Role of bentonite adsorbent sub-layer in the photocatalytic-adsorptive removal of methylene blue by the immobilized TiO2/bentonite system

An immobilized clay composite (BEN–PVAG) on a glass plate (GP) was fabricated using bentonite powder (BEN) and glutaraldehyde cross-linked polyvinyl alcohol (PVAG) as the adsorbent and adhesive, respectively. The immobilized bentonite composite (BEN–PVAG) was characterized using SEM, EDX, FTIR, and BET analysis. The adsorption capacity of BEN–PVAG was examined using methylene blue (MB) as the model pollutant. The results indicated that the adsorption of MB onto BEN–PVAG obeyed pseudo-second-order kinetics. In addition, the adsorption of MB by the immobilized BEN–PVAG was controlled by intra-particle diffusion. In contrast, the adsorption of MB by the suspended BEN–PVAG composite was dominated by film diffusion. The immobilized BEN–PVAG was then applied as the adsorbent sub-layer for the fabrication of P-25TiO2/BEN–PVAG/GP bilayer system where P-25TiO2 was deposited as the top layer. The fabricated bilayer system exhibited synergistic photocatalytic-adsorptive removal of MB upon irradiation with a light source, while experiment in the dark yielded only adsorption process. The rate of the synergistic photocatalytic-adsorptive removal of MB by the P-25TiO2/BEN–PVAG/GP was 5.3 times faster than the suspended P-25TiO2. The result implied the positive impact of the BEN–PVAG adsorbent sub-layer on the immobilized P-25TiO2 photocatalyst. Most important, the immobilized P-25TiO2/BEN–PVAG/GP provided a convenient reuse of the catalyst over time where the treated water could be directly discharged without the need of filtration.