Fluoride recovery using porous calcium silicate hydrates via spontaneous Ca2+ and OH- release

In the present study, porous calcium silicate hydrates (P-CSH) was prepared from CaO and soluble sodium silicate via a hydrothermal method and used for fluoride ions recovery. In comparison with the crystalline calcium silicate hydrates (C-CSH), P-CSH, which has large specific surface areas of 148.11 m(2)/g, a high Ca/Si molar ratio of 2.28 and low crystallinity, exhibited a high capacity for fluoride recovery from synthetic and practical electroplating wastewater. Fluoride recovery by P-CSH continued spontaneously at pH range of 6.4-7.4 without an extra pH adjustment. The fast Ca2+ dissolution rate of P-CSH was favorable for the fluoride precipitation with free Ca2+. Compared with the C-CSH, P-CSH exhibited massive Ca-OH linkages, which enhanced the fluoride removal capacity by forming insoluble CaF2. Based on the analysis of the P-CSH samples before and after fluoride recovery by SEM, XRD, FTIR techniques, it was concluded that the surface Ca-OH was active sites and the quantitative substitution of Ca-OH groups by F- played a key role in F- ion exchange. The fluoride recovery efficiency of P-CSH is 79.6%. The present study demonstrated a good potential of P-CSH for the fluoride recovery. (C) 2016 Elsevier B.V. All rights reserved.