Chromium Isotope Fractionation During Cr(?) Oxidation By Manganese Oxides

The evolution of redox state on earth surface has long been a hot topic.Recently,chromium isotopic system has been developed as an effective proxy for atmospheric oxygenation,based on that oxidative weathering of Cr(?)is accompanied by Cr isotopic fractionation.However,currently,the researches on Cr isotopic fractionation during oxidation are limited.The manganese oxides,are the most common natural chromium oxidants,but the direction,magnitude,and mechanism of Cr isotopic fractionation during Cr(?)oxidation by manganese oxides are still unclear.Therefore,we performed experiments of Cr(?)oxidation by two different manganese oxides,to investigate the chemical behavior of Cr isotopes during oxidation.The experiments with birnessite were conducted at pH value of 3.9,and those with pyrolusite were conducted at three different pH conditions(4.2,5.6,and 10.4).During experiments,the reactants and products were sampled from the reactors at different reaction extents,and then Cr(?)and Cr(?)were separated and purified by column chromatography.Finally,the Cr isotopic compositions of reactants and products at different reaction extents were measured by thermal ionization mass spectrometry(TIMS).The experimental results show that,the reaction rates of the experiments with birnessite are about 200 times faster than those of the experiments with pyrolusite under the same experimental condition.This difference in reaction rate may be caused by the different intrinsic structures between pyrolusite and birnessite.Compared to pyrolusite with compact structure,birnessite is characterized by loose layered structure,which can provide much more reactive sites in the interlayers.Thus,the experiments with birnessite have faster oxidative reaction rates.The Cr isotopic fractionation observed in the experiments are controlled by both the intrinsic fractionation of the oxidation reaction and the diffusion effect,and the diffusion effect is more obvious when the reaction rate is fast.The experimental results show that,the fractionation trend of the experiments with pyrolusite at different pH values are similar,with a fractionation factor(?)of 3.2‰.Considering the reaction rate is slow and the diffusion effect is small,this ? value is expected to approximate to the intrinsic fractionation factor of the oxidation reaction.We fitted the data of the experiments with birnessite according to Rayleigh fractionation model.The result shows that the fitted ? value is 0.6‰,notably smaller than that of the experiments with pyrolusite.This is probably due to that the experiments with birnessite have relatively faster reaction rates,and the observed fractionation factor is significantly masked by the diffusion effect.Bimessite is the most common manganese oxide in soil and regolith.Therefore,Cr(?)oxidation by birnessite should be the main oxidation process that causes the Cr isotopic fractionation in nature.Compared to the previous studies on Cr isotopic fractionation during Cr(?)reduction(? ranges from-1.5‰ to-4.3‰),Cr isotopic fractionation during Cr(?)oxidation in nature is expected to be small(?=0.6‰),which is consistent with previous observations on natural weathering profiles.Thus,our study complements and develops the isotopic behavior model of chromium cycle in natural environments.