We elucidate the removal mechanism of Cd by birnessite (δ-MnO2) in adsorption and coprecipitation processes in the context of acid mine drainage (AMD) treatments. The removal mechanism was studied through batch removal experiments at different initial Cd/Mn molar ratios (0–2) by zeta potential measurements, X-ray diffraction (XRD), small-angle X-ray scattering (SAXS), and X-ray absorption fine structure (XAFS) analysis. The sorption isotherm and zeta potential measurements suggest that surface complex formation is the dominant mechanism, and that surface precipitation and/or intercalation also occur in the coprecipitation process when the initial Cd/Mn molar ratio is high (1–2). Increasing the initial Cd/Mn molar ratio to above 0.5 decreased the particle size of δ-MnO2 and shifted its (001) XRD peak to lower angles, suggesting that the δ-MnO2 interlayer ((001) and (002) planes) was expanded and the growth of δ-MnO2 crystals was inhibited in the coprecipitation process. The results of XAFS analysis revealed the production of Mn(III) precipitates and surface complex formation with Cd at high Cd/Mn molar ratio condition (<1). No significant changes in the crystalline structures of δ-MnO2 over the entire range of initial Cd/Mn molar ratios were observed in the adsorption process, confirming that Cd could be adsorbed by triple-corner-sharing at neutral pH.
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