Functionalization of reduced montmorillonite as an adsorbent toward aqueous anion: role of layer charge density

Montmorillonite (Mt) is widely used in various fields due to its abundant resources and excellent adsorption performance, while the hydration and adsorption properties of raw or functionalized Mt are affected by its intrinsic property, layer charge density (LCD). Herein, a series of reduced-charge montmorillonites (RC-Mt) with high, medium, and low layer charges were prepared by the method of lithiation and microwaving Ca-Mt. Meanwhile, the effects of artificially regulated layer charge density on Mt's structure, hydration properties, and the removal ability of aqueous anions by the poly (diallyl dimethylammonium chloride) functionalized Mt (PDDA-Mt) were also investigated. Results showed that the LCD of Mt decreased with the increasing microwave time or power. In addition, the physicochemical properties of RC-Mt which included cation exchange capacity (CEC), colloidal index, and swelling properties decreased as the LCD decreased. Furthermore, the adsorption capacity of Mt for NO3- was significantly improved following the PDDA functionalization. The kinetic modeling suggests that the Pseudo-first-order model better characterized the adsorption of Ca-Mt and Li-Mt than the Pseudo-second-order model. On the other hand, the good fit of the Pseudo-second-order model implied that the adsorption process of nitrate on RC-Mt and PDDA-Mt is mainly driven by chemisorption control. The adsorption thermodynamics indicated that the adsorption process was spontaneous. Furthermore, the FTIR and XPS results suggested that in addition to diffusion and electrostatic interactions, the adsorption mechanism of PDDA-Mt was mainly attributed to the ion exchange between chloride and nitrate on the quaternary ammonium group of PDDA.