Innovative eco-friendly methyl orange removal: Mechanism, kinetic, and thermodynamic study using starch cryogel-integrated mesoporous silica nanoparticles

This study introduces a novel, eco-friendly composite, uncalcined mesoporous silica nanoparticles incorporated into a starch cryogel (MSNs-Cry), designed for the effective removal of methyl orange (MO) from water. MSNs-Cry integrates uncalcined mesoporous silica nanoparticles (MSNs) within a starch cryogel network, leveraging the high adsorption capacity of MSNs. The composite achieved a maximum adsorption capacity of 18.98 mg g⁻¹ and demonstrated high removal efficiencies of 99.00 % ± 0.21 % in synthetic water (10 mg L^-1 MO) and 92.77 % ± 1.76 % in real wastewater containing 0.43 mg L^-1 MO. The Langmuir isotherm model provided a superior fit (R² = 0.9930) compared to the Freundlich model (R² = 0.9180), and the adsorption kinetics followed a pseudo-second-order model (R² = 0.9917). The primary adsorption mechanisms included electrostatic attraction, hydrophobic interactions, and hydrogen bonding. The process was endothermic (ΔH° = 31.3 kJ mol^-1), spontaneous, and more favorable at higher temperatures (ΔG° = -34.2 to -38.6 kJ mol^-1 at 298-318 K). In the presence of sodium silicate at 13.1 times the MO concentration, removal efficiency drops by 35.77 %, and with sodium sulfate and urea at 100 times the MO concentration, it decreases by 8.65 %. Despite these challenges, MSNs-Cry effectively removes MO in the presence of the anionic dye Congo Red and metal ions, demonstrating its selective adsorption capabilities. The tablet form of MSNs-Cry prevents the loss of uncalcined MSNs, mitigating potential environmental and operational impacts. Additionally, the composite's effectiveness at a natural pH of 6.65 eliminates the need for pH adjustment, offering a cost-effective solution for real-world applications. This study establishes MSNs-Cry as a promising material for sustainable water purification.