Quantitative insight into Cr(VI) adsorption and reduction by ferrihydrite-humic acid co-precipitates: Development of multistep kinetic model integrating synergistic/antagonistic factors

Abstract

Ubiquitous amorphous ferrihydrite (Fh) and humic substances (HSs) combining into co-precipitates fundamentally alters the processes by which they reduce the mobility and toxicity of Cr(VI) contaminants in groundwater and soil. However, the kinetic characteristics of the interactions between Fh–humic acid (HA) co-precipitates and Cr(VI) remain poorly understood. In this study, the kinetics of Cr(VI) adsorption and reduction by Fh–HA co-precipitates were examined under mildly acidic experimental conditions (pH 4–5). Compared to Fh and HA in their pure phases, Cr(VI) adsorption by Fh in co-precipitates was inhibited (antagonistic adsorption), whereas Cr(VI) reduction by HA in co-precipitates was enhanced (synergistic reduction). A successful multistep kinetic model was developed to simulate this complex reaction process involving both adsorption and reduction and effectively overcome the limitations of classical kinetics. The antagonistic adsorption effect ranged between 2 %–11 % mainly because of HA competing for adsorption sites on Fh, whereas the synergistic reduction effect varied between 116 %–518 % mainly because the highly reductive active components within the HA molecules preferentially bind to Fh during co-precipitate formation (molecular fractionation). Both the antagonistic adsorption and synergistic reduction effects are influenced by the structural types of the Fh–HA co-precipitates, being particularly pronounced in Fh-like co-precipitates characterized by a low C/Fe ratio. Our findings provide a comprehensive perspective for quantitatively assessing the role of Fh–HS co-precipitates regarding Cr(VI) contamination in groundwater, contributing to its evaluation, management, and remediation.