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 S2-, MnO2 and Cd2+ were visualized by atomic force microscopy (AFM). The dissolution of MnO2 was in-situ studied by AFM in the S2-containing environments. Moreover, in the ternary system (S2-, 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 S2- 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 S2-, MnO2 and Cd2+ ternary system and shed light on the development of Cd pollution remediation methods for paddy soils.