Adsorption of Synthetic Organic Contaminants by Carbon Nanotubes: A Critical Review PDF

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w a t e r r e s e a r c h 6 8 ( 2 0 1 5 ) 3 4 e5 5 Available online at www.sciencedirect.com ScienceDirect journal homepage: www.elsevier.com/locate/watres Review Adsorption of synthetic organic contaminants by carbon nanotubes: A critical review Onur Guven Apul, Tanju Karanfil* Department of Environmental Engineering and Earth Sciences, Clemson University, 342 Computer Court, Anderson, SC 29625, United States article info abstract Article history: In last ten years, a large number (80þ) of articles regarding aqueous phase adsorption of a Received 27 May 2014 variety of synthetic organic compound (SOC) by CNTs were published in peer-reviewed Received in revised form journals. Adsorption depends upon the physicochemical properties of the adsorbates 23 September 2014 and CNTs as well as the background water chemistry. Among all properties reported in the Accepted 24 September 2014 literature, no parameter was reported as solely controlling SOC adsorption by CNTs. In this Available online 5 October 2014 article, these contributing parameters were reviewed and the associated explanations were discussed. This comprehensive literature survey provides (i) a thorough CNT character- Keywords: ization summary, (ii) a discussion of adsorption mechanisms of SOCs by CNTs and (iii) a Adsorption summary of the statistical adsorption model development efforts. It also includes dis- Carbon nanotube cussions of agreements and differences in the literature, and identifies some research Nanoparticle needs. Predictive model © 2014 Elsevier Ltd. All rights reserved. Sorption Synthetic organic compound Contents 1. Introduction................................................................................................. 35 2. General features of SOC adsorption by CNTs.................................................................... 35 3. Characterization of CNTs..................................................................................... 36 4. Adsorption of SOCs by CNTs.................................................................................. 42 4.1. Influence of CNT properties............................................................................. 42 4.2. Influence of SOC properties............................................................................. 42 4.3. Influence of background solution properties.............................................................. 45 5. Predictive models for adsorption of SOCs by CNTs.............................................................. 47 6. Comparison of carbon nanotubes with activated carbon......................................................... 49 7. Future research needs........................................................................................ 50 8. Conclusions.................................................................................................. 51 Acknowledgments............................................................................................ 51 * Corresponding author. Tel.: þ1 864 656 1005; fax: þ1 864 656 0672. E-mail address: [email protected] (T. Karanfil). http://dx.doi.org/10.1016/j.watres.2014.09.032 0043-1354/© 2014 Elsevier Ltd. All rights reserved.  w a t e r r e s e a r c h 6 8 ( 2 0 1 5 ) 3 4 e5 5 35 Supplementary data.......................................................................................... 51 References................................................................................................... 51 adsorbents to conventional activated carbons in water and 1. Introduction wastewater treatment systems. The main objective of this article was to critically review Graphitic carbon is sp2 hybridized solid phase of pure carbon the published peer-reviewed literature, and to articulate some where three of the four valance electrons are covalently future research needs on the aqueous phase adsorption of shared in a two-dimensional plane and the fourth valance SOCs by CNTs with (i) a thorough CNT characterization sum- electron is delocalized among all atoms present as a weak p mary, (ii) a discussion of adsorption mechanisms of SOCs by bond in the third dimension (Ajayan, 1999; Terrones, 2003, CNTs and (iii) a summary of the statistical adsorption model 2004). Carbon nanotubes (CNTs) can be visualized as development efforts. graphitic carbon sheets rolled into hollow cylinders with nanometer scale diameters and micrometer scale lengths (Terrones, 2003, 2004; Iijima, 1991). There are two types of CNTs: single-walled (SWCNT) and multi-walled (MWCNT) 2. General features of SOC adsorption by (Ajayan, 1999). Owing to their unique properties, the produc- CNTs tion and use of CNTs has been growing rapidly (Lam et al., 2006). The CNT market estimates were approximately 90.5 Adsorption of SOCs from water by activated carbon and other million dollars in 2010, and global revenues are projected to porous carbon materials has been