Uptake of aqueous heavy metal ions (Co2+, Cu2+ and Zn2+) by calcium-aluminium-silicate-hydrate gels

Calcium-aluminium-silicate-hydrate (C-A-S-H) gels containing heavy metal ions (Me: Co2+, Cu2+ or Zn2+) were precipitated at different Me/Si molar ratios to study the uptake mechanism and immobilization capacity. Aqueous solution chemistry and solid-phase characterization using XRD, FTIR, ESEM-EDX, TEM-HAADF and TGDSC methods reveal threshold values for ion substitution of -55-60 mol% for Zn, -30-40 mol% for Co and -25-30 mol% for Cu in defective tobermorite-type C-A-S-H with (Ca + Me)/(Si + Al) molar ratios from 0.86 to 1.04. At higher aqueous Me concentrations, Me (chloride) hydroxides start to co-precipitate. The uptake mechanism of Me ions by C-A-S-H is based on (i) isomorphous substitution of Me for Ca in the CaO layer, (ii) ion exchange in the interlayer, (iii) tetrahedral substitution (Zn) within the dreierketten chains and (iv) surface adsorption onto the C-A-S-H gel structure. Systematics in crystal-chemistry of Me-bearing C-A-S-H and implications for application and performance in natural and man-made settings are discussed.

Automated summary

The precipitation of Calcium-aluminium-silicate-hydrate (C-A-S-H) gels containing heavy metal ions at different Me/Si molar ratios was studied to investigate the uptake mechanism and immobilization capacity. The study revealed threshold values for ion substitution and discussed the uptake mechanism based on isomorphic substitution, ion exchange, tetrahedral substitution, and surface adsorption onto the C-A-S-H gel structure, concluding with implications for application and performance in various settings.