Shifting paradigms: the crucial role of zeolite acid sites in dye adsorption mechanisms.

Tailoring Brønsted acid sites (BASs) and Lewis acid sites (LASs) in zeolites has become a feasible method to improve the efficiency of the adsorptive removal of dyes. Generally, in conventional approaches, textural properties, such as pore size, pore volume, and surface area, are tuned to achieve superior adsorption performances. However, these factors alone cannot guarantee robust interactions or selectivity across a broad range of dye molecules. The experimental evidences from the kinetic and isothermic data presented in this review show that adsorption is primarily influenced by the density and accessibility of acid sites. Generally, to introduce acid sites into zeolites, conventional methods, such as dealumination, acid treatment, and tuning the Si/Al ratio, are employed. However, these methods result in non-uniform distributions of acid sites, loss of crystallinity, and collapse of the framework structure under harsh treatment conditions. Modern techniques, such as atomic-layer deposition (ALD), vapor-phase metalation, and templated crystallization, can be used to overcome these limitations. This review highlights the importance of tuning acidity along with tuning textural features in zeolites for achieving effective performance in selective dye adsorption, paving the way for future developments.