Fe Redox Cycling Research Articles

Effects of tide and season changes on the iron-sulfur-phosphorus biogeochemistry in sediment porewater of a mangrove coast

To better understand the biogeochemical processes of iron, sulfur, and phosphorus and their coupled relationships, we used the in situ high-resolution diffusive gradients in thin films (DGT) technique to characterize labile Fe, S, and P in the sediment porewater of a mangrove coast in the Jiulong River Estuary, China. The distributions of DGT-labile Fe and S indicated that microbial sulfate reduction (MSR) and microbial iron reduction (MIR) exhibited an intense competitive relationship and temperature-dependent behavior. Thus, in addition to the expected MSR, MIR is an important or even predominant biogeochemical process for organic matter (OM) mineralization in this Fe-rich mangrove coast. Labile Fe, S, and P distributions and variations demonstrate that Fe redox cycling plays a crucial role in regulating P behavior and P cycling. An additional new finding is that the flux of labile S in the intertidal zone increases during flood tides and decreases during ebb tides, indicating that the formation and reoxidation of sulfide in surface layers are regulated by tide-induced redox changes. The net P flux associated with recirculated seawater was estimated to range from −0.306 ± 0.001 to 0.343 ± 0.013 mmol m−2 d−1 in the winter survey, suggesting that sediment porewater tends to be a P source in the subtidal zone and a P sink in the intertidal zone for the overlying seawater in winter. The continuous accumulation of labile P in surface sediment could be intensively released into the overlying seawater through prevailing MIR process in the subsequent riverine flood season, which may promote eutrophication of the adjacent Xiamen Bay.

Sedimentology and oceanography of Early Ordovician ironstone, Bell Island, Newfoundland: Ferruginous seawater and upwelling in the Rheic Ocean

The Bell Island and Wabana groups together comprise a ca. 150-m-thick succession of interbedded clastic and chemical sedimentary rocks composed of eight distinct lithofacies that accumulated along the northern margin of the Rheic Ocean. Lithofacies stacking patterns indicate that deposition occurred during a marine transgression with superimposed small-scale sea level fluctuations producing at least six parasequences. Parasequences containing ironstone are 10 to 20-m-thick and composed of hummocky cross-stratified sandstone interbedded with organic-rich mudstone and phosphatic Fe-silicate-bearing siltstone, which is overlain by hematitic granular ironstone capped by an erosive flooding surface.This lithofacies association is interpreted to record the deposition of upwelling-related ironstone on a storm-dominated shelf. The close relationship between Fe-silicates and phosphorite typical of upwelling systems suggests that Fe was delivered from deep, anoxic, nutrient-rich seawater that also stimulated high surface productivities. The result was the precipitation of authigenic sedimentary apatite in anoxic organic-rich sediments that accumulated near the upwelling front. The gradual advection of Fe-rich waters away from the upwelling front, initiated precipitation of Fe-silicate coated grains and cements in suboxic pore-waters. Iron pumped into shallower environments through advection and Fe-redox cycling is interpreted to have precipitated Fe-(oxyhydr)oxide grains in sediment of the oxygenated middle shelf. These coated grains were subsequently concentrated by fairweather and storm currents on the shoreface to create granular economic Fe deposits.We challenge conventional models of Paleozoic ironstone deposition that rely on a continental source of Fe by proposing a hydrothermal source that supplied Fe2+ to the shelf through upwelling. It also highlights the potential connection between the delivery of anoxic, ferruginous seawater to the margins of the Rheic Ocean and the Early Ordovician extinctions that punctuated the beginning of the Great Ordovician Biodiversification Event.

Investigating the Composition and Metabolic Potential of Microbial Communities in Chocolate Pots Hot Springs.

