Sustainable upcycling of industrial slag into highly porous silica for wastewater treatment

Upcycling industrial slag, a byproduct of ferroalloy production, offers a sustainable approach to producing high-value, porous silica materials. Porous silicas with tailored porosity were synthesized via selective acid leaching using 3 M and 6 M HCl to remove metal oxides from slag. As the slag is composed of oxide nanoparticles of silicon, aluminum, iron, etc., the acid treatment in an optimized condition removed metal oxides other than silica, resulting in a porous structure. The structural and surface properties were characterized by N₂ adsorption-desorption isotherms, XPS, and TEM analysis. Silica prepared with a high acid concentration exhibited a significantly larger specific surface area (570.8 m² g⁻¹) than that prepared with a low concentration of acid (112.5 m² g⁻¹). Furthermore, the silica obtained with high or low acid concentrations showed microporous or mesoporous structures, respectively. According to the adsorption isotherm study utilizing a model pollutant, methylene blue, from water, it was proven that the porous silicas could remove methylene blue from water in a multilayer manner. Despite the smaller specific surface area, the mesoporous silica obtained from low-concentration acid treatment was effective in methylene blue adsorption, in terms of adsorption enthalpy and entropy.