Optimization of Binary Adsorption of Metronidazole and Sulfamethoxazole in Aqueous Solution Supported with DFT Calculations

Sulfamethoxazole [SMX] and metronidazole [MNZ] are emergent pollutants commonly found in surface water and wastewater, which can cause public health and environmental issues even at trace levels. An efficient alternative for their removal is the application of adsorption technology. The present work evaluated single and binary adsorption processes using granular activated carbon (CAG F400) for SMX and MNZ in an aqueous solution. The binary adsorption process was studied using a Box–Behnken experimental design (RSD), and the results were statistically tested using an analysis of variance. Density functional theory (DFT) modeling was employed to characterize the interactions between the antibiotics and the CAG F400 surface. For the individual adsorption process, adsorption capacities (q_e) of 1.61 mmol g⁻¹ for SMX and 1.10 mmol g⁻¹ for MNZ were obtained. The adsorption isotherm model that best fit experimental data was the Radke–Prausnitz isotherm model. The adsorption mechanism occurs through electrostatic and π-π dispersive interactions. For the binary adsorption process, the total binary adsorption capacity achieved was 1.13 mmol g⁻¹, evidencing competitive adsorption. The significant factors that determine the removal of SMX and MNZ from a binary solution were the solution pH and the initial concentration of antibiotics. From DFT studies, it was found that SMX adsorption on CAG F400 was favored with adsorption energy (E_ads) of −10.36 kcal mol⁻¹. Finally, the binary adsorption results corroborated that the adsorption process was favorable for both molecules.