Iron Isotope Fractionation during Fe(II) Oxidation Mediated by the Oxygen-Producing Marine Cyanobacterium Synechococcus PCC 7002

In this study, we couple iron isotope analysis to microscopic and mineralogical investigation of iron speciation during circumneutral Fe(II) oxidation and Fe(III) precipitation with photosynthetically produced oxygen. In the presence of the cyanobacterium Synechococcus PCC 7002, aqueous Fe(II) (Fe(II)aq) is oxidized and precipitated as amorphous Fe(III) oxyhydroxide minerals (iron precipitates, Feppt), with distinct isotopic fractionation (ε56Fe) values determined from fitting the δ56Fe(II)aq (1.79‰ and 2.15‰) and the δ56Feppt (2.44‰ and 2.98‰) data trends from two replicate experiments. Additional Fe(II) and Fe(III) phases were detected using microscopy and chemical extractions and likely represent Fe(II) and Fe(III) sorbed to minerals and cells. The iron desorbed with sodium acetate (FeNaAc) yielded heavier δ56Fe compositions than Fe(II)aq. Modeling of the fractionation during Fe(III) sorption to cells and Fe(II) sorption to Feppt, combined with equilibration of sorbed iron and with Fe(II)aq using published fractionation factors, is consistent with our resulting δ56FeNaAc. The δ56Feppt data trend is inconsistent with complete equilibrium exchange with Fe(II)aq. Because of this and our detection of microbially excreted organics (e.g., exopolysaccharides) coating Feppt in our microscopic analysis, we suggest that electron and atom exchange is partially suppressed in this system by biologically produced organics. These results indicate that cyanobacteria influence the fate and composition of iron in sunlit environments via their role in Fe(II) oxidation through O2 production, the capacity of their cell surfaces to sorb iron, and the interaction of secreted organics with Fe(III) minerals

Interactions between Fe and organic matter and their impact on As(V) and P(V)

Iron (Fe) speciation is important for many biogeochemical processes. The high abundance and limited solubility of Fe(III) are responsible for the widespread occurrence of Fe(III) minerals in the environment. Co-precipitation and adsorption onto mineral surfaces limits the free concentrations of compounds such as arsenate (As(V)), Fe(III) and, phosphate (P(V)). Mineral dissolution, on the other hand, might lead to elevated concentrations of these compounds. Fe speciation is strongly affected by natural organic matter (NOM), which suppresses hydrolysis of Fe(III) via complexation. It limits the formation of Fe(III) minerals and Fe(III) co-precipitation. This thesis is focused on interactions between Fe(III) and NOM as well as their impact on other elements (i.e. As(V) and P(V)). X-ray absorption spectroscopy (XAS) was used to obtain molecular scale information on Fe and As speciation. This was complemented with infrared spectroscopy, as well as traditional wet-chemical analysis, such as pH and total concentration determinations. Natural stream waters, soil solutions, ground water and soil samples from the Krycklan Catchment, in northern Sweden, were analyzed together with model compounds with different types of NOM. A protocol based on ion exchange resins was developed to concentrate Fe from dilute natural waters prior to XAS measurements. Iron speciation varied between the stream waters and was strongly affected by the surrounding landscape. Stream waters originating from forested or mixed sites contained both Fe(II, III)-NOM complexes and precipitated Fe(III) (hydr)oxides. The distribution between these two pools was influenced by pH, total concentrations and, properties of NOM. In contrast, stream waters from wetland sites and soil solutions from a forested site only contained organically complexed Fe. Furthermore, the soil solutions contained a significant fraction Fe(II)-NOM complexes. The soil samples were dominated by organically complexed Fe and a biotite-like phase. Two pools of Fe were also identified in the ternary systems with As(V) or P(V) mixed with Fe(III) and NOM: all Fe(III) was complexed with NOM at low total concentrations of Fe(III), As(V) and/or P(V). Hence, Fe(III) complexation by NOM reduced Fe(III)-As(V)/P(V) interactions at low Fe(III) concentrations, which led to higher bioavailability. Exceeding the Fe(III)-NOM complex equilibrium resulted in the occurrence of Fe(III)-As(V)/P(V) (co-)-precipitates

