The reductive dissolution of phosphated ferrihydrite and strengite

Abstract

The reduction of ferrihydrite, HFe5O8.4H2O, at three levels of phosphate saturation of its surface, and of strengite, FePO4.2H2O, was studied in flooded sand-straw mixtures. At equal redox levels, the overall effect of adding phosphate to ferrihydrite was to decrease its solubility, even when the solutions were supersaturated with respect to vivianite, Fe,(PO4),.8H2O. It was concluded that the addition of phosphate at levels less than the sorption capacity to reduced systems containing ferrihydrite will result in its sorption, and an increase in the stability of the oxide, rather than an increase in the dissolution of the oxide caused by vivianite formation. The reductive dissolution of strengite was congruent for the first 11 days of flooding, then phosphate accumulated in excess of Fe2+, probably because of vivianite precipitation. Strengite reduction occurred at levels of Eh and pH readily obtained in waterlogged soils. Strengite was always less soluble than phosphated ferrihydrite.