Figures (7)  Tables (0)

    • Figure 1. 

      Dissolved Mn concentration ([Mn]dis) during the reduction of δ-MnO2 by HA at (a) pH 5, (b) pH 8, and (c) FA at pH 8 with R = 0.1 and 1 under anoxic or oxic conditions. The errors resulted from repeated concentration measurements using ICP-OES.

    • Figure 2. 

      E2/E3 of HA remained in the reactions with δ-MnO2 at (a) pH 5 and (b) pH 8 with R = 0.1 and 1 and FA remained at pH 8 with (c) R = 0.1 and (d) R = 1. Data for FA under anoxic conditions were adopted from Wang et al.[12].

    • Figure 3. 

      (a), (c), (e) Concentrations of acetate and (b), (d), (f) formate/pyruvate at different reaction times at pH 5 and 8 with R = 0.1 and 1 under anoxic or oxic conditions. Note: pyruvate had a retention time similar to formate, which makes them indistinguishable by ion chromatography.

    • Figure 4. 

      (a), (c), (e) The MnL–MnL distance, and (b) (d), (f) the area of the MnL–MnIL peak at ~5.3 Å derived from PDF for δ-MnO2 reacted with HA or FA at pH 5 and 8 with R = 0.1 and 1. The results for the FA systems under anoxic conditions were reported in Wang et al.[12].

    • Figure 5. 

      Synchrotron-based XRD patterns of δ-MnO2 reacted with humic acid (HA) at (a), (b) pH 5 and (c), (d) pH 8 with R = 1. Labels H and G represent hausmannite and groutite, respectively. Note that the inverse peaks in the patterns were caused by subtracted by ceria (CeO2) standard signal remaining in the empty Kapton capillary.

    • Figure 6. 

      Synchrotron-based XRD patterns of δ-MnO2 reacted with FA at (a), (b) pH 8 with R = 0.1, or (c), (d) R = 1, under oxic or anoxic conditions. The arrow and the label H indicate the position of the dip and the characteristic peak of hausmannite, respectively. Data for anoxic conditions were adopted from Wang et al.[12].

    • Figure 7. 

      Mn K-edge XANES LCF-estimated molar fractions of Mn(II) and Mn(III) in δ-MnO2 reacted with DOM at pH 5 and 8 with (a), (c), (e) R = 0.1 or (b), (d), (f) R = 1 under oxic or anoxic conditions. The molar fraction of Mn(IV) is omitted for clarity of Mn(II) and Mn(III) changes. Refer to Supplementary Fig. S4 for LCF analysis.