Metal behavior in sediment associated with acid mine drainage stream: Role of pH

Metal mobilization from sediments in streams, which are severely impacted by the acid mine drainage (AMD), around Makum coalfield in North-eastern India was studied. Sediments in stream close to the collieries show low pH (up to 2.5) and very high electrical conductivity (up to 1816 mu S/cm). The stream sediments mainly consist of quartz, clay minerals, plagioclase with minor amounts of schwertmannite, goethite and jarosite. Concentrations of Cd, Cr, Mn, Ni and Zn exceed their respective crustal abundances in most of the areas. Fractionation study of metals reveals that in moderate to slightly acidic sediments, Cd and Cu are mainly associated with exchangeable and organic fractions respectively; Zn fractionated into Fe-Mn oxyhydroxides while Cr, Ni, Mn, Pb mostly into residual part. Higher concentrations of all metals are available in exchangeable fraction under strongly acidic environment; partitioning of Mn, Ni, Pb, Cu and Zn into exchangeable part is significantly controlled by pH. More Mn and Ni get partitioned into carbonate and Fe-Mn oxyhydroxides phases with increase in pH up to near-neutral value. Cadmium appears to be the most mobile elements. Apparent mobility and potential bioavailability of metals are in the order of Cd>Pb>Mn>Ni >= Zn>Cu>Cr. (c) 2012 Elsevier B.V. All rights reserved.