Visualization the fixation of cadmium on manganese dioxide in sulfur reduction environments

Manganese oxides as common soil components were considered as an important sink for the cadmium pollution, which, however, would be affected by the reductive sulfide introduced during the flooding period of paddy soil. In this study, the phase transitions caused by the reactions among S^2-, MnO₂ and Cd²⁺ were visualized by atomic force microscopy (AFM). The dissolution of MnO₂ was in-situ studied by AFM in the S^2-containing environments. Moreover, in the ternary system (S^2-, MnO₂ and Cd²⁺), the pre-adsorption of Cd²⁺ by the MnO₂ nanosheets would promote the subsequent precipitation of CdS on the surface of MnO₂, while the pre-formed CdS nanoparticles in the aquatic phase would tend to suspense rather than precipitating on MnO₂. The kinetic study results indicated that the CdS crystallite generation rate was faster than the MnO₂ dissolution rate in the aquatic environments with different sulfide contents. In the macroscopic Cd²⁺ fixation test, the introduction of S^2- dramatically improved the fixation of the pre-adsorbed Cd²⁺ on the MnO₂ nanosheets by forming the CdS precipitate. This study provided a fundamental understanding of the interactions among the S^2-, MnO₂ and Cd²⁺ ternary system and shed light on the development of Cd pollution remediation methods for paddy soils.