Iron (Fe) redox-based metabolisms likely supported life on early Earth and may support life on other Fe-rich rocky planets such as Mars. Modern systems that support active Fe redox cycling such as Chocolate Pots (CP) hot springs provide insight into how life could have functioned in such environments. Previous research demonstrated that Fe- and Si-rich and slightly acidic to circumneutral-pH springs at CP host active dissimilatory Fe(III) reducing microorganisms. However, the abundance and distribution of Fe(III)-reducing communities at CP is not well-understood, especially as they exist in situ. In addition, the potential for direct Fe(II) oxidation by lithotrophs in CP springs is understudied, in particular when compared to indirect oxidation promoted by oxygen producing Cyanobacteria. Here, a culture-independent approach, including 16S rRNA gene amplicon and shotgun metagenomic sequencing, was used to determine the distribution of putative Fe cycling microorganisms in vent fluids and sediment cores collected along the outflow channel of CP. Metagenome-assembled genomes (MAGs) of organisms native to sediment and planktonic microbial communities were screened for extracellular electron transfer (EET) systems putatively involved in Fe redox cycling and for CO2 fixation pathways. Abundant MAGs containing putative EET systems were identified as part of the sediment community at locations where Fe(III) reduction activity has previously been documented. MAGs encoding both putative EET systems and CO2 fixation pathways, inferred to be FeOB, were also present, but were less abundant components of the communities. These results suggest that the majority of the Fe(III) oxides that support in situ Fe(III) reduction are derived from abiotic oxidation. This study provides new insights into the interplay between Fe redox cycling and CO2 fixation in sustaining chemotrophic communities in CP with attendant implications for other neutral-pH hot springs.

Heterogeneous activation of peroxymonosulfate by LaFeO3 for diclofenac degradation: DFT-assisted mechanistic study and degradation pathways

A perovskite oxide, LaFeO3 (LFO), was synthesized and evaluated as a heterogeneous catalyst to activate peroxymonosulfate (PMS) for the oxidative degradation of diclofenac (DCF), a non-steroidal anti-inflammatory drug. It was observed that the catalytic activity of LFO was much higher than that of Fe2O3. LFO catalyzed PMS to degrade DCF with a turnover frequency (2.02 × 10−3 min−1) which is 17-fold higher than that of Fe2O3. Both sulfate and hydroxyl radicals were identified during LFO-activated PMS process by electron spin resonance (ESR). Radical competitive reactions indicate sulfate radicals played a major role in DCF degradation by LFO/PMS process. The PMS decomposition can be attributed to the formation of an inner-sphere complexation between the Fe (III) sites on LFO surface and PMS. Theoretical calculations illustrated the strong interaction between PMS and Fe (III) and electron transfer from PMS to Fe (III). Hydrogen temperature-programmed reduction (H2-TPR) indicates that the LFO perovskite oxide is capable of facilitating an easier reduction of Fe (III) to mediate a redox process. Oxygen temperature-programmed desorption (O2-TPD) suggests much more oxygen vacancies exist in LFO than in Fe2O3. Oxygen vacancies are favorable for the formation of chemical bond between Fe (III) and PMS and the activation of PMS. In situ ATR-FTIR analysis of LFO surface during PMS decomposition implies Fe (III)–Fe(II)–Fe (III) redox cycle was believed to account for the generation of sulfate radical. The intermediates generated during DCF degradation were identified and the possible degradation pathways were advanced in LFO/PMS system.

Anoxic ammonia removal using granulated nanoscale oxyhydroxides of Fe (GNOF) in a SBR

Microbial mediated anoxic ammonia removal in the presence of granulated nanoscale oxyhydroxides of Fe (GNOF) was investigated in a SBR, as a novel alternative to conventional nitrification and denitrification. Activated sludge flocs collected from a SBR treating sewage was used as seed biomass to develop the process. After confirming the feasibility of anoxic ammonia removal, SBR operations were carried out for 267 days in 9 phases by varying the initial NH4+-N concentrations from 100 to 500 mg/L and cycle time from 2 to 0.5 day. Effective anoxic ammonia removal was achieved in 0.5 day cycle time with the removal rates of NH4+-N and TN as 130 mg/L/d and 115 mg/L/d, respectively. Batch studies showed that the optimum COD/NH4+-N ratio for effective NH4+-N removal was less than or equal to 2. The kinetics of NH4+-N removal, confirmation studies for GNOF coupled nitrification-denitrification and Fe redox cycling involved in the process were also performed and discussed. The developed process is economical as there is no need for aeration and alkalinity.

Stable Isotope Probing for Microbial Iron Reduction in Chocolate Pots Hot Spring, Yellowstone National Park.

