Ultrafiltration and composite microfiltration biocatalytic membrane activity and steroid hormone micropollutant degradation at environmentally relevant concentrations.

Biocatalytic degradation of micropollutants has been extensively explored in both batch and membrane reactors in µg/L to mg/L concentrations and variable water compositions. The degradation of micropollutants by biocatalytic membranes at environmentally relevant concentrations of ng/L range found in natural surface water matrices has not yet been investigated, presumably because of the challenging concentration analysis. This study investigated the limitations of biocatalytic degradation of estradiol (E2) micropollutant at environmentally relevant concentrations by a biocatalytic membrane. The contributions of solute flux, hydraulic residence time (HRT) and water matrix composition on reaction kinetics, the apparent rate of disappearance (or reaction rate) and enzyme activity were examined. Two biocatalytic membranes were used: i) laccase entrapped in an ultrafiltration (UF) membrane support (namely UF-SNPs) and, ii) laccase covalently bound to the nanofiber matrix of a composite microfiltration (MF) membrane. The three main findings are reported. Firstly, the apparent rate of E2 disappearance decreases significantly by four orders of magnitude at a low micropollutant concentration of 0.1 µg/L, resulting in undetectable degradation during filtration, irrespective of the biocatalytic membrane. Secondly, the solute mass transfer and HRT control the biocatalytic degradation through the membranes resulting in different E2 removal. For the UF-SNPs membrane, a removal of 31 % is achieved only by increasing the concentration to 3000 µg/L and at a flux of 60 L/m².h (HRT of 4.5 s) due to an increase in solute flux by an order of magnitude similar to the apparent rate of disappearance. In contrast, the nano-MF membrane is ineffective in achieving biocatalytic degradation regardless of E2 concentration, as the HRT is approximately seven times lower (0.6 s) than that of the UF-SNPs, and thus insufficient for E2 to reach the catalytic site. Thirdly, the composition of the aqueous matrix plays a crucial role in the control of laccase activity irrespective of the membrane. Indeed, laccase is inactivated predominantly by chloride ions in synthetic carbonate buffer, since the typical NaCl concentration is about two orders of magnitude higher than E2 concentration. This study highlights that the slower kinetics achieved in the biocatalytic UF-SNPs and MF membranes are ineffective in removing steroid hormone micropollutants at realistic concentrations in surface water matrices. Further research is suggested to accelerate the reaction kinetics at such low concentrations and prolong the residence time within the membrane.