Inhibition mechanisms of Fe²⁺/Fe³⁺ and Mn²⁺ on fungal laccase-enabled bisphenol a polyreaction

Bisphenol A (BPA) is regarded as an endocrine disruptor associated with negative health effects in animals and humans. Laccase from white-rot fungus can enable BPA oxidation and auto-polymerization to circumvent its biotoxicity, but the work concerning the effect mechanisms of divalent and trivalent metal ions (MIs) on BPA polyreaction have rarely been reported. Herein, Trametes versicolor laccase-started BPA conversion within 1 h followed pseudo-first order kinetics, and the rate constant (k(prcs)) and half-life were respectively 0.61 h(-1) and 1.14 h. The presence of Ca2+, Mg2+, Cu2+, Pb2+, Cd2+, Zn2+ and Al3+ exhibited insignificant impact on BPA removal, whereas Fe2+, Fe3+ and Mn2+ had a strong inhibiting effect. Compared with MI-free, the k(prcs) values of BPA respectively lowered 34.4%, 44.3% and 98.4% in the presence of Fe2+, Fe3+ and Mn2+. Enzymatic activity and differential absorption spectrum disclosed that the inhibitory actions were accomplished by two different mechanisms. One is Fe2+ was preferentially oxidized into Fe3+ that restrained laccase activity at the initial stage of reaction, and subsequently, the formed Fe3+ complex bound with laccase T1-Cu site and thus impeded the single-electron transfer system. The other is Mn2+ was instantly oxidized by laccase to generate Mn3+-citrate complex, which completely consumed the dissolved O-2 in solution and consequently terminated BPA removal. Considering environmental bioremediation, T. versicolor laccase-enabled auto-polymerization is a simple and convenient candidate to eliminate BPA in enzymatic wastewater treatment, however the effects of Fe2+/Fe3+ and Mn2+ on BPA decontamination should be cautiously assessed.