Preparation Of ZnO Nanocomposites For Bacterial Inactivation Under Visible Light Irradiation

The spread of microorganisms in the environment,especially pathogenic bacteria,usually results in the outbreak of serious waterborne diseases,which poses high risks to human life and health.Therefore,developing antibacterial materials to control the growth and reproduction of pathogenic microorganisms is of great significance.As a photocatalyst with low cost,simple production process and low toxicity,nano ZnO has a broad application prospect in antibacterial field.However,nano ZnO also has some drawbacks,such as narrow photoresponse range,high recombination rate of photogenerated carriers,photocorrosion effect and recycling difficulty,which limit its practical application.In this dissertation,nano ZnO was doped with different metal ions,and the influence of doping on the structures,morphologies and optical properties of nano ZnO was systematically studied.Also,magnetic ZnO nanocomposites were prepared,which could be simply collected by a magnet.And the visible-light-driven photocatalytic bacterial inactivation efficiencies of as-prepared samples against E.coli were explored.The details are as follows:?1?ZnO nanoparticles doped with different metal ions(Li⁺,Mg²⁺,Al³⁺and Ti⁴⁺)were synthesized by a sol-gel method.The influence of different metal ion doping on the structures,morphologies,optical properties and photocatalytic bacterial inactivation properties of ZnO was systematically studied.XRD and TEM results demonstrated that Li doping had a role of facilitating the crystallization and grain growth of ZnO particles,while Al doping and Ti doping exhibited an inhibitory action.The BET surface areas of doped ZnO and ZnO followed an order:Ti-ZnO>Al-ZnO>Mg-ZnO>ZnO>Li-ZnO,which was opposite to the order of particle size.Al-doped ZnO exhibited the best visible-light-driven photocatalytic activity among the as-prepared samples.25 mg of Al-doped ZnO could completely inactivate 7-log of E.coli cells within 4 h under the irradiation of Xe lamp??>400 nm,light intensity:200 mW/cm²?.The bacterial inactivation mechanisms of Al-doped ZnO could be explained as follows.1)Al-doped ZnO exhibited a long absorption onset wavelength?433.3 nm?,and a small band gap energy?2.86 eV?,which indicated that Al-doped ZnO could use visible light efficiently.2)Al-doped ZnO possessed more oxygen vacancies,which could adsorb more oxygen,so more reactive oxygen species was formed during the process of photocatalysis.3)Abundant surface defect sites on Al-doped ZnO could act as electron traps or hole traps,which promoted the separation of photogenerated electrons and holes.Moreover,after 4 h of Xe lamp irradiation,5.23 mg/L of Zn²⁺was released from ZnO,while only 0.76 mg/L of Zn²⁺was released from Al-doped ZnO,which revealed that Al doping could efficiently suppress the photocorrosion of ZnO.?2?Magnetic Fe@Al-ZnO nanocomposite was prepared by two-step precipitation method.The phase structure,morphology,elemental composition and magnetic property of Fe@Al-ZnO composite were analyzed by XRD,SEM,XPS and VSM.The results indicated that Fe@Al-ZnO had a particle size of about 50 nm.And its saturation magnetization was 35.2 emu/g,which was high enough to be magnetically separated.Under the irradiation of Xe lamp??>400 nm,light intensity:200 mW/cm²?,40 mg of Fe@Al-ZnO could completely inactivate 7-log of E.coli cells in 5 h.The XRD pattern of used Fe@Al-ZnO revealed that the peaks of Fe@Al-ZnO stayed unchanged,and some new diffraction peaks were found,corresponding to bacterial mineralization products(Zn_0.71Al_0.29?OH?₂?CO₃?_0.145·xH₂O).Also,the used Fe@Al-ZnO still exhibited good ferromagnetic behavior?32.0 emu/g?.The above results proved excellent stability of Fe@Al-ZnO photocatalyst.?3?Magnetic Fe@ZnO_0.6S_0.4 nanocomposite was prepared by liquid phase coprecipitation method.The phase structure,morphology,elemental composition and magnetic property of Fe@ZnO_0.6S_0.4 composite were analyzed by XRD,TEM,SEM,XPS and VSM.The results indicated that Fe@ZnO_0.6S_0.4 had a small particle size of about 10-20nm,and the saturation magnetization of Fe@ZnO_0.6S_0.4 was 5.8 emu/g.UV-Vis DRS results showed that the absorption onset wavelength of Fe@ZnO_0.6S_0.4 was about 550 nm,and the band gap energy of Fe@ZnO_0.6S_0.4 was 2.25 eV,which indicated that Fe@ZnO_0.6S_0.4 could use visible light efficiently.Under the irradiation of white LED lamp?light intensity:15mW/cm²?,30 mg of Fe@ZnO_0.6S_0.4 could inactivate 7-log of E.coli cells in 5 h.The photocatalytic bacteria inactivation performance of Fe@ZnO_0.6S_0.4 was much better than Fe@ZnO and Fe@ZnS.In addition,Fe@ZnO_0.6S_0.4 photocatalyst had good stability and reusability.Almost no decrease of photocatalytic efficiency in bacterial inactivation was observed even after five consecutive cycles.Moreover,no Fe²⁺/Fe³⁺was released from Fe@ZnO_0.6S_0.4 even after 5 h of LED lamp irradiation.The release of Zn²⁺was 1.16 mg/L,far below the release of Zn²⁺from nano ZnO.The bacterial inactivation mechanism analysis showed that the reactive oxygen species generated during the photocatalytic process were the main factors that inactivate the E.coli,rather than the bactericidal effect of the released metal ions.