Nonsingularity of Naphthalene Sorption in Soil

Abstract

Realistic models for transport of organic chemicals in soil require accurate predictions of the adsorption‐desorption kinetics. In this study, a physically based two‐compartment one‐rate (TCOR) sorption model was evaluated by comparison of model simulations to measured adsorption‐desorption isotherms of naphthalene (C10H8) for five different soils on a short‐term (48 h) and a longer‐term (504 h) time scale. Two soils exhibited minor adsorption‐desorption nonsingularity (labeled Type I soils), two soils pronounced nonsingularity (Type II soils), and the fifth soil pronounced nonsingularity only on the longer‐term time scale (Type I/II soil). The TCOR sorption model fitted, measured adsorption‐desorption isotherms well on both short‐term and longer‐term time scales. However, the TCOR sorption model parameters varied for each soil between short‐term and longer‐term data, especially for Type II soils. The uniqueness of the TCOR model fit was tested by varying the number of desorption data used, resulting in markedly changed parameter values. The TCOR sorption model was evaluated by using model parameters obtained from short‐term data to predict longer‐term sorption results. The TCOR prediction of adsorption‐desorption behavior was good for Type I soils, satisfactory for the Type I/II soil, and poor for Type II soils. Model parameters obtained from short‐term and longer‐term experiments were used to predict independently measured adsorption kinetics, showing decreasing prediction accuracy with increasing sorption nonsingularity. The results imply that the TCOR sorption model description of the diffusion process at the grain scale is oversimplified, and that sorption nonsingularity is not well explained by kinetic factors alone.