Hexavalent chromium removal in contaminated water using reticulated chitosan micro/nanoparticles from seafood processing wastes
Introduction
In the coastline of Patagonia-Argentina (42–43°S, 64–65°W), seafood processing industry discards large amounts of crustaceans shellfish wastes; exoskeletons are converted in a solid residue, which accumulate in landfills becoming an environmental pollutant. The exoskeleton of the crustaceans represents approximately 50–60% of the total weight in crabs and more than 35% in shrimps. However these residues contain chitin, which is a natural, abundant and non-toxic material. Chitosan is obtained cost-effectively by the derivation of chitin (2-acetamido-2-deoxy-β-d-glucose through β(1–4) linkage) and represents an attractive alternative to other biomaterials because of its physico-chemical characteristics, chemical stability, high reactivity, and excellent chelation behavior (Laus et al., 2010, Wu et al., 2010). Chitosan has three types of reactive functional groups, an amino group as well as both primary and secondary hydroxyl groups at the C-2, C-3 and C-6 positions, respectively (Schmuhl et al., 2001, Qi and Xu, 2004). Deacetylation degree and molecular weight are important properties to characterize this biopolymer when it is obtained from natural sources. Chitosan is a well known cationic, biocompatible, biodegradable, non-toxic biopolymer which makes it attractive for numerous applications including waste water treatments.
Water contamination with heavy metals is a critical problem because metals tend to persist and accumulate in the environment. Industrial and mining wastewaters are important sources of pollution of heavy metals. One of the heavy metals that have been a major focus in wastewater treatment is hexavalent chromium (Aydın and Aksoy, 2009, Hena, 2010). The use of chromate and dichromate has many industrial applications such as in textile, electroplating, leather tanning, cement preservations, paints, pigments and metallurgy industries (Aydın and Aksoy, 2009, Bhatnagar and Sillanpää, 2009). Cr(VI) is a toxic metal and must be removed from wastewater before it can be discharged (Hena, 2010). Cr(II), Cr(III) and Cr(VI) are the three oxidation states for chromium in nature, but only the last two are stable (Aydın and Aksoy, 2009). Cr(III) is stable and less toxic or nontoxic, and is considered an essential element for the good health and nutrition of many organisms.
Cr(VI) is 500 times more toxic, mutagenic and carcinogenic than Cr(III) (Dubey and Gopal, 2007). The United States Environmental Protection Agency has laid down the maximum contaminant level for Cr(VI) into inland surface waters as 0.1 mg/L and in domestic water supplies to be 0.05 mg/L (Sivakami et al., 2013).
Different techniques have been used to remove Cr(VI) from the industrial wastewater, such as chemical precipitation, ion exchange, reduction, electrochemical precipitation, solvent extraction, membrane separation, cementation, electrodialysis and adsorption (Sivakami et al., 2013). In the last years very interesting works have been published concerning the removal of hexavalent chromium using adsorbent materials that can reduce Cr(VI) to the less or nontoxic Cr(III) and simultaneously adsorb the reduced Cr(III) (Bellú et al., 2010, Gu et al., 2012, Huang et al., 2013, Park et al., 2008, Qiu et al., 2014a, Qiu et al., 2014b, Qiu et al., 2014c, Qiu et al., 2014d, Qiu et al., 2015, Sankararamakrishnan et al., 2006, Sivakami et al., 2013, Xu et al., 2014, Zhang et al., 2010, Zhang et al., 2013, Zhu et al., 2012, Zhu et al., 2014). These adsorbents not only act as electron donors for the Cr(VI) reduction, but also have active sites for Cr(III) adsorption. Different materials were analyzed and techniques such as X-ray photoelectron spectroscopy (XPS), X-ray absorption spectroscopy (XAS) were applied to determine the oxidation state of the adsorbed chromium species (Bellú et al., 2010, Park et al., 2008, Qiu et al., 2014a, Qiu et al., 2014b, Qiu et al., 2015, Xu et al., 2014).
Adsorption using biopolymers, is one of the most recommended processes, due to economical and technical advantages (Bhatnagar and Sillanpää, 2009). The major advantages of biosorption over conventional treatment methods include: low price, high effectiveness, minimization of chemical and/or biological mud, restoration of biosorbent and possibility of metal recovery.
Chitosan, is a natural cationic polyelectrolyte that has been used for remove cationic and anionic metals such as copper (Laus et al., 2010, Yu et al., 2013), mercury and lead (Ng et al., 2003, Qi and Xu, 2004), from aqueous solutions: it combines with metal ions by three forms: ion exchange, adsorption and chelation (Qi and Xu, 2004). The adsorption of a metal ion on chitosan depends strongly on the pH of the solution (Schmuhl et al., 2001). Several authors analyzed the adsorption of Cr(VI) onto chitosan and modified chitosan particles (Hena, 2010, Huang et al., 2013, Sankararamakrishnan et al., 2006, Sivakami et al., 2013, Zhang et al., 2013).
