Contrasting effects of phosphate on the rapid transformation of schwertmannite to Fe(III) (oxy)hydroxides at near-neutral pH

Schwertmannite is a metastable Fe(III)-bearing mineral that can rapidly transform to more stable Fe(III) oxides upon Fe(II) sorption, via a dissolution-reprecipitation pathway. Phosphate (PO₄³⁻) is a nutrient found in varying quantities in natural systems, but the influence of PO₄³⁻ on the Fe(II) induced transformation of schwertmannite is not well characterized. Here, we aim to quantify the aqueous and mineralogical effects of varied PO₄³⁻ loading, and subsequent Fe(II) loading, to schwertmannite at circumneutral pH. Phosphate was presorbed to schwertmannite at loadings of 0, 80, 400 and 800 μmoles g⁻¹ in anoxic artificial groundwater. Fe(II) was then added at a final concentration of 12.5 mM and aqueous and mineralogical products were observed over 10 days. No transformation occurred before Fe(II) addition in the zero PO₄³⁻ treatment. In contrast, the three highest PO₄³⁻ treatments partially transformed to a microcrystalline Fe(III) oxyhydroxide before Fe(II) addition, possibly due to exchange of sulfate with the added PO₄³⁻. Within 1 h of subsequent Fe(II) addition in the zero PO₄³⁻treatment, all schwertmannite had transformed to goethite via the Fe(II)-induced transformation pathway. Increases in the level of PO₄³⁻ loading attenuated goethite precipitation via Fe(II)-induced transformation of the precursor schwertmannite and microcrystalline Fe(III) oxyhydroxide, with 65%, 34%, and 21% solid phase Fe as goethite at day 10 in the low (80 μmoles g⁻¹), medium (400 μmoles g⁻¹) and high (800 μmoles g⁻¹) PO₄³⁻ treatments. Under low PO₄³⁻ loading, the addition of Fe(II) induced the transformation of the microcrystalline Fe(III) oxyhydroxide and schwertmannite to a mixture of lepidocrocite and goethite. In the medium and high PO₄³⁻ treatments, schwertmannite and the microcrystalline Fe(III) oxyhydroxide were instead stabilized by surface-complexation of PO₄³⁻ and did not extensively transform to more crystalline phases. Overall, the results show that PO₄³⁻ loadings can both drive rapid schwertmannite transformation to a new microcrystalline Fe(III) oxyhydroxide phase, and subsequently inhibit the Fe(II)-induced formation of lepidocrocite and goethite.