Anaerobic degradation of brominated flame retardants in sewage sludge

Tetrabromobisphenol A (TBBPA), hexabromocyclododecane (HBCD), and decabromodiphenyl ether (DecaBDE) are high production volume chemicals used as flame retardants in plastics for products such as electronic equipment, insulation panels, and textiles. The environmental safety of brominated flame retardants, especially their persistence, bioaccumulation, and toxicity is a controversial topic. Here, we studied and compared the degradation of TBBPA, HBCD, and DecaBDE under anaerobic conditions in digested sewage sludge. The half-lives of TBBPA and a technical HBCD mixture were 0.59 and 0.66 d, respectively. The fact that (±)-α-HBCD exhibited an almost doubled half-life compared to (±)-β-HBCD and (±)-γ-HBCD is an important finding with respect to the discussion on the persistence of individual HBCD stereoisomers and the recent reports on strong relative enrichment of α-HBCD in biota. We found no statistically significant enantioselective degradation of α-, β-, or γ-HBCD. Half-lives of TBBPA and a technical HBCD mixture were not dependent on the presence of additional nutrients or primers. Concentrations of TBBPA and a technical HBCD mixture decreased also in sterile control samples, however, at a rate that was more than a factor of 50 smaller than in incubations under non-sterile conditions. Compared to TBBPA and a technical HBCD mixture, DecaBDE exhibited a much longer half-life of 7 × 102 d in the same system. Pseudo-first-order degradation rate constants decreased according to the following sequence: TBBPA congruent with (±)-γ-HBCD congruent with (±)-β-HBCD > (±)-α-HBCD much greater-than DecaBDE. Preliminary investigations suggest that degradation of TBBPA, HBCD, and DecaBDE occurs in full-scale anaerobic digesters, as well.