Heavy metal partitioning in sediments from rivers flowing through coal fields in Mpumalanga, South Africa

Abstract

The association of elements Cd, Co, Cr, Fe, Mn, Pb, Ti and V with various geochemical phases in the sediments from the Olifants, Klein Olifants, Wilge rivers and a tributary of the Olifants River was studied using a four step sequential extraction scheme. By employing enrichment factors these elements were found to be contaminating the sediments. Sequential extraction enabled partitioning of the metals into exchangeable, reducible, oxidizable and residual fractions. Most of the elements were found to exist in the residual fraction, characterized by stable compounds. Application of risk assessment code (RAC) to the exchangeable fraction revealed that most of the elements posed a medium risk to aquatic life, with the exception of Co, Pb, and Mn which were classified into the high risk category. Non-residual/more bioavailable fractions were examined using statistics. Correlation analysis was employed to understand the interaction between the more bioavailable fractions of the metals with the reducible phase consisting of oxides of Fe-and Mn. These oxides contribute to the adsorption of trace metals onto sediments. Elements Co, V, Pb, Cr and Cd in the reducible fraction were found to be associated with Fe-oxides, while some V, Cr and Ti were associated with Mnoxides, as indicated by significantly high correlation coefficients. Through cluster and factor analysis three anthropogenic activities associated with mining and use of coal and iron and steel manufacturing were found to be contributing metals to the sediments.