Removal mechanisms of phosphate from water by calcium silicate hydrate supported on hydrochar derived from microwave-assisted hydrothermal treatment

The discharge of phosphorus in water has led to the problems of eutrophication and waste of phosphorus resources. Here, calcium silicate hydrate supported on hydrochar derived from microwave-assisted hydrothermal treatment (w-CH/CSH) was developed for phosphate removal. The characterizations of w-CH/CSH were examined through scanning electron microscope, X-ray diffraction, X-ray photoelectron spectroscopy, Fourier transform infrared spectrometer, and specific surface area analyzer. Batch adsorption experiments were conducted to identify the adsorption characteristics of phosphate onto w-HC/CSH. Results showed that the best w-HC/CSH adsorbent was successfully prepared. The CSH was uniformly distributed on the hydrochar surface and the specific surface area (SSABET) of w-HC/CSH increased to 98.76 m2/g. The phosphate adsorption was governed by chemisorption on heterogeneous surfaces and could be better fitted by the pseudo-second-order and Freundlich model. The film diffusion and intraparticle diffusion were the rate-limiting process of phosphate adsorption. The underlying mechanisms were mainly attributable to the surface microprecipitation including Ca3(PO4)2, Ca8H2(PO4)6, Ca(H2PO4)2, CaHPO 4 and Ca5(PO4)3Cl, and the ligand exchange with the CaCO 3 and oxygen-containing functional groups.