Novel mesoporous carbon-silica composites from vinasse for the removal of dyes from aqueous silk dyeing wastes

Novel mesoporous carbon-silica composites were prepared from vinasse, a by-product from ethanol production, as the carbon source, and sodium silicate (Na2SiO3) and potassium silicate (K2SiO3) as low-cost silica source alternatives to tetraethyl orthosilicate (TEOS). The composites were characterized for their surface area and porous properties using nitrogen adsorption-desorption porosimetry. The composites possessed a high mesopore volume (45-77%) with moderate specific surface areas (343-656 m2/g) and pore size of 3.12-5.58 nm. The adsorption behaviour of carbon-silica composites for the removal of four silk dyes (Green 41, Blue 32, Dark red 34 and Dark gold brown 35) from aqueous solution was investigated and compared with a commercial activated carbon. The effects of adsorption time, dye concentration, pH and temperature were analyzed. The dye adsorption kinetics of all four dyes followed the pseudo-second order kinetic model, suggesting chemisorption as the dominant mechanism. Moreover, the intraparticle diffusion model showed that both internal diffusion and external diffusion were rate-limiting. The maximum adsorption capacities for all dyes were found at pH ~ 2. The equilibrium data for silk dyes adsorption were best described by the Langmuir equation, thus indicating monolayer adsorption. Thermodynamic parameters indicated that the dye adsorption was an endothermic process (ΔH > 0). The negative values of free energy (ΔG) confirmed that dye adsorption was spontaneous at the investigated temperatures (303-323 K). For all dyes, the maximum adsorption capacities of carbon-silica composites were comparable to the commercial activated carbon. The combination of vinasse with low-cost silica sources is a promising approach to produce inexpensive carbon-silica composites for application as adsorbents for dye removal from aqueous waste solutions generated during silk manufacture and dyeing.