Sustainable microwave-hydrothermal synthesis for in-situ solidification of heavy-metal zinc in tobermorite

Microwave hydrothermal synthesis can provide a sustainable way to prepare tobermorite, which can potentially efficiently solidify heavy metals by replacing calcium or silicon within its structure of tobermorite. In this paper, the feasibility of microwave hydrothermal process in solidifying heavy metal zinc (Zn) was investigated. The tobermorite was prepared from fumed silica and calcium hydroxide at Ca/Si of 0.83. The Zn-substituted tobermorite (Zn-tobermorite) was then prepared by adding Zinc chlorine under the Zn/Ca ratios of 0.05, 0.1, 0.15, 0.2, respectively. Additionally the characterisation of hydration product and the leaching behaviour of immobilised Zn was detected. Results indicated that the primary reaction products under both microwave and conventional hydrothermal conditions were Zn-tobermorite, tobermorite, gyrolite, hemimorphite, and reinhardbraunsite. During the synthesis of Zn-tobermorite, Zn²⁺ ions substituted for Ca²⁺ at an optimal Zn/Ca ratio of 0.15, achieving a Zn immobilization efficiency of 99.9 %. The microwave-assisted hydrothermal process reduced the synthesis time by approximately 85 % from 14 h to 2 h compared to conventional methods, while maintaining similar immobilization performance. This work offers a sustainable and time-efficient approach for heavy metal immobilization, showing significant potential for large-scale environmental remediation in industrial settings.