Iron oxi(hydroxi)des are known to be efficient sorbents toward arsenic species, thanks to the formation of complexes of the inner sphere, through ligand exchange of surface -OH₂ and -OH^. with the arsenic species in the coordination sphere of the structural Fe atoms.[1] Recently, we compared the sorption ability of different iron oxi(hydroxi)des, namely akaganeite (β-FeOOH), ferrihydrite (referred to as Fe₅HO₈∙4H₂O), and maghemite (γ-Fe₂O₃, as a bare and silica-based composite), toward arseniate/arsenite species in spiked aqueous solutions.[2] Akaganeite was proved to be the best sorbent for arseniate anions (89 mg g -1 at pH₀ 3, 52 mg g^-1 at pH₀ 8) and also an efficient one for arsenite species (91 mg g^-1 at pH₀ 3–8), thanks to its high positive surface charge (measured by Electrophoretic Light Scattering, ELS, ≈30 mV) and high surface area (determined by N₂ physisorption, ≈200 m² g^-1 ). To investigate the adsorption process and the interaction between arseniate/arsenite and akageneite, different methods, such as powder X-ray diffraction (PXRD), infrared spectroscopy (FTIR-ATR), transmission electron microscopy also in high resolution (HRTEM) and coupled with chemical probes (EDX-EELS), X-ray photoelectron spectroscopy (XPS), X-ray absorption (XAS), and DC magnetometry, were adopted. None of these techniques were able to detect any changes in the sorbent properties upon adsorption, except for FTIR-ATR and DC magnetometry. In particular, the interaction between akageneite and arseniate species was found to produce a distinctive change in the magnetism of the sorbent, with the appearance of a second band in the temperature profile of the magnetisation, whose associated intensity increased with increasing adsorbate amount. This work demonstrates the high sensitivity of DC magnetometry in exploring surface phenomena in iron-bearing materials.

[1] A. Jain, et al Environ. Sci. Technol. 1999, 33, 1179–1184.

[2] M. Sanna Angotzi, et al, Nanomaterials 2022, 12, 326, 1-22.