Effective removal of Cr(VI) from aqueous solution through adsorption and reduction by magnetic S-doped Fe-Cu-La trimetallic oxides

In this work, magnetic S-doped Fe-Cu-La trimetallic oxides (S-FeCuLaO) were prepared and adopted as an adsorbent to remove Cr(VI) from aqueous solutions. The S-FeCuLaO material was characterized by SEM, XRD, FTIR, XPS and VSM, and batch adsorption experiments were employed to investigate the effects of main interfering factors on Cr(VI) adsorption. The results demonstrated that almost 100% of Cr(VI) was removed at initial pH 3–4 and Cr(VI) removal efficiency sharply declined to 25.3% with initial pH increasing to 10. Cr(VI) adsorption process abided by pseudo-second-order kinetic model and Langmuir model, indicating that Cr(VI) adsorption onto S-FeCuLaO was monolayer chemical adsorption. The maximum adsorption capacity of Cr(VI) achieved from Langmuir model at initial pH 4 reached 157.98 mg/g. XPS analysis indicated 36.7% of adsorbed Cr(VI) anions was converted to non-toxic Cr(III) by Cu(I) and S2−. Furthermore, the removal mechanism of Cr(VI) at pH < pHPZC was proposed as electrostatic attraction, ion-exchange reaction, reduction of partial Cr(VI) and formation of Cr(OH)3. Among the five coexisting anions studied, only SO42− suppressed Cr(VI) adsorption. Moreover, the adsorbent displayed high stability at pH above 4 and great reusability with the removal of above 96% Cr(VI) in the fifth cycle. Therefore, the magnetic S-FeCuLaO composite is a low-cost material for the efficient removal of Cr(VI) from water environment.