Polyaniline-based adsorptive removal of sodium salicylate from water: a combined experimental and theoretical study

Sodium salicylate, a derivative of aspirin, is an important contributor to pharmaceutical contamination in the environment, as it is widely used for its anti-inflammatory properties and serves as a preservative in food products. The sodium salicylate molecules may persist through conventional wastewater treatment processes, leading to their discharge into natural aquatic ecosystems, where they can pose ecotoxicological risks to microorganisms and aquatic organisms. Several advanced wastewater treatment processes, including physicochemical ones (e.g., adsorption), are used to reduce the harmful effects of these pollutants. In this study, polyaniline (PANi) was synthesized and utilized as a cost-effective adsorbent for the removal of sodium salicylate from aqueous solutions. The effects of key physicochemical parameters (e.g., initial pH, solution temperature, PANi dosage, initial sodium salicylate concentration, and contact time) were systematically investigated. The optimal adsorption conditions were determined as follows: equilibrium time of 60 min, adsorbent dosage of 0.66 g/l, pH of 4.60, initial sodium salicylate concentration of 10 mg/L, and temperature of 25 °C. Kinetic analysis revealed that the adsorption process follows the pseudo-second-order model. The adsorption behavior of sodium salicylate was described by the Langmuir isotherm model with a calculated maximum monolayer adsorption capacity of 185.18 mg/g. Thermodynamic analysis indicated that the process is spontaneous (ΔG° < 0) and endothermic (ΔH° = 16.01 kJ/mol). Furthermore, the adsorption mechanism was explored through Density Functional Theory (DFT) calculations at the B3LYP/6–31G(d) level, providing deeper insight into the interactions of sodium salicylate with PANi surface.