1. 1Department of Ecology and Resource Management, School of Environmental Science and Engineering, University of Venda for Science and Technology, P/Bag X5050, 0950, Thohoyandou, Limpopo Province, South Africa; School of Chemistry, University of the Witswatersrand, Private Bag 3, Wits 2050, Johannesburg, South Africa Department of Analytical Chemistry, Lund University, P.O.Box 124, S-22100 Lund, Sweden