Fe-organic Complexes Research Articles

Role of Source, Mineralogy, and Organic Complexation on Lability and Fe Isotopic Composition of Terrestrial Fe sources to the Gulf of Alaska.

Iron (Fe) is a key trace nutrient supporting marine primary production, and its deposition in the surface ocean can impact multiple biogeochemical cycles. Understanding Fe cycling in the subarctic is key for tracking the fate of particulate-bound sources of oceans in a changing climate. Recently, Fe isotope ratios have been proposed as a potential tool to trace sources of Fe to the marine environment. Here, we investigate the Fe isotopic composition of terrestrial sources of Fe including glacial sediment, loess, volcanic ash, and wildfire aerosols, all from Alaska. Results show that the δ56Fe values of glaciofluvial silt, glacial dissolved load, volcanic ash, and wildfire aerosols fall in a restricted range of δ56Fe values from -0.02 to +0.12‰, in contrast to the broader range of Fe isotopic compositions observed in loess, -0.50 to +0.13‰. The Fe isotopic composition of the dissolved load of glacial meltwater was consistently lighter compared to its particulate counterpart. The 'aging' (exposure to environmental conditions) of volcanic ash did not significantly fractionate the Fe isotopic composition. The Fe isotopic composition of wildfire aerosols collected during an active fire season in Alaska in the summer of 2019 was not significantly fractionated from those of the average upper continental crust composition. We find that the δ56Fe values of loess (<5 μm fraction) were more negative (-0.32 to +0.05‰) with respect to all samples measured here, had the highest proportion of easily reducible Fe (5.9-59.6%), and were correlated with the degree of chemical weathering and organic matter content. Transmission electron spectroscopy measurements indicate an accumulation of amorphous Fe phases in the loess. Our results indicate that Fe isotopes can be related to Fe lability when in the presence of organic matter and that higher organic matter content is associated with a distinctly more negative Fe isotope signature likely due to Fe-organic complexation.

Redox cycling of straw-amended soil simultaneously increases iron oxide crystallinity and the content of highly disordered organo-iron(III) solids

Iron speciation in soils is influenced largely by its redox state, but the extent of and controls on Fe speciation during recurrent reduction and oxidation events are not fully understood. To investigate the effects of organic matter (OM) inputs and the frequency and duration of redox oscillations on soil Fe speciation, we conducted redox-oscillation experiments with topsoil from a Fluvisol mixed with rice straw (0, 10, 50 g/kg organic carbon, OC). The soil was initially dominated by short-range ordered (SRO) Fe(III) solids and subjected to 14- and 28-day reduction–oxidation cycles for 112 days, with the time spent under anoxic and oxic conditions maintained at 6:1. Reduction was initiated by flooding reactors with artificial river water. To simulate leaching conditions, soil re-oxidation was achieved by air-drying soil after removal of reacted solutions. Fresh river water was then added for each new redox cycle. We monitored changes in solution composition (Eh, pH, Fe(II), total Fe, OC, and Si) and assessed changes of solid-phase Fe speciation by selective extractions, X-ray absorption spectroscopy, and 57Fe Mössbauer spectroscopy. Dissolved OC and Fe increased with increasing straw addition, but decreased in each treatment through consecutive reduction intervals. Release rates of dissolved Fe and OC were highly correlated, implying that microbial reduction of soil Fe(III) solids was fostered by straw amendments. Reduction-induced losses of OC and Fe from straw amended soil were amplified at high redox frequency. Ferrous Fe did not detectably accumulate in the solid phase upon repeated soil oxidation. Although Fe(III)-poor phyllosilicates gained in relative importance in redox-cycled soils, their fraction was hardly affected during redox cycling. Instead, straw additions led to an enhanced depletion of ferrihydrite during soil redox cycling and a relative enrichment of highly disordered Fe(III) species [‘very SRO (vSRO) Fe(III) solids’], which remained only partially ordered in 5-K Mössbauer spectra and likely consisted predominantly of polynuclear organic Fe complexes. The depletion of ferrihydrite in straw-amended soils after 112 days was greater in the 14-day cycle than in the 28-day cycle experiment and accompanied by a less pronounced enrichment of vSRO Fe(III) solids. The crystallinity of distinct Fe oxides (ferrihydrite, lepidocrocite, and hematite) increased during soil redox cycling especially in straw-amended soils, but without noticeable ferrihydrite conversion into crystalline Fe oxides. The increase in the crystallinity of distinct Fe oxides after 112 days was greater at low redox frequency in straw-free soil, however this frequency effect was suppressed by straw additions. Longer soil redox cycling (112 vs. 56 days) increased the crystallinity of distinct Fe oxides, which was most pronounced at high straw levels and low redox frequency. Our results imply that redox changes in SRO Fe oxide- and OM-rich soils can cause a relative enrichment of more crystalline Fe oxides, while still maintaining a pool of vSRO Fe(III) solids. We conclude that soil redox oscillations can lead to divergent transformation pathways of Fe oxides, which concomitantly increase bulk Fe-oxide crystallinity and generate increasing fractions of highly disordered Fe(III) solids on comparatively short time scales. In addition, our study suggests that faster redox cycling in soils with ample electron donor supply and water leaching leads to higher element exports (e.g., OC, metal(loid)s) from soil due to weekly redox pulsing than more slowly alternating redox conditions.

