# Degradation of Trace Organic Contaminants by a Membrane Distillation--Enzymatic Bioreactor
## 3 Results and Discussion
### TrOC Degradation in Enzymatic Bioreactor  
Degradation of a substrate by lacase involves the transfer of an electron from the substrate to lacase with concomitant reduction of atmospheric oxygen to water. The extent of degradation depends on, among others, the molecular properties (e.g., EWGs, EDGs or phenolic moiety) of the target substrate [11; 39]. In this study, high degradation (87-99%) of 10 out 13 phenolic TrOCs was achieved by the MD-EMBR (Figure 2). These included five steroid hormones (estriol, estrone, 17\(\beta\)-estradiol, 17\(\alpha\)-ethinylestradioliol and 17\(\beta\)-estradiol-17-acetate (95-99%)), two industrial chemicals (4-tert-butylphenol, and 4-tert-octylphenol (87-99%)) and two personal care products (oxybenzoate  
Figure 2: Overall removal and enzymatic degradation of 30 TrOCs in the MD-EMBR. Error bars indicate the standard deviation of duplicate samples. MD-EMBR operating conditions: the initial TrOC concentration and lacase activity was 20 \(\mu\)g/L and 95–100 \(\mu\)M\({}_{\rm(DMP)}\)/min, respectively; temperature of the enzymatic bioreactor and the permeate tank were kept at 30 and 10 \({}^{\circ}\)C, respectively; and cross-flow rate of media from the enzymatic bioreactor and distillate was 1 L/min (corresponding to a cross-flow velocity of 9 cm/s). and triclosan (89-98%)). On the other hand, enzymatic degradation of some phenolic compounds, namely pentachlorophenol, enterolactone and salicylic acid, ranged from 55 to 75%. Their incomplete degradation, despite the presence of a strong EDG (i.e., hydroxyl group), can be attributed to the concomitant presence of an EWG (e.g., halogen) in their molecular structure (see Supplementary Data Table S1) [39]. Depending on the medium ORP, increase can also degrade non-phenolic compounds. However, the reaction kinetics can be slow [17; 39]. In this study, the enzymatic degradation of 17 non-phenolic TrOCs varied from 40-99% (Figure 2). Laccase catalyzed degradation of 13 compounds fell in the range of 40-65%, while the degradation of the remaining four TrOCs was in the range of 94-98%. The well degraded non-phenolic TrOCs include metronidazoleacle, benzophenone, amitriptyline and octocrylene. High laccase-catalyzed degradation (80-99%) in continuous flow UF-EMBR has been previously reported [18; 37] for benzophenone, amitriptyline and octocrylene, but not for metronidazoleacle. Metronidazole contains both EWGs (i.e., -NO\({}_{2}\)) and EDGs (i.e., methyl and hydroxyethyl) in its molecule (see Supplementary Data Table S1). High enzymatic degradation of metronidazoleacle following its complete retention by the MD membrane in MD-EMBR can be attributed to the prolonged contact time that may have promoted the interaction of lacase with the EDGs of metronidazoleacle. An overall degradation of only 40-65% was achieved by the MD-EMBR for a number of non-phenolic TrOCs (Figure 2), however, these removal efficiencies in fact compare favorably with those reported in the literature [12; 15; 37]. For instance, laccase catalyzed degradation of carbamazepine, clofibric acid, fenoprop and atrazine has been reported to be less than 10% in both batch and continuous flow ultrafiltration based enzymatic bioreactors [15; 18; 40]. By contrast, 40-45% degradation of these TrOCs by the MD-EMBR was observed in this study. Since most of the selected non-phenolic TrOCs contain both EWGs and EDGs in their structure (see Supplementary Data Table S1), complete retention of these TrOCs in the enzymatic bioreactor may have facilitated the interaction of EDGs with nearby redox centers, thereby providing higher possibility of electron transfer to enzyme [17]. Previously, [PERSON] et al. [20] reported that dosing of GAC into an UF-EMBR led to simultaneous adsorption of laccase and TrOCs on GAC, yielding significant improvement in the degradation of four non-phenolic TrOCs, namely atrazine, sulfamethoxazole, diclofenac and carbamazepine.