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 leading method for restoring contaminated ecosystems using biological tools on a global scale. The purpose of this work is to immobilize the laccase enzyme to enhance the biocatalyst′s ability to remove bisphenol A. Initially, the nanocatalyst was prepared by immobilizing laccase on chitosan using carbon nanotubes decorated with zinc oxide. Subseuentally, Fourier Transformed Infrared (FTIR) spectroscopy, X-ray diffraction (XRD), Field Emission Scanning, and Transmission Electron Microscopy (FESEM and TEM respectively), Energy-dispersive X-ray spectroscopy (EDX), and dot mapping were employed to characterize the prepared nanocomposites. Furthermore, laccase activity, immobilization yield (%), thermal and storage stability and reusability of laccase immobilized on chitosan-coated CNTs/ZnO nanocomposites were measured. Finally, the performance CS-CNTs/ZnO@laccase nano biocatalyst in different operational conditions (initial concentration of bisphenol A, amount of nano biocatalyst, pH, and irradiation time) on the photodegradation process of bisphenol A was 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, nano biocatalyst 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 recycles, immobilized laccase retained 61% of its original activity, indicating that the nano biocatalyst can be used in purification processes as a durable and robust catalyst.