Photocatalytic Performance Of Iron Doped ZnIn2S4and Intergrated Dynamic Membrane System

Many researches about photocatalytic mechanism, photocatalytic materials and pollutants degradation have been investigated during the development of photocatalytic technology, which provide sufficient theoretical basis and solid experimental basis for the application of photocatalysis. However, the practical application of photocatalysis is still limited by the low photocatalytic efficiency especially for visible light, the separation and reuse of photocatalyst and long-term stability of photocatalyst. In this paper, zero valent iron (Fe0), semiconductor Fe2O3and FeOOH were used as dopants and intergrated with ternary semiconductor ZnIn2S4. It is anticipated to improve photocatalytic degradation of halogenated phenol under both UV illumination and visible light based on different reaction mechanism.There also existed two main limiting factors that hindered the implementation of efficient and practical membrane separation technology in separating suspended photocatalyst.1). Photocatalyst particles can cause the membrane fouling in nanofiltration (NF), ultrafiltration (UF), or reverse osmosis (RO) and there is a danger of degradation and destruction of membrane structure by UV light or hydroxyl radicals.2). In most studies, photocatalysis and membrane separation were two independent operation processes, which only realized the separation of photocatalyst but didn’t realize the recycle and reuse of photocatalyst. Therefore, a photocatalysis and rotating dynamic helical membrane reactor system was established for continuous photocatalytic degradation of2,4,6-Tribromophenol (2,4,6-TBP) and2,4-Dichlorophenol (2,4-DCP). Rotating helical membrane module can realize retention and recycle of photocatalyst and at the same time can serve as a stirrer to ensure the uniform suspension of photocatalysts. Different dynamic layer materials such as graphite oxide, graphene oxide and CaCO3were applied to enhance the filtration performance and prevent fouling and destruction of supporting membrane. In this way, serval works have been done and obtained corresponding results.(1) Zero valent iron doped ZnIn2S4was prepared through a facile hydrothermal synthesis process at low temperature without templates and followed chemical reductive deposition of Fe0. UV-vis diffuse reflectance spectra showed that the absorption edge of Fe0-ZnIn2S4red-shifted slightly comparing to the Znln2S4. The corresponding band gaps of Fe0-ZnIn2S4and ZnIn2S4calculated from the onset of the absorption edges were2.12eV and2.05eV, respectively. Based on the auxiliary dehalogenation of zero valent iron under UV illumination, the debomination of2,4,6-TBP was enhanced by Fe0-ZnIn2S4. After1h reaction, the debromination efficiency was up to61%on Fe0-ZnIn2S4, which was1.11times higher than ZnIn2S4. The repeated tests proved that the synthesized Fe0-ZnIn2S4was stable, and could be reused for many times without significant loss in photocatalytic activity.(2) There might be iron oxides formed when Fe0-ZnIn2S4composite photocatalyst was used, so iron oxides doped ZnIn2S4were investigated. On account of different semiconductor band structure and taking advantage of the powerful photo-oxidation of iron (hydr)oxides and the outstanding reducing power of ZnIn2S4, Fe2O3or FeOOH doped ZnIn2S4showed excellent photocatalytic activity in degradation of2,4,6-Tribromophenol (2,4,6-TBP). The degradation rate constants of Fe2O3-ZnIn2S4and FeOOH-ZnIn2S4were1.72and1.21times higher than ZnIn2S4under visible light,4.54and4.34times higher under UV illumination, respectively. The Fe2O3-ZnIn2S4was found more active than FeOOH-ZnIn2S4in debromination and mineralization of2,4,6-Tribromophenol. After2h photocatalytic reaction on Fe2O3-ZnIn2S4, the released Br-concentration was26.3mg L-1corresponding88%debromination, which was1.41times higher than debromination on ZnIn2S4. The tentative photocatalytic degradation pathways on Fe2O3-ZnIn2S4and FeOOH-ZnIn2S4were proposed based on the LC-MS detected intermediates.(3) To achieve in situ separation and circulation of photocatalyst, a rotating helical membrane was designed by taking example of a propeller. The turbulence was enhanced through ingenious spirals and fluid motion caused by rotating helical membrane, which can result in membrane fouling reduction in separating solid particles. Short-term filtration experiment showed that about27%enhancement of stable flux can be maintained by rotating a360°helical membrane, comparing with rotating a same size flat membrane. The particle tracer study visually emphasized that rotating a helical membrane could improve vortex mixing, which was advantageous to uniform suspension of particles. Comparing with general stirring, rotating helical membrane was more conducive to mass transfer, especially for separation, stirring and mass transfer in a single device, which can be used as a practical photocatalytic membrane reactor (PMR).(4) In order to enhance the filtration performance and improve separation efficiency of photocatalyst, a photocatalysis and dynamic membrane reactor system, with pre-coated dynamic layer on helical membrane, was studied for continuous photocatalytic degradation of halogenated compounds2,4,6-Tribromophenol and2,4-Dichlorophenol in water. Fe0-ZnIn2S4was used as photocatalyst. Comparing with the PMR system with graphite oxide and graphene oxide dynamic layer, the debromination efficiency of2,4,6-Tribromophenol was increased by27%and11%, COD removal rate was increased by20%and30%in the PMR system with CaCO3dynamic layer, respectively.85%-90%debromination and72%COD removal were obtained in CaCO3dynamic membrane hybrid system. The batchwise experiment indicated that CaCO3could scatter incident light and enhance light utilization. And the enhanced photocatalytic degradation efficiency was accomplished by increased direct UV photolysis efficiency in the solution and on CaCO3surface. The long-term experiment demonstrated that photocatalysis dynamic membrane hybrid system was very steady in the long-term operation and the debromination efficiency was maintained at about85%during the photocatalytic debromination reaction.Iron dopants such as zero valent iron (Fe0), FeOOH and Fe2O3can significantly improve photocatalytic degradation of halogenated phenol. The research on degradation mechanism offered experimental and theoretical basis for iron-doped composite photocatalyst and the treatment of halogenated phenol. Rotating helical membrane provided a new alternative for membrane fouling reduction. The multifunctional performance of photocatalysis and rotating dynamic helical membrane hybrid PMR system indicated great application prospect in suspended photocatalytic system.