Research articleNovel alginate/polyethyleneimine hydrogel adsorbent for cascaded removal and utilization of Cu2+ and Pb2+ ions
Graphical abstract
Introduction
Water pollution caused by the toxic heavy metal ions is a serious environmental problem originated from the anthropogenic sources. Particularly, the discharge of effluents emitting from the batteries, metal finishing, textile, mining, plating, ceramic and glass industries into the water resources is continuously increasing, which can contain toxic heavy metal ions and cause water pollution. The dissolved metal ions are not degradable in nature unlike organic contaminants and can interact with the living organisms through the food chain, e.g. they can easily bind with proteins, nucleic acids, and small metabolites to block their biological functions, leading to severe health problems (He et al., 2014; Ozay et al., 2010). For instance, copper (Cu2+) and lead (Pb2+) ions are the two heavy metal ions commonly found in the industrial wastewater, which can damage the gastrointestinal system, kidney, liver and central nervous system even at a trace amount (Awual et al., 2014; Tahtat et al., 2017). Once the wastewater containing heavy metal ions is dumped into the rivers, lakes, or oceans without any treatment, it may permanently damage the water-based biological system. Therefore, the removal of such metal ions from the wastewater is of great importance and deserves the immediate attention. Various techniques, such as chemical precipitation (Alvarez et al., 2007), ion exchange (Wong et al., 2014), membrane separation (Zhu et al., 2015), and adsorption (Cui et al., 2015) have been committed to the removal of the metal ions in the wastewater. Among them, adsorption is considered to be an effective and economic one because of its relative simplicity, high efficiency, and recyclability to remove the heavy metal ions from the industrial wastewater (Magnacca et al., 2014; Nayab et al., 2014; Yan et al., 2014).
Traditional adsorbents, such as activated carbon, zeolites, and resins are effective ones for the removal of the heavy metal ions (Kulkarni et al., 2018; Li et al., 2018; Macías-García et al., 2017). However, these materials are either expensive or non-biodegradable. Recently, the exploration of low-cost adsorbents derived from natural resources, e.g. modified polysaccharide, alginate, cellulosic materials, and waste biomass have been pursued for the metal ion removal (Godiya et al., 2019a; Wang et al., 2018; Zhuang et al., 2016). Among those natural adsorbents, alginate (ALG) is a promising material due to its abundance, cost-effectiveness and biodegradability (Esmat et al., 2017; Shao et al., 2018). The ALG monomer unit is composed of one α-L-guluronic acid (G units) and one β-D-mannuronic acid (M units) linked by a β-1,4-glycosidic bond, thus providing hydrophilicity and biocompatibility (Rhein-Knudsen et al., 2017). The ALG can easily be crosslinked by divalent ions (e.g. Ca+2 ions) by the interaction between G-blocks, which tightly hold the junctions and thus form a hydrogel (Sikorski et al., 2007; Zhuang et al., 2016). The ionically crosslinked ALG hydrogel is rich in the hydroxyl (OH) and carboxyl (COOH) groups, which can be chelated with various metal ions (Stewart et al., 2009). However, the poor mechanical property, easy bacterial degradation in the wastewater, and partial solubility in the acidic water limit the application of such ionically crosslinked ALG hydrogel in the wastewater treatment (Feng et al., 2018). To tackle this problem, the ALG can be functionalized by introducing other functional materials via covalent crosslinking, thus resulting in the mechanical robustness and excellent adsorbability. For instance, Wang and his coworkers have prepared an ALG-attapulgite foam adsorbent and used for the adsorption of Cu2+ and Cd2+ ions in the wastewater and the adsorption capacity was found 119 mg·g−1 and 160 mg·g−1, respectively (Wang et al., 2018). Feng and coworkers have prepared the ALG-melamine sponge composite for the removal of Cu2+ ions, and found the adsorption capacity 90.1 mg·g−1 (Feng et al., 2018). Besides, ALG/graphene oxide (GO) aerogel was examined to remove the Cu2+ and Pb2+ ions with the adsorption capacities of 98.0 mg·g−1 and 267.4 mg·g−1, respectively (Jiao et al., 2016). However, improvement in the adsorption capacity and mechanical strength for the ALG based materials is still a challenge.
