A Freundlich‐type multi‐component approach for modeling the sorption of nickel and phosphate in soil

Abstract

AbstractIn this study, the effect of phosphate on Ni sorption by Mahan soil was investigated. In batch experiments, an initial solution concentration of 1 mM phosphate increased Ni sorption by 15.1%, and initial concentrations of 1 mM Ni increased phosphate sorption by 12.8%. The effect of phosphate on Ni sorption, and vice versa, was dependent on initial concentrations of either solute, with greater effects being observed with greater initial concentrations. Sequential extractions suggest that the increased sorption of either species in the presence of the other is due to a combination of electrostatic interactions and the formation of ternary complexes. A novel Freundlich‐type multi‐component isotherm (FMC) was developed and described the sorption of Ni both in the absence and presence of phosphate with a single set of parameters. The effect of phosphate on the sorption kinetics of Ni was evaluated using the stirred‐flow approach. The presence of phosphate increased the maximum sorbed concentration of Ni by 7.3% and increased the rate of Ni sorption. A novel Freundlich‐type multi‐component kinetic model (FKM) was developed and described the stirred‐flow data well. The FKM was able to describe the sorption‐desorption of both Ni and phosphate applied individually as well as simultaneously. The FMC and FKM offer advantages over previous modeling approaches in that they require far fewer parameters than surface complexation models and have greater descriptive capabilities than traditional empirical models. However, both models are empirical, and their ability to describe the observed data should not be taken to imply specific molecular‐level mechanisms.