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Abstract
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Heavy metals can be introduced into natural waters by wastewater effluent from different industries. Among various methods for removal of heavy metals, adsorption is more versatile. Types of adsorbents are sawdust, clays, polymers, biomass and activated carbon. Recently, carbon nanotubes (CNTs) have demonstrated high capacity for adsorbing a wide variety of heavy metals. CNT exists as single-walled (SWCNTs) or multi-walled (MWCNT). Both kinds of CNTs are used in adsorption, but MWCNTs became popular because of its ease of synthesis. However, the experimental results for adsorption of divalent metal ions showed that SWCNTs has over 18% higher adsorption capacity than MWCNTs. The surface nature of CNT can be modified via chemical methods, including purely chemical processes (i.e., chemical oxidation and deposition) and extended chemical process (i.e., electrochemical, sol-gel, microemulsion, and hydrothermal methods). The large number of experimental studies has been devoted on bare and functionalized CNTs. Adsorption of Cd (II) by pristine CNTs and oxidized CNTs, Pb (II) and Ni (II) by carboxylic-functionalized CNTs, Cu (II) by chitosan/poly (vinyl) alcohol membranes modified with amino- functionalized CNTs, Ni (II) and Sr (II) by CNTs/iron oxide magnetic composites and Pb (II) by acid-modified CNTs were investigated. These studies showed that the heavy metal adsorption capacity on CNT surface is improved after modification because of high surface area and adsorption sites of functionalized CNTs.
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