Mesoporous carbon-silica composite from low-cost vinasse for the effective adsorption of methyl orange and methylene blue dyes from aqueous systems

The adsorption behaviour of anionic and cationic dyes, namely methyl orange (MO) and methylene blue (MB), respectively, has been studied on carbon-silica composites. These composites were produced through the valorisation of waste vinasse from industrial sugar production as the carbon feedstock, while the silica production was achieved via a simple sol-gel method using low-cost sodium and potassium silicate solutions (Na2SiO3 and K2SiO3) or higher-cost tetraethyl orthosilicate (TEOS). Use of Na2SiO3, K2SiO3 and TEOS, led to composites with specific surface areas of 375, 343 and 656 m2/g and % mesoporosity of 79.45, 94.35 and 82.93%, respectively. Adsorption of MO is reduced at higher pH, but MB removal was enhanced under basic pH. Kinetic results demonstrated a strong correlation with the pseudo-second order model, and it was noted that intraparticle diffusion had a key role to play in the mechanism of mass transport. Adsorption data fitted the Sips isotherm model, with excellent adsorption capacity of MO (844.63 mg/g for Na2SiO3, 979.06 mg/g for K2SiO3, 1161.65 mg/g for TEOS) and MB (514.30 mg/g for Na2SiO3, 385.34 mg/g for K2SiO3, 294.88 mg/g for TEOS). Elevated temperatures led to preferential removal of the acidic and basic dyes, suggesting an endothermic adsorption process. Carbon-silica composites derived from vinasse are promising materials for the remediation of MB and MO from aqueous systems, exhibiting comparable or higher surface area and adsorption capacities for both dyes relative to other reported materials, while being synthesized through simple preparation processes from low-cost agricultural by-products.