Alginate-Based PVA–Bentonite Hydrogel Beads Immobilizing Fungal–Bacterial Mixed Culture for Rapid Methylene Blue Removal

Functional biomacromolecule-based hydrogels developed for environmental applications are of growing interest due to their biocompatibility and structural tunability. However, limited studies have investigated alginate-based hydrogel systems immobilizing fungal–bacterial mixed cultures for methylene blue (MB) removal. Therefore, this study aimed to fabricate and evaluate alginate-based polyvinyl alcohol–bentonite hydrogel beads as a biomacromolecular matrix for immobilizing a fungal–bacterial mixed culture ( Aspergillus oryzae and Bacillus subtilis ). Structural and functional characteristics of beads were analyzed using SEM, FTIR, and XRD, confirming successful integration of alginate with reinforcing components. The immobilized bead system (B-AOBS) achieved 95.29% MB removal (50 mg/L) within 24 hours under static conditions, which significantly higher than by free cells of B. subtilis and A. oryzae , reaching only 33.81% and 40.08%, respectively. In addition, downscaled system using 2.5 g of B-AOBS achieved 97.01% MB removal under agitation, showing slightly higher performance than empty beads (B-APB). Adsorption kinetics followed a pseudo-second-order model, suggesting possible involvement of chemisorption-related interactions. Meanwhile, equilibrium data were best fitted by the Langmuir isotherm with a maximum adsorption capacity of 1.776 mg/g, suggesting monolayer adsorption on homogeneous active sites. In addition, B-AOBS maintained high removal efficiency at pH 5.5 and 30°C, achieving 98.57% within 120 min incubation time. Furthermore, the hydrogel retained substantial activity after four reuse cycles. These results demonstrate that the engineered alginate biomacromolecule hydrogel provided a stable and reusable platform for microbial immobilization and represented a promising material for dye-contaminated wastewater treatment.