Iron speciation in peats: Chemical and spectroscopic evidence for the co-occurrence of ferric and ferrous iron in organic complexes and mineral precipitates

Journal Article · 31 October 2017 · Organic Geochemistry

DOI: https://doi.org/10.1016/j.orggeochem.2017.10.012 · OSTI ID:1426466

Bhattacharyya, Amrita ^[1]; Schmidt, Michael P. ^[2]; Stavitski, Eli ^[3]; Martínez, Carmen Enid ^[2]

---

1. Lawrence Berkeley National Lab. (LBNL), Berkeley, CA (United States)
2. Cornell Univ., Ithaca, NY (United States). Soil and Crop Sciences, School of Integrative Plant Science
3. Brookhaven National Lab. (BNL), Upton, NY (United States). National Synchrotron Light Source II (NSLS-II)

The speciation of iron (Fe) in organic matter (OM)-rich environments under in situ variable redox conditions is largely unresolved. Peatlands provide a natural setting to study Fe–OM interactions. Utilizing chemical, spectroscopic and theoretical modeling approaches, we report the chemical forms, oxidation states and local coordination environment of naturally occurring Fe in the vertically redox-stratified Manning peatlands of western New York. In addition, we report dominant carbon, sulfur and nitrogen species that can potentially stabilize the various Fe species present in these peatlands. Our results provide clear direct and indirect evidence for the co-occurrence of ferrous (Fe²⁺) and ferric (Fe³⁺) iron species in peats under both oxic and anoxic conditions. Iron is mostly present within the operationally defined organic and amorphous (i.e., short range ordered, SRO) fractions; ferric iron primarily as magnetically isolated paramagnetic Fe³⁺ in Fe(III)-organic complexes, but also in mineral forms such as ferrihydrite; ferrous iron in tetrahedral coordination in Fe(II)-organic complexes with minor contribution from pyrite. All of the Fe species identified stabilize Fe(III) and/or Fe(II) in anoxic and oxic peats. Fundamental differences are also observed in the relative proportion of C, S and N functionalities of OM in oxic and anoxic peats. Aromatic C=C, ester, phenol and anomeric C (R-O-C-O-R), as well as thiol, sulfide and heterocyclic N functionalities are more prevalent in anoxic peats. Collectively, our experimental evidence suggests iron forms coordination complexes with O-, S- and N-containing functional groups of OM. We posit the co-occurrence of organic and mineral forms of Fe(II) and Fe(III) in both oxic and anoxic peat layers results from dynamic complexation and hydrolysis-precipitation reactions that occur under variable redox conditions. In conclusion, our findings aid in understanding the crucial role OM plays in determining Fe species in soils and sediments.

• Research Organization: Brookhaven National Laboratory (BNL), Upton, NY (United States)
• Sponsoring Organization: USDOE Office of Science (SC), Basic Energy Sciences (BES); USDOE
• Grant/Contract Number: SC0012704; AC02-98CH10886
• OSTI ID: 1426466
• Alternate ID(s): OSTI ID: 1549526
• Report Number(s): BNL-203360-2018-JAAM
• Journal Information: Organic Geochemistry, Vol. 115, Issue C; ISSN 0146-6380
• Publisher: ElsevierCopyright Statement
• Country of Publication: United States
• Language: English