Effects of Fe(II) on Cd(II) immobilization by Mn(III)-rich δ-MnO2

Manganese (Mn) oxides have high Cd(II) sorption affinities and could effectively immobilize Cd(II) in soil and aquatic systems. However, coexisting Fe(II) can react with Mn oxides at oxic-suboxic interfaces, thereby affecting Cd(II) immobilization by Mn oxides. Mn(IV)-rich Mn oxides in the environment are readily to be reduced to Mn(III)-rich Mn oxides by natural organic matter. Herein, we determined the effects of Fe(II) on Cd(II) immobilization by Mn(III)-rich delta-MnO2 (denoted as HE-MnO2) at pH 5.5 and 7.5. Results show that Fe(II) decreased Cd(II) retention on HE-MnO2 at pH 5.5 but had no effects at pH 7.5 due to the high Cd(II) adsorption affinity of HE-MnO2 at high pH. Poorly crystalline Fe oxides, likely ferrihydrite, uniformly precipitated on HE-MnO2 surfaces upon Fe(II) addition at both pHs. beta-MnOOH formed at the high initial Fe(II) concentration at pH 7.5. Cd (II) was mainly adsorbed on HE-MnO2 rather than the newly formed Fe oxides and beta-MnOOH which had low Cd (II) adsorption capacities. The decrease of Cd(II) sorption in the presence of Fe(II) could be explained by the reduction of HE-MnO2, the precipitation of Fe oxides on HE-MnO2, and the competition of generated Mn(II) for the sorption sites. Cd(II) formed double-corner sharing (DCS) and double-edge sharing (DES) complexes with Mn (III) edge sites on HE-MnO2. After the addition of Fe(II), Cd(II) formed only DCS complexes with Mn(III) edge sites. The alternation of the surface complexes caused by Fe(II) promoted Cd(II) desorption from HE-MnO2. This work suggests that Fe(II) can decrease the removal efficiency of Cd(II) by Mn(III)-rich delta-MnO2 at oxic-suboxic interfaces in the environment.