pH-Controlled Tailoring of Multiscale Magnetite Structure for Enhanced H₂S Capture

Iron oxide-based adsorbents offer strong potential for hydrogen sulfide (H₂S) removal due to their cost-effectiveness and environmental compatibility. However, how to precisely control multiscale microstructural features that govern adsorption performance remains inadequately understood. Here, a pH-controlled hydrothermal synthesis strategy using ferrous sulfate heptahydrate as the iron precursor was developed to elucidate the relationship between the microstructure and the adsorption performance of Fe-based adsorbents. A distinct functional pH threshold (≥7) was identified, above which a pure-phase Fe₃O₄ was formed and the surface Fe²⁺/Fe³⁺ redox couple became stabilized/activated, enabling effective H₂S capture. H₂S fixed-bed tests were conducted at ambient temperature with an inlet concentration of 1000 ppm. The adsorbent synthesized at pH = 13.0 integrated mixed-valence Fe²⁺/Fe³⁺ sites, defect-rich surfaces with enhanced basicity, mesoporosity, and exposed {111} facets, delivering a breakthrough capacity of 85 mg/g (H₂S outlet concentration < 0.01 ppm), which was 7 times higher than that at the threshold (pH = 7), and an estimated total sulfur uptake of 250 mg/g. This work clarifies how multiscale microstructural features govern H₂S capture and provides a design strategy for tuning redox-active sites, defects, and surface basicity.