1. Department of Environmental, Water and Earth Sciences, Faculty of Science , Tshwane University of Technology , Private Bag X680 , Pretoria , South Africa
2. Department of Chemical, Metallurgical and Materials Engineering and Institute of NanoEngineering Research (INER) , Tshwane University of Technology , Pretoria West Campus, Staatsartillerie Rd , Pretoria 0183 , South Africa
3. Laboratory for Microscopy and Microanalysis, Faculty of Natural and Agricultural Sciences , The University of Pretoria , Room 1-31, Building NS-2, Private Bag X20 , Hatfield 0028 , South Africa