Dependence of pesticide degradation on sorption: nonequilibrium model and application to soil reactors

The effect of sorption on degradation of the pesticide 2, 4-dichlorophenoxy acetic acid (2,4-D) was studied in a soil amended with various amounts of a ctivated carbon (AC). The relationship between sorption and decay of 2,4-D was analyzed using analytical solutions for equilibrium sorption and to a t wo-site nonequilibrium adsorption model coupled with two first-order degrad ation terms for the dissolved and sorbed pesticide, respectively. The sorpt ion parameters in the latter model were determined based on data obtained f rom batch sorption experiments, while those for degradation were obtained f rom incubation experiments. The adsorption coefficients, ranging from 0.811 to > 315 ml g(-1), increased at higher AC, and were negatively related to degradation as measured by the first-order rate constant, implying that deg radation is faster from the liquid phase than from the sorbed phase. A nonl inear fit of the decay curves to the nonequilibrium model revealed that deg radation rate constants were 0.157 and 0.00243 day(-1) for the liquid and s orbed phases, respectively, differing by a factor of 65. Similar results we re also obtained using the equilibrium model. A parameter sensitivity analy sis of the nonequilibrium model indicates that nonequilibrium sorption will initially favor degradation; however, over the long term, will decrease de gradation when desorption kinetics becomes the limiting factor in the degra dation process. In the presence of a lag phase that allows appreciable amou nts of chemical to diffuse into kinetic sorption sites, nonequilibrium sorp tion will only impede degradation. (C) 2000 Elsevier Science B.V. All right s reserved.