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

Abstract

In this study, the layer-lattice calcium silicate hydrate mineral, tobermorite, was synthesized from waste green or amber container glass and separately ion-exchanged with Ag+ or Zn2+ ions under batch conditions. Hydrothermal treatment of stoichiometrically adjusted mixtures of waste glass and calcium oxide in 4 M NaOH(aq) at 125 °C yielded tobermorite products of ~75% crystallinity with mean silicate chain lengths of 17 units after one week. Maximum uptake of Zn2+ ions, ~0.55 mmol g−1, occurred after 72 h, and maximum uptake of Ag+ ions, ~0.59 mmol g−1, was established within 6 h. No significant differences in structure or ion-exchange behavior were observed between the tobermorites derived from either green or amber glass. Composite membranes of the biopolymer, chitosan, incorporating the original or ion-exchanged tobermorite phases were prepared by solvent casting, and their antimicrobial activities against S. aureus and E. coli were evaluated using the Kirby–Bauer assay. S. aureus and E. coli formed biofilms on pure chitosan and chitosan surfaces blended with the original tobermorites, whereas the composites containing Zn2+-substituted tobermorites defended against bacterial colonization. Distinct, clear zones were observed around the composites containing Ag+-substituted tobermorites which arose from the migration of the labile Ag+ ions from the lattices. This research has indicated that waste glass-derived tobermorites are functional carriers for antimicrobial ions with potential applications as fillers in polymeric composites to defend against the proliferation and transmission of pathogenic bacteria.

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

“…Irrespective of color, the typical major oxide components of soda-lime-silica container glasses are SiO 2 (70-75 wt%), Na 2 O (12-16 wt%), CaO (5-15 wt%), and Al 2 O 3 (1-5 wt%), which provide a potentially reactive source of amorphous calcium, silicate, and aluminate species in the following molar ranges: 0.13 < [Ca]/[Si+Al] < 0.23 and 0.015 < [Al]/[Si+Al] < 0.078 [17][18][19][20][21]39,64]. Optimal [Ca]/[Si+Al] ratios for tobermorite formation have been obtained by supplementing the WCG reaction mixture with lime [17][18][19]21,39] or cement bypass dust (CBD) [20]. Mixtures of WCG and lime require additional alkalinity, such as sodium hydroxide, to depolymerize and dissolve the silicate network [17][18][19]21,39], whereas CBD possesses sufficient intrinsic alkalinity to activate the glass [20].…”

“…Optimal [Ca]/[Si+Al] ratios for tobermorite formation have been obtained by supplementing the WCG reaction mixture with lime [17][18][19]21,39] or cement bypass dust (CBD) [20]. Mixtures of WCG and lime require additional alkalinity, such as sodium hydroxide, to depolymerize and dissolve the silicate network [17][18][19]21,39], whereas CBD possesses sufficient intrinsic alkalinity to activate the glass [20].…”

“…The discrepancy between the two estimates of crystallinity from XRD analysis (~67 wt%) and 29 Si MAS NMR spectroscopy (~75 % of the silicate species) is attributed to the extensive structural disorder of tobermorite and to the formation of a semi-crystalline calcium (alumino)silicate hydrate (C-A-S-H) precursor that is not effectively detected by XRD analysis. In this respect, XRD analysis tends to overestimate the amorphous-content of poorly crystalline calcium silicate hydrate materials synthesized at modest temperatures [13,17].…”

“…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) .…”

“…Many studies have been carried out to upcycle this WCG into value-added products such as sorbents, ion-exchangers, catalysts, ceramics, geopolymers, and construction materials [57,[59][60][61][62][63][64][65]. WCG has also been used as a feedstock for the preparation of tobermorite in alkaline media [17][18][19][20][21]39].…”