JOURNAL OF ENVIRONMENTAL SCIENCES ISSN 1001-0742 CN 11-2629/X www.jesc.ac.cn Available online at www.sciencedirect.com Journal of Environmental Sciences 2012, 24(6) 1133–1141 Phosphine functionalised multiwalled carbon nanotubes: A new adsorbent for the removal of nickel from aqueous solution Muleja Anga Adolph, Yangkou Mbianda Xavier ∗ , Pillay Kriveshini, Krause Rui Department of Chemical Technology, University of Johannesburg, Doornfontein Campus P.O Box 17011, Johannesburg, South Africa. E-mail: mbianday@uj.ac.za Received 09 July 2011; revised 07 November 2011; accepted 15 November 2011 Abstract Synthesised triphenylphosphine-linked multiwalled carbon nanotubes (Tpp-MWCNTs) were used to study the adsorption of nickel in aqueous solutions and their adsorption capabilities were compared with purified MWCNTs. The adsorption capacity increased with an increase in pH for all adsorbents. The adsorption equilibrium was reached in 40 and 30 min for purified MWCNTs and Tpp-MWCNTs, respectively. Both Freundlich and Langmuir isotherms used to investigate the adsorption process fitted the experimental data well with the correlation coefficient R 2 close to 1 for all adsorbents. On the other hand, the experimental data fitted well with a pseudo second- order model. The speciation of nickel also influenced the adsorption on the purified and Tpp-MWCNTs. The adsorbents used in this study showed superior adsorption capacity when compared to other adsorbents reported in the literature. Key words: phosphine functionalised multiwalled carbon nanotubes; adsorbent; XPS remediation; nickel DOI: 10.1016/S1001-0742(11)60880-2 Introduction The presence of nickel in the environment originates from several sources. The sources include the production, the consumption; the recycling and the disposal of nickel (Ni) and Ni compounds. Nickel and nickel compounds are widely used in various industries (metallurgical, electronic, electrical, medical) as catalysts, alloys etc. (Rosenberg, 1968; Moskalyk and Alfantazi, 2002; Denkhaus and Sal- nikow, 2002). Ni salts are often used in the metal plating industry and constitute another significant source of Ni in the environment. This widespread and varied use has led to an increase in the release of nickel into the environment, resulting in envi- ronmental, pollution (Denkhaus and Salnikow, 2002). For instance, the concentrations of Ni in industrial wastewaters range from 3.40 to 900 mg/L( ¨ Ozbay, 2009). This concen- tration is higher than the maximum limit for Ni in terms of the regulation, which is 0.15 mg/L for wastewater. The accumulation of Ni and Ni compounds in the environment causes serious hazards to the plants, animals and human health. Some of the adverse effects are skin allergies, lung fibrosis, and cancer of the respiratory tract (Denkhaus and Salnikow, 2002; IARC, 1997). Several forms of nickel, including its water soluble compounds, have been found to be carcinogenic in humans (IARC, 1997). The World Health Organisation (WHO) and the European Economic Community have restricted the concentration of nickel in * Corresponding author. E-mail: mbianday@uj.ac.za drinking water to 0.02 and 0.05 mg/L respectively ( ¨ Ozbay, 2009; WHO, 1996, 1998). Several methods have been developed to remove nickel from effluent solutions. These methods include chemical precipitation, solvent extraction, ion exchange, adsorption using activated carbon, and membrane processing (Fu and Wang, 2011; Ahalya et al., 2003; Crini, 2005). Howev- er, their application is sometimes restricted because of environmental, technical and/or economic constraints (Fu and Wang, 2011; Ahalya et al., 2003; Crini, 2005). It is therefore necessary to improve these removal methods. Nowadays, adsorption is the most used technique for the removal of heavy metals because of its cost-effectiveness and simplicity (Qu, 2008). Several adsorption processes and adsorbents have been applied for the removal of heavy metals such as copper, zinc, cadmium and lead from aqueous solutions including alginate immobilized kaolin (Li et al., 2011), magnetic particles modified with amino groups (Lin et al., 2011), resin (Zhu et al., 2007), chitosan (Wan Ngah and Fatinathan, 2010), micelle-template silica (Panadda et al., 2009) and tourmaline (Jiang et al., 2006). However, new and more effective adsorbents are still required. Carbon nanotubes (CNTs) are newly developed adsor- bents that prove to be efficient in the adsorption of metals such as: Ni, Cd, Zn, Mn, Co, Cu, and Cr from solutions (Lu et al., 2008; Safavi et al., 2010; Pillay et al., 2009; Lu et al., 2006). MWCNTs have demonstrated a larger potential ad- sorption capacity towards metals ions than the commonly