Chocolate Pots hot springs (CP) is a circumneutral-pH Fe-rich geothermal feature located in Yellowstone National Park. Previous Fe(III)-reducing enrichment culture studies with CP sediments identified close relatives of known dissimilatory Fe(III)-reducing bacterial (FeRB) taxa, including Geobacter and Melioribacter However, the abundances and activities of such organisms in the native microbial community are unknown. Here, we used stable isotope probing experiments combined with 16S rRNA gene amplicon and shotgun metagenomic sequencing to gain an understanding of the in situ Fe(III)-reducing microbial community at CP. Fe-Si oxide precipitates collected near the hot spring vent were incubated with unlabeled and 13C-labeled acetate to target active FeRB. We searched reconstructed genomes for homologs of genes involved in known extracellular electron transfer (EET) systems to identify the taxa involved in Fe redox transformations. Known FeRB taxa containing putative EET systems (Geobacter, Ignavibacteria) increased in abundance under acetate-amended conditions, whereas genomes related to Ignavibacterium and Thermodesulfovibrio that contained putative EET systems were recovered from incubations without electron donor. Our results suggest that FeRB play an active role in Fe redox cycling within Fe-Si oxide-rich deposits located at the hot spring vent.IMPORTANCE The identification of past near-surface hydrothermal environments on Mars emphasizes the importance of using modern Earth environments, such as CP, to gain insight into potential Fe-based microbial life on other rocky worlds, as well as ancient Fe-rich Earth ecosystems. By combining stable carbon isotope probing techniques and DNA sequencing technology, we gained insight into the pathways of microbial Fe redox cycling at CP. The results suggest that microbial Fe(III) oxide reduction is prominent in situ, with important implications for the generation of geochemical and stable Fe isotopic signatures of microbial Fe redox metabolism within Fe-rich circumneutral-pH thermal spring environments on Earth and Mars.

An evaluation of sedimentary molybdenum and iron as proxies for pore fluid paleoredox conditions

Iron speciation and trace metal proxies are commonly applied together in efforts to identify anoxic settings marked by the presence of free sulfide (euxinia) or dissolved iron (ferruginous) in the water column. Here, we use a literature compilation from modern localities to provide a new empirical evaluation of coupled Fe speciation and Mo concentrations as a proxy for pore water sulfide accumulation at non-euxinic localities. We also present new Fe speciation, Mo concentration, and S isotope data from the Friends of Anoxic Mud (FOAM) site in Long Island Sound, which is marked by pore water sulfide accumulation of up to 3 mM beneath oxygen-containing bottom waters. For the operationally defined Fe speciation scheme, ‘highly reactive’ Fe (FeHR) is the sum of pyritized Fe (Fepy) and Fe dominantly present in oxide phases that is available to react with pore water sulfide to form pyrite. Observations from FOAM and elsewhere confirm that Fepy/FeHR from non-euxinic sites is a generally reliable indicator of pore fluid redox, particularly the presence of pore water sulfide. Molybdenum (Mo) concentration data for anoxic continental margin sediments underlying oxic waters but with sulfidic pore fluids typically show authigenic Mo enrichments (2–25 ppm) that are elevated relative to the upper crust (1–2 ppm). However, compilations of Mo concentrations comparing sediments with and without sulfidic pore fluids underlying oxic and low oxygen (non-euxinic) water columns expose non-unique ranges for each, exposing false positives and false negatives. False positives are most frequently found in sediments from low oxygen water columns (for example, Peru Margin), where Mo concentration ranges can also overlap with values commonly found in modern euxinic settings. FOAM represents an example of a false negative, where, despite elevated pore water sulfide concentrations and evidence for active Fe and Mn redox cycling in FOAM sediments, sedimentary Mo concentrations show a homogenous vertical profile across 50 cm depth at 1 to 2 ppm. A diagenetic model for Mo provides evidence that muted authigenic enrichments are derived from elevated sedimentation rates. Consideration of a range of additional parameters, most prominently pore water Mo concentration, can replicate the ranges of most sedimentary Mo concentrations observed in modern non-euxinic settings. Together, the modern Mo and Fe data compilations and diagenetic model provide a framework for identifying paleo-pore water sulfide accumulation in ancient settings and linked processes regulating seawater Mo and sulfate concentrations and delivery to sediments. Among other utilities, identifying ancient accumulation of sulfide in pore waters, particularly beneath oxic bottom waters, constrains the likelihood that those settings could have hosted organisms and ecosystems with thiotrophy at their foundations.

