Modeling sorbing chemicals: Considering the nonsingular adsorption/desorption isotherm

A simple model to predict the time dependent concentration of a sorbing hydrophobic pollutant in a lake was used to compare modeling results with laboratory measurements. The modeling results were obtained for three different forms of describing the pollutants equilibrium between the dissolved phase and the phase adsorbed on sediments while the pollutant is desorbing from the sediments. First a linear desorption isotherm that is equal to the adsorption isotherm was considered, then a nonlinear desorption isotherm was included in the model, and finally it was assumed that the observed nonlinear desorption isotherm is in fact linear and the nonlinearity was exclusively an effect of nonsettling particles. In this last case the appropriate adjustments were made in the model's parameter estimation to compensate for the nonsettling effect. Laboratory studies were conducted using Malathion as the sorbing compound. The adsorption/desorption isotherms as well as the kinetics of evaporation of Malathion from solution in a completely mixed system with and without sediments were determined. The results of the modeling and laboratory studies showed that the best agreement between the predicted and the observed values was obtained when the nonlinear desorption isotherm was used to describe the equilibrium between the adsorbed and dissolved phases.