Kinetics and mechanism of adsorption of phosphate on fluorine-containing calcium silicate

The nanowires-reticulated calcium silicate with a specific surface area more than 100 m²/g was prepared by a hydrothermal process using hydrated lime (Ca(OH)₂, HL) and silica containing soluble fluoride, which was a by-product of fluorine industry, and the soluble fluoride in raw silica was fixed as CaSiF₆ at the same time. The kinetic characteristics and mechanism of adsorbing phosphate by fluorine-containing calcium silicate were investigated in the experiments of phosphorus (P) removal from aqueous solution. The results show that the prepared fluorine-containing calcium silicate has excellent performance for adsorbing phosphate, the adsorption process appears to follow pseudo-second-order reaction kinetics and the process is mainly controlled by chemisorption. The product resulted from P adsorption is mainly composed of hydroxyapatite (HAP) and fluorapatite (FAP), which are further used as adsorbents of heavy metal ion Cd²⁺ in aqueous solution and display excellent performance.