Porewater inputs drive Fe redox cycling in the water column of a temperate mangrove wetland

Iron is a vital micronutrient within coastal marine ecosystems, playing an integral role in the scale and dynamics of primary production and carbon cycling in the world's oceans. We investigated the relative importance of in situ Fe(II) production from photochemical, microbial and thermal Fe reduction in the surface water column as well as advective porewater inputs in a temperate saline wetland in Australia containing mangrove and saltmarsh vegetation. The diel average concentration of Fe(II) (0.63 ± 0.21 μM, accounting for >70% of the total dissolved Fe present in surface water) was much higher than commonly reported in oxygenated marine waters despite high dissolved oxygen concentrations (81–112% saturation), pH (7.7–7.8) and salinity (33–36) that favor Fe oxidation. In situ production of Fe(II) in the surface water column was primarily driven by microbial processes rather than photochemical and thermal reduction, with a maximum production rate of 4.9 × 10−3 nM s−1. Advective porewater Fe(II) inputs to the wetland averaged over a diel cycle (3.0 × 10−1 nM s−1) were an order of magnitude greater than the combined Fe(II) production rate from autochthonous water column processes (1.0 × 10−2 nM s−1). A bottom up model based on the estimated individual fluxes was used to explain the high Fe(II) concentrations measured during a 24 h time series experiment. Combined, different lines of evidence suggest that advective porewater exchange provides significant quantities of Fe(II) to the estuarine wetland.

Reinventing Fenton Chemistry: Iron Oxychloride Nanosheet for pH-Insensitive H2O2 Activation

This study intends to reinvent classical Fenton chemistry by enabling the Fe(II)/Fe(III) redox cycle to occur on a newly developed FeOCl nanosheet catalyst for facile hydroxyl radical (•OH) generation from H2O2 activation. This approach overcomes challenges such as low operating pH and large sludge production that have prevented a wider use of otherwise attractive Fenton chemistry for practical water treatment, in particular, for the destruction of recalcitrant pollutants through nonselective oxidation by •OH. We demonstrate that FeOCl catalysts exhibit the highest performance reported in the literature for •OH production and organic pollutant destruction over a wide pH range. We further elucidate the mechanism of rapid conversion between Fe(III) and Fe(II) in FeOCl crystals based on extensive characterizations. Given the low-cost raw material and simple synthesis and regeneration, FeOCl catalysts represent a critical advance toward application of iron-based advanced oxidation in real practice.

Impact on the Fe redox cycling of organic ligands released by Synechococcus PCC 7002, under different iron fertilization scenarios. Modeling approach

Abstract The kinetics of Fe redox transformations are of crucial importance in determining the bioavailability of iron, due to inorganic Fe(II) and Fe weakly organic complexes being the most easily assimilated species by phytoplankton. The role played by the natural organic ligands excreted by the cyanobacteria Synecococcus PCC 7002 on the iron redox chemistry was studied at different stages of growth, considering changes in the organic exudation of the cyanobacteria, associated with growth under two different scenarios of iron availability. The oxidation/reduction processes of iron were studied at nanomolar levels and under different physicochemical conditions of pH (7.2– 8.2), temperature (5– 35 °C) and salinity (10– 37). The presence of natural organic exudates of Synechococcus affected the redox behavior of iron. A pH-dependent and photo-induced Fe(III) reduction process was detected in the presence of exudates produced under Fe-Low conditions. Photolytic reactions also modified the reactivity of those exudates with respect to Fe(II), increasing its lifetime in seawater. Without light mediated processes, organic ligands excreted under iron deficient conditions intensified the Fe(II) oxidation at pH A kinetic modeling approach to describe the speciation and the contribution of individual Fe(II) species to the overall oxidation rate was applied, considering the experimental data and delimiting the equilibrium and redox constants between iron and the major ligands present in solution. Two organic type ligands for the exudates of Synechococcus PCC 7002, with different iron-chelation properties were included in the model. The Fe(II) speciation was radically affected when organic ligands were considered. The individual contributions to the overall Fe(II) oxidation rate demonstrated that these organic ligands played a key role in the oxidation process, although their contributions were dependent on the prescribed iron conditions. The study, therefore, suggests that the variability in the composition and nature of organic exudates released, due to iron availability conditions, might determine the redox behaviour of iron in seawater.

