Uptake of inorganic orthophosphate by greek fly ashes characterized using various techniques

Abstract

Four different fly ash samples collected from thermal power plants in Greece burning lignite of coal-lignite mixtures, were characterized using various physicochemical techniques, including specific surface area and particle size distribution determination, X-ray diffraction analysis, potentiometric titration and electrophoretic mobility measurements. The interaction of inorganic orthophosphate at the water/solid interface of these samples was investigated. It was found that the uptake of phosphate is extensive on fly ash surfaces and exceeds by far that corresponding to a monolayer coverage. Pretreating the samples by prewashing with potassium nitrate solutions did not cause any difference in adsorption behavior. Only one sample, containing relatively high amounts of soluble calcium, showed enhanced phosphate uptake. The increased uptake was not shown when the excess of calcium ions was removed from the supernatant by pre-equilibration. Although this suggested that this particular sample would cause precipitation of some calcium phosphate phase, this process could not be identified. Thermodynamic calculations based on the solubility data for calcium, infrared spectra and the shift of the isoelectric point at lower pH values, as phosphate is taken up, suggest that inorganic phosphate was specifically adsorbed on the fly ash particles examined.