Efficient adsorption of Hg2+ ions on chitin/cellulose composite membranes prepared via environmentally friendly pathway
Highlights
► In this work, the chitin/cellulose composite membranes were prepared by directly dissolving chitin and cellulose in 7 wt% NaOH/12 wt% urea aqueous solution at low temperature. ► The membranes exhibited high efficiency to remove heavy metal ions from wastewater, and indicated a superior adsorption of Hg2+. ► The chitin/cellulose sorbents had good mechanical properties, and could be recovered completely. ► The major mechanism of the adsorption of metal ions by chitin/cellulose composite membranes was described as multi-interactions.
Introduction
Concerning health and environmental problems, water pollution represents major challenges facing the global society [1]. Availability of clean and uncontaminated water is a prime requirement for human life [2]. The increasing level of toxic metals that are discharged into the environment as industrial wastes, represent a serious threat to human health, living resources, and ecological systems [3], [4]. The wastewater contaminated with heavy metal ions are produced in the clothing industry, petrochemical industry, mechanical and metallurgical industries that discharge to rivers, lakes, and seas [5], [6]. Some of these water may be used as raw sources for drinking water production, so the presence of such heavy metal pollutant might have a negative impact on the quality of drinking water [7]. The main techniques, which have been utilized to reduce the heavy metal ion content of effluents, include membrane separation, filtration, chemical oxidation or reduction, evaporative recovery, ion exchange and reverse osmosis, lime precipitation [8], [9], [10], [11], [12], [13]. Adsorption technique is one of the most efficient methods of cleaning the environment of heavy metal pollutant [14]. Moreover, membrane biosorbent materials have been widely used in the industrial separation technology for removing of the hazardous materials in wastewater. Its application for water treatment has increased dramatically in the past decade with the improvement in membrane quality and the decrease in membrane costs [15]. Biosorption is an innovative process regarding the biological methods of treatment and has been used to remove toxic metals from wastewater applying inactive or dead biomass [16], [17], [18], [19]. Surface functional groups of adsorbents not only affect the sorption behavior, but also dominate the sorption mechanism [20].
Chitin is an abundant biomacromolecule existing in exoskeletons of crab and shrimp, and it has been first identified in 1884. It is a linear polysaccharide of N-acetyl-d-glucosamine (GlcNAc) connecting through αβ(1 → 4) glycosidic linkage [21]. Due to the advantages of being biocompatible, biodegradable, antibacterium and almost non-toxic, chitin has attracted more and more attention [22]. Chitin has been extensively investigated as adsorbents for the removal of metal ions from wastewater, and its efficient adsorption potential can be attributed to high hydrophilicity and high chemical reactivity due to large number of functional groups [23]. This biopolymer shows an attractive alternation for many materials because of its physico-chemical characteristics, chemical stability, high reactivity, excellent chelation behavior and high selectivity toward pollutants [24], [25], [26]. Chitin as adsorbents for the removal of metal ions has been reported in relatively few. Only little report deals with the efficient adsorptions of chitin on heavy metal ions [27], [28]. Chitin is very difficult to dissolve in common solvents, so the research and development of chitin as sorbents are limited.
There are new solvents, which have been developed to dissolve the most intransigent of molecules cellulose and chitin. Ionic liquids are a group of new organic salts that exist as liquids at a relatively low temperature (<100 °C) and have many attractive properties, as well as they can dissolve cellulose and chitin. [29], [30], [31]. In our laboratory, a new solvent, NaOH/urea aqueous solution, has been used to dissolve cellulose at low temperature [32], [33]. It is worth noting that NaOH/urea aqueous solution is an economical and environmentally friendly solvent of cellulose [34]. The biodegradable cellulose/chitin beads have been prepared by coagulating a blend of cellulose and chitin in 6 wt% NaOH/5 wt% thiourea aqueous solution with 5% H2SO4 as coagulant, showing good adsorption of Pb2+ [28]. Recently, we demonstrated that chitin can be dissolved directly in NaOH/urea aqueous solution via the freezing/thawing method to obtain a transparent chitin solution, and we prepared blend membranes from a mixture of chitin and cellulose in 7 wt% NaOH/12 wt% urea aqueous solution by coagulating with 5% Na2SO4 aqueous solution [35], [36]. Cellulose membrane has good mechanical properties, but its uptake capacity of heavy metal ions is relatively weak. However, the inadequacies of pure chitin membrane is very brittle, low chemical affinity and poor porosity. To combine chitin binding force to heavy metal ions with porous structure and chemical affinity of wet cellulose membrane, blend of cellulose and chitin in a good solvent should be an efficient pathway. Moreover, utilization of natural polymers to produce functional materials has the great importance for a sustainable development and environmental conservation. In present work, novel membranes were prepared from cellulose and chitin in NaOH/urea aqueous system. Their uptake capacity and adsorption dynamics of heavy metal ions, particularly Hg2+, which led to terrible minamata disease, on chitin/cellulose membranes were studied and discussed.
Section snippets
Materials
Cotton linter pulp (α-cellulose >95%) was provided by Hubei Chemical Fiber Group Ltd. (Xiangfan, China). Its viscosity-average molecular weight (Mη) was determined by using an Ubbelohde viscometer in LiOH/urea aqueous solution at 25 ± 0.05 °C and calculated according to the Mark–Houwink equation [η] = 3.72 × 10−2 Mw0.77 to be 8.1 × 104. Chitin was supplied by Zhejiang Golden-Shell Biochemical Co., Ltd., China. The weight-average molecular weight (Mw) of chitin, measured by dynamic light scattering (DLS,
Miscibility and structure of chitin/cellulose composite membranes
Fig. 1 shows photographs of chitin/cellulose composite membranes at dry and wet states. Optical transmittance is often used as an empirical method for determining the compatibility between various components in polymer blend materials. Obviously, the dry blend membrane was transparent, indicating that chitin and cellulose had good compatibility.
Fig. 2 shows the XRD profiles of the chitin, cellulose, RC, RCT and composite membranes with different chitin/cellulose ratios. The native cellulose had
Conclusions
The chitin/cellulose composite membranes were successfully prepared in 7 wt% NaOH/12 wt% urea aqueous solution via a freezing/thawing method to dissolve chitin, and then by coagulating with 5 wt% Na2SO4. The blend between cellulose and chitin was certain miscible, as a result of the strong interaction between chitin and cellulose. The composite membranes exhibited microporous structure, large surface area and affinity on metals, leading to the efficient uptake capacity of heavy metal ions.
Acknowledgements
This work was supported by National Basic Research Program of China (973 Program, 2010CB732203), and the National Natural Science Foundation of China (20474048 and 20874079).
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