Pharmaceutical adsorption on NaOH-treated rice husk-based activated carbons: Kinetics, thermodynamics, and mechanisms

This study aims to explore the adsorption behavior of four pharmaceuticals: ciprofloxacin (CIP), sulfamethoxazole (SUL), diclofenac (DIC), and acetaminophen (ACE) on activated carbons produced from rice husk using different NaOH-based protocols at 800 °C. The main objective is to assess and compare the efficiency of activated carbons synthesized by solid-solid and wet treatment methods on pharmaceutical adsorption. All three activated carbons displayed strong affinity towards pharmaceutical adsorption, but the material produced with wet protocol after the carbonization process demonstrated the highest adsorption capacity and the lowest equilibrium time (15 min) due to its superior textural properties and higher surface area (1671 ± 31 m2 g−1). Further investigation of pharmaceutical adsorption in synthetic urine was carried out using the mentioned material. The results revealed that in both mono and multicomponent systems, CIP, SUL, and DIC did not exhibit any significant competition with each other or with the urine matrix. However, ACE showed competition for adsorption sites. For both CIP and ACE, the diffusion boundary layer and the adsorption process were identified as the velocity-limiting steps for adsorption in urine. To better understand the adsorption mechanism, both experimental and computational tools were employed. The results revealed that ACE exhibited typical Langmuir-like adsorption behavior with an exothermic nature on the mentioned material with an adsorption capacity of 209.6 ± 5.6 mg g−1. Regarding CIP, its adsorption displayed a non-typical endothermic nature, which was attributed to steric hindrance due to its zwitterionic structure and the porosity in the adsorbent. Analyzing the combination of Redlich-Peterson and Langmuir models with computational analysis supported this finding. However, the material exhibited an impressive adsorption capability of 584.4 ± 21.5 mg g−1. The parameter φ (cncreased cost associated with enhancing the adsorption process) revealed an augmentation in adsorption capacities of 45.1 and 125.6 mg USD−1 for ACE and CIP, respectively, compared to the raw rice husk. This research underscores the prospect of employing agricultural and industrial byproducts to create activated carbons capable of efficiently extracting pharmaceuticals from intricate mixtures, such as urine.