Formation of Cd precipitates on γ-Al2O3: Implications for Cd sequestration in the environment
Apart from surface complexation, precipitation of minerals also plays an important role in reducing the mobility and transport of heavy metals in the environment. In this study, Cd(II) sorption species on surfaces of γ-Al 2 O 3 at pH 7.5 were characterized using multiple techniques. Results show that in addition to adsorption complexes, Cd hydroxide phases (Cd(OH) 2 precipitates and Cd x (OH) y polynuclear complexes) were formed at the initial stages of Cd(II) sorption and gradually transformed to CdCO 3 with time. In addition, Cd(II) formed Cd Al layered double hydroxide (LDH) on γ-Al 2 O 3 under various conditions, independent of temperature and Cd loadings. The formation of Cd hydroxide phases and Cd Al LDH could be ascribed to surface-induced precipitation because the bulk solution was undersaturated with respect to hydroxides. Cd Al LDH formation on the Al-bearing mineral here is rather surprising because typically this occurs with elements of ionic radii similar to that of Al 3+ ; this formation is unknown for metals such as Cd(II) with a much larger ionic radius. The thermodynamic feasibility of Cd Al LDH formation was further confirmed by laboratory synthesis of Cd Al LDH and density function theory (DFT) calculations. These results suggest that Cd precipitation on Al-bearing minerals can be an important mechanism for Cd immobilization in the natural environment. Additionally, the finding of Cd Al LDH formation on Al-bearing minerals and the thermodynamic stability of Cd Al LDH provides new insights into the remediation of Cd-polluted soils and aquatic systems.
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