Chitosan/titanium oxide composite: a photocatalyst and bacteriocide

A chitosan/titanium oxide composite exhibited high photocatalytic activity against harmful dyes as well as antibacterial properties, and it was easier to recover from the environment than titanium oxide alone. Over the past few decades, synthetic dyes in industrial effluents have…

A chitosan/titanium oxide composite exhibited high photocatalytic activity against harmful dyes as well as antibacterial properties, and it was easier to recover from the environment than titanium oxide alone.

Over the past few decades, synthetic dyes in industrial effluents have posed a major threat to the environment. Worldwide, approximately 10–15% of the dyes are wasted into the environment after their use in dyeing units.1 Since the dyes are stable, recalcitrant, colorant, and even potentially carcinogenic and toxic, their release into the environment poses a major issue.1 Methylene blue (MB) is the most commonly used substance for dying cotton, wood, and silk. On inhalation, it can give rise to short periods of rapid or difficult breathing while ingestion through the mouth produces a burning sensation and may cause nausea, vomiting, profuse sweating, mental confusion, and methemoglobinemia. Therefore, the removal of MB dye from waste effluents is environmentally important. Many techniques are employed to accomplish this task. Among these, photocatalysis is a promising method already used in the degradation of organic pollutants in wastewater.2Recently, titanium dioxide (TiO2) has garnered huge attention because of its abundance, low cost, good chemical stability, environmental benignity, high photocatalytic activity, and antimicrobial properties. Photocatalytic applications of TiO2 are widely used in the decomposition of aqueous pollutants because of the strong resistance of TiO2 to photocorrosion, the low operating temperature required, and its very low energy consumption.3 Unfortunately, it is difficult to recover and recycle TiO2 powder at industrial scale. Immobilization of TiO2 onto a solid support could solve the problem. Various attempts have been undertaken to develop a new, efficient solid support. Recent research demonstrates polysaccharides are a good choice for supporting the metal oxide photocatalyst.4 The advantages of the approach include low-cost, high catalytic activity, and extensive potential for reuse.4Chitosan (CS) is a naturally occurring alkaline polysaccharide composed of β-1,4-linked glucosamine. It is easily obtained by N-deacetylation of chitin. CS is a high-capacity adsorbent for efficient removal of contaminants from wastewater, better than activated carbon.5 The presence of plentiful amino and hydroxyl groups in the macromolecular chains is highly advantageous for conducting modification reactions and for providing distinctive biological functions. Its other outstanding characteristics include biocompatibility, biodegradability, nontoxicity, and potent antimicrobial activity.6We prepared a new kind of multifunctional CS/TiO2 composite using a chemical precipitation method.7 The composite was designed to meet the needs of high adsorption, self-regeneration, easy separation, and cost-effective water treatment with enhanced antimicrobial properties.Figure 1.(a) UV–visible spectra of the methylene blue (MB) solution in the presence of chitosan-titanium dioxide (CS/TiO2) composite at different UV irradiation times, and (b) photodegradation of the MB dye. Co: initial concentration of the dye. Ct: concentration of the dye at time t.We recorded the UV–visible spectra of MB solution with the photocatalyst CS/TiO2 at varying durations of UV irradiation (see Figure 1a). The intensity of the absorption peak corresponds to the MB dye, which decreased with increasing UV exposure time, indicating the dye degradation. During degradation, the dye molecule is completely destroyed, with conversion of all carbon atoms into gaseous carbon dioxide and the nitrogen and sulfur atoms into inorganic ions such as nitrate, ammonium, and sulfate. The evolution of MB dye degradation by the photocatalyst is shown in Figure 1(b). Approximately 90% of the dye was degraded by the composite within 3 hours.7 The photocatalytic process is based on the generation of electron–hole pairs by band-gap radiation, which can give rise to redox reactions with the species adsorbed on the surface of the catalysts. In the photocatalytic process, the active species is generally considered to be the hydroxyl free radicals (•OH), which originate from the oxidation of OH− or H2O by the photogenerated electron-hole pairs in the presence of oxygen. These are strong oxidants and can oxidize the organic dye, leading to dye fading. We also tested the stability and reusability of the photocatalyst. Our results showed that the repeated use of recycled composite (five times) did not affect its photocatalytic activity significantly.7Furthermore, we examined the antibacterial activity of the CS/TiO2 composite. The composite inactivated the bacteria by 98% within 6 hours, as shown in Table 1.7 The high antibacterial activity of the composite is due to the synergistic effect of CS and TiO2. The positively charged composite interacts with the negative charged microbial surface and distorts the surface’s permeability, thus blocking nutrient uptake and negatively influencing the cell’s growth and viability.8 It is also worth noting that the bacteria were killed by the reactive species generated by the photocatalytic process of TiO2.Table 1.Antibacterial activity of CS/TiO2composite against E.coli. CFU: colony-forming units.Incubation time (h)CFU (ml−1)Survival fraction N/NoReduction in viability %04.0×10610669×10317.3×10−398.271245×10311.3×10−398.871835×1038.8×10−399.92244.0×1021.0×10−4100In summary, we showed the efficient photocatalytic and antibacterial activities of CS/TiO2 composite. The CS helped in recollection and reuse of TiO2. We believe the composite could be a cost effective, highly efficient, reusable, and environmentally-friendly photocatalyst. The superior antibacterial activity with complete reduction in growth of bacteria showed its efficiency for sterilization of bacteria in ordinary living spaces. The next stage of our research will be to fabricate CS/MgO and CS/Fe3O4 composites for various applications, especially heavy metal adsorbents and biosensors.AuthorYuvaraj HaldoraiYeungnam UniversityYuvaraj Haldorai is an international research professor in the school of chemical engineering. His research interests include organic/inorganic hybrids and nanomaterials for energy applications.ReferencesG. Moussavi and M. Mahmoudi, Removal of azo and anthraquinone reactive dyes from industrial wastewaters using MgO nanoparticles, J. Hazard. Mater. 168, pp. 806, 2009. C. Sahoo, A. K. Gupta and I. M. Sasidharan Pillai, Photocatalytic degradation of methylene blue dye from aqueous solution using silver ion-doped TiO2 and its application to the degradation of real textile wastewater, J. Environ. Sci. Health A 47, pp. 1428, 2012. B. Ohtani, Y. Ogawa and S. Nishimoto, Photocatalytic activity of amorphous-anatase mixture of titanium (IV) oxide particles suspended in aqueous solutions, J. Phys. Chem. B 101, pp. 3746, 1997. E. Guibal, Heterogeneous catalysis on chitosan-based materials: a review, Prog. Polym. Sci 30, pp. 71, 2005. W. S. Wan Ngah, L. C. Teong and M. A. K. M. Hanafiah, Adsorption of dyes and heavy metal ions by chitosan composites: A review, Carbohyd. Polym. 83, pp. 1446, 2011. E. I. Rabea, M. E. T. Badawy, C. V. Stevens, G. Smagghe and W. Steurbaut, Chitosan as antimicrobial agent: applications and mode of action, Biomacromolecules 4, pp. 1457, 2003. Y. Haldorai and J. J. Shim, Novel chitosan-TiO2 nanohybrid: Preparation, characterization, antibacterial, and photocatalytic properties, Polym. Compos, 2013. L. H. Li, J. C. Deng, H. R. Deng, Z. L. Liu and X. L. Li, Preparation, characterization and antimicrobial activities of chitosan/Ag/ZnO blend films, Chem. Eng. J. 160, pp. 378, 2010. DOI:  10.2417/spepro.005202

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