A highly efficient polyampholyte hydrogel sorbent based fixed-bed process for heavy metal removal in actual industrial effluent
Graphical abstract
Introduction
Heavy metal wastewater is the current outstanding problem in the field of water pollution. Due to the high toxicity and non-biodegradability of heavy metals (Liu et al., 2013), it is very urgent to seek effective methods to remove heavy metals from wastewater (Bian et al., 2012). Various treatment technologies have been applied for the removal of toxic metal ions pollutants from wastewater, including ion exchange, filtration, chemical precipitation, solvent extraction, sorption and so on (Jin et al., 2014a). Although various techniques for the removal of toxic metal ions have been developed, sorption is considered a facile effective technique due to its cost-effective, versatile and simple to operate for removing trace-levels metal ions from aqueous systems (Ali, 2012).
Recently, the research on adsorbents for the removal of toxic metal ions has been focused on nanomaterial-based adsorbents which possess large surface areas, easy modification, and large number of active sites for binding metal ions (Sun et al., 2013b). Still, the separation and reusability of nanomaterials is a tedious job (Ali, 2012), and the residual nanomaterials in water will undoubtedly bring new security concerns as well (Lowry et al., 2012, Zhao et al., 2014). Therefore, the adsorbents with good separation and regeneration properties show incomparable advantages over nanomaterial-based adsorbents. Bulk materials used as sorbents are in favor of separation, but the low adsorption capacity and slow adsorption rate of them is a real problem (Cao et al., 2012).
Hydrogel, a three-dimensional cross-linked network of polymers, exhibits a distinctive feature of high swelling in water (Wang et al., 2010b). They have been considered as a type of mechanically weak materials (Calvert, 2009), and thus little attention is focused on them as possible structural materials. Recently several tough hydrogels with good mechanical properties, such as double-network hydrogels (Sun et al., 2012, Chen et al., 2013, Nakajima et al., 2012), polyampholyte hydrogels (Sun et al., 2013a), and hydrogel composites (Huang et al., 2007), have revealed the potential as structural materials (Shibayama, 2012). Among these hydrogels, polyampholyte hydrogels bear randomly dispersed cationic and anionic repeat groups, forming tough and viscoelastic hydrogels with multiple mechanical properties. Also, the supramolecular structure of polyampholyte hydrogels can be tuned to change multiple function properties over wide ranges by diverse functional unit combinations (Sun et al., 2013a). Additionally, excellent water penetration of hydrogel is highly accessible to foreign molecules (Kudaibergenov and Ciferri, 2007). This uniqueness of polyampholyte hydrogel motivates us to exploit this kind of materials as adsorbents for removing heavy metals in water. To our knowledge, there is no report about sorbents based on polyampholyte hydrogel.
This study presents a novel polyampholyte hydrogel strengthened with graphene oxide, which also provided a framework for the formation of 3D structure. This polyampholyte hydrogel possesses some advantages: good mechanical property for easy separation and durable recycling, large amount of carboxyl and amino groups for high sorption capacity, and high swelling for fast sorption. As a result, the polyampholyte hydrogel as adsorbents was successfully used in the treatment of actual industrial wastewater. The experimental parameters of the adsorption characteristics, fixed-bed column sorption and regeneration/recycling tests were investigated. This work is expected to potentially develop a step change in adsorbent materials for removing heavy metal ions in wastewater.
Section snippets
Materials
Graphene oxide (GO) was prepared by oxidation of natural graphite powder (320 mesh, Qingdao Tianhe Graphite Co. Ltd) according to a modified improved Hummers' method (Marcano et al., 2010). Acrylic acid (AA) and methyl methacrylate (MMA) were distilled under reduced pressure before use. Ammonium persulfate (APS), N,N-methylenebisacrylamide (MBA), N,N,N′,N′-tetramethylethylenediamine (TEMED), sodium dodecylsulfonate (SDS) and triethylene tetramine (TETA) were purchased from commercial sources
Characterization of polyampholyte hydrogel
The polyampholyte hydrogel shows a massive texture (inset in Fig. 1a). Obviously, the massive materials are in favor of fast separation from water. The SEM image of the polyampholyte hydrogel reveals a loose and porous structure (Fig. 1a). The interpenetrating network channels are accessible to foreign molecules including metal ions. The good mechanical strength is demanded for a qualified sorbent. The recovery or fatigue resistant behavior of the polyampholyte hydrogel was investigated by
Conclusions
In this study, a polyampholyte hydrogel was well designed and prepared via a simple radical polymerization procedure. The polyampholyte hydrogel sorbent could be easily regenerated and highly reused. The sorption capacities were as high as 216.1 mg/g for Pb(II) and 153.8 mg/g for Cd(II). The adsorption could be conducted in a wide pH range of 3–6 and the equilibrium fast reached in 30 min. The polyampholyte hydrogel was effective in removing heavy metals from actual industrial effluent with
Acknowledgments
This work was supported by Hunan Provincial Natural Science Foundation of China (14JJ1015), the National Natural Science Foundation of China (51178173, 51238002, 51272099, 51378187, and 51478171), and Program for Innovation Research Team in University (IRT1238).
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