Removal of tetracycline from water using immobilized laccase on polyvinyl alcohol-gum Arabic hydrogel nanocomposites

Abstract

This study explored the synthesis of the hydrogel nanocomposite based on gum Arabic (GA) and polyvinyl alcohol (PVA) reinforced with Fe₂O₃ nanoparticles incorporated onto nanocellulose (NC-Fe). The water absorbencies of the hydrogels varied from 33.1 to 217.1 g/g, depending on Fe/NC and cross-linker content, as well as PVA molecular weight. The SEM images of the hydrogels revealed that the presence of Fe/NC led to smaller pores in the hydrogel matrix (i.e., pore size range of 6.9–50 µm in hydrogel nanocomposite vs. 8.3–63.3 μm in the pristine hydrogel). The immobilization of a metagenomic laccase (PersiLac3) onto the hydrogels provided an efficient, innovative, and multi-functional system for the removal of the antibiotic tetracycline (TC) from water. Immobilizing PersiLac3 on both the pristine hydrogel (PVA-GA-hydrogel) and the hydrogel nanocomposite (PVA-GA-hydrogel/NC-Fe) remarkably boosted its performance in TC removal compared to the free enzyme. At an initial TC concentration of 500 mg/L, the maximum removal reached 83 %, 67 %, and 38 % for PersiLac3@PVA-GA-hydrogel/NC-Fe, PersiLac3@PVA-GA-hydrogel, and the free PersiLac3, respectively, within 60 min. Moreover, the TC removal performance of the neat hydrogels (48 % for PVA-GA-hydrogel/NC-Fe and 17 % for PVA-GA-hydrogel) suggested that the antibiotic was eliminated through the combined processes of adsorption by the hydrogels and enzymatic degradation by PersiLac3. Addition of NC-Fe in the hydrogel played a bi-functional role in TC removal: it reduced the pore size, thereby increasing TC sorption by the hydrogel, and it acted as a Fenton-like catalyst in TC degradation. Finally, the immobilized enzyme demonstrated superior stability in harsh environments compared to the free enzyme, removing 70 % of TC after six consecutive runs. This is the first report on the potential application of the bio-based PVA–GA/Fe–NC hydrogel as a robust, reusable, and eco-friendly carrier for metagenomic laccase in antibiotic degradation, offering a novel strategy for sustainable water treatment.