A density functional theory study of Fe(II)/Fe(III) distribution in single layer green rust: a cluster approach

Weichao Sun,Dominique J. Tobler,Martin P. Andersson

doi:10.1186/s12932-021-00076-0

Weichao Sun, Dominique J. Tobler + Show 1 more

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https://doi.org/10.1186/s12932-021-00076-0

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Abstract

Green rust (GR) is a potentially important compound for the reduction of heavy metal and organic pollutants in subsurface environment because of its high Fe(II) content, but many details of the actual reaction mechanism are lacking. The reductive capacity distribution within GR is a key to understand how and where the redox reaction occurs and computational chemistry can provide more details about the electronic properties of green rust. We constructed three sizes of cluster models of single layer GR (i.e., without interlayer molecules or ions) and calculated the charge distribution of these structures using density functional theory. We found that the Fe(II) and Fe(III) are distributed unevenly in the single layer GR. Within a certain range of Fe(II)/Fe(III) ratios, the outer iron atoms behave more like Fe(III) and the inner iron atoms behave more like Fe(II). These findings indicate that the interior of GR is more reductive than the outer parts and will provide new information to understand the GR redox interactions.

Highlights

Green rusts (GR) are a family of Fe(II), Fe(III) layered double hydroxides (LDH) that frequently form in oxygenpoor, Fe(II)-rich soils and waters [1,2,3,4,5]
GR structure model The GR structure was based on crystallographic data for sulphate GR (GRSO4) provided in Christiansen et al [3] with following formula: NaFe(II)6Fe(III)3(SO4)2(OH)18. 12H2O. Sulphate green rust (GRSO4) particles form hexagonal platelets, which consist of hydroxide layers where all octahedral sites are occupied, and the interlayer spaces are filled with octahedrally hydrated sodium and sulphate ions, along with additional water
The calculations showed that a high spin state is favored thermodynamically for the single layer GR model and that a minimum amount of Fe(III) are required, i.e., Fe(II)/ Fe(III) ratios between 0.2 and 5, to maintain the hexagonal shape of the GR structure

Summary

Introduction

Green rusts (GR) are a family of Fe(II), Fe(III) layered double hydroxides (LDH) that frequently form in oxygenpoor, Fe(II)-rich soils and waters [1,2,3,4,5]. GRs are classified into two types based on the anion they intercalate: [1] GR type 1 has a rhombohedral unit cell and intercalates planar or GRs have been widely investigated for removal of organic and inorganic contaminants from waters and soils [12,13,14,15,16,17,18,19,20,21,22,23,24,25,26,27] (e.g., SeO42− [13], U6+ [15], TcO4− [17], Ag+, Au3+, Cu2+ and H g2+ [18], CCl4 [16, 22, 23], NO3− [14], CrO42− [24,25,26,27]) due to their excellent reducing capacity In these studies, researchers have proposed several mechanisms to explain redox reactions by GRs. Thomas et al [26] proposed that chromate is directly reduced at sulphate GR particle surface sites by electrons

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