Materials Today Chemistry
Silver nanoparticles embedded cation-exchange membrane for remediation of Hg species and application as the dip catalyst in organic transformation
Graphical abstract
Introduction
Mercury (Hg) is ubiquitous in the global environment and is considered one of the highest priority toxic elements in the environment [1]. Monitoring ultra-trace amounts of Hg in natural waters that resulted from industries, refineries, mining, and domestic discharges is a demanding task due to its toxicity, bioaccumulation and persistent character in the environment. Hg strongly accumulates in the food chain and, according to the Environment Protection Agency guidelines, Hg2+ ions concentrations should be below 2 ppb in drinking water [2]. Hg salts are highly toxic compare with Hg metal and amalgams because of their solubility in water. Solid-phase extraction of Hg using carbon nanotubes [[3], [4], [5]], activated carbon [6], mesoporous silica [7], nanocomposites [8,9], magnetic nanoparticles [10], and membrane technologies are emerging frontiers in the continuous monitoring of toxic contaminants such as Hg from the environment and their subsequent remediation. The solid supports such as the membrane are easy to handle. Therefore, the design of sensitive and discriminating membrane techniques for quantitative and selective determination of Hg2+ ions in complex matrices are necessary in terms of environmental protection, economical consideration and safe handling of toxic metals [11,12]. Recently, Albatrni et al. [13] have reviewed the adsorption and membrane technologies for the removal of Hg. The comparison given in this review seems to suggest that adsorption processes offer several advantages over membrane-based technologies such as lower capital and operating costs, ease of operation, and potential applications to a wide range of the removal of hazardous substances. Some of the most common adsorbents used for the Hg removal are carbon materials, biomass, polymers, nanomaterials, and other sorbents such as natural minerals, metallic organic frameworks, and so on [13]. The nitrogen-enriched tailored microporous carbons have been found to be potential CO2 adsorbents [[14], [15], [16], [17], [18]]. However, their efficacies for Hg removal are not fully explored. It is also interesting to note that the nanoparticles embedded membranes can also be used as selective adsorbents for Hg sensing and remediation.
The poly(perfluorosulfonic) acid membrane (Nafion) is a unique cation-exchange membrane without formal crosslinking and pores. Nafion has received much attention as a host for a variety of nanoparticles because of its numerous advantages [19,20]. The noble metal nanoparticles particularly silver nanoparticles (Ag NPs) have emerged to open new opportunities for many environmental applications because of their cost-effective and ease to monitor by spectrophotometer because of the high molar extinction coefficient of their surface plasmon resonance absorption band (1010 M−1 cm−1) [21], which provides better sensitivity [22]. These nanoparticles having unique optical, electrical, adsorptive, catalytic and magnetic properties, as well as high surface-to-volume ratio have emerged as powerful sorbents for sensing, detection and speciation of toxic elements [23]. The potential sorbents based on Ag NPs are now obtaining increase attention. The embedment of silver nanoparticles in polymer membranes can produce good mechanical strength in the pre-concentration techniques and could act as a potential sorbent in the quantitative determination of toxic elements. The major works have been reported for Hg2+ ions detection using Ag NPs. The selective colorimetric sensors based on the monitoring of unmodified silver nanoparticles reduction for the determination of Hg [24], dye-coated silver nanoparticle-based probes were developed for the ultrasensitive and selective detection of Cu2+ ions and Hg2+ ions [2], calorimetric detection of Hg2+ ions by using silver nanoparticles in the presence of glutathione [25], silver and gold nanoparticles impregnated in nylon membrane filters have been proposed as a new solid phase for pre-concentration of Hg from natural waters [26], the reaction between biologically green synthesized silver nanoparticles and Hg2+ ions was introduced as a new and potential sensor for the selective recognition of Hg2+ ions in aqueous samples [27]. Recently, a silver nanoparticle imprinted sodium alginate/polyvinyl alcohol nanocomposite thin films have been fabricated for the sensitive and selective detection of Hg2+ ions in the aqueous phase [28]. The Ag NPs possess antimicrobial activity and can be used for catalyzing the reduction of toxic organic pollutants [29]. As the toxic element Hg exists in metallic, inorganic and organic forms [30], it is necessary to detect and measure the level of Hg2+ ions in environmental samples with high sensitivity and selectivity, without the interference of other metal ions. In our previous work, we have shown that galvanic replacement of Ag NPs with noble metal ions in the ionomer matrix is possible and could be used for forming different noble metal NPs [19]. Thus, the Ag NPs embedded membrane would have multitasking capability for various applications.
