Desorption resistance of organic compounds in wetland soils

Reversible sorption of organic compounds in soils and sediments is a common assumption in assessing risk and determining remedial endpoints. However, the presence of a desorption-resistance phase has been widely reported in the literature. Recent work has suggested that the quality of organic matter is important in the magnitude of the desorption-resistant effect. For example, a series of wetland peats demonstrated little or no desorption resistance for phenanthrene (Huang and Weber, 1997). The goal of the present study was to determine if desorption-resistance is observed in recent deposited organic matter by studying wetland soils explicitly. Sorption/desorption characteristics for both surface marsh soil (top 0-2 cm, <5 years old) and deeper marsh soil (below 10-cm, >20 years old) were investigated to see whether soil age can cause differences in sorption/desorption in wetlands. Sorption-desorption of organic compounds (chlorobenzene and phenanthrene) in recently deposited freshwater marsh soils was determined using the batch desorption procedure of Kan et al. (1998). Adsorption was biphasic with about 30-62% (chlorobenzene) and 46.7-82.9% (phenanthrene) of the adsorbed mass residing in the desorption-resistant fraction after 7-14 desorption steps. Sorption/desorption parameters were determined for several of the currently proposed models for explaining the desorption resistance including the Tomson-Kan model, slow sorption model and dual reactive domain model (DRDM). All models indicated the presence of a sizeable desorption-resistant phase in wetland soils and an increase in the size of the desorption-resistance in "older" organic matter. Implications for wetland remediation are discussed.