ABSTRACT
Due to the rapid urbanization in the last decade, most water bodies are becoming contaminated with untreated industrial effluents containing heavy metals and other toxic materials. On the other hand, heavy metals are important strategic resources, as well as essential raw materials in the field of advanced functional materials. For example, copper compounds are widely used in the manufacturing of fungicides, metal plating, wood pulp production, antifouling paints, and electrical and semiconductor devices. Nickel compounds are used as high-performance catalysts. Living organisms also require trace amounts of essential heavy metals (Co, Cu, Fe, Mn, Mo, Sr, and Zn), but their excessive levels can be detrimental when they get passed up the human food chain. Also, nonessential heavy metals (Hg, Cr, Cd, As, Pb, and Sr) originating from natural sources, mining, industrialization, and so on are considered to be a greater threat to public health when mixed into fresh water supplies. For instance, excess copper would cause gastrointestinal distress in short-term exposure, and liver or kidney damage in long-term exposure. The World Health Organization (WHO) reported that drinking water containing zinc at levels above 3 mg L−1 can cause an undesirable astringent taste, and the U.S. Environmental Protection Agency has set a maximum allowable level of zinc at 5 mg L−1. In general, the toxicity of a heavy metal is attributed to its inhibition and reduction of various enzymes, complexation with certain amino acid ligands, and substitution of essential metal ions from enzymes (Hall et al. 2002). Toxicological studies have found that the degree of heavy metal toxicity depends on its oxidation state (Babel and Kurniawan 2003). In spite of government regulations, many industries still allow their effluents into the public sewers or drinking water sources like rivers and lakes without adequate treatment to save their operational costs. Contamination of drinking water with toxic metal ions such as Ag+, Hg2+, Pb2+, Ni2+, Co2+, Cu2+, Cd2+, As3+, Cr3+, Cr4+, and As5+ is fast becoming a severe environmental and public health problem, and there is an urgent need to find efficient and cost-effective methods for their removal from water resources.
