Iron and Phosphate Dissolution during Abiotic Reduction of Ferrihydrite‐Boehmite Mixtures

Excessive phosphorus loss from soils poses a threat to surface‐water quality. Soils comprise assemblages of multiple minerals, with Fe‐ and Al‐oxides being important for phosphate sorption. Our objective was to measure reductive dissolution of an Fe‐oxide and sorbed orthophosphate as affected by the presence of an Al‐(hydr)oxide mineral. Aqueous suspensions containing 0.5 g ferrihydrite kg−1 and up to 0.7 g boehmite kg−1 and KH2PO4 added at 750 mmol kg−1 of ferrihydrite were abiotically reduced at pH 6.0 for 72 h using 0.5% H2(g) in the presence of a Pt catalyst. A sharp decrease in zero‐order Fe(II) dissolution rate coefficients was observed between 0 and 0.008 g kg−1 of added boehmite, whereas net Fe(II) dissolution was essentially null for boehmite additions ≥ 0.02 g kg−1 Although net dissolution of PO4 occurred over time in the absence of boehmite, a net uptake occurred in the presence of boehmite. Auxiliary experiments suggested that Al(III) dissolved from boehmite decreased Fe(II) dissolution during reduction by sorbing to the ferrihydrite surface and blocking electron transfer. Because PO4 was taken up in excess of the maximum boehmite sorption capacity in systems with ≤ 0.008 g boehmite kg−1, results suggested the formation of Al‐phosphate or an Al(III)–PO4 complex on ferrihydrite surfaces. Phosphorus K‐XANES spectroscopy of samples collected during reduction of a 1:1 ferrihydrite/boehmite mixture showed no consistent change in sorbed PO4 associated with Fe(III) versus Al(III).