Enhanced decolorization of malachite green by a magnetic graphene oxide-CotA laccase composite

In this study, a CotA laccase from Bacillus subtilis cjp3 was successfully immobilized onto magnetic graphene oxide (MGO) nanomaterials via covalent bonding with hydrochloride/N-hydroxysuccinimide (EDC/NHS). The morphology, structure, and properties of the MGO-laccase were then characterized by scanning-electron microscopy (SEM), energy-dispersive X-ray spectroscopy (EDX), transmission electron microscopy (TEM), Fourier-transform infrared spectroscopy (FT-IR), X-ray-photoelectron spectroscopy (XPS), and a magnetic-property-measurement system (MPMS). The magnetic composite exhibited an extremely high binding capacity of ~145.04   mg/g and maintained maximal relative enzyme activities at 25   °C, pH   7, and a reaction time of 2   h. The pH, thermal, operational, and storage stabilities of MGO-laccase were significantly improved over those of free laccase. Moreover, MGO-laccase exhibited a higher tolerance than that of free laccase in the presence of organic solvents , inhibitors, metal ions, and salts. Furthermore, MGO-laccase showed good decolorization performance of malachite green (MG), with decolorization rates reaching 99% after 5   h of reaction at 30   °C and pH   6. In addition, the maximum saturation magnetization of MGO-laccase was 27.7   emu/g, allowing for rapid magnetic separation. Accordingly, magnetic separation allowed MGO-laccase to maintain 75% of its activity after ten consecutive decolorization cycles.