Kinetics of Sulfate Retention on Soil as Affected by Solution pH and Concentration

Knowledge of the time dependence of chemical retention by soil is crucial to a full understanding of how retention is affected by solution variables and to accurately simulate the fate of soil‐applied chemicals. The kinetics of SO 2‐ 4 retention by and release from soil were examined in a continuously stirred reactor tank. The concentration‐time histories were processed with a number of commonly used kinetic equations. Of those tested, an equation that was first order in available retention sites consistently gave the best fit to the data. Sulfate‐retention kinetics were strongly influenced by SO 2‐ 4 concentration and the presence of PO 3‐ 4 in solution, but unaffected by a change in pH from 3 to 4. For example, when SO 2‐ 4 concentration was doubled from 0.313 to 0.625 m M in the retention phase of the experiment, the average first‐order forward rate constant increased from 0.07 to 0.24 min −1 . The addition of PO 3‐ 4 to the solution increased the forward rate constant to 0.13 min −1 and decreased equilibrium retention from 8.1 mmol/kg when PO 3‐ 4 was not present to 5.2 mmol/kg. In the release phase of the experiment, the presence of PO 3‐ 4 increased the estimated reverse rate constants by an order of magnitude, and an even greater increase in the reverse rate constant was estimated when the initial sulfate concentration was increased from 0.313 to 0.625 m M . Although the data was closely approximated by first‐order kinetics, changes in the rate coefficients for the different influent solutions indicate that commonly determined kinetic parameters have limited applicability beyond the particular conditions of the experiment.