Kinetics of Mn(II) adsorption and catalytic oxidation on the surface of ferrihydrite

Abstract

Mn(II) adsorption-oxidation on iron (Fe) oxides (e.g., ferrihydrite) occurs in various soils and sediments, significantly affecting the toxicities and bioavailabilities of Mn and other associated elements. However, the detailed processes of Mn(II) adsorption-oxidation on ferrihydrite remain elusive. In this study, the Mn(II) (2 mM) adsorption-oxidation kinetics on different masses of ferrihydrite (0.25, 0.50, 1.00, and 1.25 g) at pH 7 were determined using batch kinetic studies combined with X-ray diffraction, transmission electron microscopy, and wet chemistry analyses. The results indicated that the low-concentration Mn(II) adsorption-oxidation on ferrihydrite occurred in two steps. First, Mn(II) was adsorbed onto ferrihydrite, where it was partially oxidized by the catalytic effect of ferrihydrite, within ~0–60 min; subsequently, the remaining Mn(II) underwent autocatalytic oxidation on the previously generated Mn (oxyhydr)oxides. The initial adsorption-oxidation behaviors of Mn(II) on the ferrihydrite surface determined the kinetics of Mn(II) removal and oxidation, and therefore the amounts and types of Mn (oxyhydr)oxides formed. Furthermore, the specific characteristics of Mn(II) adsorption-oxidation on ferrihydrite showed a strong dependence on the Fe/Mn molar ratio. When this ratio was below 16.35, the initial process was dominated by Mn(II) adsorption onto ferrihydrite, with slight oxidation generating hausmannite (~0–60 min), followed by the catalytic oxidation of Mn(II) on the formed hausmannite, generating manganite or groutite. Conversely, when the Fe/Mn molar ratio was above 32.7, the reactions primarily involved Mn(II) adsorption onto ferrihydrite with minor oxidation to form Mn(III/IV) (oxyhydr)oxides (~0–60 min), followed by the autocatalytic oxidation of Mn(II) on the freshly-generated Mn(III/IV) (oxyhydr)oxides, forming Mn(III) (oxyhydr)oxides, i.e., feitknechtite. These results provide further insight into the interaction between Fe and Mn, Mn(II) removal, and Mn (oxyhydr)oxide formation in the environment.