Steam-activated carbon from malt bagasse: optimization of preparation conditions and adsorption studies of sunset yellow food dye

Abstract In this work the malt bagasse was used as precursor for preparation of activated carbon (AC) via physical activation with steam. The variables involved in the preparation process as temperature and time of activation, and steam flow, were evaluated and optimized from rotatable central composite design, response surface methodology and desirability function. The variables showed significant effects on the responses BET surface area (SBET) and yield of the AC. The activated carbon obtained in the optimal conditions (ACMop) of temperature 841 °C, time 82 min and steam flow 164 cm3 min-1, showed SBET of 917 m2 g-1 and yield 9.45%, in addition to surface acid groups and pHpzc 3.92. The ACMop was applied in the adsorption of food dye sunset yellow (SY) from the aqueous solution, showing fast adsorption kinetics and the adsorption equilibrium around the 240 min. The kinetic data were better described by Elovich model, while the fit of Weber and Morris model showed that intraparticle diffusion is not the rate-limiting step. The Redlich-Peterson isotherm model was that better fitted to experimental data, being that the g parameter near to 1 suggested the Langmuir assumptions as the more suitable to describe the adsorption of SY onto ACMop. Additionally, monolayer adsorption capacity of ACMop was found to be 199.7 mg g-1 at temperature of 55 °C. The thermodynamic study showed the spontaneity of adsorption process, with increase of randomness on the solid/liquid interface, nevertheless occurring from an endothermic process.