Production of iron-peptide complexes from spent yeast for nutraceutical industry

Iron (Fe) deficiencies are a major health condition concern and over the years many solutions in the form of Fe supplementation have been investigated. Organic Fe-complexes are the most promising for Fe deficiencies remediation. The aim of present study was to value peptide rich waste streams from β-glucan and mannan production from spent yeast (Gpep and Mpep, respectively) as Fe-peptide complexes for Fe supplementation. These waste streams were first subjected to ultrafiltration treatment before assessing the capacity of these fractions to complex Fe was evaluated, without, and with nitrogen. Results have shown that Gpep> 1 kDa was the best fraction with a optimal pH of 6.0 and a time of 30 min. The resulting Fe-peptide complex was characterized using powder XDR, fluorescence, FTIR, SEM and Mastersizer Laser Diffraction. Results have shown that Gpep and Mpep waste streams have potential application as Fe supplementation in the form of Fe peptide complexes.

Iron ligands and isotopes in hydrothermal plumes over backarc volcanoes in the Northeast Lau Basin, Southwest Pacific Ocean

Deep-sea hydrothermal venting is an important source of dissolved iron (dFe) to the oceans. Fe isotopes can be used as a potential tool to trace the dispersal of hydrothermal plumes. However, Fe isotope fractionation and its relation with Fe speciation as hydrothermal plumes disperse is still poorly constrained. In this study, we determined the Fe speciation and total and dissolved Fe isotope composition (δ56tFe, δ56dFe) for several hydrothermal plumes from backarc volcanoes in the Northeast Lau Basin. This combined approach provides important insights into the evolution of Fe isotopes in hydrothermal plumes. The results suggest δ56tFe variation in plumes is related to the loss of particulate Fe-sulfides or Fe-oxyhydroxides (FeOOH), both of which are dependant on the H2S concentrations and Fe/H2S in the source hydrothermal fluids. δ56dFe compositions in the hydrothermal plumes increase during plume dispersal/dilution and can be as high as 0.85 ‰, demonstrating that hydrothermal plumes can export dissolved Fe with a significantly heavier δ56dFe than hydrothermal fluids. The reasons may be ascribed to the organic Fe complexes (FeL) and colloidal FeOOH in the dissolved phase. Another interpretation might be associated with the low pH in volcanic arc hydrothermal systems rich in magmatic CO2 and SO2, which decreases the Fe(II) oxidation rate. Further, we demonstrate for the first time that the δ56dFe is positively correlated with the conditional stability constants of FeL (logKʹFeL). A Rayleigh distillation model is presented based on the mass balance of the determined FeL, and colloidal FeOOH in hydrothermal plumes, which can explain the observed Fe isotope compositions in hydrothermal plumes. Our data show how Fe isotopes are transformed within a hydrothermal plume above arc volcanoes and how these may differ from that of the original vent fluids. It adds to our understanding of the processes that have an impact on the Fe speciation and isotope composition in deep-sea hydrothermal plumes.

