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Recently, photocatalytic processes in the presence of bismuth-based nanomaterials have been widely used as an efficient and low-cost method to treat pharmaceutical wastewater, including antibiotics. Among these photocatalysts, the Bi2WO6 semiconductor due to its structure and unique physico-chemical properties, activity in the visible light region and high stability against photocorrosion has attracted much attention. However, the photocatalytic efficiency of these nanomaterials in the visible region is not very favorable due to relatively high band gap energy, relatively low light absorption ability, fast electron-hole recombination, low absorption capacity, tendency to accumulate in solution, and difficult recovery. The effective strategies to overcome these problems are the stabilization of nanoparticles on support and the formation of a heterojunction structure. In this regard, sepiolite clay mineral is considered as a favorable choice for loading and immobiliing nanoparticles due to its low cost, abundance, fibrous and porous morphology, high surface area, and high absorption capacity. On the other hand, CuO with very narrow band gap energy and high oxidation power is a suitable semiconductor for forming an efficient heterojunction structure with Bi2WO6. Therefore, in this research, the effectiveness of different loadings of Bi2WO6 nanoparticles as well as the various compositions of CuO-Bi2WO6 heterojunction structure synthesized by precipitation method over sepiolite in the photocatalytic degradation of tetracycline under simulated solar light were investigated. The synthesized samples were characterized using XRD, FESEM, N2 adsorption-desorption, DRS, and PL analyses. The performance results showed the significant effectiveness of the sepiolite and copper oxide existence on the photocatalytic performance of Bi2WO6 semiconductor. According to the obtained results, it was found that 20 wt.% is the optimal content of bismuth tungstate photocatalyst for loading over raw sepiolite both economically and efficiently. With higher loading, the photocatalytic removal efficiency does not change significantly mainly due to the intense and multi-layered surface coverage of raw sepiolite and the formation of agglomerates. The evaluation results of 20 wt.% loading of CuO-Bi2WO6 heterojunction structure with different compositions showed that the use of more than 10 wt.% of copper oxide, despite the further reduction of the band gap energy and better activation in the visible region, causes intense surface coverage of the sepiolite fibers and noticeable loss of texture properties and thereupon, increase in the recombination rate of charge carriers as well as a decrease in the number of active sites for reaction with pollutants and reduce the effectiveness of copper oxide. Kinetic studies also showed that the degradation rate of the contaminant over Bi2WO6-CuO/Sepiolite nanocomposite containing 10wt.% of each of copper oxide and bismuth tungstate nanoparticles is higher and follows the second-order kinetics reaction.Also, the effect of operating parameters on the efficiency of the photocatalytic process was evaluated using a central composite design with four factors at five levels. According to how effective operational factors and interactions between them affect the response, the maximum photocatalytic degradation of tetracycline (97.4%) under optimal operating conditions of pH =5.02, photocatalyst dosage of 0.62 g/L, initial tetracycline concentration of 14.97 ppm and irradiation time of 133.78 min was obtained.
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