Identification of the Chromate Sorption Mechanism Conversions in a Quartz–Montmorillonite–Ferrihydrite System

The fate of Cr­(VI) in soils/sediments largely depends on its sorption by iron oxides/hydroxides. But, their combination with surrounding minerals (e.g., quartz (Qu), clays) evidently changes some properties. Since ferrihydrite (Fh) is the precursor for most iron oxides, the 10 days sorption experiments studying Cr­(VI) mobility were performed using the reference Qu–montmorillonite (Mt)–Fh system. From the results of batch experiment, ζ potential, Fourier transform infrared spectroscopy, X-ray diffraction, and scanning electron microscopy equipped with energy dispersive X-ray spectroscopy, chromium­(VI) sorption mechanisms are sensitive in Qu–Mt–Fh mixtures and closely relate to pHs. At acidic conditions, the deposition of Fh on Qu/Mt surfaces decreases the sorption sites for Cr­(VI) while the formation of outer-sphere complexation driven by electrostatic attraction increases Cr­(VI) mobility in contrast to inner-sphere complexation. At alkaline conditions, the situations are different in Qu–Fh and Mt–Fh systems, even though both increase sorption compared to that with only Fh. Sorption is subject to free Fh in Qu–Fh mixtures, while considering the high loading of the Mt–Fh surface due to the strong association of Si–OH with Fe–O­(H), CrO₄^2– captures are favorable by hydrogen bonding with water as outer-sphere complexation in Mt–Fh mixtures, resulting in high mobility especially at Mt/Fh ratios of 2/3.