Valorization of spent coffee grounds through pyrolysis as adsorbent for the removal of Vivizole Red 3BS dye from aqueous solution

Ever-increasing coffee consumption results in the generation of a significant amount of solid residue in the form of spent coffee ground and its subsequent disposal cause environmental pollution. Valorization of spent coffee ground (SCG) through pyrolysis could be one of the solutions to this challenge. Pristine biochar of SCG shows less efficiency to remove dyes from aqueous solutions. Herein, iron(III) salt was used as a catalyst during the carbonization of SCG and has a good graphitization efficiency and thus enhanced the formation of aromatic structures which provide adsorption sites for the dye. The physical characteristics of the prepared biochar were analyzed by FTIR, XRD, and BET. A predictive model for the removal of the dye was investigated with the Design Expert 11.0 software through the central composite design (CCD) - response surface methodology (RSM) by conducting a batch adsorption study and the suggested optimum values of the CCD were 10 ppm initial dye concentration, 1 g per 100 ml adsorbent dose, and contact time of 101 min with optimum predicted dye removal of 99%. Langmuir model was the most fitted isotherm model with an adsorption capacity of 2.07 mg/g and the adsorption kinetic equilibrium data was better described by pseudo-second-order model and from the thermodynamic study, it has been suggested that the adsorption process was spontaneous, favorable, endothermic, and a pysicochemisorption in nature. The possible adsorption mechanisms governing the adsorption process of the dye with biochar are π – π electron donor-acceptor interactions and hydrogen bonding.