Reductive Transformation of Birnessite by Acetate under Visible Light at pH 8: Clues on Mn Cycling in Marine Euphotic Zone

Abstract:

Birnessite is a layered-structure Mn oxide vastly distributing in marine euphotic zone. As a typical semiconducting mineral, it plays important photochemical roles in driving the marine element cycle. Previous studies have demonstrated the rapid dissolution of birnessite by marine organics under illumination. However, little is known about the detailed photo-reduction process as well as the fine structural evolution of birnessite. Our study mainly focused on the reductive transformation of hexagonal birnessite by acetate under visible light irradiation. The pH condition was maintained at 8.0, so as to simulate the alkaline environment of seawater. As measured by ICP-OES, the releasing efficiency of Mn²⁺ reached ~70% after 12 hours of reaction under illumination, whereas in dark condition only ~32% Mn²⁺ was dissolved. Mn K-edge XANES spectra were collected at 3 h, 6 h, 9 h and 12 h during the reaction. And the linear combination fitting of the spectra revealed that, as the reaction progressed, the solid-phase Mn(IV) content gradually decreased from 95% to 80%, while Mn(III) increased from 2% to 12%. Derived from the fitting results, the average Mn oxidation state was substantially lowered from 3.88 to 3.70, indicating a continually promoted reduction extent with the reaction time. XRD analysis showed the enhanced intensity of (001) and (002) band diffraction, and the enlarged d values of (100) and (110) basal planes. All these results implied the generation of large-size Mn(III/II) in the structure, which would either incorporate into vacancy to enlarge the layer sheets, or adsorb above vacancy to promote layer stacking. Our study would give new insights into the photocatalytic Mn cycling in marine euphotic zones.