Visible Light Photocatalytic/Oxidative Degradation Of Typical Aqueous Organic Dyes VIA Sodium Bismuthate Based Perovskite Materials

The production and widespread use of soluble organic dyes inevitably results in their unintended release into the aqueous environment where they pose potential threats to human and ecological health. With the growing awareness and environmental concerns, it is imperative that the hierarchy of reduction and degradation be adopted and measures be taken to remove pollutants from the organic dye wastewater. The traditional treatment techniques prove to be ineffective in removing these pollutions. The desire to develop efficient water treatment techniques is quickly becoming a major focus in the environmental research community. This work presented novel, affordable and easy-operated organic dye wastewater control technologies based on sodium bismuthate-type functional material, viz. NaBiO3initiated visible light photocatalytic (PC) technique, silver bismuth oxide (BSO) initiated direct oxidation (DO) technique and combined DO/PC technique.For PC process, the photocatalytic degradation of Rhodamine B (RhB) over NaBiO3under visible light irradiation was investigated.20mg/L of RhB solution was completely decolorized in30min with1g/L of NaBiO3·2H2O at the pseudo first-order reaction constant k=0.124min-1. The dye solution absorption maximum shifted from554to534nm during the degradation reaction. NaBiO3heating temperature significantly influenced its photocatalytic activity. The original sample exhibited higher activity than the heated samples did. The stability of NaBiO3was evaluated by repeated degradation of RhB solution for five times. All N-deethylation intermediates and several small molecular products were separated and identified. The yield distinctness between two isomer intermediates was correlated with the changes in the electron density of the dye molecule. Then two possible competitive photodegradation pathways of RhB over NaBiO3were proposed:Chromophore cleavage and N-deethylation.For DO process, the oxidative powder BSO was firstly prepared by simple coprecipitation of NaBiO3·2H2O and AgN03. The technique was evaluated for the decolorization and oxidative decomposition of RhB. The results demonstrated that mixing BSO with an aqueous solution of RhB (20mg/L) resulted in rapid decolorization (pseudo-first-order kinetic constant k=0.5594min-1) and formation of several small molecular weight products. The optimal starting weight for NaBiO3·2H2O to AgNO3is2to1for synthesizing the BSO. The reaction proceeded at ambient temperature and pressure, and required no external energy sources or light. An advantage of the technique is that BSO can be used to degrade sequential additions of dye without significant fouling or loss of activity. The characterization of BSO and its corrosion products revealed that Ag species were reduced to metallic silver and NaBiO3·2H2O was transformed into the Bi2O2CO3during the reaction process. Singlet oxygen (1O2) was identified as the major reactive species generated by BSO for the degradation of RhB.For combined DO and PC process, extensive and much enhanced organic dye degradation was achieved. In single PC treatment process, the Crystal Violet (CV) dye solution absorption maximum blue-shifted from584to576nm; whereas in single DO treatment process, it red-shifted from584to592nm. CV dye decomposition by each reagent proceeded via two distinct pathways, each involving different active oxygen species. N-demethylation of CV significantly occurred in PC but not in DO process. Same amount of NaBiO3·2H2O based DO process exhibited a superior performance to PC process with respect to the CV decolorization, particularly when high concentration dye solution was treated. A comparison of each treatment method alone and in combination demonstrated that using the combined sequential methods37%of TOC removal was achieved in30min for130mg/L of CV solution whereas only18%and15%was obtained using DO and PC alone, respectively. The combined method also manifested significantly improved mineralization of several diverse groups of dyes. In the combined process DO acts as a pre-treatment to rapidly bleach the dye solution which substantially facilitates subsequent PC processes. The integrated sequential DO and PC are complementary manifesting a>100%increase in TOC removal, compared to either isolated method. The combined process is proposed as a novel and effective technology (especially with respect of TOC removal) for treating wastewaters contaminated by high concentrations of organic dyes.In addition, as an extension of the study, copper doped sodium bismuthate (CSB) was firstly synthesized through aqueous ion exchange reaction and a simple and efficient method based on it for degrading organic dye is proposed. The Malachite Green dye degradation experiments demonstrated that mixing0.2g NaBiO3·2H2O based CSB in the dye solution (30mg/L) for15min resulted in95%of decolorization and formation of several small molecular weight products. The sodium bismuthate starting weight equivalent to that of copper nitrate is optimal to synthesize CSB. The reaction proceeded at ambient temperature and pressure, and required no external energy sources or light. The characterization suggested copper species were distributed through the external surface of CSB. CSB performed in a similar functional mode as BSO did and it could be further developed as an optional material for BSO at lower cost.