A greener one-pot synthesis of nanostructured SiO₂ for the efficient emerging contaminant removal from simulated...

Emerging contaminants are a group of chemicals that have the potential to enter the environment and cause potentially adverse effects on the ecosystems and their components. Currently, the interest in achieving the removal of emerging contaminants from water bodies and wastewater has grown considerably, which is reflected in several publications on the synthesis of nanomaterials capable of adsorbing them. Among emerging pollutants, methylene blue (MB) is a widely used model dye for the study of adsorption processes on nanomaterials. In this work, we report a facile and greener one-pot synthesis of SiO₂ nanoparticles (SiO₂NPs) than the classical Stöber method, involving a cheaper Si source than TEOS, only water as solvent, and shorter reaction times under neutral conditions at room temperature, i.e. a new sol-gel strategy with favorable greenness attributes. A multi-technical characterization of SiO₂NPs (XRD, FTIR, UV-vis DR, TEM, SEM, EDX, Z-potential, and N₂ adsorption-desorption isotherms at 77 K) confirmed the formation of spherical NPs, with amorphous and polydisperse nature, negatively charged surface, and mesoporous structure. Several batch adsorption experiments of MB were performed by varying pH, contact time, model dye concentration, and SiO₂NPs dosage, and the kinetic and thermodynamic behavior of the removal reaction was elucidated. It was determined that the adsorption process followed a pseudo-second-order kinetic model and a Langmuir isotherm model. SiO₂NPs showed high efficiency towards MB removal after 30 min of contact time (maximum adsorption capacity = 165.6 mg g^-1) and high reusability for up to seven cycles without appreciable loss of adsorption efficiency. In addition, this work reports the first successful application of SiO₂NPs as a cationic dye nanoadsorbent under simulated conditions of real textile wastewater (high pH, very high concentration of MB and dissolved salts, and high COD), proving that NPs are suitable for conditioning water resources contaminated with industrial dyes.