Coupled Effects of Nonlinear, Rate-limited Sorption and Biodegradation on Transport of 2,4-dichlorophenoxyacetic Acid in Soil

The transport and fate of many contaminants in subsurface systems can be influenced by several coupled processes, such as nonlinear, rate-limited sorption and biodegradation. We investigated the transport in soil of a model organic compound, 2,4-dichlorophenoxyacetic acid (2,4-D), influenced by nonlinear, rate-limited sorption and biodegradation. Miscible displacement and batch sorption experiments were conducted using a wide range of solute concentrations to investigate the impact of concentrationdependent behavior on transport. The sorption isotherm was approximately linear at a low concentration, and nonlinear over the extended range of concentrations. Results from the transport experiments, with the fitted N values approaching 1 at low input concentrations, were consistent with the batch sorption study. Nonlinear sorption significantly influenced the position of the breakthrough curves because of concentration-dependent retardation. However, although both nonlinear and rate-limited sorption influenced the shape of the breakthrough curves, the impact of rate-limited sorption was greater. The effective travel time of 2,4D transport is influenced by synergistic interactions between sorption and biodegradation. For example, the sequential rightward shift of the breakthrough curves with decreasing input concentration, due to nonlinear sorption of 2,4-D, is enhanced by biodegradation. Keywords—Coupled effects Sorption Biodegradation Transport