Photocatalytic degradation of some azo dyes in water

Abstract

This research aims to study the photocatalytic degradation in water of seven azo dyes using titanium dioxide as a catalyst at wavelength above 310 nm. Azo dyes used are monoazo dyes (acid orange 7, acid yellow 17, orange G, tattrazine and new coccine) and diazo dyes (acid black I and congo red). It is found that all azo dyes are photocatalytically degraded. The rate of photocatalytic degradation is higher than that of photodegradation. The degradation rate increase with the amount of titanium dioxide. Suitable amount of titanium dioxide for decomposition of seven azo dyes in this study is 75 mg. Degradation rates of high-concentration of acid orange 7 are less than those of low-concentration. Suitable dye concentration of this study is 1x10[superscript -4] M. The degradation rate of dyes dependes on the chemical structures. Diazo dyes were less degradable than monoazo dyes. The degration rate of the seven azo dyes are as follows : acid orange 7>orange G>acid yellow 17>new concine>tartrazine>acid black1>congo red. Degradation rate of acid orange 7 is nearly independent to the independent to the initial pH of dye solutions in range of 4 to 6; however, the rate increased when the initial pH is in the rang of 4 to 2. At concentration of dye solution not exceeded 1x10[superscript -4] M, azo dyes are completely mineralized within 4 hours, except congo red which took time more than 7 hours. Intermediates identified during the degradation are hydroxylated aromatics (phenol, hydroquinone and sodium sulfanilate) and organic acids (malonic acid, maleic acid, citric acid, glyoxalic acid, formic acid, glycolic acid, fumaric acid and acetic acid). These intermediates further degrade and produce nitrate ion, nitrite ion, ammonium ion, sulfate ion and carbon dioxide as final degradation products.