Reactivity of Fe from a natural stream water towards As(V)

Interactions between iron (Fe) and arsenic (As) play a vital role in aquatic and terrestrial ecosystems influencing the reactivity and transport of arsenic. A key aspect is the effect of natural organic matter (NOM) on these interactions, and previous investigations have reported the existence of ternary As-Fe-NOM species. In this study, the reactivity of Fe, from a boreal stream water, towards As(V) was investigated using Fe and As K-edge X-ray absorption spectroscopy (XAS). The native stream water was shown to contain mononuclear Fe-NOM complexes together with Fe(III) (hydr) oxides associated with the NOM. Addition of As(V) to this water at Fe to As ratios of 2.0-15.6 resulted in substantial changes in the Fe speciation; the Fe(III) (hydr) oxides were partly converted into FeAsO4(s) or a solid solution where As(V) was incorporated into Fe(III) (hydr) oxide structures. Under the same conditions no or only small effects of As(V) on the Fe-NOM complexes initially present were observed, and the concurrent existence of these complexes and free As(V) showed that a large fraction of the Fe-NOM complexes were non-reactive towards As(V). This study suggests that complexation of Fe by NOM in organic rich environments may lead to elevated free, aqueous arsenic levels as these complexes do not interact with As(V). Moreover, the formation of Fe-NOM complexes also reduce the tendency of Fe to form reactive Fe(III) (hydr) oxides particles and Fe(III)-arsenate precipitates. (C) 2015 Elsevier Ltd. All rights reserved

An Experimental Protocol for Structural Characterization of Fe in Dilute Natural Waters

The properties of iron (Fe) complexes and compounds in the environment influence several central processes, e.g., iron uptake, adsorption/desorption of contaminants and nutrients, and redox transformations, as well as the fate of of natural organic matter (NOM). It is thus important to characterize Fe species in environmental samples. Synchrotron-based extended X-ray absorption fine structure (EXAFS) spectroscopy has been used in several studies on soils and sediments, but literature is scarce on investigations of natural waters because of low Fe concentrations. In this study we have described a gentle and noninvasive preconcentration method, based on electrostatic adsorption onto ion-exchange resins, suitable for EXA.FS analysis of Fe species in dilute stream water samples. The EXAFS results of metal organic model complexes showed that no significant local structural distortions were induced by the method. We also demonstrated the feasibility for an 8 mu M Fe stream water sample. The Fe heterogeneity in this stream water was investigated via a gradient series at 28%, 42%, 77%, 84%, and 100% adsorption of total iron. The EXAFS results showed that Fe(III) in this stream water was divided into Fe(III)-NOM complexes and Fe(III) (oxyhydr)oxides associated with NOM, and that each class consisted of several subspecies

Impact of iron-organic matter complexes on aqueous phosphate concentrations

The close linkage between iron (Fe) and phosphorus (P) suggests that changes in Fe speciation may have a strong effect on the bioavailability of P. At the same time Fe speciation in natural oxic environments is known to be affected by the presence of organic matter (OM), pH and total Fe concentrations, thus these parameters should also influence the Fe-P interactions. The main objective of the present work was to study how OM affected the distribution of P(V) in the presence of Fe(III) and to address the questions if and by what mechanism(s) OM influenced the concentration of aqueous phosphate. This was accomplished by investigating the ternary P(V)-Fe(III)-OM system over a wide range of chemical conditions; [Fe]tot = 5000-50,000 μg g-1, Fe/P = 0.5-2.0 at pH 2.9-7. Iron speciation was probed via Fe K-edge X-ray absorption spectroscopy, P speciation and concentrations were analyzed via infrared spectroscopy, and chemical equilibrium modeling was conducted to simulate the distribution of chemical species of the system. The collective results showed that the dominating species were Fe(III)-OM complexes and ferric phosphate (FePO4(s)). At low concentrations, the Fe(III)-OM complexes suppressed the formation of FePO4(s), which resulted in elevated aqueous phosphate concentrations. At high concentrations, FePO4(s) was formed and co-existed with Fe(III)-OM complexes; ternary P(V)-Fe(III)-OM complexes were not detected under any experimental condition. The collective spectroscopic and equilibrium modeling results offer a mechanistic and thermodynamic consistent explanation to why OM contributes to elevated concentrations of soluble P and thereby to increased bioavailability of P in soils and waters