The major objective of the present work is to uptake and lock Hg in the membrane host by galvanic reaction of Hg2+ with Ag NPs for the safe dual applications in the quantification, as well as in the organic reactions where Hg is used as the catalyst. To attain these objectives, the Ag NPs were formed in the Nafion-211 membrane by in situ reduction of the Ag+ ions in the presence of formamide at 65 °C and sodium borohydride at room temperature. The distributions of the Ag NPs in the membrane matrix were studied by the field emission scanning electron microscopy imaging of the cross-section of the membrane. Based on the Ag NPs uniform distribution, the Ag NPs embedded membrane formed by the formamide reduction has been subjected to the sorption of Hg2+ ions by the galvanic exchange reaction. The quantitative galvanic replacement of Ag0 with Hg2+ ions in the membrane matrix has been studied by spectrophotometry and energy dispersive X-ray fluorescence (EDXRF). The sorption efficiency of Hg2+ ions as the Hg0 in the Nafion-211 membrane matrix has been studied by radiotracer (203Hg). The quantification of Hg0 nanodroplet accumulated in the membrane has been obtained by cold vapor atomic absorption spectrophotometry (CV-AAS). The possible application of the Hg0-membrane as a dip catalyst in the organic transformation of phenylacetylene to acetophenone has been studied for its safe application in catalysis.
Section snippets
Experimental section
All reagents were of analytical grade, and all aqueous solutions were prepared in a high-purity water system using Merck Millipore-Q, India. Perfluorosulfonic acid Nafion membranes (Nafion-211) having thickness 25.4 μm and density 50 g/m2 was purchased from Aldrich. Silver nitrate, sodium chloride, formamide, sodium nitrate, and purified Hg (II) nitrate monohydrate (mercuric nitrate) were purchased from Merck (Mumbai, India). Radiotracer 203Hg was obtained from the Board of Radiation and
Results and discussion
Nafion membrane is a cation-exchange membrane ionomer having negative fixed charged sites (SO3−) (co-ions) and exchangeable cations (counterions). Therefore, the anions such as BH4− ions do not enter the matrix of the membrane because of the Donnan exclusion. However, neutral species such as formamide can enter the matrix of the ion-exchange membrane and can reduce the Ag+ ions in situ to form Ag0 atoms. In the present work, the Ag+-loaded Nafion-211 membrane samples were reduced using anionic
Conclusions
Ag NPs embedded Nafion-211 membrane was developed for the quantitative preconcentration and determination of Hg2+ ions in aqueous solutions. The UV–vis spectrophotometry of the developed membrane produced a blue shift at ~375 nm and, thereafter, gradually decrease in the intensity of the surface plasmon absorption band of Ag NPs with increasing concentrations of Hg2+ ions was observed. With equilibration in Hg2+ ions aqueous solutions, the Ag NPs embedded Nafion-211 membranes changed their
Credit author statement
Kalyan Yakkala: Writing-original and major draft, methodology, Investigation, conceptualization and Review. Sankararao Chappa: Investigation. Prakash B. Rathod: Investigation. Ramakrishna Naidu Gurijala: Supervision. Ashok K. Pandey: Writing, conceptualization and Review.
Declaration of competing interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgments
The author Dr. Kalyan Yakkala is grateful to the Science and Engineering Research Board (SERB), Department of Science and Technology (DST), Government of India for the financial support given under the Fast Track Scheme for Young Scientists in Earth and Atmospheric Sciences vide letter reference no: SR/FTP/ES-43/2013.
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Present address: Department of Environmental Studies, University of Delhi, Delhi-110007.