Photocatalytic adsorption-driven removal of rhodamine B by silver Tetratungstate-modified bentonite: Kinetic, isotherm, and thermodynamic analysis

The influence of zwitterionic surfactant intercalation into photocatalyst-impregnated clay composites on textile dye removal has not been fully explored. In this study, three types of photoadsorbent composites: acid-activated bentonite (AB), a silver tetratungstate (Ag₈W₄O₁₆) photocatalyst-doped version of AB known as Ag@AB, and a surfactant-intercalated variant called Ag@OAB, which incorporates dodecyl dimethyl betaine (BS12) were investigated on Rhodamine B (RhB) removal. The preparation of the AB composite involved treating natural bentonite with hydrochloric acid. The Ag@AB was created by depositing the photocatalyst into AB through a wet impregnation technique. Subsequently, BS12 was intercalated into the Ag@AB to form the Ag@OAB composite. The structural properties of these composites were analyzed using Scanning Electron Microscopy (SEM), X-Ray Diffraction (XRD) and Fourier Transform Infrared Spectroscopy (FTIR). The kinetics of Rhodamine B (RhB) removal by all three composites were best described by the pseudo-second-order kinetic model, suggesting that chemisorption is the main mechanism of removal, rather than photocatalytic. Adsorption isotherm analysis showed that the AB composite best fit the Langmuir model, while Ag@AB and Ag@OAB followed the Redlich-Peterson model. Thermodynamic analysis indicated that RhB adsorption onto all three composites takes place spontaneously as a physical adsorption process and is exothermic. Although kinetic data suggests that electrostatic attraction is a driving force behind chemisorption, isotherm and thermodynamic data indicate that physisorption is the dominant mechanism. This discrepancy may be due to the repulsive interactions between the zwitterionic functional groups of RhB and the positively charged surfaces of the composites, which could diminish overall electrostatic attraction. The addition of the zwitterionic surfactant BS12 in the Ag@OAB composite likely increases these repulsive interactions, leading to a lower adsorption capacity compared to AB. Overall, the findings underscore the intricate balance of attractive and repulsive forces and highlight the enhanced photocatalytic degradation involved in RhB removal by these composites.