Synergistic FeCl3/sodium dodecyl sulfate functionalization of diatomite for enhanced methylene blue adsorption: from surface engineering to mechanistic insights

The removal of persistent cationic phenothiazine dyes from wastewater remains a major challenge because of their high stability and resistance to conventional treatments. Although diatomite is an abundant and low-cost adsorbent, its limited surface reactivity restricts its practical application. In this study, diatomite was surface-engineered through sequential FeCl3 functionalization followed by sodium dodecyl sulfate (SDS) treatment. The novelty of this approach lies in the sequential incorporation of iron-containing species and an anionic surfactant onto diatomite to tailor its surface chemistry and dispersion behavior, thereby enhancing its adsorption performance. Comprehensive characterization revealed the formation of poorly crystalline iron oxyhydroxide species following FeCl₃ treatment, while subsequent SDS modification promoted a more homogeneous distribution of surface species and altered the surface charge. Nitrogen adsorption revealed an increase in the BET surface area from 31.5 m² g⁻¹ for raw diatomite (D) to 227.2 m² g⁻¹ for FeCl3-modified diatomite (DF) and 242.5 m² g⁻¹ for FeCl3/SDS-modified diatomite (DFS). The adsorption performance toward methylene blue was evaluated. At pH 7 and 55 °C, the adsorption capacities reached 80.1, 106.2, and 160.3 mg g⁻¹ for D, DF, and DFS, respectively, with DFS exhibiting an approximately twofold increase in adsorption capacity compared with raw diatomite. The adsorption kinetics were well described by the pseudo-second-order model, while the Langmuir–Freundlich model provided the best fit to the equilibrium data. The most efficient adsorbent, DFS, retained a high adsorption capacity over three adsorption–desorption cycles using methanol as the desorbent. A combined interpretation of the characterization results, adsorption data, and FTIR spectra suggest that surface Fe–OH groups contribute to hydrogen bonding, whereas SDS-derived negatively charged sulfate groups enhance electrostatic attraction toward cationic dye molecules. These findings show that controlled surface functionalization of natural diatomite transforms a natural material into an efficient and multifunctional adsorbent for sustainable water treatment.