Advances in the persistent contamination and microbial degradation of hexabromocyclododecanes (HBCDs)

Hexabromocyclododecanes (HBCDs), a class of persistent organic pollutants (POPs), have been extensively used as additive flame retardants in building materials, textiles, and electronic equipment for decades. Although HBCDs have been gradually banned from production and application, their continuous release from existing products is expected to persist for more than a century, resulting in long-term environmental concern. Recent studies indicate that regulatory measures have been effective in most regions, yet HBCD contamination remains severe in certain areas, e.g., e-waste recycling zones. The reported half-lives of HBCD biotransformation in environmental media range from several to over 100 days, which are affected by contamination levels, redox potential, pH, and hydrodynamic conditions. Organohalide-respiring bacteria (OHRB) constitute a critical functional group in environmental microbial communities for anaerobic degradation of HBCDs, while the enzymatic and genetic mechanisms remain poorly characterized. Biotransformation of HBCDs by microbial strains and their enzymes has been reported, including Pseudomonas aeruginosa HS9, Citrobacter sp. Y3, and Alcanivorax sp. SZ2-4 and the corresponding enzymes CYP168A1, HBCD-hd-1, DadAH, and DadBH. However, several critical questions remain insufficiently addressed, including the stepwise dehalogenation mechanisms of the implicated enzymes and the metabolism of debrominated intermediates. This review summarizes current knowledge on the environmental persistence of HBCDs, microbial community responses to HBCD stress, and microbial degradation of HBCDs. It also identifies major knowledge gaps for future researches, aiming to promote the bioremediation of HBCD contamination.