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

Abstract

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 O2– 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.

Waste Glass-Derived Tobermorite Carriers for Ag+ and Zn2+ Ions

“…On average, approximately 5.8 MJ kg −1 of energy is required to produce new container glass, and the mining of raw materials gives rise to problematic erosion, flooding, land degradation, and pollution [38][39][40]. Accordingly, alternative upcycling of unwanted post-consumer container glass into functional valueadded materials has been explored in many recent studies [3][4][5][6][7][8][9][10][11][12][13][14][15][16][17][18][19].…”

“…Despite the theoretical potential to recycle up to 90% of container glass, approximately 200 Mt of post-consumer soda-lime-silica glass is landfilled per annum [1]. The failure to effectively recycle container glass arises from various technical, geographical, and economic barriers, particularly compositional heterogeneity, wide dispersal coupled with poor collection infrastructure, inadequate separation from other wastes, and lack of regional demand [2][3][4][5].…”

“…Recent research to explore alternative options to upcycle waste container glass (also known as cullet) into value-added products includes the production of zeolites [2][3][4][6][7][8][9][10], silicate minerals [5,[11][12][13][14][15][16], geopolymers [17][18][19], and ceramics [20][21][22] for applications in catalysis, sorption/separation technology, and construction. In this respect, several studies have utilized a facile one-pot hydrothermal method to synthesize the layer-lattice calcium silicate phase, 11Å tobermorite (Ca 5 Si 6 O 16 (OH) 2 •4H 2 O), from a mixture of waste glass cullet and lime or other calcium-bearing wastes [2,5,[11][12][13][14]23]. The waste glass-derived tobermorite products have demonstrated high cation exchange capacities for a range of heavy metal contaminants [5,[11][12][13] and basic catalytic properties for organic condensation reactions [23].…”

“…In this respect, several studies have utilized a facile one-pot hydrothermal method to synthesize the layer-lattice calcium silicate phase, 11Å tobermorite (Ca 5 Si 6 O 16 (OH) 2 •4H 2 O), from a mixture of waste glass cullet and lime or other calcium-bearing wastes [2,5,[11][12][13][14]23]. The waste glass-derived tobermorite products have demonstrated high cation exchange capacities for a range of heavy metal contaminants [5,[11][12][13] and basic catalytic properties for organic condensation reactions [23].…”

Calcium Silicate Hydrate Cation-Exchanger from Paper Recycling Ash and Waste Container Glass

“…Previous studies have considered the hydrothermal synthesis of tobermorites from a wide range of commercial and industrial waste materials under relatively mild conditions (i.e., 80-125 • C in water or 0.5-4 M NaOH (aq) ) in order to appraise the reactivity of the feedstock components and to understand the relationship between the structure and properties of the product [16][17][18][19][20]25,29,30,35,36,39,56,73]. Accordingly, the present study investigated the hydrothermal synthesis of Al-substituted tobermorite from a mixture of paper recycling ash (PRA), waste container glass (WCG), and lime at 100 • C in 4 M NaOH (aq) .…”