Application of cellulose acetate to the selective adsorption and recovery of Au(III)
Introduction
Precious metals are of great importance currently because of their widespread applications in high-tech industries. Gold and palladium are especially indispensable in the manufacture of mobile phones and computers. The frequent replacement of these electronic devices causes the accumulation of large amounts of electronic and electrical waste, offering an important recycling opportunity for the secondary supply of precious metals. For example, the gold concentration in mobile phone handsets is 300–350 g/t and in computer circuit boards is 200–250 g/t, which is tens of times higher than that in gold ores (5–30 g/t) (Hageluken & Corti, 2010). Accordingly, the separation and recovery of gold from e-waste has attracted much interest (Mack, Wilhelmi, Duncan, & Burgess, 2007).
Conventional methods for metal recovery such as solvent extraction and ion exchange have been applied to the separation and recovery of gold. Several extractants have been designed for the recovery of gold (Baker et al., 2011, Lu et al., 2011). In addition, various ion exchange resins have also been studied and evaluated for their adsorption performance toward Au(III) ions (Alguacil et al., 2005, Nguyen et al., 2010a, Nguyen et al., 2010b). However, the capacity and selectivity of these conventional methods are still low. Moreover, secondary waste can be generated during these recovery processes, and post-treatment is always costly. From an environmental perspective, these petroleum-dependent reagents, solvents, and resins should be replaced by alternatives derived from renewable biomass resources.
Recently, natural polymers have attracted interest as alternatives to synthetic adsorbents. After treatment or modification, natural polymers such as cellulose, chitosan (Bratskaya et al., 2011, Chen et al., 2011, Ramesh et al., 2008), chitin (Schleuter et al., 2013), tannin (Huang et al., 2010, Ogata and Nakano, 2005) and proteins (Kiyoyama et al., 2008, Maruyama et al., 2007) have been studied, and are reported to be effective in the removal and recovery of metal ions. Despite its crucial importance, the adsorption selectivity of biosorbents toward metal ions has hardly been discussed, especially adsorption selectivity toward precious metal ions. Chitosan and its derivatives have been widely used for the adsorption of precious metal ions because of their nitrogen containing structure. Although amine groups and other nitrogen containing groups have been shown to have a large adsorption capacity for Au(III), no separation ability for Au(III) has been reported. On the other hand, oxygen containing functional groups shows better selectivity toward gold (Saitoh, Suzuki, & Hiraide, 2005).
Among all the natural polymers, cellulose has great potential because of its abundance and low cost. Cellulose has overwhelmingly been developed as an adsorbent by blending with chitosan (Qu et al., 2009) or upon modification by functional groups (Tasdelen, Aktas, Acma, & Guvenilir, 2009). Despite several reports on cellulose-based adsorbents for the removal of heavy metal ions and the recovery of precious metal ions, few reports exist on the use of cellulose acetyl derivatives as an adsorbent.
Cellulose acetyl derivatives are produced by the esterification of acetic acid with naturally derived cellulose from wood pulp or cotton linter. The glucose unit that makes up the raw material polymer cellulose has three hydroxyl groups, and triacetate is the cellulose derivative in which most of the hydroxyl groups have been substituted with acetyl groups. Cellulose acetate is obtained by the partial hydrolysis of triacetate. These polymers, which are widely used in photographic film, cigarette filters, plastic materials and as a fiber in the textile industry, are thus industrially produced and a large amount of waste products are released. If cellulose acetyl derivatives can be used as an adsorbent without further modification, a cost-effective adsorption process is possible.
In this study, for the first time, we used cellulose acetyl derivatives as adsorbents for Au(III) and examined its practical use in the adsorptive separation of Au(III) from other metal ions.
Section snippets
Materials
Cellulose acetate powders (CAP, degree of substitution DS: 2.4) and cellulose acetate fibers (CAF, DS: 2.4) derived from a cotton linter were provided by Daicel Chemical Industries, Ltd. (Tokyo, Japan), and were used as received.
Commercial reagent grade microcrystalline cellulose Avicel PH101 (Avicel) (Sigma–Aldrich, St. Louis, MO, USA), cellulose acetate (CA) and cellulose triacetate (CTA) (DS: 2.4 and 2.9, respectively, Wako Pure Chemical Industries, Ltd., Osaka, Japan) were used for
Characterization of adsorbents used
Fig. 1 shows the XRD patterns of Avicel, CA, CAP and CTA, from which morphological differences were determined. Fig. 1(a) refers to the diffraction pattern of Avicel. The peaks at 15° and 22.5° are characteristic of highly crystalline cellulose (Cheng et al., 2011). In Fig. 1(b) and (c), the main peak at approximately 8° in the XRD patterns of CA and CAP is the principal characteristic peak of a semicrystalline acetylated cellulose. The position of this peak indicates the generation of a
Conclusions
Cellulose acetate fibers that originate from cotton linters are shown to be effective in the selective adsorption of Au(III). Under the experimental conditions used in this study, the degree of Au(III) adsorption onto CAF increased with an increase in the hydrochloric acid concentration to around 2 M. The adsorption may occur by a proton-mediated electrostatic interaction between cellulose acetate and a Au(III) chloro-complex. The adsorption equilibrium was reached within 1 h. The relatively fast
Acknowledgements
The authors thank Dr. S. Shimamoto and Dr. T. Nakamura of the DAICEL Corporation for providing the cellulose derivatives and for useful discussions. The authors express sincere thanks to Prof. Syouhe Nishihama of the University of Kitakyushu and Mr. Keiji Horio of BEL Japan Inc., for the BET surface area measurement and valuable suggestions. This work was supported by a Grant-in-Aid for Science Research (No. 25420806) from the Ministry of Education, Culture, Sports, Science, and Technology
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