Electrochemical Analysis of Changes in Iron Oxide Reducibility during Abiotic Ferrihydrite Transformation into Goethite and Magnetite

Dynamics of Fe, Mn, and potentially harmful elements under reducing conditions in iron ore tailings after dam collapse in the Doce River basin-Brazil: flooded and non-flooded rice cultivations

Potentially harmful element (PHE) bioavailability is important to environmental contamination and must be checked under several soil conditions. This study aimed to assess Fe, Mn, and PHE uptake by rice (Oryza sativa) grown on flooded and non-flooded Fe tailings collected from the Doce River basin after its collapse in Brazil. After 65 days of sowing, shoots and roots were harvested to determine PHE concentrations. The mean concentrations of Mn in shoots and Fe in the roots of rice grown on the flooded tailings were 2140 mg kg-1 and 15,219 mg kg-1, respectively. Mn was extensively translocated from roots to shoots (translocation factor (TF) = 2). Conversely, Fe accumulated in roots (TF = 0.015) and caused morphological damage to this rice organ. The application of macro and micronutrients lessened Fe toxicity in the roots of rice cultivated on the flooded tailings. The flooding of tailings influenced more Fe accumulation than Mn accumulation by rice plants. The PHE Ag, As, Cd, Ni, Hg, Pb, and Sb exhibited low total concentrations (maximum of 9 mg kg-1 for Ni and a minimum of 0.2 mg kg-1 for Cd, Hg, and Sb), and it was not observed an increase in their availability under tailings flooding conditions.

Iron Oxyhydroxide Transformation in a Flooded Rice Paddy Field and the Effect of Adsorbed Phosphate

The mobility and bioavailability of phosphate in paddy soils are closely coupled to redox-driven Fe-mineral dynamics. However, the role of phosphate during Fe-mineral dissolution and transformations in soils remains unclear. Here, we investigated the transformations of ferrihydrite and lepidocrocite and the effects of phosphate pre-adsorbed to ferrihydrite during a 16-week field incubation in a flooded sandy rice paddy soil in Thailand. For the deployment of the synthetic Fe-minerals in the soil, the minerals were contained in mesh bags either in pure form or after mixing with soil material. In the latter case, the Fe-minerals were labeled with 57Fe to allow the tracing of minerals in the soil matrix with 57Fe Mössbauer spectroscopy. Porewater geochemical conditions were monitored, and changes in the Fe-mineral composition were analyzed using 57Fe Mössbauer spectroscopy and/or X-ray diffraction analysis. Reductive dissolution of ferrihydrite and lepidocrocite played a minor role in the pure mineral mesh bags, while in the 57Fe-mineral-soil mixes more than half of the minerals was dissolved. The pure ferrihydrite was transformed largely to goethite (82-85%), while ferrihydrite mixed with soil only resulted in 32% of all remaining 57Fe present as goethite after 16 weeks. In contrast, lepidocrocite was only transformed to 12% goethite when not mixed with soil, but 31% of all remaining 57Fe was found in goethite when it was mixed with soil. Adsorbed phosphate strongly hindered ferrihydrite transformation to other minerals, regardless of whether it was mixed with soil. Our results clearly demonstrate the influence of the complex soil matrix on Fe-mineral transformations in soils under field conditions and how phosphate can impact Fe oxyhydroxide dynamics under Fe reducing soil conditions.

