Effect of Two Humic Acids on Laccase-Driven 17β-Estradiol Conversion Kinetics and Polymerization Products at Varying pH Levels

Abstract

Estrogens with tremendous ecological risks are proverbially found in water. Laccase can drive humification of estrogens to reduce their ecotoxicity and removability, but little investigation existed in exploring the effect of humic acids (HAs) on E2 conversion kinetics and polymerization products at different pH conditions. Here, Trametes versicolor laccase (Tvlac) was capable of efficiently converting a representative estrogen, 17β-estradiol (E2) with two different HAs, and the process followed a pseudo-first-order kinetics. The velocity constants were respectively 0.048, 0.022, and 0.020 min− 1 for HA-free, peat-derived HA, and commercial HA at pH 5.0. The changing pH not only impacted E2 coupling kinetics in Tvlac-evoked humification, but altered the aromaticity and humification degrees of HAs. A total of five intermediate species including estrone (E1), E2 dimer, trimer, and tetramer, as well as E1-E2 cross-linked products were tentatively detected, in which the dominating species were E2 self-oligomers resulting from radical-centered carbon-carbon/oxygen stepwise polymerization routes. Yields of dimeric, trimeric, and tetrameric species with increased molecular sizes were the highest at pH 5.0 in the given pH values, and they were readily handled by precipitation and filtration. Especially E2 was able to be covalently incorporated into humic constituents to generate new humified co-polymers, thereby accelerating E2 humification and detoxification. These findings demonstrate that pH exhibits a far-reaching influence on the conversion kinetics, humification degrees, and products distribution of E2 and HAs in Tvlac-reinforced polyreaction. Thus, there is need to reappraise the fate and transport of estrogens with HAs present in natural water at varying pH levels.