The effect of fluid ionic strength and system temperature on naphthalene sorption to and desorption from Lincoln Fine Sand

Contaminant sorption to soils has two components: (1) instantaneous sorption, and (2) time dependent sorption. Three coefficients can be used to describe the sorption/desorption of contaminants to soils. (1) The partition coefficient relates the contaminant concentration in the soil to the contaminant concentration in the aqueous phase. (2) The fraction of instantaneous sorption is the ratio of the time independent sorption to the total sorption. (3) The sorption rate constant quantifies the kinetics of the time dependent sorption. This study challenges the long accepted premise that the values of the sorption coefficients are equal to the values of the desorption coefficients and examines the ionic strength and temperature dependence of those coefficients. The sorption/desorption of naphthalene on Lincoln Fine Sand was examined with ionic strengths ranging from 0.001M to 0.5M. The partition coefficients varied as predicted with ionic strength but were about half to a third the predicted magnitude. An apparent minimum partition coefficient was observed at 0.06M. The fraction of equilibrium sorption was a maximum in the range of 0.01M to 0.1M. The sorption rate constants, $\alpha$, decreased with increasing ionic strength. The $\alpha$ for the sorption data was 0.5 hr$\sp{-1}$ at 0.001M decreasing to 0.05 hr$\sp{-1}$ at 0.5M. $\alpha$ for desorption only decreased from 0.13 hr$\sp{-1}$ to 0.09 hr$\sp{-1}$ over the same ionic strength range. Temperature has little effect on the sorption/desorption parameters over the temperature range studied (12 to 30$\sp\circ$C). There is a significant difference between the sorption coefficients and the desorption coefficients. Latent contamination of the soil pore water can be caused by the comparatively slow desorption of naphthalene from the soil. A hitherto uninvestigated mechanism for delayed contaminant release is proposed. Soil organic matter (SOM) may not be immobile on the sediment grain surface. The dynamic nature of the SOM may control the slower desorption