DC-magnetometry is proposed as an ultrasensitive probe to directly study surface spin ordering induced by chemical interactions. As a proof-of-concept, we investigated the adsorption of arsenic species onto akaganeite nanorods, a nanostructured iron oxyhydroxide particularly effective in the removal of AsV/AsIII species. DC-magnetometry unequivocally revealed the presence of arsenate through a new and distinct magnetic signature (a second peak in the ZFC curve), highlighting the effect of the adsorbate-adsorbent interaction. This peculiar magnetic feature, strictly dependent on the adsorbate speciation (AsIII vs AsV), amount, and type of interaction, was found to be directly and linearly correlated with the amount of AsV adsorbed on the surface. The comparison between the results obtained by different conventional and advanced techniques further highlighted the unique ability of DC magnetometry in qualitatively and quantitatively revealing the adsorption phenomenon. The suggestion of a ligand exchange process by XPS for the AsV allowed us to interpret the observed surface spin ordering in terms of specific chemical interactions. This evidence establishes DC-magnetometry as a powerful and unconventional analytical tool for monitoring and quantifying complex surface phenomena on magnetically responsive materials suitable as heterogeneous catalysts, sorbents, and sensors.
Angotzi et al. (Fri,) studied this question.