Removal of polychlorinated biphenyls from aqueous solutions using β-cyclodextrin grafted multiwalled carbon nanotubes

Abstract

Cyclodextrins have excellent ability in the preconcentration of organic pollutants from aqueous solutions by forming inclusion complexes. Multiwalled carbon nanotubes (MWCNTs) possess high adsorption capacity in the removal of organic pollutants through the formation of conjugated complexes. In this paper, β-cyclodextrin (β-CD) was grafted on the surfaces of MWCNTs by using plasma technique. The β-CD grafted MWCNTs (MWCNT-g-CD) were characterized by using Raman spectroscopy, X-ray photoelectron spectroscopy, X-ray diffraction, thermo gravimetric analysis-differential thermal analysis, and scanning electron microscopy in detail. The prepared MWCNT-g-CD were used to remove polychlorinated biphenyls (PCBs) from aqueous solutions under ambient conditions. The results suggest that MWCNT-g-CD have much higher adsorption capacity than MWCNTs in the removal of PCBs from aqueous solutions. MWCNT-g-CD are suitable materials in the preconcentration and immobilization of PCBs from large volumes of aqueous solutions in environmental pollution cleanup.

Introduction

The occurrence of polychlorinated biphenyls (PCBs) in soils and water systems is currently a major problem of global concern because of their harmful impact on ecosystem health and on the safety of human food supplies (Sawicki and Mercier, 2006). Moreover, PCBs are the typical persistent organic toxins present in the natural environment ubiquitously, and they are highly toxic and resist to degradation and have high bioaccumulation property. Therefore, the remediation of PCB pollutants from natural environment has become a major global problem of the world. Among all kinds of PCBs, the knowledge on the properties of low chlorinated PCBs in the environment are significant for the evaluation of the transport and overall fate of PCBs because of their relatively high aqueous solubility in wastewater. Among the low chlorinated PCBs, dichlorobiphenyls and trichlorobiphenyls are the primary congeners of PCBs in the natural environment (Adeel et al., 1997). Thereby, 4,4′-dichlorobiphenyls (4,4′-DCB) and 2,3,3′-trichlorobiphenyl (2,3,3′-TCB) herein are selected as the models of PCBs.

Cyclodextrins (CDs) are cyclic oligosaccharides formed from the degradation of starch by bacteria and have hydrophobic interior cavity and hydrophilic periphery face, which have the tendency to form inclusion complexes with typical organic pollutants, such as PCBs, chlorinated hydrocarbons, polycyclicaromatic hydrocarbons (PAHs), and explosives (Wang et al., 1998, Tick et al., 2003, Wu et al., 2004, Sawicki and Mercier, 2006, Villaverde et al., 2006, Galia et al., 2007, Kida et al., 2008). The hydrophilic periphery of CDs increases the apparent aqueous solubility of low polarity organic compounds (such as PCBs, PAHs, and organic solvents) as well as enhancing their desorption properties and transport in soil (Brusseau et al., 1997, Boving et al., 1999, Ko et al., 2000, Villaverde et al., 2005, Field and Sierra-Alvarez, 2008). The hydrophilic periphery is really convenient for the need of carrying out the applications of CDs in the removal of organic molecules from large volumes of aqueous solutions (Wu et al., 2004). Moreover, CDs can decrease the microbial toxicity of organic compounds (such as pesticides, phenanthrene, and polychlorinated biphenyls), which results in the enhancement of the biological detoxification (Bizzigotti et al., 1997, Wang et al., 1998, Field and Sierra-Alvarez, 2008). Due to these favorable physicochemical properties, CDs have recently been used in environmental applications to improve the remediation of contaminated soils and groundwater (Wang et al., 1998, Villaverde et al., 2006). The adsorption behaviors of PCBs on CDs are important for the understanding and evaluation the remediation effect of CDs in the removal of PCB pollution in the environment. Kida et al. (2008) reported that γ-CD was an effective adsorbent to remove chlorinated aromatic compounds (including PCBs) from insulating oil via the formation of inclusion complexes. However, few literatures are available to understand the adsorption of PCBs with small molecules by CDs (Hunt et al., 2007).

Carbon nanotubes (CNTs) are relatively new adsorbents and they have been proven to possess excellent adsorption capacity to eliminate many kinds of organic pollutants (Long and Yang, 2001, Gotovac et al., 2006, Chen et al., 2007) and heavy metal ions (Chen and Wang, 2006, Tan et al., 2008, Chen et al., 2009) from large volumes of wastewaters. The results suggested that CNTs are very suitable material in the removal of organic and inorganic pollutants from large volumes of aqueous solutions. However, the inherent insolubility of CNTs in most organic and aqueous solvents limits the manipulation of CNTs in solution phase, which greatly hinders the application of CNTs in real work (Qin et al., 2004, Hong et al., 2006). Therefore, researches have been extended to include the modification of CNTs by introduce special functional groups on the surfaces of CNTs to enhance the adsorption capacity of CNTs and to improve the dispersion property in aqueous solutions. As mentioned above, CDs can remove organic pollutants effectively through the formation of inclusion complexes. However, CDs can not be used directly to remove organic pollutants because of the excellent solubility in aqueous solutions. If the CD molecules are grafted on the surfaces of CNTs, the solubility of CDs in aqueous solutions is negligible because the CD molecules are connected together with the insoluble CNTs. Plasma induced grafting treatment is a promising method to introduce functional groups to CNT surfaces without altering the properties of CNTs, and the plasma grafted functional groups to the substance surfaces can enhance the chemical functionality. Comparing to the chemical modification method, plasma technique is also an environmental friendly method to graft functional groups on CNTs because no chemicals are used in the process (Shao et al., 2009). Therefore, CNTs modified with CDs can improve the compatibility of CNTs and extend the application of CNTs and CDs in the removal of organic pollutants from large volumes of aqueous solutions in the environmental pollution cleanup.

Herein, we firstly synthesized the multiwalled carbon nanotubes (MWCNTs) grafted with β-CD (denoted as MWCNT-g-CD) by N₂ plasma induced grafting technique. The prepared MWCNT-g-CD were applied to remove 4,4′-DCB and 2,3,3′-TCB from aqueous solutions to evaluate their potential application in the removal of PCBs in the environmental pollution cleanup.