ABSTRACT
The heavy metals are grouped into three main categories: toxic metals, radionuclides, and precious metals (Wang and Chen, 2006, 2009). Mercury, chromium, lead, copper, nickel, cadmium, and cobalt are listed under the category of toxic metals. The metals such as uranium, thorium, radium, and polonium are included under radionuclides. Precious metals include platinum, palladium, gold, and silver. There are three methods for the removal of metal ion from aqueous solution: physical, chemical, and biological technologies. Conventional methods have been practiced for many years for the removal of metal ions from various aqueous solutions (such as chemical precipitation, chemical and electrocoagulation, filtration, ion exchange, electrochemical treatment, membrane technologies, adsorption on activated carbon, zeolite, and evaporation). However, its operation is restricted nowadays due to various limitations and drawbacks. The membrane technologies and activated carbon adsorption process cannot be adopted for large-scale operations because they are extremely expensive and only a low concentration of heavy metals can be treated. The various other advantages and disadvantages of the conventional metal removal technologies are summarized by Volesky (2001). In order to improve the competitiveness of industrial operation processes, it is essential to develop and implement a cost-effective process for removal/recovery of metals. The various drawbacks such as high cost, less efficiency, release of secondary pollutants, etc. have led to the evolution of novel and sophisticated separation technologies (Volesky and Naja, 2007).