Microfacies and composition of ferruginous beds at the platform-foreland basin transition (Late Albian to Turonian Natih Formation, Oman Mountains): Forebulge dynamics and regional to global tectono-geochemical framework

Two forebulge successions exist in the Oman Mountains at the platform-foreland basin transition between the Late Albian to Turonian Natih Formation and the foreland basin shales of the Late Cretaceous Muti Formation. Forebulge creation is suggested by limestone microfacies analyses and analyses of ferruginous crusts and oolites, showing rapid changes in bathymetry and relative sedimentation rate. Each succession displays basal shallow subtidal limestone, passing upward directly to ferruginous crusts and then to Fe-rich oolites deposited in shallower and agitated water. Each succession is topped by clayey layers. Microfacies and lithological evolution of the two successions are alike, suggesting repetitively similar depositional and tectonic conditions. As both sequences occur at the same site, lateral forebulge migration possibly did not occur, suggesting an overall stationary vertical and stepwise forebulge development. The compositional evolution of the ferruginous crusts was complex and includes postdepositional diagenetic effects of the rocks. Both ferruginous crusts once consisted of iron sulfides, implying at least slightly reducing conditions during their formation, associated with water-deepening events. Both oolite levels contain chlorite, hematite, quartz, calcite and apatite. They also contain fragments of chlorite and hematite as nuclei, suggesting that these fragments derived from preexisting ferruginous crusts. Iron oxyhydroxides and clinochlore within the oolites reflects bathymetric changes to more oxidizing aqueous conditions, associated with water-shallowing events. Fe-rich anoxic to sub-oxic sea water of the marine foreland basin was the source for the crusts and oolites which coincided with a high rate of global Cretaceous ocean crust production and related hydrothermalism as well as the regional proximity of an active spreading axis. Fe was probably stabilized in ocean water as organic Fe complexes and Fe colloids.

Rhizosphere priming regulates soil organic carbon and nitrogen mineralization: The significance of abiotic mechanisms

The effect of living roots on the co-mineralization of soil organic carbon (SOC) and nitrogen (SON) and driving mechanisms of the rhizosphere priming effect (RPE) remains unclear. Moreover, it is still poorly understood whether the abiotic mechanisms, whereby roots accelerate soil organic matter (SOM) loss by destabilizing organo-mineral associations, are involved. Biotic and abiotic processes involved in the RPE and gross N mineralization (GNM) were investigated using three paddy soils (C3) under maize (C4 plant) cultivation. The soils had high and low total N (1.79 and 3.3 g kg−1 soil) as well as iron- (Fe-) (hydr-) oxide (1.1 and 2.2 g kg−1 soil) contents, which gave the following combinations: high-Fe/low-N, low-Fe/low-N, and low-Fe/high-N. The RPEs and GNM were measured using a 13C-natural abundance approach and 15N-pool dilution technique, respectively, on day 86 of maize cultivation. Living roots enhanced native SOC mineralization by 70.4–204% and GNM by 118–382%. As expected, biotic mechanisms contributed to RPEs (‘microbial activation’ and ‘microbial N-mining’) that was supported by the increase of soil microbial biomass C and extracellular enzyme activities in presence of plant, and the lower C/N ratio of primed SOM than the original SOM in unplanted soils. Higher RPEs and relative primed N mineralization via stronger ‘microbial N-mining’ were found in low-Fe/low-N vs low-Fe/high-N soils. In contrast to expectations, the RPEs and relative primed N mineralization were greater in high-Fe/low-N versus low-Fe oxide soils. The strongest decrease in SOM-derived Fe-bound C and increase in root-derived Fe-bound C were observed in the high-Fe soil, because root exudates liberated more C from Fe-bound SOM and co-precipitated with Fe oxides. This shows that the abiotic process was involved in RPEs whereby root exudates promoted soil C loss by releasing it from Fe-organic complexes. Thus, this study demonstrated that coupled biotic-abiotic processes could regulate RPEs.

Fortification of bread wheat with iron through soil and foliar application of iron-organic-complexes

Increasing iron (Fe) concentration in food crops is a global challenge that depend on the type and methods of fertilizer application in agriculture practices. Fe-organic complexes (Fe-OCs) including Fe-Glycine (Fe-Gly), Fe-Tyrosine (Fe-Try), and Fe-Chitosan (Fe-Chi) were synthesized and characterized by different analytical techniques. A greenhouse experiment was conducted to evaluate the role of soil, fertigation, and the foliar application of Fe-OCs in comparison to Fe-EDDHA to improve Fe status in wheat (Triticum aestivum, cv. N91-8). The finding of elemental analysis supported the formation of Fe-OCs. The soil application of Fe-OCs increased the yield and yield components of the wheat. Further, the shoot Fe concentration and other macronutrients increased due to the Fe-OCs with the soil application, however, Fe-Gly and Fe-Tyr more than Fe-Chi resulted in an increase in Fe concentration. Fe-OCs improved nitrogen (N) status in the shoot and grain, and Fe in the grain. The foliar application of Fe-Gly, Fe-Tyr, and Fe-Chi increased Fe concentration in the grain compared to Fe-EDDHA. Fe-OCs can serve as an important agronomic practice to improve Fe grain concentration.

