Sorption Isotherms and Kinetics of Sediment Phosphorus in a Tropical Reservoir

The sorption of phosphorus, in the form of dissolved phosphate, on aquatic sediments collected from the tropical Kranji reservoir in Singapore was investigated under oxic and anoxic conditions. Kinetic experiments on sediment samples, collected from three monitoring stations, show that the sorption process consists of fast and slow adsorption stages followed by an equilibrium stage in which further adsorption or desorption may take place. The periods for each stage are in the scale of minutes, hours, and days, respectively. About 67.3–96.6% of the sorption is completed in the first stage under oxic and anoxic conditions. First-order kinetic constants are estimated to be from 4.60 to 12.26 h⁻¹ for the fast adsorption stage, from 0.15 to 0.71 h⁻¹ for the slow adsorption stage, and from 0.005 to 0.028 h⁻¹ for the equilibrium stage. There were no significant differences observed between the sorption kinetic rates under oxic and anoxic conditions. The sorption isotherms of dissolved phosphate are found to be approximately described by the Langmuir equation, taking into consideration the native adsorbed phosphorus. The sorption capacities in different stations are 4.77–10.34 mg/g of dry sediment for oxic conditions and 2.28–6.01 mg/g of dry sediment for anoxic conditions. System redox potential has an apparent effect on the sorption capacity. Estimations of the natively sorbed phosphorus show that the sediments of the Kranji reservoir have initial amounts of phosphorus, ranging from 6.29 to 33.46% of the maximum sorption quantity. The sorption constants obtained are comparable to those obtained in temperate regions but have higher adsorption capacities.