Development of a sustainable adsorption system based on alginate-encapsulated clay for pharmaceutical pollutant capture in aqueous solution using Box–Behnken methodology

Abstract

Our study focused on the removal of metformin in aqueous solution. Metformin, an antidiabetic drug, is increasingly detected in wastewater. This study explored the feasibility of using bentonite encapsulated in sodium alginate as a sustainable and efficient sorbent for metformin removal. The adsorbent was characterized by RAMAN, Fourier transform infrared (FTIR), X-ray diffraction (XRD), scanning electron microscope (SEM), electron-dispersive spectrometer (EDS), and point of zero charge (pHzc). The adsorbent contains various functional groups including hydroxyl (-OH) groups from both bentonite and sodium alginate, carboxyl (-COOH) groups from sodium alginate, and silanol (-Si-OH) groups from bentonite. Batch kinetic studies were carried out as a function of pH, metformin concentration, sorbent amount, and solution temperature. The Freundlich and Langmuir models were used to describe the isotherm data. The maximum monolayer adsorption capacity of the sorbent material was 19,19 mgg−1. The pseudo-second order kinetic model adequately describes the kinetic data. The negative ΔH (−35,253 kJmol−1) confirms an exothermic process, which is further supported by the negative Gibbs energy values (−0,489 to −3,668 kJmol−1), indicating spontaneity and reversibility. Moreover, ΔS° = −0.105 kJ/mol−1.K−1. Response surface methodology was employed to identify the optimal parameters for metformin removal. These parameters were found to be: pH of 6.34, metformin concentration of 5.4 mgL−1, and adsorbent dosage of 3 g L−1 resulted in a remarkable efficiency of 94%. Additionally, bentonite beads (BBs) maintained a significant adsorption capacity for up to three cycles, demonstrating promising reusability. This study highlights that the encapsulation of clay in sodium alginate is a sustainable and effective approach for removing metformin during water treatment.