Spring flood induced shifts in Fe speciation and fate at increased salinity

Rivers have traditionally been viewed as negligible sources of iron (Fe) to marine waters, as most Fe gets lost during estuarine mixing. However, recent findings demonstrate that Fe from boreal rivers display a higher resistance towards salinity-induced aggregation, presumably due to stabilizing interactions with organic matter. Previous studies have shown that Fe (oxy)hydroxides are selectively removed by aggregation processes, and that organic Fe complexes are less affected by increasing salinity. It has been further proposed that Fe speciation varies in response to seasonal differences in hydrology. In this study X-ray absorption spectroscopy (XAS) was used to determine the temporal variation in Fe speciation and the connection to Fe stability in response to increasing salinity in two boreal rivers (Kalix and Råne River), with the purpose to better understand the fate of riverine Fe export. Sampling was done from winter pre-flood, over the spring flood, to post-flood conditions (early April until mid June). In addition, parallel analyses for Fe speciation and isotope composition (δ56Fe relative to IRMM-14) were made on river samples, as well as salinity-induced aggregates and the fraction remaining in suspension, with the main objective to test if δ56Fe reflect the speciation of Fe.The contribution of organically complexed Fe increased during spring flood compared to the pre- and post-flood, as did Fe transport capacity. However, since Fe (oxy)hydroxides were dominating throughout the sampling period, the seasonal variability was small. Interestingly, salinity-induced aggregation experiments revealed that Fe (oxy)hydroxides, which dominated aggregates, displayed lower δ56Fe than in the river samples Fe, while organic Fe complexes in suspension had higher δ56Fe values. The seasonal variability in Fe isotope signature could not be simply linked to Fe speciation, but was probably also influenced by variation in source areas of Fe and processes along the flow-path that alter both Fe speciation and isotopic composition.

The Size Fractionation and Speciation of Iron in the Longqi Hydrothermal Plumes on the Southwest Indian Ridge

AbstractThe distribution and composition of Fe in hydrothermal plumes are crucial for understanding the contribution of hydrothermal venting to the oceanic Fe inventory. In this study, we conduct the first complete investigation of the size fractions of Fe and Fe‐binding ligands in the Longqi hydrothermal plumes on the Southwest Indian Ridge, combining the approaches of the size partitioning and reverse titration‐competitive ligand exchange‐adsorptive cathodic stripping voltammetry. Concentrations of all the Fe fractions were very high in the plume samples, and dissolved Fe (&lt;0.2 μm) constituted a significant portion (48.7 ± 7.9%) of total Fe, which might be related to the existence of colloidal Fe oxyhydroxides and sulfides, as well as organic Fe complexes. Dissolved Fe consisted of colloidal Fe (10 kDa to 0.2 μm, ~70.2%) and soluble Fe (&lt;10 kDa, ~29.8%). Colloidal Fe mainly contained Fe oxyhydroxides and sulfides (86.4 ± 6.2%), while most of the soluble Fe were organic Fe complexes (65.7 ± 5.4%).Fe speciation analyses showed that dissolved ligand concentrations were high in the hydrothermal plumes but were significantly lower than the dissolved Fe concentrations. Inferring from previous studies in the Longqi hydrothermal field, the ligands were most likely derived from the diffuse flow adjacent to the hydrothermal vents. The conditional stability constants (logK′FeL) of dissolved FeL (20.4 ± 0.5) were slightly higher than those of soluble FeL (19.6 ± 0.4), although they were not statistically different. Based on the ligands' size partitioning, the soluble ligands stabilized ~8.8 ± 3.8% of the hydrothermal Fe, which is over 2 times that of the colloidal ligands, ~4.0 ± 1.8%.

