Efficient removal of sulfonamides and tetracyclines residues by the laccase-mediator system employing a novel laccase from Lysinibacillus fusiformis

The laccase-mediator system (LMS) is an important solution to eliminate veterinary residues. In this study, a novel laccase from Lysinibacillus fusiformis (Lyfu-Lac) showed promising removal efficiency towards sulfonamides and tetracyclines residues in the presence of syringic acid (SA) and 2'-Azinobis-3-ethylbenzthiazoline-6-sulphonate (ABTS). The optimum temperature of Lyfu-Lac against ABTS was 80 °C, which is much higher than that of most laccases. At 40 °C, more than 85% of sulfamethazine (SMZ), sulfamethoxazole (SMX), and sulfadiazine (SDZ) were removed by the Lyfu-Lac-SA system after 12 h. Meanwhile, tetracycline (TC), and oxytetracycline (OTC) were significantly reduced to 0% and 16.9% by the Lyfu-Lac-ABTS system after 6 h. Based on the analysis of transformation products (TPs), a two-step reaction was reasonably proposed in the removal of sulfonamides: (i) the oxidation of mediators by Lyfu-Lac to radicals, (ii) the adductive reaction between radicals and sulfonamides. Moreover, the removal mechanism of tetracyclines was proposed, including the cleavage of the N-N bond after the adductive reaction between ABTS radicals and tetracyclines.