Adsorption-Reduction Behavior of Cr(VI) by Ferrihydrite-Fulvic Acid Complexes with Different C/Fe Mole Ratios

Ferrihydrite and fulvic acid are prevalent components of soils and can significantly influence the environmental behavior of Cr(VI) in these matrices. Prior research has investigated the sorption behavior of Cr(VI) by ferrihydrite and fulvic acid independently; however, the sorption-reduction capacity of Cr(VI) by the ferrihydrite-fulvic acid complex, which is ubiquitously present in soils, has been less explored. In this study, ferrihydrite-fulvic acid complexes (Fh-FA) with varying C/Fe mole ratios were synthesized, and the adsorption-reduction behaviors of Cr(VI) on Fh-FA were examined using batch experiments, FTIR, BET, XRD, SEM-EDS, and XPS. The results demonstrate that the pseudo-second-order kinetic model can accurately describe the adsorption process of Fh-FA on Cr(VI), which can be delineated into three stages: rapid adsorption (0-30 min), slow adsorption (30-120 min), and reaction equilibrium (>120 min). The adsorption of Cr(VI) on Fh-FA primarily occurs through chemisorption. FTIR and XPS analyses revealed that Cr(VI) is initially adsorbed by Fe-OH on the Fh-FA surface. However, as the C/Fe mole ratio of Fh-FA increases, more Fe-OH is complexed by -COOH from FA, resulting in a decrease in the adsorption capacity of Cr(VI) by Fh-FA. The number of phenolic hydroxyl groups from FA also increases, providing additional electrons to reduce Cr(VI) to Cr(III) with increasing C/Fe mole ratio. This study not only emphasizes the adsorption-reduction behavior of Cr(VI) by ferrihydrite-fulvic acid complexes but also uncovers the interplay between C/Fe mole ratios and Cr(VI) adsorption-reduction, contributing to a more comprehensive understanding of the relative roles of Fe hydroxides and natural organic matter in soil environments. These findings have significant implications for Cr(VI) management in soils, enhancing our capability to protect and sustain the environmental quality.