Effect of Natural and Artificial Light on Fe(III) Organic Complexes Photolysis: Case of Fe (III)-Malonate and Fe(III)-Malate

Abstract The photolysis of iron(III) organic complexes in the presence of natural and simulated light was a concern for the role it plays within the environment in terms of producing hydroxyl radicals able to degrade organic products. In the present work, the study focused on two iron(III) organic complexes. The characterization of these complexes by spectrometry allowed to show their stability in environment with a stability constant equal to Log β = 13.13 for the complex Fe(III)-malonate, and Log β = 20.85 for the Fe(III)-malate complex. Photolysis of these complexes was carried out using a lamp at 365 nm at different pHs, and followed by several assay Fe(II), H2O2 and •OH. The complexes present different photo-reactivities, in the case of Fe(III)-malate, the photolysis is favored by the Fenton process and generates hydroxyl radicals, contrarily to Fe(III)-malonate photolysis which generates an oxidizing •CH2COOH radical without formation of H2O2 nor of •OH. The presence of oxygen and the addition of isopropanol and chloroform show different effects on the complex photolysis. Complexes mineralization at natural irradiation was also studied using chemical oxygen demand (COD).

Ligands representing important functional groups of natural organic matter facilitate Fe redox transformations and resulting binding environments

Abstract Functional groups in natural organic matter have the potential to interact with and stabilize iron (Fe) redox species (ferric, ferrous) through complexation reactions. In this study, iron complexes with cysteine, arginine and histidine are used to investigate the extent to which specific functional groups present in these amino acids might hinder the oxidation of Fe2+ or promote the reduction of Fe3+. Iron complexes are synthesized by addition of ferrous or ferric salts to cysteine, arginine and histidine solutions at pH 7. The Fe-amino acid complexes are analysed using X-ray Absorption Spectroscopy (XAS), theoretical CTM4XAS (Charge Transfer Multiplet for XAS) calculations, and vibrational spectroscopy (FTIR and Raman). In addition, oxidation of the amino acids is determined by linear sweep voltammetry and chemical equilibrium modeling is used to predict the speciation of Fe in complexes with the amino acids. It is observed the extent of Fe redox transformation is affected by the electron donating capability of the ligand with which Fe reacts. In particular, the extent of Fe3+ reduction is related to the oxidation of the ligand with 80, 14 and 0% Fe3+ reduction by cysteine, arginine and histidine, respectively. Conversely, Fe2+ is preserved by cysteine (80%) > arginine (77%) > histidine (62%). The chemical forms of Fe in these mixed Fe oxidation-state systems include Fe-organic complexes and Fe precipitates. XAS and vibrational spectroscopy indicate thiol-S and amino-N bind Fe in complexes with cysteine. While amino-N and guanidyl-N functional groups of arginine bind Fe, carboxylate-O binding and the formation of Fe precipitates are only observed when Fe3+ is the predominant redox species. Whereas Fe precipitates seem to prevail in Fe(III)-histidine, distinct organic (Fe O/N C) and mineral (Fe O Fe) or mixed organic-mineral (Fe(O/N C)x(O Fe)y) coordination environments occur in Fe(II)-histidine with ∼60% Fe2+; in either case, Fe N(imidazole) binding is not evident. The ligand atoms to which Fe binds and the inherent reduction capacity of the ligand appear to mediate Fe redox transformations. The proportion of ferric iron (Fe3+) seems to determine whether a precipitate forms. Consistent with spectroscopic data, chemical equilibrium modeling using experimental solution conditions and fractional Fe2+ and Fe3+ concentrations obtained from CTM4XAS predict the formation of Fe(II)- and Fe(III)-amino acid complexes and hydrolyzed Fe species; these calculations however also predict the formation of Fe oxide precipitates in all Fe complexes. The results presented in this study help to explain the co-occurrence of Fe2+ and Fe3+ in natural environments where organic matter is present. They also highlight the role specific organic functional groups play on the formation and stabilization of Fe(II, III)-organic complexes and precipitates, and thus on the Fe redox cycle which affects Fe bioavailability and mobility in terrestrial and subsurface environments.