studied for several decades exceed 1 billion dollars by 2015 (The Global Market for Car, (Dobbs and Cohen, 1980; Giusti et al., 1974; Moreno-Castilla, 2010). CNTs are now synthesized at larger scales (i.e. more 2004; Kutics and Suzuki, 1993). On the other hand, adsorp- than 3000 metric tons/year), and used in many electronic, tion of SOCs by CNTs has been rapidly growing (Iijima, 1991; medical, space and military applications (Klaine et al., 2008; Yang et al., 2006b). More than 80 articles have been pub- Mauter and Elimelech, 2008; Keller et al., 2013). In addition, lished in peer-reviewed journals on SOC adsorption by CNTs superior hydrophobicity, high specific surface area, and hol- over the last ten years. These studies included the adsorption low and layered structures of CNTs make them also particu- of a multitude of organic contaminants by CNTs: polycyclic larly promising adsorbents (Upadhyayula et al., 2009; Zhang aromatic hydrocarbons (PAHs) (Yang et al., 2006a, 2006b; Yang et al., 2011). Adsorption of many compounds in water such and Xing, 2007; Zhang et al., 2010a), benzene derivatives as various classes of synthetic organic contaminants (Gotovac (Wang et al., 2008; Chen et al., 2007, 2008a; Peng et al., 2003), et al., 2006, 2007a, 2007b; Yang et al., 2006a; Yang et al., 2006b; phenolic compounds (Lin and Xing, 2008a; Shen et al., 2009; Cho et al., 2008; Wang et al., 2008; Chen et al., 2007; Lin and Yang et al., 2008; Salam and Burk, 2008; Chen et al., 2009a), Xing, 2008a; Oleszczuk et al., 2009; Ji et al., 2009a; Gupta pharmaceuticals (Chen et al., 2011; Ji et al., 2010a; Peng et al., et al., 2013; Pyrzynska et al., 2007) (see Table S1 in 2012; Yang et al., 2012; Zhang et al., 2010b; Fu et al., 2011; Ji Supplementary Information), natural organic matter (NOM) et al., 2010b; Cai and Larese-Casanova, 2014; Li et al., 2014; (Su and Lu, 2007; Yang and Xing, 2009; Ajmani et al., 2014; Yu et al., 2014), polychlorinated biphenyls (Velzeboer et al., Smith et al., 2012), and metallic ions (Li et al., 2005, 2003; Rao 2014; Beless et al., 2014), dialkyl phthalate esters (Wang et al., 2007) by CNTs has been widely reported. et al., 2014), proteins (Kharlamova et al., 2013), insecticides From a natural system perspective, the remarkable in- (Peng et al., 2009), herbicides (Pyrzynska et al., 2007; Chen crease in production and use of CNTs raises health and envi- et al., 2009b), organic dyes (Gupta et al., 2013), aliphatics (Lu ronmental concerns, particularly upon release into the et al., 2006; Wang et al., 2010a) and dioxin (Long and Yang, environment (Lam et al., 2006; Powell and Kanarek, 2006; 2001). Johnston et al., 2010; Petosa et al., 2010). CNTs may enter the Multiple mechanisms, of varying relative importance, have environment through either intentional or unintentional re- been proposed to control the adsorption of SOCs by CNTs. The leases (e.g. atmospheric emissions and solid or liquid waste quantification of these individual contributions is a chal- streams) from production facilities, causing damage to plant lenging task, and it has yet to be addressed in any significant and animal life at the cellular level (Lam et al., 2006; Klaine fashion in the current CNT adsorption literature. However, et al., 2008; Petosa et al., 2010). The toxicity of CNTs may some previous studies indicated that certain parameters are also be enhanced by the adsorbed organic contaminants in more predominant in controlling adsorption than others. the environment (Ferguson et al., 2008; Xia et al., 2010a; Yang According to Yang et al. (2006b), Kow of SOCs were strongly and Xing, 2007). Therefore, it is of critical importance to un- correlated with adsorption capacity of CNTs. Evidently, hy- derstand adsorption of SOCs by CNTs to adequately assess drophobic driving forces play important roles; however they their environmental impact. From an engineering perspective, cannot completely explain adsorption. Electrostatic in- understanding the adsorption of SOC by CNTs is also impor- teractions, pep interactions and hydrogen bonding also in- tant for evaluating the feasibility of using CNTs as alternative fluence adsorption interactions considerably (Chen et al., 36 w a t e r r e s e a r c h 6 8 ( 2 0 1 5 ) 3 4 e5 5 2007; Wang et al., 2014). According to the mechanism pro- SOC molecules for adsorption sites (Long and Yang, 2001; Liu posed by Chen et al. (2007), p-electron donor and acceptor et al., 2014). interactions influence adsorption of aromatic SOCs by CNTs. The aqueous phase adsorption of SOCs by CNTs depends Similarly, among CNT parameters, specific surface area was on the physicochemical properties of the adsorbate and CNTs reported to be a controlling parameter of adsorption capacity, as well as the background water chemistry (Ma et al., 2011). In though not the only controlling factor (Zhang et al., 2010a). order to simplify the complicated nature of intermolecular Pore volume, pore size distribution and functional groups of adsorption interactions, to gain a more comprehensive insight