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-25TiO(2)/BEN-PVAG/GP bilayer system where P-25TiO(2) 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-25TiO(2)/BEN-PVAG/GP was 5.3 times faster than the suspended P-25TiO(2). The result implied the positive impact of the BEN-PVAG adsorbent sub-layer on the immobilized P-25TiO(2) photocatalyst. Most important, the immobilized P-25TiO(2)/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.