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

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, SO42- release and speciation of SO42- 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 SO42- 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 SO42- 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 SO42- was linear with pH and X-ray photoelectron spectroscopy measurements confirming that the surface bound SO42- was released before bulk SO42-. Zeta potential measurements indicate a pHIEP of 7.2 for the schwertmannite sample. Prior to conversion into goethite, the SO42- 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 SO42- was shown to vary with pH and two predominating species were detected. As pH increases, SO42- is increasingly coordinated in an outer sphere mode whereas a stronger, possibly inner sphere, complex dominates at low pH.