The aim of this study was to elucidate the electrochemical conditions prevailing in aqueous solution during the oxidation of different green rusts (GR), containing both ferrous (Fe(II)) and ferric (Fe(III)) ions. Suspensions of hydroxy-chloride green rust (GR1(CI )) and hydroxy-sulfate green rust (GR2(SO42-)) were synthesized and subsequently oxidized by injecting nitrogen gas containing oxygen. During oxidation, the pH and oxidation-reduction potential (ORP) of the aqueous solutions were measured, and X-ray diffraction (XRD) measurements were used to identify the solid particles precipitated. The results show that GRl(Cr) and GR2(SO42-) were transformed into α-FeOOH and γ FeOOH, and that the amount of α-FeOOH increased with decreasing reaction temperature. Two reaction stages were observed in plots of pH and ORP versus oxidation time. These stages were interpreted as representing the dissolution of GRl(Cl-) and GR2(SO42-) and the subsequent precipitation of α-FeOOH and α-FeOOH. The electrochemical conditions—pH, ORP, and temperature—in aqueous solutions were found to be crucial for the transformation of GRl(Cr) and GR2(SO42-) into α-FeOOH and γ-FeOOH.
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