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

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 LiNO 3 or LiNO 3 -NaNO 3 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 LiNO 3 , 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 LiNO 3 . On the other hand, the partial replacement of LiNO 3 by NaNO 3 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 LiNO 3 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.