readily extracted from the woodland soil than the agricultural soil. Solute extractability decreased with contact time, indicating that sorbent-solute interactions proceeded long after apparent sorption equilibrium at the particle or aggregate scale. Results from this study show that "slow" sorption processes occurring at the molecular scale continue long after apparent sorption equilibrium at the particle scale. This "aging effect" was operational over longer periods for DPP than the parent DCP, thereby ensuring long-term reductions in contaminant mobility in soils treated with peroxidase enzyme.