The effect of CO2 and oxidation rate on the formation of goethite versus lepidocrocite from an Fe(II) system at pH 6 and 7

Abstract

To investigate the influence of carbonate on the formation of goethite and lepidocrocite, ∼200 samples were synthesized by oxidizing FeCl₂ solutions with air/CO₂ gas mixtures at ambient temperature and pH 6 and 7. The proportion of lepidocrocite in the lepidocrocite/goethite mixtures (Lp/(Lp + Gt)) decreased from 100 to 0% with increasing in solution and with decreasing average oxidation rate (AOR). These two parameters explained 81% of the variation of Lp/(Lp + Gt)). At a given , more goethite was formed at pH 6 than at pH 7. The Lp + Gt mixtures contained 0–8 mg g⁻¹ carbon (Ct) which could not be removed by washing. Ct reached apparent saturation at a equilibrium concentration of ∼6–8 and 60–80 mmol l⁻¹ at pH 6 and pH 7, respectively. In a plot of Ct vs. Lp/(Lp + Gt) all data fell on the same line irrespective of oxidation parameters (pH, AOR). IR spectra showed two broad bands at ∼1300 and 1500 cm⁻¹ which can be assigned to distorted carbonate adsorbed at the goethite surface. Identical bands were also found in a young, poorly crystalline goethite formed from coal mine drainage in Ohio. It is suggested that carbonate anions direct the polymerization of the double bands of FeO₃(OH)₃ octahedra common to both minerals toward a corner sharing arrangement, and thereby to goethite, whereas chloride permits edge-sharing as in lepidocrocite.