Internal phosphorus loading from sediments causes seasonal nitrogen limitation for harmful algal blooms

It is proposed that the internal loading of phosphorus (P) from sediments plays an important role in seasonal nitrogen (N) limitation for harmful algal blooms (HABs), although there is a lack of experimental evidence. In this study, an eutrophic bay from the large and shallow Lake Taihu was studied for investigating the contribution of internal P to N limitation over one-year field sampling (February 2016 to January 2017). A prebloom-bloom period was identified from February to August according to the increase in Chla concentration in the water column, during which the ratio of total N to total P (TN/TP) exponentially decreased with month from 43.4 to 7.4. High-resolution dialysis (HR-Peeper) and diffusive gradients in thin films (DGT) analysis showed large variations in the vertical distribution of mobile P (SRP and DGT-labile P) in sediments, resulting in the SRP diffusion flux at the sediment-water interface ranging from −0.01 to 6.76mg/m2/d (minus sign denotes downward flux). Significant and linear correlations existed between SRP and soluble Fe(II) concentrations in pore water, reflecting that the spatial-temporal variation in mobile P was controlled by microbe-mediated Fe redox cycling. Mass estimation showed that the cumulative flux of SRP from sediments accounted for 54% of the increase in TP observed in the water column during the prebloom-bloom period. These findings are supported by the significantly negative correlation (p<0.01) observed between sediment SRP flux and water column TN/TP during the same period. Overall, these results provide solid evidence for the major role of internal P loading in causing N limitation during the prebloom-bloom period.

Fine-scale LA-ICP-MS study of redox oscillations and REEY cycling during the latest Devonian Hangenberg Crisis (Moravian Karst, Czech Republic)

Trace elementsand REEY distribution of thinly laminated limestone (laminite) and underlying pre-event calciturbidite (Siphonodella praesulcata conodont Zone) deposited in association with the Hangenberg Black Shale Event were analyzed using laser ablation-inductively coupled plasma-mass spectrometry. These deposits were also analyzed by carbonate petrography and scanning electron microscopy. Our study documents the origin of authigenic microbial limestones during the latest Devonian Hangenberg extinction event, when unusual oceanic conditions appear to have enhanced precipitation of calcite within microbial carbonates. Detrital contamination of the calciturbidite and laminated limestone is very low, as demonstrated by low contents of Al, Zr, Ti and Th. These deposits display only limited degree of early diagenetic overprint of limestones, and some of them fall within or very close to the seawater field. Elevated U and Mo enrichments and covariation patterns reflect the episodic establishment of oxic to euxinic bottom condition during deposition of the laminite. The oxic conditions that existed during accumulation of the underlying calciturbidite were likely the result of turbidity current introduction of oxygen into the bottom environment. REE contents of both the laminite and calciturbidite are nearly ten times higher than those of most modern carbonates and similar to Devonian microbialites from the Canning Basin. Calciturbidite display a prominent MREEN bulge. Shale-normalized REE patterns of the overlying laminite are similar to those of modern seawater and Devonian microbialites from the Canning Basin (LREE depletion and progressive enrichments in heavier REE) reflecting the dominance of authigenic carbonate phases. Minor contamination is bound to apatites, siliciclastic minerals, pyrites and organic matter. The element content of authigenic phases benefited from Fe and Mn oxyhydroxide redox cycling. Negative Ce and positive Eu anomalies, low Y/Ho ratios, and very low Al/(Al+Fe+Mn) ratios suggest an influence of diluted high- temperature hydrothermal fluids perhaps related to latest Devonian rift magmatism in the eastern part of the Rhenohercynian zone. The proliferation of anachronistic facies following the Devonian-Carboniferous (D-C) extinction event recognized in Moravia and traced worldwide seems to share some features in common with anachronistic carbonate sequences documented in the aftermath of the end-Permian mass extinction.

