A novel route of synthesis of sodium hexafluorosilicate two component cluster crystals using BF4− containing ionic liquids

Recovery of Fluoride-Rich and Silica-Rich Wastewaters as Valuable Resources: A Resource Capture Ultrafiltration–Bipolar Membrane Electrodialysis-Based Closed-Loop Process

“…The SEM images showed a smooth pristine membrane without foulants in Figure S6d. In contrast, the hexagonal crystals formed a scaling layer on the membrane surface (Figure S6e), which was consistent with the Na 2 SiF 6 crystals . The EDS results showed that the main elements of crystals were fluoride, calcium, and silica (Table S3).…”

“…The XRD analysis indicated that the generated particles are Na 2 SiF 6 (Figure S2a). In addition, the different pH values of 5, 6, and 7 resulted in Na 2 SiF 6 yields at 3.8, 4.9, and 4.8%, respectively (Figure a). The low Na 2 SiF 6 yield was mainly ascribed to the Na 2 SiF 6 solubility in aqueous medium, and the high residual fluoride concentration in the UF feed indicated an incomplete reaction of fluoride with silica.…”

“…On the contrary, as the n­(F/SiO 2 ) increased to 12:1, the residual silica completely reacted with HF to form a larger size of Na 2 SiF 6 macrocrystals with a loose and porous cake layer. The ATR-FTIR spectra of the fouled UF membrane (n­(F/SiO 2 ) = 12:1) indicated an intense line at 730 cm –1 corresponding to the Si–F stretching vibration (Figure S6c), which was a main feature of Na 2 SiF 6 . The SEM images showed a smooth pristine membrane without foulants in Figure S6d.…”

“…Na 2 SiF 6 is an odorless and white hexagonal structure crystalline salt, which has been widely used in water fluoridation, as well as the photovoltaic and frosted glass industry. 18 Previous studies have reported that the chemical reaction of hydrofluoric acid (HF) and silica generates silicofluoride (SiO 2 + 6HF → SiF 6 2− + 2H 2 O + 2H + ). 19 Moreover, the precipitation reaction of silicofluoride with sodium ions can be used to produce Na 2 SiF 6 (SiF 6 2− + 2Na + → Na 2 SiF 6 ↓).…”

“…On the contrary, capturing fluoride and silica from different wastewaters (e.g., fluoride-rich wastewater and silica-rich wastewater) as well as recovery of the ionic products such as sodium silicofluoride (Na 2 SiF 6 ) will eliminate pollutants from the environment and bring economic values simultaneously. Na 2 SiF 6 is an odorless and white hexagonal structure crystalline salt, which has been widely used in water fluoridation, as well as the photovoltaic and frosted glass industry . Previous studies have reported that the chemical reaction of hydrofluoric acid (HF) and silica generates silicofluoride (SiO 2 + 6HF → SiF 6 2– + 2H 2 O + 2H + ) .…”