Testing a mechanistic model. II. The effects of time and temperature on the reaction of zinc with a soil

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Abstract
SUMMARY: Samples of a soil were mixed with zinc nitrate solutions and incubated from 1 to 30 days at temperatures from 4 to 60°C. The solution concentration of zinc, which would not have changed on brief mixing with the soil at 25°C, was measured. Background electrolytes for this measurement of null‐point concentration were both calcium and sodium nitrate. The effect of the temperature at which null‐point concentration was measured was also investigated. After incubation with zinc nitrate, desorption of zinc, and sorption of further zinc, were measured. Null‐point concentration of zinc decreased with increasing period of incubation, with the rate of decrease greatest at high temperatures of incubation. The effects of both temperature and time were closely described by a model which postulated an initial rapid adsorption of ZnOH + ions onto heterogenous charged surfaces, followed by a diffusive penetration. Increasing the temperature of incubation increased the rate of diffusive penetration and led to low solution concentrations. In contrast, increasing the temperature at which null‐points were measured increased the concentration of ZnOH + ions. This was shown to be consistent with a change in position of the equilibrium of the initial, rapid, adsorption reaction. Curves for desorption of zinc were continuous with curves for sorption of further zinc, but neither desorption nor further sorption coincided with the position of the curves relating retention of previously added zinc to concentration. This result was consistent with the model and occurred because desorption must reverse diffusive penetration. However, the model under‐predicted the magnitude of both desorption and sorption of further zinc. Desorption in calcium solutions was greater than in sodium solutions, even when the solution concentration of zinc approached zero. This was consistent with exchange diffusion of calcium ions for some of the penetrated zinc.

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