Abstract
Water and wastewater studies in Malawi have revealed very high levels of heavy metals in most streams and other water bodies particularly within urban areas. The metals are produced and released during industrial and agricultural activities, and also in vehicular emissions. These metals may pose serious threats to both human health and the environment. This study investigated the potential of mixed clay, obtained from the Tundulu area, in removing, Cd2+, Cr3+, Cu2+, Hg2+, Pb2+ and Zn2+ cations and AsO43- anions from aqueous solutions using batch equilibrium technique. Qualitative mineralogical characterisation of the clay revealed that the clay contains illite, distorted kaolinite, mixed layer minerals and non-clay mineral carbonate fluoroapatite. pHpzc for the raw clay, as determined by potentiometric titrations, was 9.66 while pHpzc of pretreated clay was 9.63. Pretreatment of the clay involved removal of carbonates, iron oxides and organic matter. Initial total metal concentrations ranged from 3 to 5 mg/l. pH metal sorption dependence of the clay revealed Cr3+ removal from pH of 3 to complete removal at pH 5 with over 90% of the removal attributable to adsorption on the clay while the remaining 10% attributable to both adsorption and Cr(OH)3 precipitation. Zn2+ complete removal occurred at pH above 7 with 92% attributable to adsorption while the rest could be from both adsorption and hydroxide precipitation. Cu2+ was removed from pH 4 and completely above pH 6.8 with 50% due to adsorption. Cd2+ removal was between pH of 6 and 9 with 85% due to adsorption to the clay. Lead was completely removed at pH greater than 7.67. Removal of Hg2+ at total Hg2+ concentration of 0.023 mM was pH independent fluctuating between 30 and 60%. No effective removal of AsO43- anion was observed. Water SA Vol.32 (4) 2006: pp.519-526
Highlights
Heavy metal water pollution is an area of major concern today in both developed and developing countries
This study investigated the potential of mixed clay, obtained from the Tundulu area, in removing, Cd2+, Cr3+, Cu2+, Hg2+, Pb2+ and Zn2+ cations and AsO43- anions from aqueous solutions using batch equilibrium technique
Qualitative mineralogical characterisation of the clay revealed that the clay contains illite, distorted kaolinite, mixed layer minerals and non-clay mineral carbonate fluoroapatite. pHpzc for the raw clay, as determined by potentiometric titrations, was 9.66 while pHpzc of pretreated clay was 9.63
Summary
Heavy metal water pollution is an area of major concern today in both developed and developing countries. Heavy metals occur naturally in soils, and they are widely used in industry especially in electronics and the rapidly growing information technology sector. Common anthropogenic sources include agricultural activities, atmospheric deposition, road run off, discharges from industrial plants and sewage works, acidic mine effluents and building of reservoirs. Agricultural activities provide important non-point sources of heavy metal pollutants such as cadmium, lead, and iron from phosphatic fertilisers. The presence of high levels of these metals in the environment may cause long-term health risks to humans and ecosystems. It is mandatory that their levels in drinking water, wastewater, and water used for agricultural and recreational purposes must be reduced to within the maximum allowable concentrations recommended by national and international health authorities such as the World Health Organisation (WHO, 2004)
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