Due to the increasing awareness of toxic heavy metals contamination to the environment, studies of metal accumulation from the view point of metal removal from contaminated water have been performed (Kuyucak and Volesky, 1988;Darnall et al., 1986). Conventional methods including precipitation, oxidation, reduction, ion exchange, filtration, electrochemical treatment, membrane technologies and evaporation recovery are expensive or ineffective, especially when the metal concentrations are very low in the order of 1 to 100 mg/L (Volesky, 1990). The use of biological systems for removing metals from low metal solution has the potential to achieve greater performance at lower cost.Aquatic plants and/or algae are known to accumulate metals and other toxic elements from contaminated water (Wolverton and McDonald, 1975;Muramoto and Ohi, 1983;Green and Bedell, 1990;Maeda and Sakaguchi, 1990;Wilde and Benemann, 1993). The bioremoval process using aquatic plants often exhibits a two-stage uptake process: an initial fast, reversible, metal-binding process (biosorption), followed by a slow, irreversible, ion-sequestration step (bioaccumulation). The initial metal biosorption by different parts of cells can occur via complexation, coordination, chelation of metals, ion exchange, adsorption and microprecipitation. The bioaccumulation process is an active mode of metal accumulation by living cells. This process is dependent on the metabolic activities of the cell, which in turn can be affected by the presence of the metallic ions (Wilde and Benemann, 1993).The bioremoval application merits consideration when comparing with other methods (Kuyucak, 1990).The advantages of bioremoval of metals are:1. Metals at low concentration can be selectively removed to meet regulatory level. This could serve as a polishing step after the treatment of effluents from conventional process, or for treatment of contaminated surface or groundwater with low levels of contamination.
