The mobility and fate of Cr during aging of ferrihydrite and ferrihydrite organominerals

Abstract

The fixation of environmentally hazardous Cr(VI) in soils is largely attributed to its retention by iron (Fe) (oxyhydr)oxides in the soil environment, which are prevailingly associated with organic carbon (OC). The effect of OC with different binding strengths on Cr(VI) adsorption and the mobility and fate of Cr(VI) during aging of Fe (oxyhydr)oxide however, are unknown. Here we investigate how OC binding strength influences Cr(VI) adsorption to ferrihydrite (Fh) organominerals, the aging of Fh organominerals and the subsequent retention or release of Cr(VI) as aging proceeds. We conduct Cr(VI) adsorption experiments onto Fh organominerals, apply surface complexation modelling to study the relative binding strengths between OC and Cr(VI), and then conduct aging experiments to track the redistribution of Cr(VI). We show based on the relative binding strength between OC and Cr(VI), that weakly-bound OC at different C loadings has almost no discernable influence on Cr(VI) adsorption, while strongly-bound OC significantly suppresses Cr(VI) adsorption via surface site blocking and/or electrostatic repulsion. The mobility and fate of Cr(VI) with Fh organominerals during the aging process is strongly influenced by the presence of OC. Weakly-bound carboxyl-poor OC not only near doubles the rate of Cr(VI) retained (i.e., Cr(VI) redistribution from a weakly-bound to a strongly-bound fraction) in neoformed Fe minerals at pH 5.0, but also increases the final proportion of Cr(VI)strongly-bound, which we attribute to the loose and porous structure of Fh organominerals and easier replacement of weakly-bound OC by Cr(VI). More strongly-bound carboxyl-rich OC and higher pH however, reduces the influence of OC, such that the rate of Cr(VI) retained is similar to or lower than that for the pure Fh system, which we attribute to the fact that the strongly-bound OC is less easily replaced by Cr(VI) and hence Cr(VI) is less able to access surface sites or surface pores and thus to be retained inside neoformed minerals. Overall we suggest that OC with higher carboxyl-richness will further suppress the rate of Cr(VI) retained and overall proportion of Cr(VI)strongly-bound, and thus that the mobility and fate of Cr(VI) in soils is strongly controlled by the carboxyl-richness of OC in Fe organominerals.