Investigation of the Interactions Occurring Between Cr(VI) and Citric Acid-Schwertmannite Composites: A Macroscopic and In Situ ATR-FTIR Study

Organo-mineral composites play an important role in the mobility and bioavailability of toxic metals in natural environments. However, the interactions between heavy metals and composites remain ambiguous. In this study, citric acid-schwertmannite (CA-Sch) composites were synthesized to investigate the interfacial behaviors of Cr(VI) adsorption via macroscopic experiments (adsorption kinetics, adsorption iso-therms, and pH adsorption edge) coupled with in situ ATR-FTIR spectroscopy and XPS analysis. Macroscopic experimental results showed that the adsorption capacities and rate of Cr(VI) on CA -Sch composites presented a distinct decrease compared with those on Sch, wherein the former had a negative linear correlation with carbon loading content and the pHIEP of the composites. The structural sulfate and adsorbed CA could be substituted by Cr(VI) ions. In situ ATR-FTIR spectra of CA-Sch showed different responses after Cr(VI) adsorption, including a continuous release of structural sulfate and a slight weakening in the vas(COO-) and vs(COO-) of adsorbed CA, further revealing that Cr(VI) mainly coordinated to the CA-Sch composite surface via anion exchange with the tunnel structural sulfate, followed by an outer-sphere CA complex on the external surface. The coordination modes of Cr(VI) on the composite included monodentate (C3v) and bidentate (C2v) complexes. In addition, the analysis of element species content and XPS showed that partially adsorbed Cr(VI) on the CA-Sch composite was reduced to Cr(III) without being released into the solution. These findings advance our understanding of the interfacial behaviors of Cr(VI) on the CA-Sch composite surface and contribute to the evaluation of the fate of Cr(VI) in AMD-contaminated environments.