The removal of phosphate ions from aqueous solution by fly ash, slag, ordinary Portland cement and related blends

Phosphate ions have been removed from aqueous solution by fly ash, slag, ordinary Portland cement (OPC) and related cement blends. The rate and efficiency of PO43- removal were found to increase in the order: fly ash, slag, OPC, apparently mimicking the order of increasing percent CaO in the adsorbents. Blending OPC with fly ash or slag evidently results in diminished PO43- removal efficiency. Better removal was obtained at higher solute concentration, acidic pH and higher temperature. The effect of particle size and the speed of mixing were found not to be significant. A first-order kinetic model was used to obtain values for overall sorption rate constants and intraparticle diffusion constants. The Frumkin isotherm was found to be the appropriate equation for modelling isotherms from the experimental adsorption data, and values have been obtained for the isotherm constants. A 400-mg/l PO43- (as P) solution was fed at a steady velocity of 2.0 cm/min through a 2.0-cm fixed-bed column (at pH 9.0 and 25 degreesC), and breakthrough curves were constructed to obtain estimated adsorption capacity values of 32, 60, 75, 78 and 83 mg PO43- adsorbent for fly ash, slag, OPC + fly ash, OPC + slag and OPC, respectively. (C) 2002 Elsevier Science Ltd. All rights reserved.