Removal of heavy metals (Co, Cr, and Zn) during calcium-aluminium-silicate-hydrate and trioctahedral smectite formation

Abstract

Hydrated aluminosilicates were synthesized with and without aqueous heavy metals (Me), such as cobalt (Co), chromium (Cr), and zinc (Zn), by a sol-gel process at different initial molar ratios of Ca/(Si + Al) (0.6-1.6) and Me/Si (0.0-2.0), and constant Al/Si ratio (0.05) using equilibrium-approaching experiments. The chemical composition of the reactive solutions during aluminosilicate precipitation and maturation was monitored by ICP-OES. The mineralogy, nanostructure, and chemical composition of the precipitates were studied by XRD and high-resolution TEM. At Me/Si ratios ≤ 0.2, calcium-aluminium-silicate-hydrates (C-A-S-H) with a defect 14 A tobermorite-like structure formed, whereas at a Me/Si ratio of 2.0, either trioctahedral Co-and Zn-smectite or amorphous Cr gels precipitated, independent of the initial Ca/(Si + Al) ratio used for gel synthesis. The immobilization capacities for Co2+, Cr3+, and Zn2+ by C-A-S-H, Cr gel, and trioctahedral smectite are 3-40 mg/g, 30-152 mg/g, and 122-141 mg/g, respectively. The immobilization mechanism of heavy metals is based on a combination of isomorphous substitution, interlayer cation exchange, surface (ad)sorption, and surface precipitation. In engineered systems, such as underground concrete structures and nuclear waste disposal sites, hydrated aluminosilicates should exhibit a high detoxication potential for aqueous heavy metals.