Dual Fe–Ce pillared bentonite for highly efficient Congo Red removal: Synthesis, characterization, and adsorption mechanisms

Abstract

The development of economical and effective, environmentally friendly adsorbents is essential for treating wastewater, particularly for removing dangerous dyes like Congo red (CR). This study describes the synthesis of iron-cerium pillared bentonite (Fe-Ce/Bent.) and examines its capacity to adsorb CR from aqueous solutions. The Fe-Ce/Bent. adsorbent was thoroughly analyzed using multiple characterization techniques, confirming the successful pillaring of bentonite. In comparison to raw bentonite (90.68 m²/g), Fe-Ce/Bent. exhibited a larger specific surface area (116.02 m²/g), indicating improved porosity. XRD revealed an increase in basal spacing from 15.14 to 17.28 Å, while FTIR showed N–H vibration shifts from 3424 to 3430 cm⁻¹ and azo group vibrations at 1573 and 1382 cm⁻¹ . SEM images demonstrated enhanced structural integrity and uniform particle dispersion, with the layered morphology preserved. Elemental analysis confirmed Fe (16.7 wt%) and Ce (15.0 wt%) incorporation, whereas TGA showed lower mass loss compared to Sodium bentonite (Na-B) and Raw bentonite (RB), signifying improved thermal stability. Additionally, the pH PZC decreased from 7.96 to 5.33, reflecting the creation of additional acidic sites and modified surface charge properties. Batch adsorption experiments assessed the influence of initial pH, adsorbent dosage, dye concentration, and contact time on Congo Red (CR) removal. Enhanced adsorption effectiveness was observed under acidic conditions (pH < pH pzc ), where CR molecules exhibit neutral or slightly negative charges. Adsorption kinetics followed a pseudo-second-order model with k₂ = 0.003 g·mg⁻¹ ·min⁻¹ , qₑ₂ = 117.43 mg/g, R² = 0.988, χ² = 3.868, and Δq(%) = 0.688. Equilibrium data were best described by the Freundlich isotherm (R² = 0.989), indicating multilayer adsorption on a heterogeneous surface. Thermodynamic analyses revealed negative ΔG° values (-4.044 kJ/mol), confirming spontaneity, with increased favorability at higher temperatures. The positive ΔH° (<40 kJ/mol) demonstrated an endothermic, physisorption-driven process dominated by weak van der Waals and electrostatic interactions, while the positive ΔS° (98.0 J/mol·K) indicated greater randomness at the solid–liquid interface. Regeneration trials confirmed good reusability, with removal efficiency decreasing moderately from 96% to 78% over five cycles. These results highlight Fe-Ce/Bent. as an efficient, stable, and sustainable adsorbent for CR dye removal from wastewater. • Fe–Ce pillared bentonite synthesized and structurally confirmed by XRD, FTIR, SEM, and TGA. • Pillaring increased surface area to 116.02 m²/g and expanded basal spacing to 17.28 Å. • pH PZC decreased from 7.96 to 5.33, indicating enhanced surface acidity and charge modification. • CR adsorption followed pseudo-second-order kinetics and fit the Freundlich isotherm model. • Adsorption was spontaneous, endothermic, and showed good reusability over five regeneration cycles.