A partial air-freeze drying strategy for TiO2-immobilized chitosan beads: Balancing swelling and stability for improved photocatalysis

We here investigated for the first time how the surface morphology, pore volume, swelling, mechanical stability, and photocatalytic performance of the TiO2 photocatalyst-immobilized chitosan beads (TCB) change with the hot air drying and fully freeze drying. The fully lyophilized (freeze-dried) TCB exhibited the presence of wider surface cracks and fissures (8.45 ± 3.04 μm), and had 2.73- and 11.67-times higher surface area and pore volume as compared to the air-dried TCBs. The internal porous structure destruction during hot air drying led to the absence of surface pores, cracks, and fissures, resulting in only a 1.32 % water uptake capacity. However, the freeze-dried TCB had very low mechanical strength (100 % breakage and mass loss) compared to highly stable air-dried ones (15 % mass loss and 33 % breakage). We proposed a hybrid partial air drying (50 ± 10 %) followed by freeze drying for the TCB, which retained surface openings and cracks (6.3 ± 2.27 μm), good surface area (115.15 m2/g, 4.5 times higher pore volume), good mechanical stability, and a higher swelling capacity (227 %), alongside better photocatalytic performance. The optimum beads, prepared with 25 % TiO2 loading, a 1:1 crosslinker ratio, and air-freeze drying, were able to remove 75 % of the anionic dye within 120 min of operation with comparatively low catalyst doses. These beads showed excellent re-swelling capacity and photocatalytic performance in repeated recycles with air drying after each run. The estimated catalytic potential, synergy index, and relative contribution suggest that the synergistic nature of biopolymers and photocatalysts leads to higher degradation than the constituent processes.