The interaction between aqueous Fe(II) (Fe(II)aq) and iron (hydr)oxides is an important reaction of iron cycle and plays a critical role in the environmental behavior of heavy metal in soils. The metal substitution has been reported to decrease the Fe atom exchange rates between the Fe(II)aq and metal-substituted iron (hydr)oxides as well as inhibiting the recrystallization of iron (hydr)oxides, whereas the environmental behaviors of the substituted metal during the processes remain unclear. In this study, Fe(II)aq-induced recrystallization of Cr-substituted goethite (Cr-goethite) and the sequentially release of substituted Cr were investigated under different conditions. The results of stable Fe isotopic tracer and Mössbauer characterization studies show that Fe atom exchange occurred between Fe(II)aq and structural Fe(III) in Cr-goethites, during which Cr-goethites were recrystallized. The Cr substitution inhibited the rates of Fe atom exchange and Cr-goethite recrystallization. During the processes, Cr were released from Cr-goethite and Cr-goethites with higher Cr-substituted contents released more. Highest Fe atom exchange rate and most amount of Cr release were observed at pH 7.5, while no change in mineral phase after reaction for 30 days. At a lower pH of 5.5 or a higher pH of 8.5, significantly less exchange and Cr release occurred. The releases of Cr were positively correlated with the Fe atom exchanges rates, suggesting the driving force of sorbed Fe(II) for the Fe atom exchange and Cr release. The release and re-incorporation of Cr simultaneously occurred during the Fe(II)aq-induced recrystallization of Cr-goethites, especially at the late stage of the reactions. Our findings indicate that dynamic changes of Cr forms relating to Fe(II)aq-induced recrystallization of iron (hydr)oxides should be considered when evaluating the soil Cr pollution in Fe-rich soils.
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Cr release from Cr-substituted goethite during the aqueous Fe(II)-induced recrystallization
Published: 24 July 2018 by MDPI in The 1st International Electronic Conference on Mineral Science session Nanominerals and Mineral Surfaces
Keywords: Trace metal release; Goethite; Fe atom exchange; Phase transformation