The Effect of Co2 and Oxidation Rate on the Formation of Goethite versus Lepidocrocite from an F/~(ii) System at Ph 6 and 7

To investigate the influence of carbonate on the formation of goethite and lepidocrocite, ~200 samples were synthesized by oxidizing FeC12 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+ GO) decreased from 100 to 0% with increasing [HCO~] in solution and with decreasing average oxidation rate (AOR). These two parameters explained 81% of the variation of Lp/(Lp + Gt). At a given [HCO~], more goethite was formed at pH 6 than at pH 7. The Lp + Gt mixtures contained 0-8 mg g-i carbon (CO which could not be removed by washing. Ct reached apparent saturation at a [HCO~] equilibrium concentration of ~ 6-8 and 60-80 mmol 1-~ at pH 6 and pH 7, respectively. In a plot of Ct vs. Lp/(Lp + GO all data fell on the same line irrespective of oxidation parameters (pH, AOR). IR spectra showed two broad bands at ~ 1300 and 1500 cm-I 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 FeO 3 (OH) 3 octahedra common to both minerals toward a comer sharing arrangement, and thereby to goethite, whereas chloride permits edge-sharing as in lepidocrocite. The close association of goethite and its metastable polymorph lepidocrocite in many soils has not been satisfactorily explained. A detailed study of Fe oxide accumulations around root channels (so-called pipestems) in South African soils showed, however, that goethite dominated in the inner part of the pipestem, i.e. close to the root, whereas in the outer part lepidocrocite was the main oxide (Schwertmann & Fitzpatrick, 1977). This distribution could be due to small-scale differences in chemical environment within the rhizosphere. One of the factors which may vary in the rhizosphere is Pco:, which, because of root respiration, should decrease with increasing distance from the root. Schwertmann (1959) oxidized FeCI2 solutions with O2-CO2 mixtures and found that increasing amounts of goethite were formed as the proportion of CO2 in the purge gas increased. Later, this result was substantiated by Fey & Dixon (1981). Goodman & Lewis (1981) found that the addition of NaHCO3 during oxidation of a FeC12 solution induced the formation of goethite rather than lepidocrocite. A systematic study of …