Biological Characteristics and Concentrations of Extractable Fe, Al, and Si Compounds in Spruce Rhizosphere in Podzolic Soil

Some biological properties and concentrations of extractable Fe, Al, and Si compounds in the samples of the AOEL horizon of podzolic soil taken in five replicates from the rhizosphere of 15- to 20-year-old spruce and from the nonrhizosphere soil are discussed. The soil mass of the rhizosphere is characterized by a significantly higher total number of bacteria, more abundant and diverse saprotrophic bacterial complex, greater length of fungal mycelium, and higher content of benzenecarboxylic acids dominated by benzoic acid in comparison with the nonrhizosphere soil. The soil mass of the rhizosphere and the fraction of 1–5 µm isolated from it are also characterized by significantly higher concentrations of Fe and Al extracted by the Tamm and Bascomb reagents because of the accumulation of Fe-organic and Al-organic complexes. For extractable Al compounds in the rhizosphere, this finding is confirmed by the high coefficient of correlation between the amount of Al in the extracts and the Corg content in the bulk soil mass and in the fraction 1–5 µm. Such a correlation is absent in the nonrhizosphere soil. The content of iron compounds extracted by the Mehra–Jackson reagent from the rhizosphere and nonrhizosphere soils is significantly higher than that extracted by the Tamm and Bascomb reagents; in the nonrhizosphere samples, it is significantly higher than in the rhizosphere. This difference can be explained by the fact that the content of Fe oxides and hydroxides minerals decreased in the soil mass of the rhizosphere due to more active dissolution processes under the conditions of more acid medium and higher concentration of organic ligands, so that the mobilized Fe enters Fe–organic complexes.

Iron and silicon isotope behaviour accompanying weathering in Icelandic soils, and the implications for iron export from peatlands

Incipient warming of peatlands at high latitudes is expected to modify soil drainage and hence the redox conditions, which has implications for Fe export from soils. This study uses Fe isotopes to assess the processes controlling Fe export in a range of Icelandic soils including peat soils derived from the same parent basalt, where Fe isotope variations principally reflect differences in weathering and drainage. In poorly weathered, well-drained soils (non-peat soils), the limited Fe isotope fractionation in soil solutions relative to the bulk soil (Δ57Fesolution-soil=−0.11±0.12‰) is attributed to proton-promoted mineral dissolution. In the more weathered poorly drained soils (peat soils), the soil solutions are usually lighter than the bulk soil (Δ57Fesolution-soil=−0.41±0.32‰), which indicates that Fe has been mobilised by reductive mineral dissolution and/or ligand-controlled dissolution. The results highlight the presence of Fe-organic complexes in solution in anoxic conditions. An additional constraint on soil weathering is provided by Si isotopes. The Si isotope composition of the soil solutions relative to the soil (Δ30Sisolution-soil=0.92±0.26‰) generally reflects the incorporation of light Si isotopes in secondary aluminosilicates. Under anoxic conditions in peat soils, the largest Si isotope fractionation in soil solutions relative to the bulk soil is observed (Δ30Sisolution-soil=1.63±0.40‰) and attributed to the cumulative contribution of secondary clay minerals and amorphous silica precipitation. Si supersaturation in solution with respect to amorphous silica is reached upon freezing when Al availability to form aluminosilicates is limited by the affinity of Al for metal–organic complexes. Therefore, the precipitation of amorphous silica in peat soils indirectly supports the formation of metal–organic complexes in poorly drained soils. These observations highlight that in a scenario of decreasing soil drainage with warming high latitude peatlands, Fe export from soils as Fe-organic complexes will increase, which in turn has implications for Fe transport in rivers, and ultimately the delivery of Fe to the oceans.

16 Electron Spin Resonance (ESR) Investigation on Organic-Fe complexes in Coal and Coal Extract

16 Electron Spin Resonance (ESR) Investigation on Organic-Fe complexes in Coal and Coal Extract

Can Fe isotope fractionations trace the pedogenetic mechanisms involved in podzolization?

Stable Fe isotopes have shown the potential for tracing pedogenetic processes. Large isotopic fractionations were especially observed in Podzols. Nevertheless, a clear link between isotopic fractionation and elementary processes still needs to be established. To spatially distinguish the mechanisms successively involved in pedogenesis, we studied a podzolic chronosequence from Vancouver Island (British Columbia, Canada). We analyzed depth variations in soil properties (pH, particle size fractions, organic carbon, Si/Al ratio, Zr, Ti), Fe concentrations in different Fe pools, and Fe isotopic compositions. The Si/Al ratio, Zr, and Ti demonstrated that the Cox Bay Podzols developed from the same parental material, satisfying the requirements of a chronosequence. We showed that acidification, to a pH of 4.7, was a prerequisite for the start of podzolization. This first phase took place after 270years. Furthermore, we observed that once the required pH was reached (4.7), podzolization occurred rapidly over 50years. We found that Fe isotope fractionation in the A/E horizon was linked to mineral dissolution, while in the podzolic B horizons this fractionation was clearly associated with the accumulation of Fe-organic complexes and poorly crystalline Fe oxyhydroxides.

