Transport and Retention of Molybdenum(VI) in Soils: Kinetic Modeling

Abstract

Core Ideas Mo(VI) sorption was kinetically controlled and highly nonlinear in two different soils. Mo(VI) retention mechanism in Windsor sand was different from Webster loam. Rate‐limited processes greatly influenced Mo(VI) transport in soils. Mo(VI) transport was well described using a second‐order model. Understanding the mechanisms and kinetics controlling the retention and transport of molybdenum [Mo(VI)] is a prerequisite in the effort to evaluate environmental risk of groundwater contamination. In this study, kinetic batch and miscible‐displacement experiments were performed to investigate Mo(VI) adsorption–desorption and transport behaviors on a neutral soil (Webster loam) and a slightly acidic soil (Windsor sand). Batch results indicated that adsorption of Mo(VI) was kinetically controlled and highly nonlinear in both soils. The Mo(VI) maximum adsorption and kinetic rate for acidic Windsor sand was much higher than for neutral Webster loam. Mo(VI) breakthrough curves (BTCs) were asymmetrical and exhibited extensive tailing with incomplete recovery in the effluent: these features demonstrate that rate‐limited or non‐equilibrium process influenced Mo(VI) transport behavior. To better understand the controls on the nonlinear kinetic behavior of Mo transport in these soils, a second‐order (SO) model which accounts for equilibrium, reversible, and irreversible retention mechanisms was used to describe Mo(VI) retention and transport in soils. Based on optimized and predictive modeling results, a fully reversible equilibrium‐kinetic SO model successfully described the time‐dependent Mo(VI) sorption for Webster loam. However, for Windsor sand, which exhibited strong affinity and low mobility for Mo(VI), an additional consecutive irreversible reaction phase was necessary to describe Mo(VI) retention and transport with time.