Development of industrial activities and the growth of the world population has caused serious environmental problems such as water pollution and subsequently led to limited access to clean water throughout the world 1. Due to the occurrence of such problems in recent years, scientists have made several attempts toward removal of existed pollutants such as toxic organic, chemicals substances and pathogenic microorganisms in water reservoirs 2. A lot of industries, including the textile, tanneries and other chemical factories, continuously release a great amount of polluted wastewater originated from harmful waste materials like dyes, heavy metals and surfactants, which are major health issues all around the world 3.
Some dyes, especially those use in the textile industry, show great resistance against microbiological degradation and some of them have carcinogenesis and mutagenesis properties which are regarded as the major health risk factors for human beings and all other living organisms in the world 4. One of the most commonly used dyes in industries, medicine and chemical activities is Methylene Blue (MB) which is a heterocyclic organic compound with sulfonic group and N ≡ N bonds in its molecular structure. Water contamination with MB causes serious health problems such as blindness, respiratory disorders and digestive diseases 5. In addition, the presence of MB in water, even at extremely low concentrations impacts photosynthesis due to reducing the sunlight penetration. So, refining processes for the MB dye molecule removal from industrial wastewaters is crucial before being released into the environment 6. Besides the dyes, the existence of pathogens and pollutants microorganisms in water encourages the researchers to develop new technologies to control infections and contagious diseases caused by pathogenic bacteria. These attempts resulted in introducing effective approaches against pathogenic microorganism propagation and growth 2.
There are several chemical and physical methods to refine dye-containing industrial wastewaters, which include adsorption, electrochemical precipitation, membrane separation processes, chemical precipitation, photocatalysis, coagulation flocculation, ozonation, biological methods, membrane filtration and sono-oxidation7,8. Most of these treatments are not effective in the refining of dye contaminated effluents because most of them solely transferring the pollutants from one phase to another one without more processing and therefore lead to secondary pollution.
Advanced oxidation processing (AOP) which is based on multiply structures photocatalysts has been applied as an alternative method for treatment of contaminated water samples 8. These approaches are based on platforms with lower cost, nontoxic features, facile operation, higher efficiency and nondestructive properties 9. Most of the AOP strategies generate hydroxyl radicals that cause the oxidation process and following mineralize organic and inorganic contaminants in water 8. The photocatalysis mediated with semiconductors has been regarded recently to overcome environmental challenges like removing pollutants from wastewater 7. Excitation of electrons in the valence band of semiconductors by ultraviolet or visible light results in electron transfer to the conduction band. This phenomenon leads to the production of electron-hole pairs, that in turn generates some free radicals which are involving in oxidation-reduction reactions. So far, many photocatalysts such as Fe2O3, ZnO, TiO2, Bi2O3, CdS, ZnS and WO3\(\)have been used for biological and industrial applications 10. Among them, ZnO has a critical and useful broad bandgap of 3.37 eV with essential binding energy (60 meV). Moreover, ZnO in heterogeneous photocatalysis has been extensively applied for treating wastewater due to its distinguished photocatalytic property, abundance in nature, low price, high chemical stability, suitable optical band gap, low toxicity and environmental resistance. It has been demonstrated that ZnO showed higher photocatalytic performance than common TiO2 photocatalyst. Despite mentioned advantages, ZnO photocatalyst still exhibits low photocatalytic activity due to various problems like low photon utilization efficiency, the high recombination rate of charge carriers and narrow spectrum range with a wide bandgap, etc. So, more treatment of ZnO is required to improve its photocatalytic efficiency 5.
