Acid mine drainage (AMD) causes acidification of surface waters in combination with the release of trace elements, such as As and U. The presence of iron secondary minerals can be considered of great importance in these areas, since they are naturally occurring scavengers and ultimately, they define the mobility of contaminants. Schwertmannite, Fe8O8(OH)6(SO4)2, a poorly crystalline oxyhydroxysulphate, shows high specific surface area and adsorption capacity towards contaminants such as arsenate and uranyl. We performed adsorption experiments to understand the interaction of U and Arsenic with this mineral. Our batch adsorption experiments showed high adsorption loadings of As and U on schwertmannite, compared to other iron oxyhydroxides such as goethite or jarosite. In the case of Arsenic, the adsorption was correlated to the liberation of sulphate ions to the solution. This suggests an exchange between the oxyanions and the sulphate ions present in the mineral. In the case of Uranium an inner spehere sorption mechanism seemed to be adequate to explain our experiments.<br/>We then studied the fate of As and U upon transformation of Schwermanite to other mineral phases. The long-term immobilization of the contaminants is defined by the thermodynamic stability of the mineral phase they interact with. Schwertmannite is a metastable phase, transforming eventually into more stable phases such as goethite. Wet transformations in systems containing As and U at controlled pH showed that the presence of arsenate and uranyl retarded the transformation of schwertmannite to goethite. The changes in mineral structure, as well as the Uranium and Arsenic speciation upon transformation were followed by XAFS and X-Ray Diffraction. Our results suggest the presence of Fe-U phases and As-sorbed products associated with the newly formed goethite. These results will help elucidate the fate and provide parameters to model the transport of these elements under naturally occurring conditions.

Uranium, arsenic and iron speciation in acid mine drainage environments

2019 Spring National Meeting

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