Schwertmannite precipitated from acid mine drainage: Phase transformation, sulphate release and surface properties.

Jönsson, J., Persson, P., Sjöberg, S. and Lövgren, L.
Applied Geochemistry


Schwertmannite precipitated from acid mine drainage at the Kristineberg Zn–Cu mine in northern Sweden has been characterised regarding elemental composition, phase transformation as a function of pH and time,  kemi release and speciation of  kemi associated with the solid. The elemental analysis gave the composition Fe8O8(OH)5.02(SO4)1.49 · 0.5H2O where approximately 1/3 of the  kemi is adsorbed to the surface. The conversion of schwertmannite to goethite at pH 9 was complete within 187 days; at pH 6, the conversion was still incomplete after 514 days. Lower pH and relatively high  concentration decreased the conversion even further. Also temperature was shown to be an important parameter for this process and low temperature (+4 °C) effectively stopped the transformation at pH 3. The release of  kemi was linear with pH and X-ray photoelectron spectroscopy measurements confirming that the surface bound  kemi was released before bulk kemi . Zeta potential measurements indicate a pHIEP of 7.2 for the schwertmannite sample. Prior to conversion into goethite, the  kemi associated with schwertmannite was indicated by attenuated total reflectance FTIR spectroscopy to be present both as bulk and surface species. Furthermore, the speciation of surface  kemi was shown to vary with pH and two predominating species were detected. As pH increases,  kemi is increasingly coordinated in an outer sphere mode whereas a stronger, possibly inner sphere, complex dominates at low pH.