Testing a mechanistic model. IV. Describing the effects of pH on zinc retention by soils

Samples of two soils were incubated at 60°C for 24 h with several levels of either calcium carbonate or hydrochloric acid. Zinc retention was then measured on subsamples of the treated soil over 24 h at 25°C. The results were compared with published experiments (Bar-Yosef, 1979; Harter, 1983) in which zinc retention was also measured over a range of concentrations and pH values, but using different experimental conditions. Zinc retention increased as pH increased. In all cases, this effect could be described by assuming that the ZnOH⁺ ion was retained and the effect of pH was due to the increased proportion of this ion in solution. Over most of the pH range it was not necessary to assume any contribution from a decrease in the electrostatic potential of the reacting surfaces with increasing pH. This contrasts with the effects of pH on phosphate and fluoride retention and suggests that the materials that react with zinc differ from those that react with anions. The shape of plots of zinc retention against ZnOH⁺ concentration was reproduced using a model in which it was assumed that there was a range of values of electrostatic potential. A similar model had previously been used for anions. Zinc would tend to react with the most negative end of this range and phosphate with the least negative end. This further suggests that zinc and anions may react with different materials.