Binding of Sb(V) to organic-mineral complexes: Implications for Sb mobility and fate in soils

The mobility and fate of antimony (Sb) in soil and aquatic environments are primarily gocerned its adsorption onto iron oxides (hydroxides), a process markedly influenced by natural organic matter (NOM). In this study, batch experiments (kinetic studies, isotherm analyses, and pH effect evaluations) were conducted to examine Sb(V) adsorption onto Hematite-Humic complexes (Hem-HA complexes) as representative organic-inorganic hybrid materials. HA treatment did not modify the crystal structure of Hem. However, the resulting Hem-HA complexes exhibited reduced surface area, porosity, and pore diameter compared to unmodified Hem. The adsorption of Sb(V) was significantly affected by HA concentration and pH. The maximum adsorption capacity of Hem-HA complexes was lower than that of Hem and gradually decreased as the HA treatment concentration increased. The incorporation of HA onto Hem surfaces reduced the number of reactive adsorption sites, increasing the mobility risk of Sb(V). Sb(V) adsorption was favored under acidic conditions, with Hem-HA₄ showing a decrease in maximum adsorption capacity from 4.24 to 2.05 mg/g as pH rose from 3 to 9. The adsorption process followed pseudo-second-order kinetics and was well described by both the Langmuir and Freundlich models, indicating a monolayer adsorption mechanism. Spectroscopic analyses (Fourier transform infrared and X-ray absorption spectroscopy) confirmed the formation of Fe-O-Sb monodentate, mononuclear edge-sharing inner-sphere complexes via electron sharing or transfer. Direct complexation between organic functional groups and Sb(V) was also observed. These findings enhance our understanding of Sb biogeochemical cycling and offer valuable insights for Sb remediation in complex environmental systems.