A coupled biological and photocatalysis pretreatment system for the removal of crystal violet from wastewater

• The novel coupled photo-biological system successfully removes CV.
• The optimal conditions are a 1.5-h reaction time and 2.0-min retention time.
• The coupled system efficiently mineralizes CV.
• The predominant phyla are Proteobacteria, Bacteroidetes, and Actinobacteria.
• CV degradation involves stepwise demethylation and aromatic ring opening processes.

The efficiency of a coupled photocatalytic–biological system for removing crystal violet (CV) from an aqueous solution was assessed. Initial experiments demonstrated that the optimal operating parameters for the photoreactor were a 1.5-h reaction time, pH 7.0, and a 2.0-min retention time. Under these conditions, the photocatalytic reaction reduced the toxicity of the CV solution by 94%. Subsequent evaluation of the performance and characteristics of the coupled photocatalytic–biological system in terms of CV removal revealed that the coupled system successfully removed and efficiently mineralized CV in a semi-continuous mode when the CV concentration was <150 mg L−1. Based on our analysis of the degradation products, CV degradation in this coupled system involved stepwise demethylation and aromatic ring opening. Phylogenetic analysis of the bioreactor effluent showed that the predominant phyla were Proteobacteria, followed by Bacteroidetes and Actinobacteria, suggesting that this coupled system is conducive for such processes as demethylation, aromatic ring opening, carbon oxidation, and nitrification. These results were verified in a GC–MS analysis. To our knowledge, this is the first report on CV removal using a coupled system.