Minerals and particularly reactions at their surfaces affect elemental cycling, the availability of nutrients, and the fate of contaminants in the environment. Interactions of Fe-bearing minerals with aqueous Fe(II) can result in interfacial electron transfer that increases redox reactivity and can lead to dynamic Fe atom exchange between solid and aqueous phase, which has been linked to trace element release and incorporation. Thus, a mechanistic understanding of processes at the mineral-water interface occurring at the micro- to nanoscale is crucial for predicting contaminant fate at the macroscale. Here, two examples of how processes at the Fe mineral-water interface impact contaminant fate will be showcased: interactions between As and the Fe oxide magnetite, and the effect of naturally occurring compounds on the redox reactivity of Fe-bearing clay minerals.  We investigated potential pathways for As uptake at the Fe oxide magnetite, with a particular focus on As incorporation and redistribution. While coprecipitation of As with magnetite irreversibly sequestered a high proportion of dissolved As into the magnetite structure, As sorbed to magnetite was redistributed from sorbed to incorporated over time and thus became increasingly resistant to remobilization. Interestingly, aqueous Fe(II) had only a minor effect on As redistribution. We compared the reactivity of Fe-bearing clay minerals reduced either chemically or with the natural reductant aqueous Fe(II), in the presence and absence of organic ligands and electron shuttles. Analysis of probe compound transformation kinetics showed that the reactive species in both chemically and Fe(II)-reduced clay minerals was structural Fe(II). Once organic ligands were added, their complexes with Fe(II) determined the overall redox reactivity. Similarly, electron shuttling compounds increased the reaction rate. However, the initial clay mineral Fe(II)/Fe(tot) ratio strongly influenced the reactivity in both cases, suggesting that clay mineral Fe might provide redox buffering and reduction equivalent storage in natural systems.

Reactions at the Fe mineral-water interface: Impact on contaminant fate

256th National Meeting (2018)

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