CNTs were also influential on adsorption. As a result, into the adsorption mechanism, understand the interactions describing SOC adsorption by CNTs may require considering of CNTs and SOCs and systemize the available literature, the multiple adsorption mechanisms and factors (Yang et al., influence of adsorbent (CNT), adsorbate (SOC) and background 2006b; Cho et al., 2008; Wang et al., 2008; Chen et al., 2007; solution were investigated separately, as detailed in the Lin and Xing, 2008a). following sections. A literature review of the characteristics of In addition to the already complicated nature of SOC CNTs is also provided below prior to the review of adsorption adsorption, CNTs are prone to aggregation. Aggregation mechanisms. (homo- and hetero-) is a characteristic that differentiate CNTs from other carbonaceous adsorbents, further complicating the adsorption properties. According to Saleh et al. (2008), 3. Characterization of CNTs CNTs exhibited DerjaguineLandaueVerweyeOverbeek (DLVO) interactions that are predominantly similar to most Adsorption of organic contaminants is influenced by both CNT other aqueous colloidal particles. Aggregation may reduce the morphology and surface chemistry. To examine the CNT surface area during the formation of interstitial channels be- properties, a comprehensive literature survey was conducted tween nanotubes and grooves on the periphery of the bundles. and tabulated in Table 1. The compiled information indicated The outermost surface, interstitial channels, inner cavities that the properties of two types of CNTs (i.e. SWCNT and and grooves are the four proposed adsorption sites for CNT MWCNT) varied significantly in literature. In the 57 journal bundles (Fig. 1) (Agnihotri et al., 2006, 2008). The SOCs first articles reviewed, adsorption of SOCs was investigated using attach to high-energy adsorption sites and then spread to 18 SWCNTs and 81 MWCNTs as adsorbents. Five studies, lower energy sites among all available sorption sites however, did not report the type of CNT used. The abundance (Agnihotri et al., 2008). of MWCNT in these adsorption studies was likely due to the In natural waters, the behavior of CNTs and the in- higher market prices of SWCNTs than MWCNTs. A compila- teractions between organic compounds and CNTs are further tion of prices in 2013 from 44 commercially available lab grade complicated by the presence of natural organic matter (NOM) CNT products revealed that the average market price for (Zhang et al., 2011). The suspension stability of CNTs in MWCNTs was approximately 10 $/g, and the average price for aqueous solutions may be improved by the presence of NOM SWCNTs was higher than 100 $/g. However, it should be noted due to the increased electrostatic and steric repulsion among that obtaining cheaper SWCNTs and MWCNTs (e.g., 45 $/g and NOM-coated nanotubes, while NOM molecules compete with 1.5 $/g, respectively) is possible depending on the purity and properties sought (outer diameter, oxygen content etc.). In 2008, Cho et al. reported the average price of MWCNT as 140 $/g, which shows the remarkable decrease of its price recently. In the articles reviewed, approximately 30 different CNTs were synthesized in the lab, whereas remaining 75 CNTs were obtained from manufacturers, indicating the widespread use of commercially available CNTs. It should be noted that the activated carbon is available for less than 5 $/kg (Babel and Kurniawan, 2003), thus the price of CNTs should significantly decrease to be competitive with activated carbons. The most common CNT characteristics reported in the adsorption literature were specific surface area (SSA), pore volume (PV), pore size distribution (PSD), purity, elemental analysis (EA) and morphological information such as length, the inner and outer diameter and the number of walls and types of functional groups. The mean SSA of SWCNTs and MWCNTs were 373 ± 146 m2/g and 216 ± 159 m2/g, respectively (calculated from the data tabulated in Table 1). Considering various manufacturing, purification, and surface modification techniques, high standard deviations of SSA are reasonable. The theoretical SSA of SWCNTs with open ends was calcu- Fig. 1 e Schematic representation of a typical CNT bundle lated as 2630 m2/g (Peigney et al., 2001). The theoretical SSA and its adsorption sites (1) inner cavities, (2) interstitial calculations assumed that CNTs were composed of perfect channels, (3) external grooves, (4) outermost surface sheets of carbons that covalently form hexagonal arrays. If the (Agnihotri et al., 2006). ends were closed, the inner cavity of the tube would be Table 1 e Summary of CNTs employed for SOC adsorption in the literature, their characteristics and characterization methods. Source Adsorbent Supplier Pretreat. Surface area Pore volume Inner diameter Outer diameter Length Purity name Value Mthd Value Mthd Value Mthd Value Mthd Value Mthd Value (%) Mthd (m2/g) (cm3/g) (nm) (nm) (mm) Yang et al. (2006b); Yang MWCNT 8 Chengdu, China HNO3, H2SO4 348 BET 0.816 BET 2e3 TEM