Stability, transformations and stabilizing potential of an amorphous manganese oxide and its surface-modified form in contaminated soils

A surface-modified amorphous manganese oxide (SM-AMO) was prepared to increase the stability of a previously studied promising stabilizing agent and to compare its immobilizing efficiency with respect to contaminating metals with the original material. To synthesize the SM-AMO, the AMO surface was synthetically covered with a coating of MnCO3 because newly formed rhodochrosite precipitates were previously found to increase the stability of AMO particles in soils. A preliminary experiment evaluating the long-term stability of both materials in pure water suggested higher stability for the SM-AMO particles, showing a smaller release of Mn compared to the original AMO. An adsorption kinetics study focused on As, Cd, Pb and Zn showed lower adsorption rates and adsorption capacity for Zn, probably as a result of partial surface passivation. In comparison to these results for simple controlled systems, different effects were recorded when the two materials were applied to contaminated soils. When incubated in soil, a constantly lower mass loss was recorded in the case of SM-AMO. There were no significant differences in the release of Mn and DOC into the soil solution or in the stabilizing efficiency with regard to contaminating metal(loid)s between the original and surface-modified materials. Concerning the potential solid phase transformations in soil conditions, we observed a gradual equilibration between the surface composition of both materials. While the newly formed rhodochrosite precipitated on the AMO surface, the MnCO3 coatings on SM-AMO gradually dissolved. Both amendments also effectively supported microbial activity, especially in the more contaminated soil sample. Thus, despite the smaller mass loss, the effectiveness of both materials is comparable in the long term.