Evaluation of porous calcium silicate hydrate derived from carbide slag for removing phosphate from wastewater

This study investigated the phosphate removal performance of a porous calcium silicate hydrate (P-CSH) derived from carbide slag, a kind of highly alkaline and calcium-rich industrial by-product. The P-CSH was synthesized under 150 degrees C for 6 h in an autoclave. The prepared P-CSH exhibited distinct skeleton structures and abundant pores with an average radius of 10.88 nm, which provided more active sites. Batch experiments showed that the phosphate removal capacity (PRC) could reach 95% in 4 h. The Ca: P molar ratio in reaction product was 1.49-1.64 according to ICP-OES and EDS results, and XRD patterns confirmed that the product was mainly composed of Ca-3(PO4)(2) and Ca-5(PO4)(3)OH. As for mechanism study, FTIR and XPS spectra revealed that the Ca-OH group of P-CSH was directly associated with phosphate removal, the Ca-OH will be transformed to Ca-P group after reacting with phosphate. The molar ratio of Ca: Si, equilibrium pH and coexisting ions were three important factors affecting the performance of P-CSH. Increasing Ca: Si ratio can improve the Ca2+ release amount and PRC of P-CSH. The optimal equilibrium pH was determined around 8.5 by experimental and Ksp-based theoretical calculation results. The influence of coexisting SO42- and HCO3- can be avoided by adjusting the equilibrium pH between 5.3 and 10.2. Furthermore, the reuse performance of P-CSH was investigated, the PRC could be accumulated to 14.79 mg P g(-1) after nine reuse cycles. Besides, calcination at 700 degrees C was found to exhibit a certain regeneration effect on P-CSH.