There are diverse methods to address above mentioned challenges, such as the development of novel photocatalysts with the constituents with wider bandgaps, and also metal nanoparticles as an electron sink, or stabilizing semiconductor catalysts on some materials which have much larger surfaces. The usage of stable catalysts in the photocatalyst structure has economic advantages on a large scale due to the separation and reusability of the catalyst after the process 11. The development of hybrid composites has dramatically increased since they showed a wide range of usage, which involves water treatment, antimicrobial properties and photocatalysis 12. Chitosan is a heteropolymer which composed of glucosamine and acetyl glucosamine units. The application of Chitosan and heterogeneous oxide materials is increasing because of the unique properties of its major compounds and synergistic impacts on the final product. In recent years, Chitosan is greatly used in different sections of biosensors for medical applications, as a nanoparticle protecting layer. It exhibited distinguished properties such as nontoxicity, biodegradability, biocompatibility, and high absorbance ability. Many kinds of research have depicted that Chitosan can considerably increase photocatalytic activity while integrated with a composite catalyst because of its high potential in the adsorption of different organic dyes. ZnO/Chitosan nanocomposite has extensively attracted vast interest due to its unique applications, like UV protection and antimicrobial activity in recent years 13.
The interaction of contaminant with Chitosan/metallic oxide photocatalyst happens through chelation, coordination of NH2 groups, co-precipitation and ligand exchange or electrostatic interaction with protonated NH2 groups. Adsorption of various sorts of contaminants is attributed to the unprotonated NH2\(\)group of the Chitosan as the ligand. So, protonation reduces the pollutant adsorption and the best performance of photocatalyst would be at higher pH values. Even dispersion of catalyst in water is another challenge for applied catalysts which must be addressed in the construction of photocatalysts. The modification of the Chitosan by means of physical and chemical methods can alter its physicochemical properties and further impact its adsorption behavior to overcome the limitation of a particular photocatalyst 14. An efficient method for improving the physic/chemical properties of Chitosan is its combination with inorganic fillers, which contain Silica, hydroxyapatite, Calcium phosphate cements and clay nanoparticles that are frequently use to sustain the Chitosan matrix 15.
Carbon nanotubes (CNTs) are cylindrical nanomaterials that exhibit remarkable electrical conductivity, thermal conductivity and mechanical features. The modification of CNTs with Chitosan has improved the treatment of mechanical and thermal properties compared to pure Chitosan, which may extend the application of Chitosan 15. Recently, Multi-Wall Carbon nanotubes (MWCNTs) have been extensively used as catalytic material because of their unique properties. It has been demonstrated that combining ZnO with carbonaceous nanomaterials like CNTs is more efficient than the ZnO photocatalyst 11. The reason is the inhibition effect of CNT on recombination rate between generated electron and hole pairs through accepting electrons from ZnO 8. In addition, some features of CNTs, such as good chemical stability and corrosion resistance, suppress the photo corrosion effect of ZnO in CNT/ZnO composite structure and hence improve the photocatalytic efficiency 13. It was shown that functionalization of CNTs by ZnO nanoparticles increase the photocatalyst activity via enhancing the adsorption of light by photocatalyst, improving the transport charge and increasing the surface area, which consequently inhibits the recombination of electron-hole pairs 11. So, it would be possible to enhance the efficiency of photocatalysts through the construction of nanocomposites by incorporating MWCNTs, ZnO and Chitosan.
In the present research, the ZnO nanoparticle, ZnO/Chitosan nanocomposites and MWCNTs/ZnO/Chitosan have been synthesized to determine their photocatalytic activities (Scheme 1). Morphological and properties of synthesized nanocomposite photocatalysts were characterized by FT-IR, UV-Vis, XRD, SEM, EDAX, TEM, AFM and BET analysis. Then, photocatalytic degradation and adsorption of MB as the target dye has been investigated in the presence of UV light irradiation. The antibacterial effects of photocatalysts were also surveyed on gram-negative (Escherichia coli) and gram-positive bacteria (Staphylococcus aureus and Bacillus subtilis). The performance of photocatalysts was studied through optimization of parameters including catalyst weight, dye concentration, pH of the solution and the temperature on the MB degradation. To the best of our knowledge, this is the first report of MWCNTs/ZnO/Chitosan nanocomposites used for photocatalytic degradation and adsorption of MB pollutants under UV light irradiation.