Synthesis and characterization of series of Ca-MCM-41 nanoporous structures for effective elimination of sulfate ions; insight into the used calcium precursor and realistic studies

Abstract

Three varieties of calcium-bearing mesoporous silica (MCM-41) were synthesized using different types of natural calcium carbonate precursors (limestone (L.MCM), coral reefs (C.MCM), and marble (M.MCM). The different varieties were characterized by different analytic techniques confirming their formation as Ca-MCM-41 with mesoporous properties and possessing different morphologies. They were applied in adsorption studies for sulfate ions, achieving saturation capacities of 135.9 mg/g (L MCM), 167.9 mg/g (C MCM), and 141 mg/g (M MCM). The better performance of C.MCM than the other forms is in agreement with its morphological aspects and its enhanced surface area (159.6 m²/g) in addition to the supporting theoretical findings. Modeling the uptake process based on the concepts and parameters of statistical physics declared enrichment of C.MCM surface with a higher density of active sites (Nm = 86.5 mg/g) as compared to L.MCM (64.7 mg/g) and 68.14 mg/g (M MCM). Each of these sites can accommodate up to 3 sulfate ions, donating the impact of multi-ionic interactions mechanisms in addition to their adsorption in parallel and vertical orientations. Energetic and thermodynamic assessment implies uptake of sulfate by exothermic reactions of spontaneous behaviors displaying uptake energy less than 8 kJ/mol. This energetic level corresponds to the physical adsorption mechanisms, including hydrogen bonding and van der Waals forces. The C.MCM structure (1.2 g/L) was applied effectively in realistic sequestration of sulfate from seawater along the Gulf of Suez, reducing its content (2614 mg/L) by about 42 %, adsorbing about 1103 mg of sulfate ions.