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Quantitative Measurement on Removal Mechanisms of Phosphate by Class–F Fly Ash

ABSTRACT The role of Class-F fly ash in the removal of phosphate from aqueous solutions and the removal mechanism were quantitatively investigated. The relative contributions of precipitation and adsorption with various pH conditions showed that precipitation was predominant under alkaline conditions, whereas adsorption was effective under acidic conditions. The precipitation accounted for 79% of the total phosphate removal without adjusting the pH of the solution. This result suggests that the phosphate was predominantly removed by precipitation with fly ash, and that adsorption on hydroxylated oxides may have contributed to the removal of phosphate. The precipitate was revealed to be calcium hydrogen phosphate (CaHPO4) from X-ray absorption fine structure analysis. The adsorption process was expressed well by the Freundlich isotherm and phosphate adsorption by fly ash is favorable (n = 1.308).