Continuous generation of hydroxyl radicals for highly efficient elimination of chlorophenols and phenols catalyzed by heterogeneous Fenton-like catalysts yolk/shell Pd@Fe3O4@metal organic frameworks

Core/shell Fe3O4-decorated Pd nanoparticles (NPs) hybrids (Pd@Fe3O4) are prepared through a “green”, and one-pot chemical process. The Pd@Fe3O4 hybrids consisted of faceted quasi-spherical Pd nanoparticles (NPs) cores (∼20 nm) surrounded by close-packed Fe3O4 NPs (∼7 nm). To improve the stability and avoid aggregation of Pd@Fe3O4 hybrids in water, hollow Fe-metal organic frameworks (Fe-MOFs) were applied to enwrap Pd@Fe3O4 to obtain yolk/shell structured composites. Sub-10 nm Fe3O4 and Pd NPs close to each other were distributed evenly in the MOFs shell of Pd@Fe3O4@MOFs. The yolk/shell Pd@Fe3O4@MOFs can catalyze the oxidative degradation of chlorophenols and phenols by hydroxyl radicals (OH) decomposed from H2O2. With low molar ratio of H2O2/pollutants, the pollutants are degraded and mineralized efficiently and rapidly. The outstanding catalytic efficiency of Pd@Fe3O4@MOFs is contributed by the fast and continuous generation of OH radicals in Pd@Fe3O4@MOFs suspension which is detected with the electron spin resonance spin-trap technique and a continuous-flow chemiluminescence system. Lack of consumption of hydroperoxyl radicals/superoxide radicals (HO2/O2−) in the Pd@Fe3O4@MOFs-H2O2 system might suggest that the production of OH radicals results from the electron transferring from Pd to Fe3O4 component both in the inner Pd@Fe3O4 and MOF shell, which facilitates fast Fe(III)/Fe(II) redox cycle.

Fe(III)]:[Fe(II)] Ratio and Redox Status of Red Wines: Relation to So-Called “Reduction Potential”

Wine oxidation is catalyzed by iron (Fe). Fe(II) is oxidized by O2 to produce Fe(III), which oxidizes reductants, such as polyphenols, thereby returning to the ferrous state. Therefore, Fe-redox cycles and the [Fe(III)]:[Fe(II)] ratio depend on the relative rates of Fe(II) oxidation and Fe(III) reduction. Under reducing conditions, Fe(II) dominates, but with increasing O2 exposure, the proportion of Fe as Fe(III) increases. Reduction potentials have been used for many years to determine the redox state of wines. However, it has now been realized that these potentials are mainly due to oxidation of ethanol on platinum electrodes and not due to the oxidative processes that normally occur in wine. It is proposed that [Fe(III)]:[Fe(II)] ratios may provide an alternative way of estimating the redox state. These ratios were obtained with ferrozine to determine Fe(II) and total Fe concentrations spectroscopically in white wines. However, ferrozine cannot be used in red wine because of color interference. A similar method using 2-(5-Bromo-2-pyridylazo)-5-[N-propyl-N-(3-sulfopropyl)amino] phenol disodium salt dihydrate (Br-PAPS), which is water soluble and the Fe(II) complex of which absorbs outside the red wine absorption range, was therefore developed with minimal wine disturbance so as not to alter ratios. High Fe(II) content (~97%) was observed in bottled wine with screwcap, technical, and plastic closures. Lower proportions of Fe(II) were found with natural corks and wine boxes. The [Fe(III)]:[Fe(II)] ratio increased on O2 exposure. Calculated reduction potentials of the Fe couple did not correspond to those that would be measured for wines, providing further evidence that these wine potentials are not true reduction potentials.

Surface catalyzing action of hematite (α-Fe2O3) on reduction of Cr(VI) to Cr(III) by citrate

Abstract The surface catalyzing action of hematite ( α -Fe2O3) in reduction of hexavalent chromium (Cr(VI)) by citric acid was studied. The results revealed that Cr(VI) could be reduced to Cr(III) by citrate in the presence of 1% (m/v) hematite in the darkness at pH range of 2.0–6.3. The reaction completed within a week to totally reduce 10 mg L − 1 of Cr(VI) by 20 mM citrate in the presence of 1% hematite at pH 4.2, well fitted with zero-order kinetic model ( R 2 > 0 . 99 ). Though ferric ions (Fe(III)) dissolved from hematite surface could also stimulate the Cr(VI) reduction by citrate, hematite surface played a much more important role in this process. The Fe atoms of hematite surface could transport electrons from citrate to Cr(VI) through Fe(III)-Fe(II) cycling, which was proposed as the underlying mechanism of reduction of Cr(VI) by citrate in the presence of iron oxides under certain environmental conditions. Our results suggest that Fe redox cycling may be driven Cr(VI) reduction in the soil/water with the coexist of low molecular organic matters, which could be a sustainable remediation strategy for chromate contamination.

