Dissolution of Phosphate in Mixed Fe- and Al-oxide Mineral Suspensions as Influenced by Reducing Conditions.

Abstract

The loss of soil phosphorus (P) to surface waters poses a threat to water quality. Research evaluating P dissolution and transfer among sorbents in pure mineral systems can provide basic knowledge useful for the environmental management of P. The objective of this research was to evaluate the effect of Al-oxides on the reductive dissolution of orthophosphate sorbed to Fe-oxides. Redox reactor systems containing 0.5 g ferrihydrite [Fe(OH)3] kg-1 suspension and 0.002 to 0.7 g boehmite (α-AlOOH) kg-1 suspension were equilibrated with 750 mmol P kg-1 of ferrihydrite, and abiotically reduced for 72 h with 0.5% H2 (g) in the presence of a catalyst of 10 % Pt on activated C. The kinetics of reductive dissolution of ferrihydrite, as indicated by dissolved Fe(II), followed a linear (zero-order) model. The rate coefficient showed a sharp, linear decrease (R2 = 0.61) with minor additions of boehmite (0 to 0.008 g kg-1), and net Fe(II) dissolution was essentially null for boehmite additions ≥ 0.02 g. Uptake of dissolved P occurred over time during reduction of mixed ferrihydrite-boehmite suspensions. XANES spectroscopy of samples collected during reduction of a 1:1 ferrihydrite: boehmite mixture did not detect a net transfer of P from ferrihydrite to boehmite over 168 h. Supporting experiments suggested that Al(III) dissolved from poorly crystalline boehmite caused the observed decrease in Fe(II) dissolution rate in the reduction reactors, either by sorbing to the ferrihydrite surface and blocking electron transfer, or by sorbing to Pt/C catalyst and inhibiting its catalytic activity. The results suggest that Al-oxides may affect net phosphate dissolution in soils undergoing reduction by taking up dissolved P or by inhibiting the reductive dissolution of iron oxides if Al(III) is sorbed to Fe(III)-oxide surfaces.