Complexation and precipitation reactions in the ternary As(V)-Fe(III)-OM (organic matter) system

The predominant forms of arsenic (As) in anoxic and oxic environments are As(III) and As(V), respectively, and the fate of these forms is influenced by interactions with mineral surfaces and organic matter (OM). Interactions between As(V) and OM are believed to occur mainly via iron(Fe)-bridges in ternary Fe-arsenate complexes, but direct evidence for these interactions are scarce. Furthermore, since the speciation of Fe in the presence of organic matter varies as a function of pH and Fe concentration, a central question is how different chemical conditions will affect the As-Fe-OM interactions. In order to answer this, the As(V)-Fe(III)-OM system have been studied under a large range of experimental conditions (6485-67,243 ppm Fe(III) and Fe(III): As(V) ratios of 0.5-20 at pH 3-7), with Suwannee River natural organic matter and Suwannee River fulvic acid as sources of OM, using Fe and As K-edge X-ray absorption spectroscopy (XAS), infrared (IR) spectroscopy and chemical equilibrium modeling. Our collective results showed that interactions in the ternary As(V)-Fe(III)-OM system were strongly influenced by pH, total concentrations and ratios among the reactive species. In particular, the high stability of the Fe(III)-OM complexes exerted a strong control on the speciation. The predominant species identified were mononuclear Fe(III)-OM complexes, Fe(III) (hydr) oxides and FeAsO4 solids. The experimental results also showed that at low concentrations the Fe(III)-OM complexes were sufficiently stable to prevent reaction with arsenate. The chemical equilibrium models developed corroborated the spectroscopic results and indicated that As(V) was distributed over two solid phases, namely FeAsO4(s) and Fe(OH)(1.5)(AsO4)(0.5)(s). Thus, neither ternary As(V)-Fe(III)-OM complexes nor As(V) surface complexes on Fe(III) (hydr) oxides were necessary to explain the collective results presented in this study. (C) 2014 Elsevier Ltd. All rights reserved

XAS study of iron speciation in soils and waters from a boreal catchment

Iron (Fe) is a key element, strongly influencing the biogeochemistry of soils, sediments and waters, but the knowledge about the variety of Fe species present in these systems is still limited. In this work we have used X-ray absorption spectroscopy (XAS) to study the speciation of Fe in soils and waters from a boreal catchment in northern Sweden. The aim was to better understand the controls of Fe speciation across different, but adjacent landscape elements including soil, soil solution, groundwater and stream water draining catchments with contrasting land characteristics. Our results showed that all samples contained mixtures of Fe(II) and Fe(III). The soils consisted of Fe phyllosilicates, Fe (hydr) oxides and Fe complexed by natural organic matter (NOM). All aqueous samples contained Fe(II)- and Fe(III)-NOM complexes, often in combination with Fe(III) (hydr) oxides that were associated with NOM. The variation in contribution from Fe-NOM and Fe (hydr) oxides was controlled by pH and total concentrations of NOM. The XAS spectra suggested formation of mononuclear Fe-NOM complexes consisting of chelate ring structures, but it could not be determined whether they originated solely from Fe(III)- or from a mixture of Fe(II)/Fe(III)-NOM complexes. Our collective results showed that the Fe speciation was highly variable across the different landscape elements and streams. This variation was manifested both in the distribution between mononuclear Fe-NOM complexes and Fe (hydr) oxides associated with NOM and between Fe(II) and Fe(III). These results highlight the complexity of Fe speciation in natural environmental systems and thus the challenges in interpreting Fe reactivity. (C) 2013 Elsevier B.V. All rights reserved