Al-Substituted Tobermorites: An Effective Cation Exchanger Synthesized from "End-of-Waste" Materials

The policies to meet the "zero waste" regime and transition to sustainable circular economy can no longer ignore the use of wastes in place of natural resources, and these daunting and vital societal challenges are nowadays being faced by several nations. The main objective of this work was to search waste materials suitable for a quick and environmentally friendly production of a nanoporous geomaterial able to trap toxic metals at the solid/liquid interface. More specifically, the end-of-waste from the thermal inertization of cement-asbestos and glass powder from domestic glass containers have been employed as sources for the hydrothermal synthesis of a tobermorite-rich material (TRM) successfully tested for the selective removal of Pb²⁺, Zn²⁺, Cd²⁺, and Ni²⁺ from aqueous solutions. The synthesis was carried out in alkaline solution under mild hydrothermal conditions (120 °C) within 24 h. The quantitative phase analyses of the TRM carried out by applying the Rietveld method showed the occurrence of a large amount of well-crystallized 11 Å Al-substituted tobermorites and an amorphous phase and a lower content of aragonite and calcite. Chemical analyses and thermogravimetric measurements coupled with simultaneous evolved gas mass spectrometry highlighted that Al³⁺ for Si⁴⁺ substitutions in the wollastonite-like tetrahedral chains of tobermorites are balanced by the occurrence of Ca²⁺, Na⁺, and K⁺ cations in the interlayer rather than by (OH)^- for O^2- substitutions in the CaO polyhedra. Time-dependent removal tests clearly indicated that metal cations are selectively adsorbed depending on their concentration in solution. Moreover, the kinetic curves showed that the removal of metals from solution is fast and the equilibrium is almost reached in the first 8 h.