Biogeochemical transformation of soil Fe- and Mn- mineral phases along a redox gradient: Implications for C sequestration
EMSL Project ID
44685
Abstract
Iron (Fe) and manganese (Mn) oxides, hydroxides and oxyhydroxides are well known redox-sensitive mineral components of environmental systems. Iron minerals in soils and sediments occur in a variety of phases ranging from poorly crystalline ferrihydrite to well-crystallized goethite and hematite while Mn minerals are often poorly crystalline. The crystal mineral phase, size, surface charge and bonding environment of Fe and Mn govern their influence on ecosystem biogeochemistry such as toxicity of trace elements and cycling of organic compounds. Iron oxides with short-range crystal order and small particle sizes contain more reactive high surface area compared to other bulk Fe-oxides. Association of organic matter (OM) with Fe minerals has been increasingly recognized as a major process for soil carbon (C) stabilization by decreasing its susceptibility to microbial degradation. At the same time, Mn oxides are the strongest oxidizing agents other than O2 and significantly more oxidizing at pH 7 than Fe oxides. Manganese oxides could facilitate the degradation of organics and formation of humic compounds. At the Christina River Basin-Critical Zone Observatory (CRB-CZO), one of six natural observatories funded by the National Science Foundation, we investigate how Fe- and Mn- redox coupling can affect C cycling in several field sites including floodplain, upland forest, and agriculture under varying redox conditions. We applied EXAFS and selective chemical extraction to investigate the changes in soil Fe solid-phase speciation across pasture and a forest hillslope transects, and a floodplain. Our results showed that the crystallinity of the FeIII-(oxy) hydroxides decreased with increasing anoxia while the abundance of ferrihydrite was proportionally increased. We hypothesize that the OM coating on newly formed amorphous Fe oxides may retard or inhibit Fe transformations and re-crystallization. Unfortunately, EXAFS is not a suitable tool to differentiate unambiguously ferrihydrite from goethite as well as to tease apart Fe-oxides from each other and from Fe in clays in sediment samples. Therefore, Mossbauer spectroscopy at EMSL could be the best technique for sediment analysis [20], especially with the expertise of Dr. Ravi Kukkadapu. In addition to Fe speciation, the investigation of Mn-mineral phases is important but very challenging. The HF and Pulsed/CW EPR spectroscopy at EMSL with the support of Dr. Eric Walter could be a great technique to separate Mn chemical species in different environments. Depending on the symmetry of the compound, HF-EPR may be the only choice for Mn(III) and Mn(IV) and some organic radicals. We propose to employ high precision Mossbauer and EPR spectroscopy, TOC, NMR SEM, TEM, and XRD at EMSL: (1) to investigate soil solid-phase Fe and Mn species along a redox gradient of different landscape positions and uses; (2) to assess how Fe and Mn speciation change seasonally; (3) to characterize short-range-ordered Fe in the soils; and (4) to evaluate the effect of OM association with ferrihydrite on its reduction and transformation. This study will enhance our understanding of the fate and distribution of C in soil/sediment water systems under various redox conditions.
Project Details
Project type
Large-Scale EMSL Research
Start Date
2011-10-01
End Date
2014-09-30
Status
Closed
Released Data Link
Team
Principal Investigator
Team Members
Related Publications
Chen C, RK Kukkadapu, and DL Sparks. 2015. "Influence of Coprecipitated Organic Matter on Fe2+(aq) -Catalyzed Transformation of Ferrihydrite: Implications for Carbon Dynamics." Environmental Science & Technology. doi:10.1021/acs.est.5b02448
Chen C., R.K. Kukkadapu, O.V. Lazareva, and D.L. Sparks. 2017. "Solid-phase Fe Speciation along the Vertical Redox Gradients in Floodplains using XAS and Mössbauer Spectroscopies." Environmental Science & Technology 51, no. 14:7903-7912. PNNL-SA-123910. doi:10.1021/acs.est.7b00700
Joshi SR, RK Kukkadapu, DJ Burdige, ME Bowden, DL Sparks, and DP Jaisi. 2015. "Organic Matter Remineralization Predominates Phosphorus Cycling in the Mid-Bay Sediments in the Chesapeake Bay." Environmental Science & Technology. doi:10.1021/es5059617
Starcher AN, W Li, RK Kukkadapu, EJ Elzinga, and DL Sparks. 2016. "Fe(II) Sorption On Pyrophyllite: Effect Of Structural Fe(III) (impurity) In Pyrophyllite On Nature Of Layered Double Hydroxide (LDH) Secondary Mineral Formation." Chemical Geology 439:152-160. doi:10. 1016/j. chemgeo. 2016. 06. 017
Wu Y, RK Kukkadapu, KJ Livi, W Xu, W Li, and DL Sparks. 2018. "Iron and Arsenic Speciation During As(III) Oxidation by Manganese Oxides in the Presence of Fe(II): Molecular-Level Characterization Using XAFS, Mössbauer, and TEM Analysis." ACS Earth and Space Chemistry 2(3):256-268. doi:10.1021/acsearthspacechem.7b00119
Wu Y, W Li, and DL Sparks. 2015. "Effect of Iron(II) on Arsenic Sequestration by ??MnO2: Desorption Studies Using Stirred-Flow Experiments and X?Ray Absorption Fine- Structure Spectroscopy." Environmental Science & Technology 49(22):13360–13368. doi:10.1021/acs.est.5b04087
Wu Y, W Li, and DL Sparks. 2015. "The Effects of Iron(II) on the Kinetics of Arsenic Oxidation and Sorption on Manganese Oxides." Journal of Colloid and Interface Science 457:319-328. doi:10.1016/j.jcis.2015.07.022