No abstract

Eutrophication of the River Stour in Kent and its potential remedy are related to the adsorption of phosphates by its bed deposits, which are derived largely from Gault and Weald clays. The main minerals in these clays are: kaolin, mica, vermiculite and smectite. Four reference samples of these clays and samples of Gault and Weald clays were characterised by X-ray diffraction, thermal analysis (DTA and TGA) as well as by infrared spectroscopy and BET surface area. For the reference clays, also characterised by chemical analysis, the cation exchange capacity (CEC) and the point of zero charge (PZC) were determined. A kinetic study of phosphate adsorption showed an initial fast step followed by a slower process. The effects of pH, phosphate concentration, rate of shaking and temperature, on the rate of adsorption, are reported. Desorption of phosphate also revealed two consecutive kinetic stages. Adsorption isotherms of phosphate on the reference clays are presented over a range of pH values. In the case of kaolin, the heat of adsorption was found to be 17 kJ mol-I. Electrophoretic mobilities of the mica particles were measured in the presence and in the absence of phosphate, over a pH range. The mechanisms of adsorption are discussed.

The distribution of phosphates in fresh waters is briefly discussed and the different phosphorus cycles are mentioned. The chemistry of phosphorus in lakes is reviewed with special reference to the extraction methods used for the fractionation of phosphorus in soils and sediments. The models of the sedimentwater exchange are also discussed together with their environmental implications. The relevant publications on the adsorption by and desorption from sediments, especially clays, are reviewed with special reference to the adsorption mechanism on clays and the oxides of Fe and Al. The important topic of the desorption of phosphate from the relevant minerals and soils is presented with special reference to the effects of the sediment and water parameters affecting the release of phosphate from sediments.