Experiment and simulation on soil Cr(VI) loss to surface runoff under condition of nonlinear Langmuir adsorption

Abstract: Farmland has become less and less in China, but the heavy metal pollution is very serious recently. So, the study on the loss of adsorptive solute of six-valence chromium, i.e. Cr(VI) from soil to surface runoff becomes very important, which is of great significance to understand the expansion of heavy metal pollution in the field. The problem of chromium pollution has been a very serious and worldwide concern recently. In this paper, based on the water balance and solute mass conservation, a simple two-layer incomplete mixing model is used to study adsorptive solute of Cr(VI) loss from soil into surface runoff with nonlinear Langmuir adsorption. Based on the water infiltration, the rainfall process is divided into four stages. Experimental data under the condition of laboratory simulated rainfall-runoff is used to verify the model with nonlinear Langmuir isotherm adsorption equation. Influences of model parameters on the soil solute loss to surface runoff and model sensitivity are analyzed. The results suggest that model simulations with Langmuir isotherm equation agree well with the observed data, which means that the simple two-layer incomplete mixing model is reliable to predict the adsorptive solute of Cr(VI) loss from soil into surface runoff with nonlinear Langmuir adsorption. Parameters γ, which is the incomplete mixing parameter of the solute between the infiltration water and the soil water in the mixing layer, is more sensitive than α which is the incomplete mixing parameter of the solute between the soil water in the mixing layer and the ponding-runoff water. The parameters γ and α have a greater impact on the simulated results of the model in the early stages and smaller impact in the late stages of rainfall, and the parameter B, which is related to the adsorption-desorption equilibrium constant for a reversible reaction and the saturated adsorption capacity absorbed by the single molecular layer in nonlinear isotherm equation, has different impact at different rainfall stages. And the parameter C, which is related to the saturated adsorption capacity in nonlinear isotherm equation, has a greater impact on the simulated results of the model in the later stages than the impact in the early stages of rainfall. Experimental data indicates that the solute concentration and amount is very small, which means the solute in the mixing layer does not transfer much into ponding-surface runoff layer. This is also proved by the small value of the simulated α, which also suggests that the most of lost solute of soil Cr(VI) is in the drainage water. The study results will provide references to prevent and reduce the agricultural non-point heavy metal pollution in the field.