Iron (Fe) oxides and hydroxides are ubiquitous in nature, where they profoundly impact numerous biogeochemical cycles, fate and transport of trace metals, and serve as important proxies in the geological record. Recent studies found Fe oxide recrystallization in the presence of dissolved Fe(II) under reducing conditions induces extensive mixing of Fe atoms, and also likely drives the incorporation and release of trace metals. In our study, we selected nickel and cobalt as divalent cation sorbates, and goethite and hematite as sorbents, to study the effect of trace metals on Fe atom exchange during Fe(II) sorption on iron(III) oxides. Isotopically normal goethite and hematite were exposed to solutions spiked with dissolved Fe(II) (primarily as <sup>57</sup>Fe(II)) and trace metals. Sorption of metals and exchange of Fe atoms were monitored within batch experiments over 40 days by measuring aqueous metal concentration by inductively coupled plasma mass spectrometer (ICP-MS). Results showed greatly diminished Fe atom exchange when Ni and Co are present, while negligible changes were observed for overall Fe(II) uptake. The effect of different initial Fe(II) concentrations were examined and results suggested higher Fe(II) caused more Fe atom exchange for goethite, while the opposite is true for hematite. Remarkably, at low surface Fe(II) saturation, higher surface site availability on goethite contributed to a significant increase in Fe atom exchange, as revealed by experiments with varied initial goethite loading from 1 to 3 g/l while maintaining 1 mM initial Fe(II). The suppressing effect was further explored with varying degrees of pre-sorbed Co, and results showed more diminished Fe atom exchange at higher surface saturation of Co. Divalent cations might be suppressing Fe atom exchange by occupying sites of oxide Fe release. The suppression effect has profound implications on interpreting Fe isotopic signatures in geological records, as it may be preserved at high ambient trace metal concentrations.

Effect of dissolved trace metal cations on iron atom exchange during aqueous Fe(II) - promoted iron oxide recrystallization

254th National Meeting (2017)

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