Hierarchically-structured laccase@Ni3(PO4)2 hybrid nanoflowers for antibiotic degradation: Application in real wastewater effluent and toxicity evaluation

Laccase@Ni3(PO4)2 hybrid nanoflowers (HNFs) were prepared by the anisotropic growth of biomineralized nickel phosphate. The immobilization yield was 77.5 ± 3.6 %, and the immobilized enzyme retained 50 % of its initial activity after 18 reusability cycles. The immobilized and free enzymes lost 80 % of their activity after 18 and 6 h incubation in municipal wastewater effluent (MWWE), respectively. The increase in α-helix content (8 %) following immobilization led to a more rigid enzyme structure, potentially contributing to its improved stability. The removal of ciprofloxacin from MWWE by laccase@Ni3(PO4)2·HNFs/p-coumaric acid oxidation system was optimized using a Box-Behnken design. Under the optimized conditions [initial laccase activity (0.05 U mL-1), the concentration of p-coumaric acid (2.9 mM), and treatment time (4.9 h)], the biocatalyst removed 90 % of ciprofloxacin (10 mg L-1) from MWWE. The toxicity of ciprofloxacin against some G+ and G- bacteria was reduced by 35-70 %, depending on their strain. The EC50 of ciprofloxacin for the alga Raphidocelis subcapitata reduced from 3.08 to 1.07 mg L-1 (p-value <0.05) after the bioremoval. Also, the acute and chronic toxicity of identified biodegradation products was lower than ciprofloxacin at three trophic levels, as predicted by ECOSAR software.