Trametes versicolor laccase-assisted oxidative coupling of estrogens: Conversion kinetics, linking mechanisms, and practical applications in water purification

The potential application of fungal laccase-assisted bioremediation for estrogen elimination has caused tremendous attention. Herein, naturally-occurring Trametes versicolor laccase (Tvlac) was capable of significantly removing 17β-estradiol (E2) and 17α-ethynylestradiol (EE2) at pH 5. A lower (or higher) pH caused decline in the velocity constants (kprcs) by obstructing the single-electron oxidation of estrogen at Tvlac T1-Cu site (or intercepting the intramolecular-electron transfer between Tvlac T1-Cu and T2/T3-Cu sites). Humic acid (HA) and natural phenolic compounds (PCs) are omnipresent in water and can influence the conversion kinetics of estrogen in Tvlac-triggered reactions. Compared with HA-free, the kprcs values of E2 and EE2 in presence of HA respectively reduced 58.61%–83.72% and 69.72%–95.62% at different pH levels. Additionally, the kprcs values for estrogen were also hampered obviously by O-dihydroxyphenol structure of model PCs, owning to the created monomeric/polymeric O-quinones inverted estrogen phenoxy radicals. The generated carbon‑carbon/oxygen self-linking aggregates during Tvlac-mediated estrogen oxidative coupling were characterized, and the polymerization mechanisms were speculated. A fluid-bed reactor with cross-linked-entrapped Tvlac in Ca-alginate beads was established to treat E2 and EE2, respectively. Compared with free Tvlac, immobilized Tvlac prominently exhibited a higher pH and temperature stability. Especially, the fluid-bed reactor with immobilized Tvlac could be reused, keeping its high conversion efficiencies of E2 and EE2 in natural water, only changing from 99.2% to 73.6% and from 98.5% to 70.9% after 1 to 5 cycles, respectively. These findings present an alternative immobilized fungal laccase-based clean biotechnology for continuously handling estrogen-contaminated waterbodies at a large-scale.