An inorganic anionic polymer filter disc: direct crystallization of a layered silicate nanosheet on a glass fiber filter

Abstract

An inorganic anionic polymer filter disc was successfully produced from heterogeneous nucleation reactions of layered silicate fine crystals on amorphous silica glass fibers. This hybrid material can be obtained only by immersing the silica fiber filter disc in an aqueous solution containing LiF, MgCl₂, and urea at 373 K for 48 h. Silica sol, which was partially dissolved from the silica fibers by hydrolysis of urea, was a source of the layered silicate. Firm immobilization of the layered silicate on the fiber silica was confirmed by immersing in aqueous LiCl solution (12 mM) for three weeks. The layered silicate crystals evenly covered the silica fibers while maintaining the original filter disc shape. Careful design of the layered silicate was performed by changing the molar LiF : MgCl₂ : urea : SiO₂ ratio. The layered silicate on the fibers became thick on adding increasing amounts of LiF and MgCl₂. In addition, by increasing the amount of LiF, the negative charge density of the layered silicate increased, following the trend of caffeine adsorption in water. Furthermore, we found that the amount of urea added to the starting solution was important for preventing a loss in the mechanical strength of the fibers (e.g., fracture due to unnecessary dissolution of the silica fiber) and emerging side-reactions (e.g., polymerization of silica sol to yield spherical silica particles). The resulting layers of silicate on the filter disc acted as adsorption sites for both organic (methylene blue and benzylammonium) and inorganic (sodium, calcium, and europium) cationic species in water, as exemplified by batch and flow tests.