Phosphate recovery from wastewater using calcium silicate hydrate (C-S-H): sonochemical synthesis and properties

Abstract

In this study, a novel ultrasound-assisted sol–gel method was developed for the room-temperature synthesis of single-phase calcium silicate hydrate (C-S-H). The optimum synthesis conditions and the phosphate recovery properties of C-S-H were investigated by batch experiments. Samples were characterized by scanning electron microscopy (SEM), specific surface area analysis, ²⁹Si magic-angle spinning nuclear magnetic resonance (²⁹Si MAS-NMR) spectroscopy, and X-ray diffraction (XRD). The results showed that ultrasound irradiation played a vital role in the rapid and effective synthesis of C-S-H at room temperature, and the Ca/Si ratio of 1.2 was the most appropriate raw material ratio. In the process of phosphate recovery, C-S-H acted as the seed crystal, the calcium ion donor, and the pH adjuster at the same time, and therefore phosphate could be recovered in one-step crystallization without adding other chemicals. The adsorption kinetics and adsorption isotherms of phosphate recovery could be well described by the pseudo-second-order model and the Langmuir model, respectively. The thermodynamic parameters indicated that the phosphate recovery process was endothermic and spontaneous. The phosphate adsorption capacity on C-S-H could reach 109.4 mg g⁻¹ when the initial pH was 5.0 and the temperature was 298 K. Unlike the case of using Ca(OH)₂, only a slight degree of carbonate inhibition was observed for the phosphate recovery by C-S-H. All the results indicate that the ultrasound-assisted sol–gel synthesis is a valuable method for C-S-H synthesis.