Note: ND - Not determined
Iron precipitation (yellow precipitate) was also observed in the presence of visible light when aqueous filtrates were stored in clear glass bottles. This yellow precipitate was confirmed as FeC2O4·2H2O using PXRD (Figure S3). The photoreduction of iron(III) oxalate complex to iron(II) oxalate complex has been reported by numerous researchers.30-32 Mangiante et al. described this photoreduction through eq 6.32 The observation of yellow precipitate from the complex could be attributed to the low pH (pH = 0.93-2.47) of the aqueous phase. In this pH range H2C2O4 and\(\mathrm{\text{HC}}_{\mathrm{2}}\mathrm{O}_{\mathrm{4}}^{\mathrm{-}}\)are the predominant forms of oxalate ions.24 The lack of \(\mathrm{C}_{\mathrm{2}}\mathrm{O}_{\mathrm{4}}^{2\mathrm{-}}\)combined with the low solubility of FeC2O4·2H2O (0.078 g/100 g H2O at 20 °C)26 leads to the precipitation of FeC2O4·2H2O.33The kinetics of the photoreduction of Fe3+ was slow but efficient, as shown in Figure 3. It is clear from Figure 3 and eq 6 that to avoid any Fe precipitation and decomposition of\(\mathrm{C}_{\mathrm{2}}\mathrm{O}_{\mathrm{4}}^{2\mathrm{-}}\) bauxite filtrates should be stored in the dark.