An investigation of phosphate adsorption by Clays and its relation to the problems of eutrophication of the river stour, Kent

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−1. 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.