Influence of sorbed phosphate on the stability of ferric hydrous oxide under controlled pH and Eh conditions

Abstract

The reduction of ferric hydrous oxide at three levels of phosphate saturation of its surface was studied in a chemical system at various controlled combinations of pH and Eh. Hydrogen in the presence of a catalyst was used as the reductant. Without added phosphate the reduction of the oxide, as measured by the release of Fe2+ into solution, occurred at pH levels between 4.0 and 7.0. The Eh, value at which reduction started ranged from 0.2 V at pH 4.0 to -0.1 V at pH 7.0. The addition of phosphate markedly inhibited reduction of the oxide. At each pH, the Eh required to cause reduction was lower after the addition of phosphate to the surface of the oxide than when no phosphate was added. With added phosphate, no reduction occurred at pH 7.0, even at Eh -0.3 V. The release of phosphate from the surface of the oxide was significant only at pH 4.0 and below Eh -0.2 V. It was shown that pH had a greater effect on the distribution of phosphate between the solution and the oxide than Eh. The significance of the results to the behaviour of phosphate in waterlogged soils is discussed.