Introduction of copper nanoparticles in chitosan matrix as strategy to enhance chromate adsorption
Introduction
Recently, considerable interest has been focused on metal nanoparticles due to their potential applications in diverse fields including catalysis, magnetic recording media, or microelectronics [1]. Several preparation methods of metallic nanoparticles were described, including wet and dry processes: (i) chemical reduction of metal salts, (ii) thermal, photochemical, or sonochemical decomposition, (iii) ligand reduction and displacement from organometallics, (iv) metal vapor synthesis, and (v) electrochemical reduction. The first cited method requires the use of stabilizers to prevent agglomeration of the resulting metal atoms as microscopic crystallites. Many studies reported the use of biopolymers as mediator during the reduction of the metallic salt solutions [2], [3], [4], [5].
Chitosan (CHI) can be used as mediator due to its great ability to form metallic complexes [6], [7], [8], [9], [10]. According to Murugadoss et al. [11], CHI can act as both a reducing agent and a stabilizer of metal nanoparticles. CHI with supported metal nanoparticles can be applied in environmental uses [12], [13], catalysis and for the development of biosensors [14].
In the field of Cr(VI) remediation, modified polymer structures recently retained a great attention [2], [5], [15], [16], [17]. Hexavalent chromium is considered by the United States Environmental Protection Agency (EPA) a carcinogenic and mutagenic agent [18]. It may cause damages to the kidney, lungs and ulcerations to the skin [19]. When in solution, the oxyanions CrO42− and HCrO4− are predominant at 2 < pH < 6. Hydrogendichromate ions (HCr2O7−) are prevalent only in strong acidic solutions (pH < 1). In basic solutions (pH > 8), the main formed specimen is CrO42− [20], [21].
Herein we report the introduction of copper nanoparticles on chitosan matrix as a strategy to enhance the chromate adsorption. Recently, using XPS technique, we have shown that the mechanism of Cr(VI) adsorption in CHI membrane containing CuNPs is governed by the redox reactions between CuNPs and Cr(VI) ions [22]. This research brings new insights into the development of modified polymeric sorbents. Our findings cover 3 important topics: (1) the synthesis and (2) the stability of the Cu-NPs into CHI matrix (hygroscopic behavior), moreover, (3) interpretation of the mechanism involved in the enhancement of Cr(VI) removal regarding the incorporation of Cu(NP)s (in comparison to pristine chitosan).
The Dispersive X-ray Absorption Spectroscopy (DXAS) technique was employed as a novel way to monitor the formation of metallic nanoparticles into CHI polymeric matrix. It is a powerful tool to estimate the speciation of copper during Cu(II) reduction (with NaBH4 as the reducing agent).
The hygroscopic behavior of the proposed sorbent was also studied by acquiring water vapor adsorption isotherms (dynamic method). The knowledge of how the CuNPs affect the hygroscopic behavior of the CHI membrane is a helpful data to support future studies about the corrosion of CuNPs into CHI polymeric matrix. As reported elsewhere [23], the ordinary corrosion of most common metals requires the presence of both water and oxygen, but the phenomena vary considerably according to whether the water or the oxygen is present in excess [23].
The current paper combined with our previous mechanism study using the XPS technique [22] represent an important contribution to further studies concerning the chromate anions adsorption by modified polymeric sorbents.
Section snippets
Preparation of the CHI-RED membrane
CHI-membrane containing Cu-NPs (CHI-RED membrane) was prepared as described elsewhere [22]. Firstly, Cu(II) was adsorbed to CHI membrane. Thus, the CHI-Cu(II) membrane (discs of d = 4.5 cm) was set into a container with 80 mL of ultra-pure water under constant magnetic stirring and under N2 atmosphere (ambient temperature, pH = 8). After approximately 2 min, a freshly prepared a NaBH4 aqueous solution (20 mL [NaBH4] = 26 mmol L−1) was added to the recipient until the CHI-Cu(II) membrane turns its color
Kinetics of Cu(II) reduction monitored by DXAS technique
The Cu(II) reduction was monitored using DXAS technique (to obtain the X-ray absorption spectra) in the course of the reduction reaction of Cu(II) from CHI-Cu(II) membrane (using NaBH4, as the reducing agent, Fig. 1). Each spectrum was studied over a photon energy ranging from 8960 to 9020 eV, XANES region (X-ray absorption near-edge structure), for getting information on the oxidation state of metal species. As revealed by Fig. 1, the equilibrium of the reaction was reached after approximately
Conclusion
This study provided valuable information regarding the Cu(II) reduction by NaBH4 into CHI membrane using DXAS technique. The XANES spectra recorded against time showed that at equilibrium (after ∼30 min contact time) the copper composition on the sorbent tended to the following distribution: Cu(I) (63%) and Cu(0) (37%).
The CHI membrane water vapor sorption isotherm obtained under N2 atmosphere was slightly changed in the presence of CuNPs. However, the hysteresis was extended to water activity
Acknowledgments
The authors are grateful to CAPES (Process 4575-11-3), CNPQ (Process 142247/2009-0), University of Campinas, Universidad de Málaga and Ecole des Mines d’Alès. The authors also thank the Brazilian Synchrotron Light Laboratory for the DXAS experiments.
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