Document Type : Article
Authors
Dept. of Civil & Environmental Engineering Tarbiat Modares University
Abstract
Synthetic dyes are widely used in many industries, such as, textile dyeing, leather tanning, plastic and paper. In general, more than 40 percent of initial dye mass remains in the dye bath, which means that it is highly contaminated. Azo dyes, the largest class of synthetic dyes, are distinguished by containing one or more azo groups (-N=N-) that are bounded to the aromatic rings. They not only give color to water but also have mutagenesis and carcinogenesis potential and induce the production of toxic byproducts. They are difficult to remove from wastewater using common physical, chemical and biological treatment methods, and, therefore, it is essential to look for appropriate techniques for treatment of these kinds of pollutant to reduce their environmental impact.
Recently, application of advanced oxidation methods, such as photo catalytic reactions, for removing toxic and non-bio-degradable pollutants from drinking water and textile wastewater have been expanded. It is based on the generation of hydroxyl radicals (0OH) with great oxidizing potential that can quickly and non-selectively oxidize a wide spectrum of diverse organic dyes. This method does not have the problem of residuals and, also, can be used at room temperature and under atmospheric pressure.
The main objective of this study is to examine the effect of adding oxidants, $NaIO_4$ and $NaBrO_3$, in a photocatalytic process using $TiO_2$ nanoparticles immobilized on concrete substrate using UV radiation in different systems for the removal of direct blue 71 dyes. In order to determine the optimum conditions, initial dye concentration, pH, intensity of UV-C radiation, and oxidants dosage during testing, were investigated. After 25 minutes, 39.4, 79.54 and 84.75 percent removal efficiency was obtained for three systems of $UV/TiO_2$,$UV/NaIO_4$ and UV/$TiO_2$/$NaIO_4$, in initial dye concentrations of 200 ppm, $40 gr/m^2$ of nano $TiO_2$,
light intensity of 90 watts and 0.064 gr/L $NaIO_4$, respectively. At the same time, 62.91 and 66.19 percent removal efficiency was observed for $UV/TiO_2/NaBrO_3$ and $UV/NaBrO_3$ or initial dye concentrations of 100ppm and 0.045 gr/L $NaBrO_3$. This means that $NaIO_4$ is a stronger oxidant in comparison to $NaBrO_3$.
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