Chitosan is soluble in most dilute mineral acids, (except in sulfuric acid solutions) and in dilute organic acids, such as acetic, propionic, formic and lactic acids (Laus et al., 2010), consequently, its chemical stability needs to be reinforced through treatments using crosslinking agents for its application in acidic media. Chemical and physical cross-linking techniques are usually employed to modify chitosan; crosslinking may be performed by reaction of CH with different agents such as glutaraldehyde, ethylene glycol diglycidyl ether and epichlorohydrin. Tripolyphosphate (TPP) has also been proposed as a possible crosslinking agent (Laus et al., 2010). CH can be chemically cross-linked with glutaraldehyde leading to quite stable matrixes, with strength covalent bonds, however glutaraldehyde is toxic and a strong irritant (López-León et al., 2005, Sano et al., 2005). Chitosan hydrogels can be obtained by ionic gelation, where micro or nanoparticles are formed by means of electrostatic interactions between the positively charged chitosan chains and polyanions employed as physical cross-linkers. Tripolyphosphate can be used as the polyanion for the cross-linking process; it is a condensed non-toxic phosphate suitable for water purification and also used as food additive (Calvo et al., 1997, Laus et al., 2010). The protonated amine groups in chitosan interact with the negatively charged counterions, through an electrostatic interaction creating ionic cross-linked networks. It is important to remark the need to use non-toxic reagents such as TPP for water treatments. The correct interaction between chitosan and TPP leads to the formation of micro and nanoparticles (Calvo et al., 1997); therefore defined proportions of both components must be establish in order to get adequate particle properties related to size distribution and Z potential.
The analysis of the pH effect on the performance of CH and MCH particles to remove Cr(VI) from contaminated water is complex because different factors, such as the proportion of Cr(VI) stable complexes (anions) formed, the presence of protonated groups in chitosan particles and the addition of TPP as reticulating agent are interacting; available information on this subject is scarce in literature.
The general objective of the present work was to analyze the use of contaminating crustacean shellfish wastes for water purification, determining the simultaneous effects of pH and ionic crosslinking of chitosan particles on the removal of hexavalent chromium.
The specific objectives are: (a) to obtain chitosan particles (CH) from seafood processing wastes (shrimp shells), determining the de-acetylation degree and the average molecular weight; (b) to synthesize reticulated chitosan micro and nanoparticles (MCH) by ionic cross-linking of non modified chitosan with a non-toxic reagent (TPP, tripolyphosphate) establishing the adequate proportion of both components; (c) to characterize MCH particles by Scanning Electron Microscopy (SEM,) size distribution, Zeta-potential; (d) to analyze the performance of MCH and CH particles, in the adsorption process of hexavalent chromium anions in aqueous solutions at different pH values, initial chromium concentrations and contact times; (e) to determine the oxidation state of the adsorbed chromium on CH and MCH; (f) to model mathematically the equilibrium isotherms and adsorption kinetics in the analyzed systems.
Section snippets
Chitin and chitosan production
Shrimps shells (Pleoticus muelleri) were used for the extraction of chitin and chitosan. The shells were provided by the industry “Madryn Seafood” SRL from Puerto Madryn, Patagonia-Argentina.
Chitin extraction, chitosan production and deacetylation degree determination
Crude shrimp chitin was purified using acid and alkaline treatments. The yield of purified chitin was 24.8 ± 0.4%. This value was into the range reported by different authors (Abdou et al., 2008, Cocoletzi et al., 2009). After N-deacetylation, the yield of shrimp chitosan represented the 76.9 ± 1.1% of the initial crude chitin.
Titration curves were performed to determine the degree of N-deacetylation (DD%) of the obtained chitosan by the potentiometric technique (Fig. 1a); two inflection points
Conclusions
Chitosan particles were obtained from shrimp shells and were physicochemically characterized; the degree of deacetylation (86–90.1%) was determined by potentiometric titration and FTIR. Molecular weight of chitosan was determined by intrinsic viscosity measurements. Reticulated chitosan micro/nanoparticles (MCH) were synthesized by inducing the gelation of a chitosan solution with tripolyphosphate (TPP), a non-toxic polyanion. Different mean sizes of MCH were obtained by adjusting the ratio of
Acknowledgments
The authors acknowledge the financial contributions of: Consejo Nacional de Investigaciones Científicas y Técnicas (CONICET); Centro Nacional Patagónico (CENPAT-CONICET), Centro de Investigación y Desarrollo en Criotecnología de Alimentos (CIDCA, UNLP-CONICET), Universidad Nacional de La Plata, Agencia de Promoción Cientifica y Tecnologica (ANPCYT), “Madryn Mariscos” SRL and “El Náutico” of Puerto Madryn.
The authors would also like to thank Dr. Carla Giacomelli and her research group of the
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