An environmentally-friendly chitosan-lysozyme biocomposite for the effective removal of dyes and heavy metals from aqueous solutions
Graphical abstract
Introduction
The discharge of untreated effluents into the environment from mining, chrome plating, textile, paint, and pigment industries pollutes water with heavy metals and dyes (Vilela, Parmar, Zeng, Zhao, & Sánchez, 2016). These toxic and environmentally harmful pollutants can adversely affect humans and animals, and must be remediated (Ma et al., 2016, Ma et al., 2017). Many techniques, such as adsorption (Im et al., 2017; Li, Zhou et al., 2017; Ruan, Chen, Chen, Qian, & Frost, 2016), ion-exchange (Sansuk, Srijaranai, & Srijaranai, 2016; Wang, Feng, Hao, Huang, & Feng, 2014), membrane processes (Ge, Wang, Wan, & Chung, 2012; Li, Lv et al., 2017), electrocoagulation (Heffron, Marhefke, & Mayer, 2016; Li et al., 2012), and photo degradation (Tan et al., 2017; Zhang, Dong et al., 2017) are currently used to diminish the heavy metal and dye contents of water, with adsorption technology being widely recognized as effective and inexpensive (Jagadish, Chandrashekar, Byrappa, Rangappa, & Srikantaswamy, 2016; Yagub, Sen, Afroze, & Ang, 2014).
Novel adsorbents for chromium removal are urgently required, because chromium is both abundant and highly toxic to humans as Cr(IV) (1000 times more toxic than Cr(III)) and causes allergy, skin ulceration, and damage to the liver, kidneys and blood cells (Dayan & Paine, 2001). Chitosan is a naturally abundant, linear polysaccharide that is an effective adsorbent for removing dyes and heavy metals from contaminated water. Its advantages include non-toxicity and biodegradability, making it highly compatible with environmental applications (Vakili et al., 2014; Wan Ngah, Teong, & Hanafiah, 2011). Chitosan is easily modified to improve its adsorption (Chen, Liu, Chen, & Chen, 2008; Rajiv Gandhi, Kousalya, Viswanathan, & Meenakshi, 2011), for example, by composite formation: a chitosan–polyaniline composite has shown higher adsorption capacity for Cr(VI) ions (Karthik & Meenakshi, 2014). Chen et al. prepared a graphene oxide–chitosan composite that effectively removed heavy metals and cationic and anionic dyes (Chen, Chen, Bai, & Li, 2013). Cho et al. prepared a magnetic chitosan composite, and demonstrated its removal of methylene blue and methyl orange (MO) from water (Cho et al., 2015). Magnetic chitosan–biochar (Liu et al., 2017), amine grafted chitosan (Zhang, Dang et al., 2017), chitosan–polymethylmethacrylate (Li et al., 2016), and chitosan-impregnated hexadecylamine (Vakili et al., 2016) are further examples of chitosan-containing conjugates capable of removing various pollutants from water.
Lysozyme is an enzyme that catalyses the hydrolysis of the glycoside linkage in bacterial peptidoglycan. Lysozyme can be isolated from animal sources (typically from egg white), or produced by recombinant techniques. Due to its antimicrobial properties, lysozyme is commonly used as preservative in the food industry (Yuceer & Caner, 2014). Lysozyme has a molecular weight of 14.3 kD (129 amino acid residues) and has a globular protein structure with hydrophilic side chain residues (charged and uncharged) exposed to the solvent. These hydrophilic residues might provide unique binding sites for pollutants such as organic dyes and heavy metal ions. Moreover, lysozyme contains disulphide-bridged residues that are known to bind heavy metals such as mercury and methyl mercury (Pesek & Schneider, 1988). The specific binding of heavy metals such as chromium to the protein ModA suggested that incorporating proteins or other polypeptide structures into adsorbent materials can help increase their functionality and binding capacity (Karpus, Bosscher, Ajiboye, Zhang, & He, 2017).
The present study developed a chitosan–protein conjugate adsorbent by using glutaraldehyde as a crosslinker to connect chitosan to lysozyme. The resulting chitosan–lysozyme biocomposite (CLC) was characterized and demonstrated to be an effective adsorbent for removing MO dye and hexavalent chromium (Cr(VI)) ions from aqueous solution. The adsorption process was described by investigating the parameters associated with the Freundlich isotherm, Langmuir isotherm, Dubinin–Radushkevich (D-R) isotherm, thermodynamic studies, and pseudo-first-order, pseudo-second-order, and intra-particle kinetic models. CLC showed significant removal of MO, Cr(VI) and other heavy metals, from aqueous solution, as well as the potential for regeneration and reuse. Moreover, the effectiveness of CLC for contaminant removal was maintained even in mixtures of dyes and metal ions. For example, CLC had the highest capacity for removing MO and Cr(VI), but also removed Ni(II) and Cd(II) ions to a lesser extent.
Section snippets
Materials
Chitosan (75%–85% degree of deacetylation as obtained from the manufacturer) was purchased from Sigma Aldrich (St Louis, MO, USA). Chitosan had an average molecular weight () of 28.3 kDa as was determined by the viscosity method (see description of molecular weight determination in Section 2.3). Lysozyme, glutaraldehyde and 1,5-diphenyl carbazide were also purchased from Sigma Aldrich (St Louis, MO, USA). Cadmium chloride (CdCl2), nickel chloride (NiCl2), MO, potassium dichromate (K2Cr2O7),
Preparation and characterization of CLC
Chitosan and lysozyme were crosslinked using glutaraldehyde as shown in Scheme 1. Glutaraldehyde is a low cost commercially available crosslinker that reacts readily with amine groups in proteins (mainly primary amine groups (Migneault, Dartiguenave, Bertrand, & Waldron, 2004)) at neutral pH to generate stable Schiff base conjugates, making it ideal for CLC preparation. ATR-FTIR spectroscopy of chitosan (Fig. 1A) showed characteristic peaks at ca. 1039, 1557, 1654, 2936 and 3279 cm−1, which
Conclusions
In this study, an environmentally-friendly biocomposite was developed consisting of lysozyme coupled to chitosan, and has been proven to be an effective adsorbent for the removal of heavy metals and dye molecules from water. The adsorption of MO and Cr(VI) was well described by the Langmuir and Freundlich isotherms, respectively. The maximum adsorption capacities of CLC for these adsorbates were 435 and 216 mg g−1, respectively, which are significantly higher than those of other chitosan-based
Conflicts of interest
There are no conflicts to declare.
Acknowledgements
KR thanks the Blaustein Center for Scientific Cooperation (Ben-Gurion University of the Negev) for a postdoctoral fellowship. This work was supported by the project “StepPolyMem” within the Italy–Israel Scientific and Technological Cooperation funded by the Ministry of Science, Technology and Space of the State of Israel and the Italian Ministry of Foreign Affairs and International Cooperation (to RK, grant No. 3-12401). CJA is grateful to the United States–Israel Binational Science Foundation
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