Contrasting effects of inorganic and organic fertilisation regimes on shifts in Fe redox bacterial communities in red soils

Although the precipitation and dissolution of iron (Fe)-containing-minerals are driven by microbially-mediated iron-redox cycling, we still have a limited understanding of complex response of such microbial communities to fertilisation. Here, using chemical and synchrotron-based spectral analyses, we show that the distribution of poorly crystalline Fe minerals and crystalline Fe minerals is different in organically- and inorganically-fertilised soils, and that compared to no fertilisation (Control), Fe redox cycling bacteria were present at higher abundance and diversity in organically-fertilised soils but lower in inorganically-fertilised soil. During Fe(III) reduction, Geobacter were important active Fe(III) reducers, with a higher relative abundance in both organically- and inorganically-fertilised soils than in Control, and their higher abundance was responsible for greater dissolution of ferrihydrite in inorganically-fertilised soil than in organically-fertilised soil. However, during the Fe(II) oxidation, Pseudomonas and Anaerolinea were more abundant, and produced higher levels of poorly crystalline Fe oxides under organic fertilisation. Thus, for the first time, we demonstrate that inorganic and organic fertilisation regimes have contrasting effects on the Fe redox bacterial communities, which then influence Fe cycling in soils.

Successful control of internal phosphorus loading after sediment dredging for 6 years: A field assessment using high-resolution sampling techniques

The effectiveness of sediment dredging for the control of internal phosphorus (P) loading, was investigated seasonally in the eutrophic Lake Taihu. The high-resolution dialysis (HR-Peeper) and diffusive gradients in thin films (DGT) techniques were used to measure the concentrations of soluble Fe(II) and soluble reactive P (SRP) as well as DGT-labile Fe/P in the non-dredging and post-dredging sediments. The P resupply kinetics from sediment solids were interpreted using DGT Induced Fluxes in Sediments (DIFS) modeling. The results showed no obvious improvement in water and sediment quality after dredging for 6years, due to their geographical proximity (a line distance of approximately 9km). However, dredging significantly decreased the concentrations of soluble Fe(II)/SRP and DGT-labile Fe/P in sediments, with effects varying at different depths below the sediment-water interface; More pronounced effects appeared in January and April. The diffusive flux of pore water SRP from sediments decreased from 0.746, 4.08 and 0.353mg/m2/d to 0.174, 1.58 and 0.048mg/m2/d in April, July and January, respectively. DIFS modeling indicated that the P retention capability of sediment solids was improved in April in post-dredging site. Positive correlations between pore water soluble Fe(II) and SRP as well as between DGT-labile Fe and P, reflect the key role of Fe redox cycling in regulating dredging effectiveness. This effect is especially important in winter and spring, while in summer and autumn, the decomposition of algae promoted the release of P from sediments and suppressed dredging effectiveness. Overall, the high-resolution HR-Peeper and DGT measurements indicated a successful control of internal P loading by dredging, and the post-dredging effectiveness was suppressed by algal bloom.

Iron geochemistry and organic carbon preservation by iron (oxyhydr)oxides in surface sediments of the East China Sea and the south Yellow Sea

