Synthesis of mesoporous silica and calcium fluoride nanoparticles from hexafluorosilicic acid waste: A circular economy approach

The growing demand for sustainable industrial practices has intensified the need for innovative approaches to managing hazardous waste. In this study, we propose a circular economy-driven method for the synthesis of mesoporous silica nanoparticles (MSNs) and calcium fluoride (CaF 2 ) nanoparticles from hexafluorosilicic acid (H 2 SiF 6 ), a highly corrosive and toxic by-product of the phosphate fertilizer industry. The process involves reacting H 2 SiF 6 with ammonia (NH 3 ) under controlled conditions to yield high-purity MSNs with tunable properties, including particle sizes ranging from 32 to 85 nm and pore diameters of 2-5 nm. In a second step, the ammonium fluoride (NH 4 F) solution obtained as a by-product during the MSN synthesis was treated with calcium hydroxide (Ca(OH) 2 ) to synthesize CaF 2 nanoparticles with an average particle size of 38 nm. The resulting nanoparticles were characterized using X-ray diffraction (XRD), energy-dispersive X-ray spectroscopy (EDX), scanning electron microscopy (SEM), and transmission electron microscopy (TEM), confirming their high purity and nanoscale dimensions. This dual-synthesis approach not only addresses the environmental concerns associated with H 2 SiF 6 disposal but also provides valuable nanomaterials for various industrial applications, thus contributing to a circular economy.