Mechanisms of Cr(VI) adsorption on schwertmannite under environmental disturbance: Changes in surface complex structures

Highlights: • C(VI) exchanges with two chemical structures of sulfate on schwertmannite. • pH drives bidentate Cr(VI) complexes convert into monodentate one. • Surface coverage exerts little influence on Cr(VI) interfacial complexes. • Interfacial reactions of Cr(VI) on schwertmannite were described. Hexavalent chromium (Cr(VI)) mobility, reactivity and bioavailability in the acid mine drainage (AMD) are restricted by adsorption reactions on schwertmannite. However, the Cr(VI) adsorption mechanisms remain unclear. In this study, batch adsorption/desorption experiments, X-ray photoelectron spectroscopy (XPS), and in‒situ attenuated total reflectance–Fourier transform infrared spectroscopy (ATR-FTIR) in combination with a multivariate curve resolution– alternating least squares (MCR-ALS) analysis were employed to characterize Cr(VI) adsorption on schwertmannite. The results of batch experiments suggested that two kinds of anion exchange reactions occurred on Sch surface: the outer-sphere complexes and the inner-sphere complexes of sulfate were successively substituted by aqueous Cr(VI) to form inner-sphere complexes. XPS analysis showed that the adsorbed Cr (VI) tended to exchange with sulfate rather than with surface hydroxyl groups on schwertmannite. In-situ ATR-FTIR spectroscopic results confirmed that the Cr(VI) coordination species contained bidentate inner-sphere (C_2ν) and monodentate inner-sphere complexes (C_3ν). MCR-ALS analysis revealed that monodentate complexes were dominant at pH 5.0–8.0. The proportion of bidentate complexes decreased from 47% to 25% when pH increased from 5.0 to 8.0. Thus, we concluded that a transition occurred between bidentate to monodentate complexes. In addition, the Cr(VI) concentration exerted little influence on the change of surface complexes.