Biogeochemical processes at redox interfaces generate conditions with coexisting dissolved Fe(II) and solid-phase Fe(III). In such systems, Fe(II) induces the recrystallization of iron oxides, which in turn redistributes trace metals among the mineral bulk, mineral surface, and aqueous solution. The presence of organic acids may potentially alter these reactions through a series of cooperative or competitive processes: solution complexation, ternary surface complexation, surface site competition, and ligand-promoted dissolution. The effects of organic acids on trace metal fate in such systems is unclear because these processes may involve both trace metals and iron and multiple processes may co-occur. The effect of oxalate on Ni adsorption on, release from, and incorporation into hematite and goethite in the absence and presence of Fe(II) was investigated. The addition of oxalate in macroscopic adsorption studies suppresses Ni uptake by goethite and hematite at pH 7. EXAFS spectroscopy indicates that Ni adsorption mechanisms to Fe oxides are altered in the presence of oxalate. This suggests the formation of Ni-oxalate ternary surface complexes. ATR-FTIR spectra of the oxalate adsorption mechanisms will also be discussed. The addition of oxalate also promotes the release of structurally-incorporated Ni from hematite and goethite, with the extent of release decreasing as pH increases from 3 to 7. The aqueous Fe:Ni ratio oxalate-promoted Ni release is non-stoichiometric, suggesting that mineral recrystallization, not ligand-promoted dissolution, is the primary mechanism. Fe(II)-catalyzed recrystallization of goethite and hematite at pH 4 and 7 in the presence of oxalate resulted in enhanced Ni release. The effect of oxalate on this release appears to be largely additive under most conditions. In contrast, oxalate suppresses Fe(II)-induced incorporation of Ni into hematite and goethite. This study demonstrates that organic acids may substantially alter trace metal cycling through iron oxides, enhancing micronutrient availability, inhibiting contaminant sequestration, and destabilizing trace metals associated with iron oxides in the rock record.<br/><br/>Note: Recipient of the Student Travel Award

Effects of oxalate on Ni adsorption and repartitioning during Fe(II)-promoted iron oxide recrystallization

251st National Meeting (2016)

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