Preparation and characterization of porous chitosan–based membrane with enhanced copper ion adsorption performance
Graphical abstract
Introduction
Unfortunately, every industrial process inevitably produces waste that is discharged into the environment, and a growing list of metal compounds accompanied this trend [1]. The heavy metal contaminated effluent is one of the most hazardous wastewaters [2]. The high solubility of heavy metals in aqueous environments leads to their entry into the food chain of living beings; and as heavy metal ions cannot be decomposed, consumption beyond the limit of heavy metals can cause severe health disorders [3,4]. Conventional techniques for heavy metal removal from waste streams (such as ion exchange, evaporation, reverse osmosis, etc.) have some unfavorable characteristics, for example, production of toxic sludge, incomplete removal, high investment, and operating costs [5,6]. Therefore, the development of cheaper and more efficient methods to improve the quality of treated wastewater has always been considered by researchers. Low-cost adsorbents with metal-binding capability have attracted lots of consideration recently [7]. These adsorbents may be minerals, biological materials, industrial wastes, agricultural residues, biomass, and polymers [2,8].
Adsorbent membranes may have benefits over adsorbent beads in heavy metal ions removal. The adsorbent membranes have a faster separation rate than adsorbing beads [9]. Besides, lower pressure drop, easy scale-up, high adsorption capacity for compounds with very low concentrations are the other advantages of adsorptive membranes [10,11]. Moreover, functional groups such as amine and carboxyl groups at the surface of membrane adsorbents play an essential role in determining the effectiveness, capacity, selectivity, and reuse of adsorbent materials [12,13]. One straightforward and useful way to prepare effective adsorptive membranes is the use of polymer blends containing functional groups [14]. In this method, the polymer with functional groups on polymer backbones such as chitosan (CS), ethylene vinyl alcohol, cellulose acetate, or mixtures of them with other polymers such as cellulose acetate/polyethylene amine would be used as the membrane polymer [14].
Because of the unique properties of CS, CS-based materials have attracted a lot of attention in recent years. It has been demonstrated that CS is a biocompatible, biofunctional, and biodegradable polymer with no toxicogenic aspect [15,16]. Some practical applications of CS membranes include water treatment, drug delivery, biosensors, and lithium batteries [17]. This biopolymer has lots of amino groups [18], which are responsible for heavy metal ion uptake [19]. In recent studies, there is a considerable interest in the preparation of CS films instead of powder or beads [9,20]. Therefore, CS-based membrane adsorbents have been developed to remove heavy metals. However, the range of pure CS membranes application is mainly unfavorable because of its weak mechanical strength and poor chemical stability [9]. To combat these limitations, CS is usually used with other polymers; in this regard, poly(vinyl alcohol) (PVA) is one of the most effective and compatible polymers [11,19,21,22].
Since polyethyleneimine (PEI)-based materials have been studied due to their high amount of amino functional groups and the metal bonding capability [23], they have good potential for the use in adsorptive membrane blends. PEI often binds to carrier particles such as porous magnetic powder, biomass, porous cellulose materials, etc. [23]. In our previous work, introducing PEI to the CS/PVA polymer matrix membrane increased the removal of Cu (II), Ni (II), and Cd (II) by more than 40% [10]. The micro-porous adsorbent membranes have reactive groups attached to their inner pores [24]. Therefore, adsorption would not be limited to the adsorbent surface. The method used to create porosity in CS membranes is based on the selective dissolution of a component from a polymer membrane mixture [25]. For example, pore-formers such as PEG, PVP, or some salts are water-soluble and well combined with CS [19,26]. The pore-former would dissolve and leave the membrane during alkaline treatment and washing step, which leads to pores and cavity in the membrane [25]. Another simple and useful method used to create porosity in membranes is to dissolve a polymer, or a mixture of polymer in a solvent mixture consists of a volatile solvent [27]. During evaporation step, volatile solvent evaporates faster and creates more porosity [28].
The design of CS-based membranes is an interesting concept. CS-based membranes could be used for different applications in static (batch) and dynamic adsorption processes [19]. Although the structure of CS-based membranes has an important effect on the membrane adsorption performance, only a few studies are available about the porosity and its creation methods. Thus, in fabricating these membranes, we used both aforementioned methods to improve the porosity of the CS/PVA/PEI adsorbent membrane. For the first method, PVP was added to the adsorptive membrane solution mixture, and for the second method, some amount of acetone was used to increase the membrane porosity. After comparing the two methods by FT-IR, SEM, swelling degree, porosity test, and BET analysis the adsorption properties of the affinity membranes were investigated by batch adsorption experiments.
Section snippets
Materials
CS used in this study was obtained from Acros (USA). Branched polyethyleneimine with the average molecular weight of 25,000 from Aldrich was used. Poly (vinyl alcohol) and poly (vinyl pyrrolidone) were purchased from Merck (Germany). Moreover, solvents of analytical grade such as CuSO4.5H2O, NaOH, HCl, NaCl, and Na2EDTA were obtained from Merck (Germany). Acetone (extra pure, assay = 99.5%) from Arvin Shimi Delta Chemical lab (Iran) was used. Activated carbon with 4 × 8 mesh and the size of 2.4
Characterization of the polymers
Fig. 1 indicates the FTIR spectrum of CS powder. The results of the DA value for CS that was measured from the area ratio of the amide (I) and hydroxyl bands in this figure, and the molecular weight value of CS which was determination by viscosimetric methods are listed in Table 2.
GPC results for PVA and PVP including the number average molecular weight (), weight average molecular weight (), and Z-average molecular weight () are listed in Table 3.
The ratio of / shows the
Conclusion
Two methods have been used to prepare a novel porous CS membrane. Introducing PVP into adsorptive membrane casting solution did not have a very significant effect on the porosity and adsorption capacity of the membrane. The removal percent of copper ion was increased only 8.7% by the addition of 0.1% PVP. However, adding some amounts of acetone as a volatile solvent to the solution raised the membrane porosity and increased the removal amount of copper ions by about 50%. The results of
Data availability
The raw data required to reproduce these findings are included in the article.
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.
Acknowledgements
The first author acknowledges the scholarship supported by the Azad University of Quchan in Iran. We should thank the laboratory department of this university for providing the experimental facilities and their generous help. We would also like to show our gratitude to Dr. Ehsan Salehi from Arak University for sharing his knowledge and expertise about the basics of membrane adsorbents that greatly assisted the research.
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