Arsenic removal from contaminated brackish sea water by sorption onto Al hydroxides and Fe phases mobilized by land-use

This study examines the spatial and temporal distribution patterns of arsenic (As) in solid and aqueous materials along the mixing zone of an estuary, located in the south-eastern part of the Bothnian Bay and fed by a creek running through an acid sulfate (AS) soil landscape. The concentrations of As in solution form (<1kDa) increase steadily from the creek mouth to the outer estuary, suggesting that inflowing seawater, rather than AS soil, is the major As source in the estuary. In sediments at the outer estuary, As was accumulated and diagenetically cycled in the surficial layers, as throughout much of the Bothnian Bay. In contrast, in sediments in the inner estuary, As concentrations and accumulation rates showed systematical peaks at greater depths. These peaks were overall consistent with the temporal trend of past As discharges from the Rönnskär smelter and the accompanied As concentrations in past sea-water of the Bothnian Bay, pointing to a connection between the historical smelter activities and the sediment-bound As in the inner estuary. However, the concentrations and accumulation rates of As peaked at depths where the smelter activities had already declined, but a large increase in the deposition of Al hydroxides and Fe phases occurred in response to intensified land-use in the mid 1960's and early 1970's. This correspondence suggests that, apart from the inflowing As-contaminated seawater, capture by Al hydroxides, Fe hydroxides and Fe-organic complexes is another important factor for As deposition in the inner estuary. After accumulating in the sediment, the solid-phase As was partly remobilized, as reflected by increased pore-water As concentrations, a process favored by As(V) reduction and high concentrations of dissolved organic matter.

Organically complexed iron enhances bioavailability of antimony to maize (Zea mays) seedlings in organic soils.

Antimony (Sb) is a metalloid belonging to group 15 of the periodic table. Chemical similarities between arsenic (As) and Sb produce concerns about potential health effects of Sb and enrichment in the environment. Antimony is found in oxic environments predominately as an oxyanionic species, antimonite (Sb[OH](6-)). As a result of its net negative charge, Sb[OH](6-) was not initially predicted to have strong interactions with natural organic matter. Oxyanionic species could bind the negatively charged organic matter via a ternary complexation mechanism, in which cationic metals mediate the strong association between organic matter functional groups and oxyanions. However, these interactions are poorly understood in how they influence the bioavailability of oxyanionic contaminants to plants. Iron (Fe) additions to organic soils have been found to increase the number of organically complexed Fe sites suitable for Sb exchange, resulting in a reduced bioavailable fraction of Sb. The bioavailability of Sb to maize seedlings as a function of organically complexed Fe was examined using a greenhouse study. A significant increase in plant tissue Sb was observed as organically complexed Fe increased, which was not predicted by methods commonly used to assess bioavailable Sb. Extraction of soils with organic acids common to the maize rhizosphere suggested that organic acid exudation can readily mobilize Sb bound by organic Fe complexes.

Effects of Fe fertilizer eluate on the growth of Sargassum horneri at the germling and immature stages

To aid in the restoration of coastal barren ground areas, it is important to clarify the effects of chelated iron on the growth of seaweed. In particular, for the further development of practical methods to promote seaweed growth, Fe-binding organic ligands, such as humic substances (HSs) composed of humus materials, rather than Fe-binding inorganic ligands, such as ethylenediaminetetraacetic acid (EDTA), should be investigated. In this study, the effects of an Fe fertilizer, made from HSs and steelmaking slag, on the growth of the brown alga Sargassum horneri at the germling and immature stages were examined. The addition of the Fe fertilizer eluate containing Fe organic ligand complexes clearly promoted the growth of S. horneri at the germling and immature stages. It was also clear that the effect of Fe concentration in the Fe fertilizer eluate on the growth rate was almost the same as that of Fe–EDTA. Moreover, the addition of the Fe fertilizer eluate had a great effect on the brown color of S. horneri thalli and promoted the increased content of photosynthetic pigments, such as chlorophyll a. Based on these experimental results, the application of the Fe fertilizer containing Fe organic ligand complexes is expected to become an effective method for the restoration of the barren ground phenomenon in Fe-deficient coastal areas.

