Synthesis of zero-valent copper-chitosan nanocomposites and their application for treatment of hexavalent chromium
Introduction
Metal nanoparticles are an important subject in modern researches of chemistry and material science. Although metal nanoparticles have superior catalytic and reduction properties, when used as catalyst and reducing agent, it is likely to agglomerate and adsorb on the reactor wall due to a strong van der Waals force. The addition of a dispersing agent will also affect the activity of the nanoparticles, and will produce a significant pressure drop in a packed bed reactor. Furthermore, it is not easy to recycle nanoparticles, which leads to inconvenience for many applications. Therefore, many studies focused on supporting nanoparticles, such as using silicon dioxide (Macquarrie et al., 2001, Yang et al., 2003), zeolites (Riahi et al., 2002), clay (Hossain et al., 2004), activated carbon (Bulushev et al., 2004), and polymer (Cumbal et al., 2003, Wu and Ritchie, 2006). However, using physical methods, such as ion exchange or static electricity, to adsorb ions, the ions would be desorbed when reduced to zero-valent atoms, causing secondary pollution and uneven distribution of agglomeration when the nanoparticles lose their electrical charges. Some studies have pointed out that, in comparison with metal powders of larger size, the use of metal nanoparticles in treating several pollutants gives quicker and more complete results (Hu et al., 2005, Liou et al., 2006, Cheng et al., 2007, Han et al., 2008, Shih et al., 2009). Therefore, this study attempted to use chitosan with strong adsorption and chelating properties to metal ions (Guibal, 2004, Ngah et al., 2005, Sankararamakrishnan et al., 2006, Baran et al., 2007, Ramesh et al., 2008), and apply appropriate reduction methods to synthesize the metal nanoparticles dispersed on polymeric support, in order to solve the above-mentioned problems.
Although chitosan has excellent physical and chemical properties, it is seldom used as a support for catalyst or supramolecular ligand (Quignard et al., 2000, Vincent and Guibal, 2002). Compared with inorganic supports, chitosan is inexpensive, and its removal process for the recycled metal is simpler and cleaner. For example, simple thermal treatment under oxygen environment can achieve complete pyrolysis of chitosan and obtain metal or its oxide. The amino and hydroxyl groups of chitosan are excellent functional groups, which can form various organometallic complexes that can be used as a precursor of a heterogeneous molecular catalyst (Choplin and Quignard, 1998). These functional groups allow chitosan to exhibit excellent surface hydrophilic properties, which are crucial in heterogenized biphasic water/organic catalytic systems by the supported aqueous phase methodology (Arhancet et al., 1989). Although there is a temperature limit (the reaction temperature should not be too high) in its applications, it is suitable for many organic or redox reactions.
Because chitosan can be dissolved in most mineral acids, it is necessary to increase its stability by crosslinking treatment. Therefore, this study employed multivalent counterions, tripolyphosphate to conduct crosslinking reactions (Lee et al., 2001). Chitosan has high adsorption capacity for Cu(II) ions, and the fabricated zero-valent copper is more stable in comparison with zero-valent iron. In addition, it is much less expensive than noble metals such as gold, platinum, and silver. Therefore, this study used copper and chitosan-tripolyphosphate beads to fabricate nanocomposites. Chitosan-tripolyphosphate beads can be used to adsorb Cu(II) ions, and then fabricate zero-valent copper-chitosan nanocomposites after the reduction process. The fabrication process and properties of the nanocomposites will be discussed in the following section. Comparison of the adsorption property of hexavalent chromium on chitosan-tripolyphosphate beads and nanocomposites prepared in this study will also be presented.
Section snippets
Materials
The chitosan used in this experiment was from Charming & Beauty Co. (Taiwan). The molecular weight was 35,0000 Da, and the degree of deacetylation was 95%. Sodium tripolyphosphate and copper(II) sulfate pentahydrate were purchased from Showa, and sodium borohydride was purchased from Lancaster. Other materials were reagent grade.
Preparation of chitosan-tripolyphosphate beads
First, 2 g of chitosan was dissolved in 100 ml of acetic acid solution (1% v/v), with stirring until the chitosan was completely dissolved. The tripolyphosphate solution
Adsorption of copper(II) ions on the chitosan-tripolyphosphate beads
In this study, the initial concentration of Cu(II) ions in the aqueous solution was 300 mg/l and the optimum pH for the adsorption of Cu(II) onto the chitosan-tripolyphosphate beads was 5 (the original pH value of the 300 mg/l Cu(II) ion solution). It was found that the adsorption initially took place rapidly, then slowed, and leveled off within 15 h. The maximum adsorption capacity was 65 mg Cu(II)/g chitosan-tripolyphosphate beads. It was proposed that the uptaking of metal ions on the
Conclusions
Spherical chitosan-tripolyphosphate chelating resins were used as a polymeric support to prevent copper nanoparticles from agglomerating, which greatly improved its applicability. XPS showed that most of copper was reduced to zero-valent state by using simple chemical reducing procedures. Cr(VI) uptake by these zero-valent copper-chitosan nanocomposites via surface adsorption, precipitation or redox reaction may be important mechanisms responsible for chromium remediation by zero-valent copper.
Acknowledgements
The authors gratefully acknowledge support for this study from the NSC of ROC, Project No. NSC 93-2211-E-131-002.
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