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.