Low‐Spin‐State Hematite with Superior Adsorption of Anionic Contaminations for Water Purification

Hematite attracts intensive interest as an adsorbent for water purification, but the oversized dimension and inherent high‐spin Fe(III) restrict its adsorption capability and kinetics. Herein a spatial‐confinement strategy is reported that synthesizes ultrafine α‐Fe2O3 benefiting from nanogrids constructed by predeposition of TiO2 nanodots in the MCM‐41 channel, and that tunes the spin‐state of Fe(III) from high‐spin to low‐spin induced by the strong guest–host interaction between the ultrafine Fe2O3 with SiO2 (MCM‐41). The low‐spin Fe(III) endorses strong bonding with anionic adsorbates, and significantly facilitates the electrons transfer from Fe2O3 to SiO2 to form a highly positive charged surface, and thereby shows superior electrostatic multilayer adsorption performance to different kinds of anionic contaminations. Specifically, the maximum uptake, adsorption rate, and distribution coefficient (Kd) for Rose Bengal dye reach as high as 1810 mg g−1, 1644 g (g min)−1, and 2.2 × 106 L kg−1, which are more than 8, 230, and 3700 times higher than those of commercial activated carbon, respectively. It also shows outstanding purification performance for real field water. It is demonstrated that a strong guest–host interaction can alter the spin‐state of transition metal oxides, which may pave a new way to improve their performance in adsorption and other applications like catalysis. Spin‐transition of hematite is realized via strong interaction between ultrafine α‐Fe2O3 and SiO2. The spatial confinement strategy allows uniform deposition of ultrafine α‐Fe2O3 on mesoporous MCM‐41, which produces low‐spin α‐Fe2O3 with stronger bonding strength with adsorbates, constructs a highly positive charged surface, and thus affords fast and deep adsorption ability for water purification and other appealing application prospects.