Immobilization of laccase on magnetic cellulose beads for enhanced biodegradation of bisphenol A and a metal-complex dye

Cellulose-based materials can be an ideal immobilizing support for enzymes due to their abundance in nature, non-toxicity, sustainability, and biodegradability. Furthermore, they do not leave any toxic residue behind after the process is utilized. In this work, magnetic cellulose beads (MCB) were coated with polydopamine-polyethyleneimine (PDA-HMDA). The MCB@PDA-HDMA beads were then functionalized by chelating Cu(II) ions or reacting with glutaraldehyde (GA) for the immobilization of laccase via metal chelate interaction or covalent binding, respectively. PDA-HMDA polymer pairs were coated onto MCB to enhance Cu(II) ions chelation and GA functionalization, which provided metal chelate interaction and covalent binding sites for laccase. The MCB@PDA-HDMA beads-based preparations were characterized using FT-IR, X-ray diffraction, SEM, TEM, and VSM. The amounts of immobilized laccase via metal chelate interaction, covalent binding, and adsorption on the MCB@PDA-HDMA-Cu(II)-Lac, MCB@PDA-HDMA-GA-Lac, and MCB@PDA-HDMA beads were found to be 87.9, 51.6, and 42.4 mg/g beads, respectively. The highest activity yield order of immobilized laccase preparations was MCB@PDA-HDMA-Cu(II)-Lac (81.6%) > MCB@PDA-HDMA-GA-Lac (68.4%) > MCB@PDA-HDMA (48.7%) compared to the free laccase using syringaldazine as an artificial substrate. The free enzyme, MCB@PDA-HDMA-Cu(II)-Lac, MCB@PDA-HDMA-Lac preparations exhibited their maximum activities at pH 6.0, whereas MCB@PDA-HDMA-GA-Lac showed its maximum activity at 6.5. The maximum activity for all the laccase preparations was obtained at 35°C. Furthermore, the immobilized forms of laccase displayed good performance over a broader pH range and at higher temperatures. The free laccase was wholly inactivated at 70°C after 60 min incubation in the substrate-free medium, while the MCB@PDA-HDMA-GA-Lac > MCB@PDA-HDMA-Cu(II)-Lac > MCB@PDA-HDMA preserved about 44.6, 21.3, and 19.4% of their initial activities, respectively. The biodegradation of two model pollutants, Bisphenol A (BPA) and Reactive Green 5 (RG-5, metal complex dye), with the free enzyme and MCB@PDA-HDMA-Cu(II)-Lac preparation was studied batch vise. In the presence of acetosyringone as a mediator compound in the reaction medium, biodegradation amounts of BPA (at 20 mg/L) and RG-5 dye (10 mg/mL) were detected as 96.9% and 78.2%, respectively, using MCB@PDA-HDMA-Cu(II)-Lac preparation for a 120 min reaction time. The MCB@PDA-HDMA-Cu(II)-Lac preparation displayed high biodegradation performance for both tested pollutants compared to the free laccase. Moreover, the MCB@PDA-HDMA-Cu(II)-Lac and MCB@PDA-HDMA-GA-Lac preparations were used for the degradation of BPA and RG-5 in a batch system over five consecutive cycles. These obtained results make the immobilized laccase preparations favorable candidates for many environmental applications, such as wastewaters management and remediation, where improved enzyme performance and reusability are key parameters.