1. Department of Natural Resources and Materials, Botswana Institute for Technology Research and Innovation, Gaborone, Botswana
2. Department of Chemical Engineering, University of South Africa, P/Bag X6, Florida, Johannesburg, 1710, South Africa
3. Department of Chemical Sciences, University of Johannesburg, PO Box 524, Auckland Park, 2006, South Africa
4. Department of Engineering, University of Cambridge, Trumpington Street, CB2 1PZ, UK