Mechanistic insight into Cr(VI) retention by Si-containing ferrihydrite

Hexavalent chromium [Cr(VI)] causes serious harm to the environment due to its high toxicity, solubility, and mobility. Ferrihydrites (Fh) are the main adsorbent and trapping agent of Cr(VI) in soils and aquifers, and they usually coexist with silicate (Si), forming Si-containing ferrihydrite (Si-Fh) mixtures. However, the mechanism of Cr(VI) retention by Si-Fh mixtures is poorly understood. In this study, the behaviors and mechanisms of Cr(VI) adsorption onto Si-Fh with different Si/Fe molar ratios was investigated. Transmission electron microscope, Fourier transform infrared spectroscopy, X-ray diffraction, X-ray photoelectron spectroscopy, and other techniques were used to characterize Si-Fh and Cr(VI)-loading of Si-Fh. The results show that specific surface area of Si-Fh increases gradually with increasing Si/Fe ratios, but Cr(VI) adsorption on Si-Fh decreases with increasing Si/Fe ratios. This is because with an increase in Si/Fe molar ratio, the point of zero charge of Si-Fh gradually decreases and electrostatic repulsion between Si-Fh and Cr(VI) increases. However, the complexation of Cr(VI) is enhanced due to the increase in adsorbed hydroxyl (A-OH-) on Si-Fh with increasing Si/Fe molar ratio, which partly counteracts the effect of the electrostatic repulsion. Overall, the increase in the electrostatic repulsion has a greater impact on adsorption than the additional complexation with Si-Fh. Density functional theory calculation further supports this observation, showing the increases in electron variation of bonding atoms and reaction energies of inner spherical complexes with the increase in Si/Fe ratio.

The Ferroxidase Centre of Escherichia coli Bacterioferritin Plays a Key Role in the Reductive Mobilisation of the Mineral Iron Core

Ferritins are multimeric cage-forming proteins that play a crucial role in cellular iron homeostasis. All H-chain-type ferritins harbour a diiron site, the ferroxidase centre, at the centre of a 4 α-helical bundle, but bacterioferritins are unique in also binding 12 hemes per 24 meric assembly. The ferroxidase centre is known to be required for the rapid oxidation of Fe2+ during deposition of an immobilised ferric mineral core within the protein's hollow interior. In contrast, the heme of bacterioferritin is required for the efficient reduction of the mineral core during iron release, but has little effect on the rate of either oxidation or mineralisation of iron. Thus, the current view is that these two cofactors function in iron uptake and release, respectively, with no functional overlap. However, rapid electron transfer between the heme and ferroxidase centre of bacterioferritin from Escherichia coli was recently demonstrated, suggesting that the two cofactors may be functionally connected. Here we report absorbance and (magnetic) circular dichroism spectroscopies, together with in vitro assays of iron-release kinetics, which demonstrate that the ferroxidase centre plays an important role in the reductive mobilisation of the bacterioferritin mineral core, which is dependent on the heme-ferroxidase centre electron transfer pathway.

Characteristics of iron (hydr)oxides and Cr(VI) retention mechanisms in soils from tropical and subtropical areas of China

Though both iron (hydr)oxides and soil organic matter (SOM) significantly influence heavy metal behaviors in soils, studies on the characteristics of natural minerals and the synergic effects of the two on Cr(VI) transformation are limited. This study investigated Cr(VI) retention mechanisms in four soils from tropical and subtropical regions of China based on a comprehensive characterization of Fe (hydr)oxides. These soils exhibited varying quantities of hematite, ferrihydrite and goethite, with distinct Al substitution levels and varied exposed crystallographic facets. Adsorption experiments revealed a positive correlation between Fe (hydr)oxide content and Cr(VI) fixation amount on colloid, which was influenced by the mineral types, Al substitution levels and facet exposures. Further, Cr(VI) was sequestered on soil by adsorption and reduction. In soils enriched with crystalline Fe (hydr)oxides, Cr(VI) reduction was primarily governed by SOM, while in soils enriched with poorly crystalline Fe (hydr)oxides, mineral-associated Fe(II) also contributed to Cr(VI) reduction. Aging experiments demonstrated that SOM and mineral-associated Fe(II) expedited Cr (VI) passivation and diminished the Cr leaching. These results improve our understanding of natural Fe (hydr)oxide structures and their impact on Cr(VI) behavior in soils, and shed light on complex soil-contaminant interactions and remediation of Cr(VI) polluted soils.

