1029 † To whom correspondence should be addressed. E-mail: amohebbi2002@yahoo.com, amohebbi@mail.ac.ir Korean J. Chem. Eng., 28(4), 1029-1034 (2011) DOI: 10.1007/s11814-010-0463-5 INVITED REVIEW PAPER Mercury(II) removal from aqueous solutions by adsorption on multi-walled carbon nanotubes Mehdi Jamshidi Shadbad, Ali Mohebbi † , and Ataallah Soltani Department of Chemical Engineering, College of Engineering, Shahid Bahonar University of Kerman, Kerman, Iran (Received 14 February 2010  accepted 25 October 2010) Abstract−Our aim was to test how MWCNTs can be used as a new adsorbent for mercury(II). Multi-walled carbon nanotubes (MWCNTs) have been used for removal of mercury from aqueous solutions. Mercury removal from aqueous solutions by batch adsorption was investigated. Equilibrium isotherms, such as Freundlich, Langmuir, Temkin, Harkins- Jura, were tested. Kinetic studies based on Lagergren first-order, pseudo-second-order and Elovich rate expressions were done. The batch experiments were conducted at three different temperatures (17, 27 and 37 o C) and different pHs of the initial solution. Error function analysis shows that mercury(II) removal obeys pseudo-second order kinetics and Freundlich isotherm equation. Finally, the effects of solution pH and temperature on the adsorption were studied. Key words: Adsorption, Kinetic, Mercury, Multi-walled Carbon Nanotubes, Equilibrium Isotherms INTRODUCTION Mercury(II) is one of the most toxic contaminants affecting the environment. Increasing mercury in both anthropogenic and bio- genic emissions of mercury over the last fifty years has resulted in serious potential health risks [1]. A global mercury mass balancing model showed that 34% of the atmospheric mercury originates from coal burning [2]. Consequently, removal of mercury in water and wastewater is important. Among several methods for the removal of heavy metals from solutions such as precipitation, evaporation, electroplating, ion ex- change and membrane separation, adsorption proves to be an effi- cient and cost-effective method strongly recommended for low con- centration [3], Many adsorbents such as hollow fiber supported liquid membranes (HFSLM) [4], biomaterials [5-8], Xanthate [9], Zeolites [10], low cost natural materials [11,12], activated carbon [13-15] were used for heavy metal removal from solutions. During the last few years, there has been a growing interest in the use of nanomaterials for the sorption and preconcentration of heavy metals from water. Multi walled carbon nanotubes (MWCNTs) are one of the new adsor- bents with high capacity for metal sorption due to large specific area. The MWCNTs can be thought of as cylindrical hollow micro- crystals of graphite with have strong adsorption ability, exceptional mechanical properties, a unique electrical property, high chemical stability, a large specific surface area [16,17], and high adsorption capacity. With such unique characteristics, MWCNTs have attracted researchers’ interest as a new type of adsorbent and have offered an attractive option for the removal of organic and inorganic con- taminants from water [18]. The highly developed hydrophobic sur- face of CNTs exhibits strong sorption properties toward various com- pounds. However, studies on the adsorption of heavy metals with MWCNTs are still very limited in the literature [19]. Tawabini et al. [20], with limited data, recently studied the efficiency of mercury(II) removal by multi-walled carbon nanotubes. They showed that mercury ad- sorption by MWCNTs follows a pseudo-second-order reaction and it is well described by the Langmuir isotherm. In this work, a detailed study was performed, including the equilibrium, kinetics and ther- modynamics of batch adsorption for mercury(II) removal from aque- ous solutions by MWCNTs in three different temperatures. The effect of initial pH of mercury(II) solution was also investigated [20]. METHODOLOGY 1. Materials Multi-walled carbon nanotubes, which were produced by chem- ical vapor deposition (CVD) of methane at the Research Institute of the Petroleum Industry, Iran, at a temperature of 1,000 o C using cobalt-molybdenum supported on magnesium oxide (Co-Mo/MgO), specific surface area of 280 m 2 /g, and purity above 95% with no additional groups, were selected as an adsorbent. The SEM photo- graph of MWCNT is shown in Fig. 1. As one can see, the outer diam- eter of MWCNT (d p ) is less than 10 nm (average d p =8 nm), the length Fig. 1. SEM photograph of MWCNT.