Recoverable Fe3O4@SiO2@TMC magnetic sorbent for efficient Congo red removal: performance and adsorption mechanism

Abstract

The increasing discharge of dye-laden wastewater necessitates the development of efficient and easily recoverable adsorbents. In this study, a trimesoyl chloride (TMC) functionalized Fe3O4@SiO2 magnetic sorbent was synthesized via a sol–gel route for the removal of Congo red (CR) from aqueous media. Dynamic light scattering indicated a uniform particle size distribution (∼130–140 nm). FT-IR analysis confirmed the presence of –COOH, –OH, and aromatic moieties, enabling multi-modal interactions including hydrogen bonding, electrostatic attraction, π–π stacking, and coordination with CR molecules. Structural and surface properties were further validated by XRD, FE-SEM, and zeta potential analyses. Operational parameters (initial concentration, pH, adsorbent dosage, and contact time) were systematically optimized, with the maximum adsorption capacity of 4614.62 (pH = 5, dosage = 2 mg) for CR dye. Kinetic modeling revealed an excellent fit to the Bangham model (R2 = 0.999), which yielded a calculated equilibrium adsorption capacity of 231.95 mg g−1, closely matching the experimental value of 233.85 mg g−1 and indicating a substantial contribution of pore diffusion. Isotherm analysis suggested multistage adsorption behavior. The Jovanović model provided a reasonable fit (R2 = 0.960) and a qm value of 4100 mg g−1, which was relatively close to the experimental adsorption capacity of 4614.61 mg g−1. The Koble–Corrigan model provided the best fit to the equilibrium data (R2 = 0.995), suggesting the contribution of heterogeneous adsorption sites. Reusability tests demonstrated that the magnetic adsorbent retained ∼80% of its initial removal efficiency after 4 consecutive cycles, confirming its practical applicability and facile separation. Overall, the Fe3O4@SiO2@TMC system presents a robust and magnetically recoverable platform for efficient dye remediation, offering a promising approach for future studies and wastewater treatment.