Preparation of Chitosan-Carbon Nanotubes Decorated with ZnO@laccase: Application in the Removal of Bisphenol A from Aqueous Solution

Abstract

Bioremediation with immobilized enzymes has been established as the method for restoring contaminated ecosystems with the help of biological tools on a global scale. The purpose of this work is to immobilize the laccase enzyme for increasing the biocatalyst′s ability in order to remove bisphenol A (BPA). Initially, the nanocatalyst was prepared by immobilizing laccase on chitosan using carbon nanotubes decorated with zinc oxide. Subsequently, Fourier transformed infrared (FTIR) spectroscopy, X-ray diffraction (XRD), field emission scanning microscopy (FE-SEM), transmission electron microscopy (TEM), energy-dispersive X-ray spectroscopy (EDX), and dot mapping were employed to characterize the prepared nanocomposites. Laccase activity, immobilization yield (%), thermal and storage stability, and reusability of laccase immobilized on chitosan-coated carbon nanotubes (CNTs)/ZnO nanocomposites were also measured. The chitosan (CS)-CNTs/ZnO@laccase nanocatalyst performance in different operational conditions (initial concentrations of bisphenol A, amount of nanobiocatalyst, pH, and irradiation time) on the photodegradation process of bisphenol A were also investigated and optimized using the response surface methodology (RSM)-central composite design (CCD) model. The RSM design and experiments demonstrated that 94.51% of BPA was removed under optimal conditions (initial BPA concentration of 100 mg L−1, nanocatalyst amount of 1 g L−1, pH=9, and 40 min of contact exposure). Laccase was a critical factor in this process, and after five recycling cycles, immobilized laccase retained 61% of its original activity, indicating that this nanobiocatalyst can be used in purification processes as a durable and robust catalyst.