The adsorption and release of phosphate from sediments of a river receiving sewage effluent

Abstract

Data are reported which are relevant to the possible effects of river sediments on the concentration of phosphate in its water. This is significant for the Kentish River Stour in view of discrepancies in the mass balance of phosphorus, P, downstream from Ashford Sewage Treatment Works, STW. Sediment samples were collected from the sites at Longport Bridge, LB, Godmersham Park, GP, and Shalmsford Street, SS, which are located 1, 10 and 16 km, respectively, from STW. Characterisation of the samples by chemical analysis showed that the percentage Fe and Fe-bound P decreased with increasing distance from STW whereas the percentage Ca and saloid-P increased. X-ray diffraction showed that phlogopite, a mica mineral, was the main constituent whilst calcite was apparent in the SS sediment. Phosphate adsorption or desorption kinetics exhibited an initial rapid stage followed by a slower process, whose data fitted a reversible second-order equation. The forward rate constants, k 1 were between 1 and 2 dm 3 mol -1 min -1 and were dependent on pH, calcium and nitrate ion concentrations. Adsorption rates under nitrogen at pH 8.0 and 25°C increased in the order: LB < GP < SS whereas the order was GP < LB < SS for adsorption in air. For the LB deposit, the marked depression of k 1 on excluding air by nitrogen was related to its high Fe content. Rates of native phosphate release increased with increased distance from STW and on replacing air by nitrogen especially for the SS deposit. The results suggest that the bonding of P is weaker in calcareous compared to iron-containing sediments. On the basis of the results obtained some phosphate strategies are discussed as well as the relevance of the data to other watercourses.