1. Department of Materials Science and Engineering, Nelson Mandela African Institution of Science and Technology, P.O. Box 447, Arusha, Tanzania
2. Department of Chemical Engineering, Hanyang University, 1271 Sa 3-dong, Sangnok-gu, Ansan-si, Gyeonggi-do 426-791, Republic of Korea
3. Chemistry Department, University of Dar es Salaam, P.O. Box 35091, Dar es Salaam, Tanzania