A critical review of the aniline transformation fate in azo dye wastewater treatment

Azo dyes are widely used in textile and dyeing industry. Aniline, an important raw material of dyes and auxiliaries, is always the common by-products from the azo dye degradation, which is difficult to be completely removed in dyeing wastewater treatment. The discharge of azo dyes and aniline in textile wastewater arouses many environmental issues, and aniline is listed as the priority pollutant. In recent years, degradation techniques including physical, chemical and biological processes of different dyes and aniline have gained much attention. Their degradation pathways and corresponding mechanisms have been proposed. However, most studies focus on the dye removal or decolorization capacity, the influence factors, and the by-product varieties in certain degradation process. There were very few reports on the behavior of aniline transformation during the azo dye degradation processes, which is crucial for aniline control. This paper aims at comprehensively and critically reviewing the fate of aniline formation and degradation in different degradation technologies. Besides, crucial questions on aniline control are raised and discussed. It is expected that this work will shed new insight and provide valuable information that will aid in avoiding the environmental risk of aniline in textile and dyeing industry.