Synthesis, characterization and application of magnetic nanoparticles modified with Fe-Mn binary oxide for enhanced removal of As(III) and As(V)

Arsenic contamination of drinking water sources is a widespread global problem. Of the As species commonly found in groundwater, As(III) is generally more mobile and toxic than As(V). In this work, magnetic nanoparticles (MNp) modified with Fe-Mn binary oxide (MNp-FeMn) were synthesized in order to develop a low cost adsorbent with high removal efficiency for both arsenic species which can be readily separated from water using a magnetic field. MNp-FeMn were characterized using different techniques including SEM/EDS, XRD and BET analysis. Adsorption of As(III) and As(V) on MNp-FeMn was studied as a function of initial arsenic concentration, contact time, pH, and coexisting anions. The BET specific surface area of MNp-FeMn was 109 m²/g and maghemite (γ-Fe₂O₃) was the dominant precipitated phase. The adsorption rate of As(III) and As(V) on MNp-FeMn was controlled by surface diffusion. FTIR analysis confirms that surface complexation through ligand exchange was the main mechanism for As(III) and As(V) removal on MNp-FeMn, with As(III) conversion to As(V) occurring on the adsorbent surface. The maximal adsorption capacity q_max of MNp for As(III) (26 mg/g) was significantly improved after modification with Fe-Mn binary oxide (56 mg/g), while q_max for As(V) was 51 and 54 mg/g, respectively. ${\mathrm{PO}}_4^{3-}$, ${\mathrm{SiO}}_3^{2-}$ and ${\mathrm{CO}}_3^{2-}$ reduced As(III) and As(V) uptake at higher concentrations. MNp-FeMn can be easily regenerated and reused with only a slight reduction in adsorption capacity. The high oxidation and sorption capacity of MNp-FeMn, magnetic properties and reusability, suggest this material is a highly promising adsorbent for treatment of arsenic contaminated groundwater.