The amine-groups on the adsorbent can easily be protonated in acidic solutions, and thus capture metal ions by the electrostatic interaction and ion exchange (Deng and Ting, 2005; Sun et al., 2014). The bioadsorbents modified with amine-groups showed an excellent adsorption capacity for Cu2+ and Pb2+ ions from water (Demey et al., 2018; Jin et al., 2017). Branched polyethyleneimine (PEI) is a well-known compound for its metal chelation property due to the presence of amine groups (Ma et al., 2014; Sun et al., 2011). PEI is often grafted on materials containing OH, aldehyde (CHO) or COOH groups by interacting with the amine groups (An et al., 2013; Liu et al., 2017). For instance, Liu and Huang has modified eggshell membrane with PEI and found improved adsorption of Cr6+ ion in water (Liu and Huang, 2011). Xiao and coworkers have prepared PEI-modified polystyrene/Fe3O4/chitosan magnetic composites, which showed excellent adsorbability of Cu2+ ions (Xiao et al., 2017). We believe that the designed PEI grafted ALG composite material could preserve advantages from both the ALG and PEI segments, as well as 3D network structure and a large amount of OH, COOH and NH2 groups which would provide abundant active sites for the adsorption of the heavy metal ions.
Therefore, in this paper, a functionalized ALG/PEI composite hydrogel was prepared through an effective and simplified chemical crosslinking process. The adsorption of metal ions was thoroughly investigated. A stepwise reduction and utilization of the as-prepared hydrogel provided us a sustainable route of the heavy metal recycling. This research will present an effective and practical paradigm for the cascaded treatment and recycling of heavy metal ions in wastewater.
Section snippets
Materials
Sodium alginate (ALG) and branched polyethyleneimine (PEI) (Mw = ca. 75,000) were purchased from Sigma-Aldrich Co., Ltd. Other chemicals, including calcium chloride (CaCl2), magnesium chloride (MgCl2), potassium hydroxide (KOH), ethyleneglycol diglycidylether (EGDE), 4-nitrophenol (4-NP), sodium borohydride (NaBH4), sodium hydroxide (NaOH), hydrochloric acid (HCl), nitric acid (HNO3), copper(II) sulfate pentahydrate (CuSO4·5H2O), lead(II) nitrate (PbN2O6) and the organic solvents, ethanol,
Preparation and characterization of the ALG/PEI composite hydrogel
The ALG/PEI composite hydrogel was prepared as illustrated in Scheme 1. In general, the ALG was used as a matrix to form a robust hydrogel with PEI, while the PEI can improve the adsorption capacity substantially. Typically, the hydrogels were prepared by mixing the two precursors in various molar ratios (ALG:PEI = 1:9, 3:7 and 5:5, total polymer weight content – 5.0 wt%). When the ALG: PEI ratio was increased more than 5:5, the two polymer precursors couldn't mixed well because of the high
Conclusions
We have demonstrated a simple approach to fabricate a functional, porous, 3D-hydrogel based adsorbent by the incorporation of PEI into ALG for the adsorption of the Cu2+ and Pb2+ ions in the single and binary aqueous solutions. The experimental results confirmed the excellent adsorption performance of the ALG/PEI composite hydrogel for both the ions. The adsorption followed the Langmuir adsorption model with a pseudo-second order kinetics process. Besides, the adsorbed Cu2+ ions were in situ
Conflicts of interest
There are no conflicts to declare.
Acknowledgements
This study was supported by the National Natural Science Foundation of China (Grant No. 21574020), the Fundamental Research Funds for the Central Universities (SEU and NJU), and a project funded by the Priority Academic Program Development of Jiangsu Higher Education Institutions.
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