Cr Release from Cr-Substituted Goethite during Aqueous Fe(II)-Induced Recrystallization

Jian Hua,Manjia Chen,Jian Long,Chengshuai Liu,Fangbai Li,Fei Wu,Ting Gao,Jing Lei,Minghua Gu

doi:10.3390/min8090367

Abstract

The interaction between aqueous Fe(II) (Fe(II)aq) and iron minerals is an important reaction of the iron cycle, and it plays a critical role in impacting the environmental behavior of heavy metals in soils. Metal substitution into iron (hydr)oxides has been reported to reduce Fe atom exchange rates between Fe(II)aq and metal-substituted iron (hydr)oxides and inhibit the recrystallization of iron (hydr)oxides. However, the environmental behaviors of the substituted metal during these processes remain unclear. In this study, Fe(II)aq-induced recrystallization of Cr-substituted goethite (Cr-goethite) was investigated, along with the sequential release behavior of substituted Cr(III). Results from a stable Fe isotopic tracer and Mössbauer characterization studies show that Fe atom exchange occurred between Fe(II)aq and structural Fe(III) (Fe(III)oxide) in Cr-goethites, during which the Cr-goethites were recrystallized. The Cr substitution inhibited the rates of Fe atom exchange and Cr-goethite recrystallization. During the recrystallization of Cr-goethites induced by Fe(II)aq, Cr(III) was released from Cr-goethite. In addition, Cr-goethites with a higher level of Cr-substituted content released more Cr(III). The highest Fe atom exchange rate and the highest amount of released Cr(III) were observed at a pH of 7.5. Under reaction conditions involving a lower pH of 5.5 or a higher pH of 8.5, there were substantially lower rates of Fe atom exchange and Cr(III) release. This trend of Cr(III) release was similar with changes in Fe atom exchange, suggesting that Cr(III) release is driven by Fe atom exchange. The release and reincorporation of Cr(III) occurred simultaneously during the Fe(II)aq-induced recrystallization of Cr-goethites, especially during the late stage of the observed reactions. Our findings emphasize an important role for Fe(II)aq-induced recrystallization of iron minerals in changing soil metal characteristics, which is critical for the evaluation of soil metal activities, especially those in Fe-rich soils.

Highlights

Ironoxides such as lepidocrocite, goethite, and hematite are common secondary iron minerals in soils
The results showed that neither Cr(III)aq nor Cr(III)extr was detected, indicating that all of the Cr(III) was structurally incorporated into the goethites
The release of Cr(III) from Cr-goethites was investigated during the recrystallization recrystallization process induced by Fe(II)aq under various conditions

Summary

Introduction

Iron (hydr)oxides such as lepidocrocite, goethite, and hematite are common secondary iron minerals in soils. Iron (hydr)oxides rarely occur in a chemically pure form, and usually contain. A number of trace metals, including Al(III), Ti(II), Cr(III), Mn(III), Ni(II), As(III), and Cu(II), were observed to be substituted into the structures of iron (hydr)oxides upon replacing Fe(III). 33 mol%, respectively, of Fe(III) sites within iron (hydr)oxides [3,6]. Cr-substituted goethites are common in natural soil due to the similar ionic radii of Cr(III) (61.5 pm) and. Isomorphous substitution of metals in the structures of iron (hydr)oxides can lead to metal immobilization and, to the formation of stable metal-containing mineral phases, reducing the activities of iron (hydr)oxides and the bioavailability of metals [3,7,8]

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Results

Conclusion