Insight into adsorption mechanism of Congo red dye onto Bombax Buonopozense bark Activated-carbon using Central composite design and DFT studies

Abstract

In this study, a novel activated-carbon was prepared from Bombax Buonopozense (AC-BBP) and used for the adsorption of Congo red (CR) from aqueous solution and the prepared material was characterized by FTIR, XRD, TGA, SEM, elemental analysis and BET. Central Composite Design (CCD) was applied to optimize the medium conditions for the maximum removal of CR from aqueous solution and the optimization has been able to establish a quadratic mathematical model using three main factors varied from level (-1) to (+1) such as adsorbent dose (0.005 to 0.010 g), pH (7 to 9) and contact time (25 to 45 min), with an R2=98.73, a CV% =1.05 and an AP=27.79. The optimal conditions of the adsorption system were selected to be an adsorbent dosage of AC-BBP (0.117 g/L), solution pH (7.87) and a contact time of 29 min. These optimal conditions achieve the removal efficiency of CR is 86.12% from water using AC-BBP. The Langmuir isotherm is a good predictor of the experimental adsorption of Congo red with an adsorption capacity of 510.2 mg/g. The pseudo-second-order kinetic model shows a good fit with a correlation coefficient of R2> 0.99. Thermodynamic parameters were evaluated at 293 K and the results show a negative values of Gibbs free energy (-8.82 kJ/mol), indicating that the adsorption process was endothermic in nature. Moreover, Density functional theory (DFT) calculations were applied to understand the adsorption mechanism of the Congo red dye onto AC-BBP. The results shows that the groups of SO3− and N = N in CR are donating charge parts while the CH and NH bonds are an attracting electron or charge parts from the AC-BBP.