Isotherm nonlinearity and nonequilibrium sorption effects on transport of fenuron and monuron in soil columns.

The influence of nonlinear sorption and prior sorbate/ sorbent exposure time on transport of fenuron and monuron in laboratory soil columns was studied. Observed monuron breakthrough curves (BTCs) were well described using a two-region transport model with a single mass-transfer rate coefficient, independent of prior sorbate/sorbent exposure history. However, levels of soil-resident postelution monuron increased considerably with increasing exposure time, indicating increased sorption nonequilibrium with aging. Significant sorption nonequilibrium was observed for fenuron in spite of relatively high water solubility, low sorption, and relatively short sorbate/sorbent exposure time (ca. l-2 weeks). Sorption of fenuron was measured at solution concentrations up to the aqueous solubility limit. Batch data were reasonably well described by two distinct Freundlich isotherms depending on the concentration range. Accordingly, fenuron retardation factors varied significantly with concentration, displaying a minimum at intermediate concentration values. A comparison of linear and nonlineartransport modeling approaches suggests that failure to consider isotherm nonlinearity may be a source of considerable error when estimating nonequilibrium transport parameters from experimental BTCs. Introduction Experimental investigations of sorption and transport are important for understanding the fate and redistribution of chemicals in the subsurface environment. Although commonly assumed otherwise, sorption of organic compounds in soil is often nonlinear, i.e., the relative distribution of a chemical between the solution and sorbed phases varies as a function of the concentration being considered. Relatively few studies have examined the sorption of chemicals on soil at concentrations approaching the solubility limit. Such information may be needed to assess chemical transport in the vicinity of point sources. Karickhoff et al. (1) reported sharp increases in sediment sorption of several aromatic and chlorinated hydrocarbons for equilibrium solution concentrations exceeding 60-70% of the sorbate solubility. Similarly, Walters and GuiseppiElie (2) found that sorption of 2,3,7,8-tetrachlorodibenzop-dioxin on soil from an aqueous methanol solution was linear up to approximately 50% of the solubility (-lo-’ mg L-l), but increased sharply at higher concentrations. Equilibrium sorption at the higher concentrations was described by the Freundlich isotherm