The Characteristics and Disinfection Performance of Chitosan-N-Doped TiO_2 Composite made from Agricultural Waste

摘要

The problem of food waste causes serious impacts in the economy and environment. In addition, the development of the circular economy has drawn considerable attention. The great amount of food waste, includes shrimp shells, crab shells, and oyster shells, can be transformed into safe synthetic antibacterial agents. Chitin has one hydroxyl group on each monomer substituted with an acetyl amine group. Chitosan (CTS) [poly (b-(1 →4)-2-amino-2-deoxy-D-glucose)] is the N-deacetylated derivative of chitin. CTS is a natural polymer found in the insect exoskeletons, crustacean shells, and fungal cell walls, which is the second most abundant polysaccharide after cellulose. CTS has a wide range of applications and beneficial properties including antifungal and antibacterial. The incorporation of nano-materials, such as nano-TiO_2 into natural biopolymer has been proven to improve the mechanical properties. Titanium dioxide (TiO_2) is an outstanding photocatalyst due to its photo-disinfection activity, high chemical/physical stability, cost efficiency, and environmentally friendly. However, traditional TiO_2 can only be activated by UV light. Nitrogen-doped TiO_2 (N-TiO_2) synthesized by our research group shows high photo-disinfection efficiency under visible light. We hypothesis that the incorporation of shrimp: shells CTS into N-TiO_2 will enhance photo-disinfection capacity with the synergy effect of CTS. The CTS-N-TiO_2 photocatalyst composite will be used in this study. Shrimp shells without pretreatment contain a huge amount of minerals and proteins. Therefore, the synthesis steps of CTS can be separated into three parts, demineralization (DM), deproteinization (DP) and deacetylation (DA). Various synthesis conditions result in different deacetylation degrees (DDA). The photo-disinfection kinetic model experiments are conducted under different key parameters, such as bacteria species effects (Gram-positive bacteria, S. aureus and Gram-negative bacteria, E. coli), visible light intensity effects, dosage effects, and initial bacteria concentration effects. Physical/chemical characteristics of CTS are evaluated by X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR) and scanning electron microscopy (SEM). Photo-disinfection mechanism of bacteria is investigated via SEM, transmission electron microscope (TEM) and atomic force microscope (AFM) technology for morphologies of bacteria cell in the photo-disinfection process. The characteristic results showed that the CTS synthesis from shrimp shell had two broad peaks at 2θ=10.3° and 20.0°. The FTIR spectrum of CTS had a broad band of about 3200-3374 cm~(-1) to show the stretching vibration of OH and NH groups. Antibacterial experiment results showed that CTS exhibited high antibacterial rate against E. coli and S. aureus achieved 99.9% within 2 hour reaction time. The novel CTS-N-TiO_2 photocatalysts produced from food waste, shrimp shell, is equipped with excellent antibacterial activity against both Gram-positive (S. aureus) and Gram-negative (E. coli) bacteria. CTS-N-TiO_2 is advantageous for addressing the problem of abundance food waste and keeping human health with the synergy effect of CTS and nano-material.