A Quantitative Model for the Coupled Kinetics of Arsenic Adsorption/Desorption and Oxidation on Manganese Oxides

Abstract

The ability to predict coupled kinetic reactions of arsenic (As) that occur at the manganese oxide–aqueous solution interface is crucial for management and remediation of As-contaminated sites worldwide. A quantitative understanding of the coupling between As oxidation and adsorption/desorption kinetics will enable us to more accurately predict the kinetic behavior of As. In this study, we developed a novel quantitative model for the coupled kinetics of As adsorption/desorption/oxidation on δ-MnO2. The kinetics of As­(III) adsorption and oxidation on δ-MnO2 was studied using a stirred-flow method under varying influent As­(III) concentrations. The model was able to account for the variations in As concentration, different binding behavior of As­(III) and As­(V), and flow rates. Our model quantitatively shows that the coupled As adsorption/desorption/oxidation processes are highly dependent on As mass loading rates on δ-MnO2. Our model suggests that the As­(III) adsorption rate is much higher than its oxidation rate. Both As­(III) oxidation and subsequent As­(V) desorption may proceed with similar reaction rates and control the overall reaction rates, although As­(V) desorption rates may be affected by As­(V) re-adsorption. Our kinetic model contributes to prediction of the dynamic behavior of As when manganese oxides are present.

Arsenite Depletion by Manganese Oxides: A Case Study on the Limitations of Observed First Order Rate Constants

“…Completing this model is numerically difficult, due in part to rates spanning many orders of magnitude. The model developed by Feng et al (2018) takes a reductionist approach, and does not incorporate the role of Mn III in determining depletion rates [139]. While this approach can predict As(III) oxidation rates in a limited set of conditions, the resulting model parameters lose physical significance, as it does not incorporate steps in the known reaction mechanism.…”

Predicting Kinetics of As(V) Adsorption and Desorption on Mixed Minerals of Ferrihydrite and δ‐MnO2

“…Arsenate adsorption and desorption significantly affected As redox kinetics on Mn oxides, which can be complicated by the competitive adsorption by Fe oxides. The model developed in this study can be further coupled into the As redox kinetics model as shown in our previous studies (Feng et al, 2018) to predict the coupled kinetics of As(III)/As(V) when multiple minerals are present. In addition, the structure and reactivity of ferrihydrite and MnO 2 may be affected by multiple biogeochemical factors, and our work based on the component additivity approach with mixed suspensions provided basis for understanding the As reactions with Fe‐Mn oxides formed in the field conditions.…”

“…Quantitative understanding of the kinetics behavior and mechanisms of As(V) adsorption and desorption on soil minerals is of great significance for assessing the mobility and fate of As(III)/As(V) in the environment since the redox reactions of As species on soil minerals was usually coupled with the adsorption–desorption reactions (Feng et al, 2018; Lafferty et al, 2010b; Lan et al, 2018). Therefore, kinetics of As(V) adsorption–desorption on both ferrihydrite and δ‐MnO 2 had significant implications for the reactions of As species on mineral surfaces.…”

“…Among the SCMs, the charge distribution and multisite surface complexation (CD‐MUSIC) model (Hiemstra and Van Riemsdijk, 1996), implemented in the chemical equilibrium software Visual MINTEQ (Gustafsson, 2018), is one of the advanced and mechanistically robust SCM that can successfully describe As(V) equilibrium binding to Fe(hydr)oxides (Gustafsson and Sjöstedt, 2016; Tian et al, 2017). There were few studies on developing SCMs for As(V) reactions with MnO 2 and equilibrium isotherm models have been used in modeling studies (Feng et al, 2018). Furthermore, there are much fewer studies to date for predicting the kinetics of As onto soil minerals with success (Farrell and Chaudhary, 2013; Tian et al, 2017; Zhu et al, 2009).…”

“…However, to our best knowledge, no model is available for the prediction of kinetics of As(V) adsorption and desorption on mixed soil minerals as are commonly found in a natural environment. Although MnO 2 is usually present with much smaller amounts compared with Fe minerals in soil, adsorption and desorption reactions of As(V) may significantly affect the redox reactions of As(III) on MnO 2 surfaces (Feng et al, 2018). A mechanistic model for the kinetics of As(V) adsorption and desorption on mixed minerals of ferrihydrite and δ‐MnO 2 , occurring together in the soil environment, would be a useful tool for predicting the dynamic behaviors of As(V) on different time scales in a natural soil environment.…”