Enhanced antimonate (Sb(V)) removal from aqueous solution by La-doped magnetic biochars

A novel La-doped magnetic biochar was synthesized by a co-precipitation method for efficient Sb(V) removal. In contrast to pristine biochar and un-doped magnetic biochar, the Sb(V) adsorption capacity in La-doped magnetic biochar was greatly improved, increasing from 2.22 mg/g and 4.85 mg/g to 18.92 mg/g at pH of 7.0, respectively. The enhanced Sb(V) adsorption remained over a wide pH range (2–10), despite the existence of Cl⁻, SO₄²⁻, NO₃⁻, HCO₃⁻, or H₂PO₄⁻. These competing anions had little interference with Sb(V) sorption except HCO₃⁻ and H₂PO₄⁻. The combined results of TEM, XRD, FTIR and XPS further confirmed that La atom was successfully doped into the Fe₃O₄ structure. The point of zero charge of the biochar increased accordingly with a number of hydroxyl groups (i.e. [tbnd]La–OH) formed on the surface. Although the magnetic performance decreased after La doping, La-doped magnetic biochar still showed high separation potential. The comparison of FTIR and XPS analyses before and after Sb(V) adsorption revealed that the formation of inner-sphere La–O–Sb complex was the dominant contribution for Sb(V) sorption enhancement. Meanwhile, other mechanisms such as hydrogen bonding, electrostatic attraction and ligand exchange were also involved. All the results suggested that La-doped magnetic biochars could serve as promising adsorbents for Sb(V) pollution minimization.