Adsorption mechanisms of single and simultaneous removal of pharmaceutical compounds onto activated carbon: Isotherm and thermodynamic modeling

Abstract

The simultaneous adsorption of pharmaceutical compounds is a challenge for real-life water treatment applications. Therefore, the present study aimed to investigate the simultaneous adsorption of paracetamol and nimesulide, using activated carbon (AC) as an adsorbent. Single adsorption experiments were also performed to compare the adsorption behavior changes. For instance, AC showed a higher single adsorption capacity for nimesulide (196.32 mg g−1) than paracetamol (58.21 mg g−1). Interestingly, during the binary adsorption, nimesulide molecules were adsorbed on active sites previously occupied by paracetamol molecules. This displacement phenomenon, due to the competition of pharmaceutical molecules at higher pharmaceutical concentrations, promoted the adsorption of nimesulide (199.49 mg g−1) and strongly suppressed the adsorption of paracetamol (25.80 mg g−1) on AC. The greater affinity between the AC and nimesulide was related to the more hydrophobic nature of nimesulide (log Kow = 2.60) than that of paracetamol (log Kow = 0.49). Besides, FTIR spectra, before and after adsorption, suggested that paracetamol was adsorbed only by H–bonding interactions, while nimesulide was adsorbed by H–bonding, π−π, and n−π dispersion interactions. Langmuir model (R2 ≥ 0.991) successfully adjusted the single equilibrium isotherms for both pharmaceuticals. Whereas, Extended Langmuir model (R2 ≥ 0.984) was chosen to predict the binary adsorption of paracetamol and nimesulide on AC. Finally, the thermodynamic analysis suggested exothermic, favorable, and spontaneous adsorption, with low adsorption energies that supported the physisorption.