Decolorization of reactive dyes with different molecular structures under different environmental conditions

Abstract

Reactive dyes have been identified as problematic compounds in textile wastewaters as they are water soluble and cannot be easily removed by conventional aerobic biological wastewater treatment systems. Anaerobic systems could reduce the color intensity more satisfactorily than the aerobic processes. However, the intermediate products are carcinogenic aromatic amines which need to be further decomposed by an aerobic treatment. An anaerobic/aerobic SBR system was used to treat a synthetic dye wastewater with glucose and acetic acid (1000 mg/l COD) as carbon sources, together with 20 mg/l of four different reactive dyes; i.e., bisazo vinylsulphonyl, anthraquinone vinylsulphonyl, anthraquinone monochlorotriazinyl and oxazine. The color reduction of the first three dyes was 63, 64 and 66%, respectively. The decolorization efficiency of the last or oxazine dye was not determined because of a strange phenomenon or re-appearance of the color when samples were disturbed. More color removal was achieved in the anaerobic phase than in the aerobic step. The initial decolorization rate was 11.9, 0.37 and 0.48 SU/h for the first three dyes, respectively. Though the system comprised anaerobic and aerobic tanks, the color reduction did not rely on phosphorus accumulating organisms (PAOs). High temperature and photo-oxidation through exposure to sunlight could increase the decolorization rate, and the decolorization was not possible if viable organisms were not present in the system. Nitrate, when present, could interfere with the color reduction while sulfate did not. The bisazo reactive dye was decolorized by the reductive reaction, which resulted in the cleavage of the azo bond. Meanwhile, the decolorization of anthraquinone dyes was through the adsorption of dyes on floc materials.