1. Department of Chemistry, Maseno University, P.O. Box 333-40105, Maseno, Kenya
2. Department of Geochemistry and Environmental Chemistry, School of Chemistry and Material Science, Technical University of Kenya, P.O. Box 52428-00200, Nairobi, Kenya
3. School of Chemistry and Physics, University of KwaZulu-Natal, Private Bag X01, Scottsville, Pietermaritzburg 3209, South Africa