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

Abstract

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, SO 4 2 - release and speciation of SO 4 2 - associated with the solid. The elemental analysis gave the composition Fe 8O 8(OH) 5.02(SO 4) 1.49 · 0.5H 2O where approximately 1/3 of the SO 4 2 - 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 SO 4 2 - 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 SO 4 2 - was linear with pH and X-ray photoelectron spectroscopy measurements confirming that the surface bound SO 4 2 - was released before bulk SO 4 2 - . Zeta potential measurements indicate a pH IEP of 7.2 for the schwertmannite sample. Prior to conversion into goethite, the SO 4 2 - 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 SO 4 2 - was shown to vary with pH and two predominating species were detected. As pH increases, SO 4 2 - is increasingly coordinated in an outer sphere mode whereas a stronger, possibly inner sphere, complex dominates at low pH.