Role of biotransformation, sorption and mineralization of (14)C-labelled sulfamethoxazole under different redox conditions.

(14)C-sulfamethoxazole biotransformation, sorption and mineralization was studied with heterotrophic and autotrophic biomass under aerobic and anoxic conditions, as well as with anaerobic biomass. The (14)C-radiolabelled residues distribution in the solid, liquid and gas phases was closely monitored along a total incubation time of 190 h. Biotransformation was the main removal mechanism, mineralization and sorption remaining below 5% in all the cases, although the presence of a carbon source exerted a positive effect on the mineralization rate by the aerobic heterotrophic bacteria. In fact, an influence of the type of primary substrate and the redox potential was observed in all cases on the biotransformation and mineralization rates, since an enhancement of the removal rate was observed when an external carbon source was used as a primary substrate under aerobic conditions, while a negligible effect was observed under nitrifying conditions. In the liquid phases collected from all assays, up to three additional peaks corresponding to (14)C-radiolabelled residues were detected. The highest concentration was observed under anaerobic conditions, where two radioactive metabolites were detected representing each around 15% of the total applied radioactivity after 180 h incubation. One of the metabolites detected under anoxic and anaerobic conditions, is probably resulting from ring cleavage of the isoxazole ring.

Assessing the use of nanoimmobilized laccases to remove micropollutants from wastewater.

Enzymes immobilization is a useful way to allow enzyme reuse and increase their stability. A high redox potential laccase from Trametes versicolor (TvL) and a low redox potential, but commercially available low-cost laccase from Myceliophthora thermophila (MtL), were successfully immobilized and co-immobilized onto fumed silica nanoparticles (fsNP). Enzyme loads of 1.78 ± 0.07, 0.69 ± 0.03, and 1.10 ± 0.01 U/mg fsNP were attained for the optimal doses of TvL, MtL, and co-immobilized laccases, respectively. In general, the laccase-fsNP conjugates showed a higher resistance against an acidic pH value (i.e., pH 3), and a higher storage stability than free enzymes. In addition, immobilized enzymes exhibited a superior long-term stability than free laccases when incubated in a secondary effluent from a municipal wastewater treatment plant (WWTP). For instance, the residual activity after 2 weeks for the co-immobilized laccases and the mixture of free laccases were 40.2 ± 2.5% and 16.8 ± 1.0%, respectively. The ability of the laccase-fsNP to remove a mixture of (14)C-bisphenol A (BPA) and (14)C-sodium diclofenac (DCF) from spiked secondary effluents was assessed in batch experiments. The catalytic efficiency was highly dependent on both the microbial source and state of the biocatalyst. The high redox potential TvL in free form attained a four-fold higher percentage of BPA transformation than the free MtL. Compared to free laccases, immobilized enzymes led to much slower rates of BPA transformation. For instance, after 24 h, the percentages of BPA transformation by 1000 U/L of a mixture of free laccases or co-immobilized enzymes were 67.8 ± 5.2 and 27.0 ± 3.9%, respectively. Nevertheless, the use of 8000 U/L of co-immobilized laccase led to a nearly complete removal of BPA, despite the unfavorable conditions for laccase catalysis (pH ~ 8.4). DCF transformation was not observed for any of the enzymatic systems, showing that this compound is highly recalcitrant toward laccase oxidation under realistic conditions.