Degradation of reactive dyes in a photocatalytic circulating-bed biofilm reactor.

Decolorization and mineralization of reactive dyes by intimately coupled TiO₂-photocatalysis and biodegradation (ICPB) on a novel TiO₂-coated biofilm carrier were investigated in a photocatalytic circulating-bed biofilm reactor (PCBBR). Two typical reactive dyes--Reactive Black 5 (RB5) and Reactive Yellow 86 (RY86)--showed similar first-order kinetics when being photocatalytically decolorized at low pH (4-5) in batch experiments. Photocatalytic decolorization was inhibited at neutral pH in the presence of phosphate or carbonate buffer, presumably due to electrostatic repulsion from negatively charged surface sites on TiO₂, radical scavenging by phosphate or carbonate, or both. Therefore, continuous PCBBR experiments were carried out at a low pH (4.5) to maintain high photocatalytic efficiency. In the PCBBR, photocatalysis alone with TiO₂-coated carriers could remove target compound RB5 and COD by 97% and 47%, respectively. Addition of biofilm inside macroporous carriers maintained a similar RB5 removal efficiency, but COD removal increased to 65%, which is evidence of ICPB despite the low pH. ICPB was further proven by finding microorganisms inside carriers at the end of the PCBBR experiments. A proposed ICPB pathway for RB5 suggests that a major intermediate, a naphthol derivative, was responsible for most of the residual COD, while most of the nitrogen in the azo-bonds (-N=N-) was oxidized to N₂.