Contrasting roles of phenol and pyrocatechol on the degradation of 4-chlorophenol in a photocatalytic-biological reactor.

Intimate coupling of photocatalysis and biodegradation (ICPB) provides superior treatment for the degradation of bio-recalcitrant compounds, such as chlorophenol. Photocatalytically generated intermediates can be promptly used by the enclosed biofilms. Chlorophenol degradation can theoretically be accelerated by a co-substrate or be compromised by the competition for photocatalytic reactive oxygen species (ROS); however, studies to examine the comparison are limited in number. Non-chlorinated phenols commonly co-exist in real wastewater; thus, we evaluated the influence of phenol (hard to photo-oxidize) and pyrocatechol (easy to photo-oxidize) on the degradation of 4-chlorophenol (4CP). The removal efficiency of 4CP was 51%, which increased to 62% after phenol addition. Meanwhile, the dechlorination efficiency of 4CP increased from 47 to 63%; similarly, the living/dead cell ratio increased from 49/51 to 79/21. However, pyrocatechol addition led to a decrease in 4CP removal efficiency to 32% and a reduction in living/dead cell ratio to 35/65. The differences in the results were attributed to the extra electron donors provided by the photodegraded products of phenol to bacteria, which enhanced 4CP degradation; meanwhile, pyrocatechol competed with 4CP for ROS, thus inhibiting its degradation. Competition for ROS and co-substrate properties should be considered in the treatment of phenolic wastewater by ICPB.