The effect of citric acid on the catalytic oxidation of Mn(II) on ferrihydrite surface

Abstract

Surface-catalyzed Mn(II) oxidation is an important process for abiotic formation of various Mn (oxyhydr)oxides in nature. Previous research has shown that some complex-forming ligands (e.g., citric acid, desferrioxamine B, and tartrate) can significantly bind with Mn(II) to promote its homogeneous oxidation by dissolved O2. However, there is little to no understanding of the role played by complex-forming ligands in abiotic interfacial oxidation of Mn(II). In this study, the adsorption and oxidation of Mn(II) on ferrihydrite (Fh) surface at a range of citric acid (CIT) concentrations was investigated using batch kinetic studies combined with X-ray diffraction, transmission electron microscopy, X-ray photoelectron spectroscopy, and wet chemistry analyses. The presence of CIT substantially inhibited the adsorption and oxidation of Mn(II) on Fh, whereas oxidized Mn generated certainly enhanced the degradation of CIT. However, at a high concentration of CIT (> 1 mM), the oxidation kinetics of Mn(II) were slightly accelerated due to ligand-promoted homogeneous oxidation. Moreover, the presence of CIT remarkably altered the distribution and composition of Mn(II) oxidation products by inhibiting the formation of hausmannite on Fh surface and the following auto-catalytic oxidation. At low CIT concentration (< 1 mM), the oxidation products were mostly coated on Fh surface with weaker crystallinity and higher Mn AOS, resulting in stronger reactivity. When CIT concentrations were above 1 mM, Mn(II) oxidation products were mainly distributed in the bulk solution as soluble Mn(III)-CIT/ox complexes, thereby enhancing the mobility of redox active Mn. These results suggested that CIT improved the redox activity of oxidation products and migration of active Mn in the environment although it inhibited Mn(II) oxidation on mineral surfaces.