Highly Efficient Cd²⁺ Removal Using Tobermorite with pH Self-Adjustment Ability from Aqueous Solution

Cadmium (Cd), as a type of heavy metal, can increase the incidence of many diseases, even in low concentrations. In this study, tobermorite was hydrothermally synthesized and then applied to adsorb Cd²⁺ from an aqueous solution. The physicochemical characteristics of the synthesized tobermorite were detected, and the results indicated that the well-crystallized tobermorite had a lot of mesopores and a large specific surface area of 140.92 m²/g. It acquired a pH self-adjustment ability via spontaneously releasing Ca²⁺ and OH^- into the aqueous solution. The effects of different factors on Cd²⁺ removal were investigated. For Cd²⁺, the removal efficiency could reach 99.71% and the maximum adsorption capacity was 39.18 mg/g using tobermorite. The adsorption data was best fitted with the pseudo-second-order kinetic and Langmuir isotherm models. In addition, there was no strict limit on the solution pH in Cd²⁺ adsorption because the tobermorite could adjust the solution pH to an alkaline atmosphere spontaneously. The efficient removal of Cd²⁺ using tobermorite was a result of surface complexation and ion exchange.