Bio-removal of phenol by the immobilized laccase on the fabricated parent and hierarchical NaY and ZSM-5 zeolites

Abstract

This study was performed to synthesize and characterize NaY and ZSM-5 zeolites and their hierarchical forms (HR-Y and HR-Z) through dealumination and soft templating procedure. The Brunauer-Emmett-Teller (BET) analysis showed significant increase in the average pore diameter (DP) size for HR-Y (2.43 nm) and HR-Z (5.07 nm) compared to those of the parent types (NaY and ZSM-5: 1.85 and 2.01 nm, respectively). The mesoporous pore volumes (Vmeso) exhibited notable enhancement for HR-Y (114%) and HR-Z (566%) compared to their parent forms. Laccase (Lac) was subsequently immobilized on NaY and ZSM-5 zeolites, and their hierarchical forms by immobilization efficiency of 74.4 ± 1.4%, 71.6 ± 1.0%, 98±2.9%, and 94±1.8% for Lac@NaY, Lac@ZSM-5, Lac@HR-Y, and Lac@HR-Z, respectively. Thermal and pH stability of the immobilized laccases were enhanced compared with the free enzyme. The relative activity for the immobilized laccases was kept over 50% of its original activity after about 3 weeks of storage at 4 °C. Moreover, reusability of Lac@HR-Z and Lac@HR-Y was considerably higher (68% and 92%, respectively) than those of Lac@ZSM-5 and Lac@NaY (6% and 5.6%, respectively) after 10 cycles of reuse. Lac@HR-Z and Lac@HR-Y exhibited higher efficiency (80.5% and 89.3%, respectively) in phenol bio-removal compared with those of ZSM-5 (67.5%) and NaY (76.5%). Besides, 90.4 ± 1.3% of phenol was practically removed under optimal conditions including 12 U/mg of Lac@HR-Y, 2.6 mM of phenol, and 70 min of the reaction time. Lac@HR-Y also exhibited 56% efficiency after 10 cycles of phenol removal. Furthermore, three phenolic compounds were detected by GC-mass analysis due to phenol bio-removal in the optimal conditions after removing the dark brown precipitate of polymerized metabolites. Therefore, Lac@HR-Y could be suggested for the elimination of phenol-containing pollutants that merits further investigations.