Preparation Of Nano-titanium Dioxide And Its Use In The Removal Of Roxarsone From Aqueous Solution

Roxarsone (ROX) has been widely used as an organic arsenic additive in animal industry. Most of roxarsone in the feed was excreted into manure without chemical structural change, which resulted in the contamination of the environment. As an advanced oxidation, photocatalysis has been proven to be an efficient way to mitigate pollution by generating reactive oxidant hydroxyl radicals. The removal of ROX from aqueous solution using TiO2 photocatalysis, the synthesis of TiO2 materials, and the removal mechanism were investigated in this paper. The following conclusions were obtained.Firstly, the adsorption of roxarsone on TiO2 under dark conditions, the photocatalytic decomposition of roxarsone under UV/TiO2, and the possible photocatalytic pathway were investigated. At the initial concentration of 5 mg/L-35 mg/L, the adsorption of roxarsone fitted well with the pesudo-second order kinetics. The isotherms analysis showed that the Langmuir model was better than the Freundlich and Dubinin-Radushkevich models for describing the adsorption process. After 7 hours’photocatalytic decomposition, a complete disappearance of roxarsone was achieved. The pH value has a significant effect on both adsorption and photocatalytic decomposition of roxarsone, acidic conditions favor adsorption while alkaline conditions favor the the photocatalytic. The results of high performance liquid chromatography-hydride generation-atomic fluorescence spectrometry (HPLC-HG-AFS) and gas chromatography-mass spectrometry (GC/MS) analysis proved the cleavage of the As-C bond during the photocatalytic decomposition process by TiO2 and the intermediates of the decomposition. Based on the results, a possible photocatalytic decomposition pathway was proposed.Secondly, due to the low photocatalytic efficiency of ROX by TiO2 and the generation of toxic byproduct As5+, a bifunctional photocatalyst TiO2/FeOOH was synthesized by a facile one-pot method. As compared to single-component system,10 mg/L of ROX could be completely photodegraded within 8h by TiO2/FeOOH, while the removal percentage by TiO2 within the same time could only reach 89%. Besides, TiO2/FeOOH in this paper has achieved a satisfying removal for As5+(byproduct of ROX degradation) due to the good adsorption capacity of inorganic arsenic by FeOOH. After 8 h’photocatalytic reaction, the concentration of As5+ in the FH/TiO2 system was only 46 μg/L, while the corresponding value was 1.46 mg/L in the TiO2 system. The prepared TiO2/FeOOH were characterized by transmission electron microscopy(TEM) with energy dispersive X-ray (EDX) analysis, X-ray diffraction (XRD), UV-Vis diffuse reflectancespectroscopy, Raman spectra and X-ray photoelectronspectroscopy (XPS) to understand the structure. It was shown that in the nanocomposites, TiO2 was coated well by FeOOH through situ growth. The UV light photocatalytic mechanism of TiO2/FeOOH, factor contributing to its high photostability and the removal of As5+ in solution are proposed.Lastly, the cost of visible light is cheaper than UV light. A clean, powerful and broad-spectrum photocatalyst TiO2/rGO/CdS was synthesized using a situ growth method. TiO2/rGO/CdS showed much greater visible light photocatalytic activity than pure TiO2 and TiO2/CdS. At the initial concentration of ROX was 10 mg/L and the catalysts dosage of 0.1 g/L, the removal percentage of ROX by Ti02/rGO/CdS, TiO2 and TiO2/CdS were 100%,53% and 18%, respectively. The removal percentage of TOC was 64% and the concentration of As5+ and- NO2-were 2.5 mg/L and 2.3 mg/L, respectively. The prepared catalysts were characterized by transmission electron microscopy (TEM) with energy dispersive X-ray (EDX) analysis, scanning electron microscope (SEM), UV-Vis diffuse reflectance spectroscopy and Raman spectrato for understanding the synthesis mechanism and the optical properties. A possible mechanism of visible light photodegradation of ROX was proposed.