Lignin peroxidase-catalyzed polymerization and detoxification of toxic halogenated phenols

Halogenated phenols and bisphenols are recognized as being recalcitrant in conventional biological treatments. The current research evaluated lignin peroxidase-catalyzed oxidation and polymerization as a potential alternative for their detoxification. Gel permeation-HPLC analysis demonstrated the formation of dimers, trimers and tetramers upon oxidation of the target substrates. Polymerization was accompanied by effective detoxification of the aqueous phase during oxidation of 2,4-dibromophenol, the extent of which correlated with the extent of oxidation and polymerization. Steady state kinetic measurements at a saturating concentration of H₂O₂ revealed high K_m values (270-1100 μmol dm^-3) for the target substrates, reflecting the strong electron-withdrawing properties of halogen substituents, which increase the oxidation potential of the phenols, resulting in thermodynamically less favorable reactions. However, k_cat values were not dissimilar from non-halogenated phenols and the rapid oxidation and polymerization suggests that low retention times could be expected in a continuous process for their treatment, in contrast to conventional biological methods. The operational stability of lignin peroxidase was significantly improved by inclusion of redox mediators, which resulted in enhanced oxidation and more rapid reaction rates. However, due to their inherent toxicity, the use of redox mediators impeded toxicity assays. The findings highlight the potential of lignin peroxidase as a possible alternative for the high-rate treatment of industrial wastewater when conventional methods are ineffective.