Aerobic biofilm reactor for treating a commercial formulation of the herbicides 2,4-D and dicamba: Biodegradation kinetics and biofilm bacterial diversity

A microbial community, able to use 2,4-D (2,4-dichlorophenoxyacetic acid) and dicamba (3,6-dichloro-2-methoxybenzoic acid) as carbon and energy sources, was selected in a continuously operated packed-bed column (PBC). Both active compounds are present in the Banvel herbicide. The predominant cultivable microorganisms isolated belong to the genera Burkholderia, Dyella, Mycobacterium, and Microbacterium. The presence of the catabolic genes tfdACD encoding enzymes involved in the 2,4-D metabolic pathway was evaluated for the bacterial isolates, with positive results for all of them. To evaluate the ability of the entire microbial community to degrade these herbicides in aerobic conditions a horizontal multistage-packed-bed reactor (HMPBR) was constructed. The reactor has four zones packed with fragments of volcanic rock, interspaced with four unpacked aerated zones. The continuous reactor, inoculated with the isolated microbial consortium, was run at increasing Banvel loading rates always operating in an aerobic environment and under steady-state conditions. In no case, 2,4-D or its main aromatic derivatives were detected in reactor's effluent, while dicamba was substantially degraded only at low loading rates. The highest degradation efficiencies of the herbicides were 100% for 2,4-D and 84% for dicamba. COD determinations showed that other components present in the commercial formulation of the herbicide were also removed. To evaluate changes in the diversity of the microbial consortium, massive semiconductor sequencing of 16S rDNA libraries was made. A remarkable variation in the bacterial diversity at phylum level, was observed in biofilm samples obtained from distinct packed bed zones of the reactor, and at different operating conditions.