Influence of humic acids on fungal laccase-initiated 17α-ethynylestradiol oligomerization: Transformation kinetics and products distribution. (November 2020) - Sun, Kai
Abstract: Fungal laccase has aroused great concern in rapidly removing estrogens because of its ability to accelerate humification and oligomerization. Here, the effect of two humic acids (HAs) on the reaction kinetics and products distribution of 17 α -ethynylestradiol (EE2) in laccase-initiated humification and coupling was systematically elucidated. Laccase from Trametes versicolor exhibited over 98.3% removal rate for EE2 at pH 5.0 within 120 min, while HAs invariably restrained EE2 transformation by competing with target-substrate for the enzymatic catalytic center. EE2 removal followed pseudo-ﬁrst-order kinetics, and the rate constant was decreased markedly with increasing concentration of two HAs (0–60 mg L −1 ). Additionally, laccase heightened the aromaticity and humification degrees (A250 nm /A365 nm ratio) of HAs probably due to the formation of new humic polymers such as (HA)m and/or (HA)m -(EE2)n ( m and n represent the number of HA and EE2 units, respectively). Three major EE2 oligomers were identified, in accordance with a mechanism involving the phenoxy radical-driven polymerization to yield a wide variety of self-coupling products. Notably, HAs diminished the extent of EE2 self-coupling but aggrandized the cross-coupling between EE2 and HAs, and the inhibition degree of EE2 self-coupling increased with the concentration of HAs. One major reason is EE2 could be covalently incorporated into humic molecules to produce (HA)m -(EE2)n cross-coupling products via radical-caused C–C, C–O–C, and/or C–O–C bonds, thereby reducing EE2 self-oligomerization. These findings highlight that HAs play a vital role in the fungal laccase-induced humification and oligomerization of EE2, which obviously alter the geochemical fate and transport of EE2 in natural aquatic ecosystems. Graphical abstract: Image 1 Highlights: EE2 was effectively eliminated by Trametes versicolor laccase at pH 5.0. Humification and oligomerization of HAs with EE2 were accelerated by laccase. Radical-induced step-polymerization was the primary EE2 transformation mechanism. HAs decreased EE2 self-coupling but increased the cross-coupling of EE2 and HAs. The natural water matrix constituents had no evident effect on removal of EE2.