Abstract In marine sediments factors that influence iron (Fe) geochemistry and its interactions with other elements are diverse and remain poorly understood. Here we comparatively study Fe speciation and reactive Fe-bound organic carbon (Fe-OC) in surface sediments of the East China Sea (ECS) and the south Yellow Sea (SYS). The objectives are to better understand the potential impacts of geochemically distinct sediment sources and depositional/diagenetic settings on Fe geochemistry and OC preservation by Fe (hydr)oxides in sediments of the two extensive shelf seas around the world. Contents of carbonate- and acid-volatile-sulfide (AVS)-associated Fe(II) (FeAVS + carb) and magnetite (Femag) in the ECS sediments are about 5 and 9 times higher, respectively, than in the SYS. This could be ascribed to the ferruginous conditions of the ECS sediments that favor the formation/accumulation of Fecarb and Femag, a unique feature of marine unsteady depositional regimes. Much lower total Fe(II) contents in the SYS than in the ECS suggest that lower availability of highly reactive Fe (FeHR) and/or weak Fe reduction is a factor limiting Fe(II) formation and accumulation in the SYS sediments. The ratio of FeHR to total Fe is, on average, markedly higher (2.4 times) in the ECS sediments than in the SYS, which may be a combined result of several factors relevant to different sediment sources and depositional/diagenetic settings. In comparison with many other marine sediments, the percent fractions (fFe-OC) of Fe-OC to total organic carbon (TOC) in the ECS and the SYS are low, which can be ascribed to surface adsorption of OC rather than coprecipitation or organic complexation as the dominant binding mechanisms. Based on the fFe-OC in this study, total Fe-OC estimated for global continental shelves is equivalent to 38% of the atmospheric CO2 pool, which indicates the important role of sorptive stabilization of Fe-OC in continental shelf sediments for buffering CO2 release to the atmosphere. In the SYS, consistently less 13C-depleted Fe-OC relative to 13C of non-Fe-bound OC (13Cnon-Fe-OC) suggests selective sequestration of labile marine OC in the marine OC-dominated sediments of the central SYS. In the ECS, however, efficient oxidation of OC and frequent redox cycling of Fe in the unsteady depositional regimes may complicate the isotopic compositions of Fe-OC. A combination of our results and literature data demonstrates that Fe-OC contents are strongly dependent on the availability of TOC and reactive Fe, but the fFe-OC is primarily controlled by the processes of Fe redox cycling in the sediments.

Geochemical behavior of phosphorus and iron in porewater in a mangrove tidal flat and associated phosphorus input into the ocean

A thorough understanding of the distribution and geochemical behavior of phosphorus (P) and iron (Fe) across the sediment-water interface (SWI) is essential for estimating the exchange flux between sediment porewater and overlying seawater. In this study, in situ high-resolution diffusive gradients in thin films (DGT) technique, ZrO-Chelex DGT, were applied to investigate the depth profiles of dissolved reactive phosphorus (DRP) and ferrous iron (Fe2+) in sediment porewater in a mangrove tidal flat of the Jiulong River estuary, China. The concentrations of DRP and Fe2+, which ranged from 0.014 to 0.288mgL−1 and 0.003–0.619mgL−1, respectively, were highly heterogeneous across these profiles. The positive correlation between DRP and Fe2+ concentrations in most parts of the profiles verified the key role of Fe-redox cycling in controlling DRP variations. However, in some parts of the profiles, this coupling relationship was disturbed by the enrichment of mangrove organic matter or biological activity, such as that of macrobenthos and microorganisms. The recirculated seawater fluxes driven by tides and wave setups were calculated to be 23.15m3 m−1 d−1 and 1.49 × 10−3 m3 m−1 d−1, respectively. Based on these calculated fluxes and end-member concentrations, the net DRP flux into the ocean was calculated to be approximately 1.57mmolm−2 d−1, which is much larger than the molecular diffusion flux of DRP (1.79 × 10−3mmolm−2 d−1). These results indicate that the advection of recirculated seawater is a significant pathway to deliver DRP into the ocean and that this kind of internal phosphorus from mangrove tidal flats or muddy coastal zones should be considered in future nutrient budgets of estuaries or coastal oceans.

Effects of temperature on phosphorus mobilization in sediments in microcosm experiment and in the field

The release of phosphorus (P) from sediments may have significant influences on the concentration of water P and the trophic status of lakes. This study investigated the effects of temperature on the mobilization of P in sediments and its release to the overlying water in an incubation experiment and in the field of a shallow Lake Taihu (China). The ZrO-Chelex diffusive gradients in thin films (ZrO-Chelex DGT) and dialysis sampler (Peeper) were used to simultaneously measure labile P/Fe and soluble P/Fe in sediments at vertical resolutions of 2 mm and 4 mm, respectively. The results showed that the mean DGT-labile P and soluble reactive P (SRP) concentrations at 25 °C increased by 279% and 125% in comparison with those at 7 °C in the microcosm experiment, respectively, while in the field the proportions became 460% and 189% respectively. The microbial activity in sediments also increased with the rising temperature. Moreover, positive correlations (p < 0.001) existed between DGT-labile P and labile Fe and between SRP and soluble Fe under different temperature, implying that the mobilization of P and its release to the overlying water was dominantly controlled by Fe redox cycling. This study provided strong evidence that the rising temperature enhanced the microbial reduction of P-hosted Fe (oxyhydr)oxide, while an on-site accumulation and degradation of algal material in the sediment surface may lead to a greater effect on P mobilization.