Factors influencing the measurement of equilibrium phosphate concentrations in river sediments

A river sediment was treated to obtain different oxic conditions, and the phosphorus status investigated by experiments in which KH2PO4 was added to suspensions in calcium chloride. The pH, redox and solution composition of dissolved Ca, Mg, Fe, Na, K, Mn, Cl and SO4 and soluble reactive phosphorus were measured at the end of 24 h mixing. The equilibrium phosphate concentration was calculated from these measurements using the linear, Freundlich and dual-site Langmuir isotherm equations. The chemical speciation of the solutions was evaluated using PHREEQC and a model designed specifically for phosphorus speciation. These were used to estimate the solution saturations with respect to various iron and calcium containing minerals. The results show that at least a two-fold increase in the calculated equilibrium phosphate concentration of the sediment occurs with a moderate change in the electron activity from 8 to 4 as the sediments became reducing. Chemical precipitation and dissolution reactions were also found to be important during the experiments, with dissolved iron concentrations in oxic conditions determined by equilibration with amorphous iron hydroxide. Only in the suspensions with the highest soluble reactive phosphorus concentration was the solubility limit of ferric phosphate approached. There was also evidence for the loss of calcium from the solutions and oversaturation with respect to octacalcium phosphate. Vivianite, found in the sediment, was also expected to dissolve in the most oxic conditions. It is proposed that ancillary measurements of pH, redox potential, dissolved Ca, Fe, Mn and alkalinity during equilibrium phosphate concentration determinations will provide a better understanding to the buffering reactions controlling phosphorus in solutions in contact with sediments.