Abstract

Surface waters in many cities are known to be highly polluted due to, among other reasons, incompletely treated municipal and industrial wastewaters that are discharged into streams. This thesis concerns (i) some recent physicochemical water quality data in streams (Limbe, Mudi and Nasolo) and wastewater treatment plants (WWTP) (Limbe WWTP, Soche WWTP) in Blantyre City in Malawi and (ii) heavy metal sorption using alkaline mixed clay minerals, Moringa oleifera and Moringa stenopetala seed extracts of water and sodium chloride. The levels of lead, cadmium, iron, manganese, zinc, chromium and nickel were much higher than the World Health safe limits for drinking water in all the sampled streams which passed through industrial areas. Although the nitrates and sulphates levels at all sampling points were lower than the safe limits for drinking water of 50 mg/L and 250 mg/L respectively, the phosphate levels were significantly above the safe limit (0.5 mg/L). The biochemical oxygen demand (BOD5) was also above the World Health Organisation limit (20 mg/L) at all sites except Mudi and Limbe streams before passing through industrial areas; thus, streams in the city are significantly polluted. Mixed alkaline clays from Tundulu area in Malawi have shown great potential in sorbing chromium(III), copper(II), zinc(II), cadmium(II), mercury(II) and lead(II) cations within characteristic pH ranges. Using an initial total metal concentrations of about 5 mg/L and clay dose of 0.015g per 10.5 ml of metal solution chromium(III) sorption occurred from pH of 3 to complete removal at pH 5. Complete removal of zinc(II) occurred at pH above 7 while copper(II) was sorbed at pH between 4 and 6.8. The removals of cadmium(II) and lead(II) were achieved at pH between 6 and 9, and >7.7 respectively where as the removal of mercury(II) was pH independent, ranging from 30 to 60% removal. Qualitative mineralogical characterisation of the mixed clays by Powder X-Ray Diffraction (PXRD) revealed presence of illite, distorted kaolinite, mixed layer minerals and non-clay mineral carbonate fluoroapatite. The pH point of zero charge was above 9.6. Extended X-ray Absorption Fine Structure (EXAFS) analyses of the metal sorbed clays revealed that oxygen atoms occupy the first coordination shells in all the studied metals. The metal species on the clay mineral surfaces seem therefore to be hydrolyzed as adsorbates and/or precipitates. Crude water and sodium chloride extracts of partially defatted powder of Moringa stenopetala (MS) and oleifera (MO) showed complete removal of chromium(III), copper(II), zinc(II) and cadmium(II) from single ion solution at initial metal concentrations of about 4 mg/L at different pH ranges for each metal. Using proton nuclear magnetic resonance, the Moringa extracts showed clear presence of amide (-CO-N-H), benzenoid (Ar-H), saturated alkyl and unsaturated fragments in both MS and MO. EXAFS measurements of the metal rich Moringa extracts indicated bonding between the metals and the oxygens of carboxylate and amide groups.

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School	: School of Natural and Applied Sciences
Issued Date	: 2008

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