Facet-dependent adsorption of safranin dye and phosphate ions by synthesized hematite nanorods derived from natural lateritic iron ore: steric and energetic investigations

Abstract

Natural lateritic iron was subjected to a morphological transformation process based on a simple alkaline hydrothermal treatment process, producing well-developed hematite nanorods (HM24). The rods display mesoporous properties (an average pore diameter of 4.2 nm) and significant surface area (141.5 m² g⁻¹). The developed nanorods were applied as enhanced adsorbents for phosphate ions (PO₄³⁻) and safranin (SFR) dye considering the changes in the exposed crystalline facet. The obtained results revealed that the modified form with its nanorod morphology and different exposed facets (012) displays a better uptake performance for both PO₄³⁻ (148 mg g⁻¹) and SFR (211.4 mg g⁻¹) than the raw sample and numerous investigated adsorbents in the literature. These results were obtained by considering the following experimental conditions: pH: 5 for PO₄³⁻ and 8 for SFR; temperature: 30 °C; volume: 100 mL; concentration: 350 mg L⁻¹; duration: 24 h; and amount: 0.4 g L⁻¹. The advanced isotherm assessments, based on the derived monolayer model and statistical physics concepts, reveal the enrichment of the rods with active site densities of approximately 84.8 mg g⁻¹ for SFR and 38.8 mg g⁻¹ for PO₄³⁻. Each of these sites can be filled with 3 molecules of SFR and 5 ions of PO₄³⁻, suggesting multi-ionic interactions and vertical configurations of these adsorbed pollutants. The energetic investigations (less than 40 kJ mol⁻¹) suggest that the active sites of HM24 take up both SFR and PO₄³⁻ through various physical mechanisms. This might include electrostatic attraction, hydrogen bonds, and van der Waals forces. These mechanisms involve spontaneous and exothermic processes considering the findings of the thermodynamic functions (enthalpy, entropy, and internal energy). The structure also exhibits remarkable recyclability to be reused six times with significant capacity.