Testing a mechanistic model. VI. Molecular modelling of the effects of pH on phosphate and on zinc retention by soils

Both phosphate and metal cations such as zinc can be retained by oxides of iron and aluminium. Yet the phosphate-retaining and zinc-retaining materials in soil appear to differ in their behaviour from that of pure oxides of iron and aluminium. In most soils, the electrostatic potentials of retaining materials appear to be negative at normal soil pH values. The changes in potential with pH, and consequent change in retention, also appear to differ from those of pure oxides. A model of a pure oxide was modified in an attempt to simulate the observed behaviour of soil. The most effective modification was to include a negative charge in the oxide. Such a charge could have arisen from solid–state diffusion of anions into the oxide, or by close association with permanently charged clays. The inclusion of this charge changes the direction of the effect of pH on phosphate retention. If the amount of negative charge is large enough, the paradoxical situation can arise in which the potential of a ‘variable-charge’ surface is not affected by pH. This is consistent with observed effects of pH on zinc retention by soils.