Fractionation of Fe isotopes during Fe(II) oxidation by a marine photoferrotroph is controlled by the formation of organic Fe-complexes and colloidal Fe fractions

Much interest exists in finding mineralogical, organic, morphological, or isotopic biosignatures for Fe(II)-oxidizing bacteria (FeOB) that are retained in Fe-rich sediments, which could indicate the activity of these organisms in Fe-rich seawater, more common in the Precambrian Era. To date, the effort to establish a clear Fe isotopic signature in Fe minerals produced by Fe(II)-oxidizing metabolisms has been thwarted by the large kinetic fractionation incurred as freshly oxidized aqueous Fe(III) rapidly precipitates as Fe(III) (oxyhydr)oxide minerals at near neutral pH. The Fe(III) (oxyhydr)oxide minerals resulting from abiotic Fe(II) oxidation are isotopically heavy compared to the Fe(II) precursor and are not clearly distinguishable from minerals formed by FeOB isotopically. However, in marine hydrothermal systems and Fe(II)-rich springs the minerals formed are often isotopically lighter than expected considering the fraction of Fe(II) that has been oxidized and experimentally-determined fractionation factors. We measured the Fe isotopic composition of aqueous Fe (Feaq) and the final Fe mineral (Feppt) produced in batch experiment using the marine Fe(II)-oxidizing phototroph Rhodovulum iodosum. The δ56Feaq data are best described by a kinetic fractionation model, while the evolution of δ56Feppt appears to be controlled by a separate fractionation process. We propose that soluble Fe(III), and Fe(II) and Fe(III) extracted from the Feppt may act as intermediates between Fe(II) oxidation and Fe(III) precipitation. Based on 57Fe Mössbauer spectroscopy, extended X-ray absorption fine structure (EXAFS) spectroscopy, and X-ray total scattering, we suggests these Fe phases, collectively Fe(II/III)interm, may consist of organic-ligand bound, sorbed, and/or colloidal Fe(II) and Fe(III) mineral phases that are isotopically lighter than the final Fe(III) mineral product. Similar intermediate phases, formed in response to organic carbon produced by FeOB and inorganic ligands (e.g., SiO44− or PO43−), may form in many natural Fe(II)-oxidizing environments. We propose that the formation of these intermediates is likely to occur in organic-rich systems, and thus may have controlled the ultimate isotopic composition of Fe minerals in systems where Fe(II) was being oxidized by or in the presence of microbes in Earth’s past.

Fe vs. TiO2 Photo-assisted Processes for Enhancing the Solar Inactivation of Bacteria in Water.

Batch solar water disinfection (SODIS) is a known, simple and low-cost water treatment technology. SODIS is based on the synergistic action of temperature increase and light-assisted generation of Reactive Oxygen Species (ROS) on bacteria. ROS are generated via the action of solar photons on i) Natural Organic Matter (NOM), ii) some mineral components of water (Fe oxides or Fe-organic complexes, nitrogen compounds) and iii) endogenous bacteria photosensitizers (e.g. cytochrome). SODIS has proven its effectiveness for remote settlements or urban slums in regions with high incident solar radiation. All of the internal and external simultaneous processes are often driven by photoactive Fe-species present in the cell, as well as in the natural water sources. In SODIS, a temperature of 50 °C is required and due to this temperature dependence, only 1-2 L can be treated at a time. As required exposure time strongly depends on irradiation intensity and temperature, some SODIS households could be overburdened, leading to inadequate treatment and probable bacterial re-growth. This is why TiO(2) photocatalysis and Fe photo-assisted systems (i.e. photo-Fenton reactants) have been considered to enhance the photo-catalytic processes already present in natural water sources when exposed to solar light. Both TiO(2) and Fe-photoassisted processes, when applied to water disinfection aim to improve the performance of solar bacteria inactivation systems by i) enhancing ROS production, ii) making the process independent from the rise in temperature and as a consequence iii) allowing the treatment of larger volumes than 1-2 L of water and iv) prevent bacterial (re)growth, sometimes observed after sole solar treatment.