Combustion analysis and Inductively Coupled Plasma Mass Spectrometry (ICP-MS) showed that these solids were both composed of ~50 wt% Fe, 2-6 wt% C and 1-2 wt% H and trace amounts of N (Methods S11). Both solids were prepared twice, and no significant difference was found in the elemental composition of the FeQ-solids and Fe-solids (Table ). The expected composition of γ-FeOOH is around ~63 wt% Fe, 1 wt% H and 36 wt% O. Therefore, elemental composition of the FeQ-solids and Fe-solids support the conclusion that these precipitates are composed of a mixture of iron oxide hydroxide and some organic impurities (Table ).