Determining kinetic and nonequilibrium sorption behavior for chlopyrifos using a hybrid batch/column experiment

Pesticide mobility in soil is strongly coupled to the chemical's sorption characteristics. A modified soil column batch experiment was conducted to measure the transient nature of chlorpyrifos sorption and desorption from Cecil soil. This experimental system minimizes many shortcomings associated with obtaining sorption parameters by fitting soil column data to an advective-dispersive transport equation. Several chlorpyrifos formulations were investigated to determine how formulations affect soil sorption, and if this effect is adequately described using transient sorption/desorption algorithms. Both a second-order sorption with first-order desorption kinetic model and the two-site kinetic/equilibrium model were found to yield reasonable comparisons to experimental observations. In general, the formulation temporarily decreases the sorptivity of chlorpyrifos and alters the time for equilibrium to be achieved. Care must be exercised when extrapolating sorption data for a pure molecule in a laboratory setting to formulated materials used in field applications when environmental fate predictions are sought.