Arsenic adsorption in soils: Second-order and multireaction models

Arsenic (As) is a toxic element, and the understanding of its retention in the soil environment is a prerequisite in predicting As behavior in the vadose zone. The objective of this study was to quantify the extent of the kinetics of As adsorption in soils over a wide range of concentrations and to investigate the capabilities of multireaction and a second-order modeling (MRM and SOM, respectively) approaches in describing the kinetic behavior of As in soils. Batch kinetic experiments were carried out to determine adsorption of As(V) over a wide range of concentrations by clay, loam, and sandy soils. Adsorption results indicated strongly kinetic behavior where the rate of As(V) retention was rapid initially and was followed by slow retention behavior with time. Sorption isotherms exhibited strong nonlinearity with a Freundlich reaction order much less than 1 for all soils. We tested the MRM and SOM for their capability to predict As concentration with time and found that several model versions fit the data equally well, but the sorption kinetics prediction capability varied among the soils investigated. MRM was superior to SOM, and the use of irreversible reaction into the model formulations was essential. In addition, we found that incorporation of an equilibrium-sorbed phase into the various model versions for As(V) predictions should be avoided.