One-pot synthesis of the reduced-charge montmorillonite via molten salts treatment

Abstract Layer charge is a critical factor determining the structural characteristics and physicochemical properties of clay minerals, and fixing small cations (e.g., Li+) into the tetrahedral/octahedral sheets of clay minerals is a feasible way for reducing their layer charges. However, conventional methods for preparing reduced-charge clay minerals are usually quite complex and time-consuming. Herein, a facile one-pot treatment method for preparing reduced-charge clay minerals was successfully developed using molten salts. In particular, montmorillonite (Mt) was mixed with mono LiNO3 or LiNO3-NaNO3 mixture, which was then heated above the melting points of salts. The obtained results showed that the cation exchange capacity (CEC) of Mt decreased evidently after being treated with molten LiNO3, and the CEC value was reversely related to the heating time. The X-ray diffraction results demonstrated that Mt layers collapsed upon treatment with molten LiNO3. On the other hand, the partial replacement of LiNO3 by NaNO3 could restrict the collapse of Mt layers, as the larger ionic radius of Na+ (than Li+) helped in maintaining the interlayer spaces of Mt during the process of charge reduction. As indicated by the Fourier transform infrared spectroscopy, the structural OH stretching vibration shifted to a higher wavenumber (from 3623 to 3632 cm−1) after the molten LiNO3 treatment, which suggested that some Li+ ions migrated into the hexagonal cavities in the tetrahedral sheets of Mt, and the new band at 3670 cm−1 indicated that some Li+ ions even further migrated into the octahedral vacancies of Mt. The locations of fixed Li+ ions were further confirmed by the X-ray photoelectron spectroscopy characterization. Due to the migration of Li+ ions into the layers of Mt, the Li1s peak shifted to lower binding energy. The above results showed that the molten salt treatment might be a cost-effective and environmentally friendly approach for synthesizing reduced-charge clay minerals, and using mixed molten salts can help in maintaining the interlayer structure.