Influence of iron (hydr)oxide mineralogy and contents in aquifer sediments on dissolved organic carbon attenuations during aquifer storage and recovery

Aquifer storage and recovery (ASR) is a promising approach for managing water resources that enhances water quality through biogeochemical reactions occurring within aquifers. Iron (hydr)oxides, which are the predominant metallic oxides in soil, play a crucial role in degrading dissolved organic carbon (DOC), primarily through a process known as dissimilatory iron reduction (DIR). However, the efficiency of this reaction varies depending on the mineralogy and composition of the aquifer, and this understanding is essential for adequate water quality in ASR. The objective of this study is to investigate the impact of iron (hydr)oxide on acetate, as an organic carbon source, attenuation during the ASR. To achieve this, three sets of laboratory sediment columns were prepared, each containing a different type of iron (hydr)oxide minerals: ferrihydrite, goethite, and hematite. Following an acclimation period of 28 days to simulate the microcosm within an aquifer, the columns were continuously supplied with the simulated river water spiked with acetate (DOC 40-60 mg L-1), and the acetate concentration in the effluent was monitored. The result revealed that the column containing ferrihydrite achieved 97% acetate attenuation through DIR with anoxic conditions (DO < 0.1 mg L-1), while the goethite and hematite columns exhibited limited attenuation rates of 40 and 50%, respectively. Furthermore, the efficiency of acetate attenuation in the ferrihydrite columns increased with the content of ferrihydrite but experienced a rapidly declined at higher contents (3-4%), possibly due to the partial conversion of ferrihydrite to goethite as a result of the interaction between ferrihydrite and the Fe(II) produced during DIR. Additionally, an analysis of the microbial community demonstrated that microorganisms known to possess the ability to reduce iron (hydr)oxides under anaerobic conditions were abundant in the ferrihydrite columns.

Cooperative roles of phosphate and dissolved organic matter in inhibiting ferrihydrite transformation and their distinct fates

Phosphate and dissolved organic matter (DOM) mediate the crystalline transformation of ferrihydrite catalyzed by Fe(II) in subsurface environments such as soils and groundwater. However, the cooperative mechanisms underlying the mediation of phosphate and DOM in crystalline transformation of ferrihydrite and the feedback effects on their own distribution and speciation remain unresolved. In this study, solid characterization indicates that phosphate and DOM can collectively inhibit the crystalline transformation of ferrihydrite to lepidocrocite and thus goethite, via synergetic effects of inhibiting recrystallization and electron transfer. Phosphate can be retained on the surface or transformed to a nonextractable form within Fe oxyhydroxides; DOM is either released into the solution or preserved in an extractable form, while it is not incorporated or retained in the interior. Element distribution and DOM composition analysis on Fe oxyhydroxides reveals even distribution of phosphate on the newly formed Fe oxyhydroxides, while the distribution of DOM depends on its specific species. Electrochemical and dynamic force spectroscopic results provide molecular-scale thermodynamic evidence explaining the inhibition of electron transfer between Fe(II) and ferrihydrite by phosphate and DOM, thus influecing the crystalline transformation of ferrihydrite and the distribution of phosphate and DOM. This study provides new insights into the coupled biogeological cycle of Fe with phosphate and DOM in aquatic and terrestrial environments.

Contact with soil impacts ferrihydrite and lepidocrocite transformations during redox cycling in a paddy soil

Iron (Fe) oxyhydroxides can be reductively dissolved or transformed under Fe reducing conditions, affecting mineral crystallinity and the sorption capacity for other elements. However, the pathways and rates at which these processes occur under natural soil conditions are still poorly understood. Here, we studied Fe oxyhydroxide transformations during reduction-oxidation cycles by incubating mesh bags containing ferrihydrite or lepidocrocite in paddy soil mesocosms for up to 12 weeks. To investigate the influence of close contact with the soil matrix, mesh bags were either filled with pure Fe minerals or with soil mixed with 57Fe-labeled Fe minerals. Three cycles of flooding (3 weeks) and drainage (1 week) were applied to induce soil redox cycles. The Fe mineral composition was analyzed with Fe K-edge X-ray absorption fine structure spectroscopy, X-ray diffraction analysis and/or 57Fe Mössbauer spectroscopy. Ferrihydrite and lepidocrocite in mesh bags without soil transformed to magnetite and/or goethite, likely catalyzed by Fe(II) released to the pore water by microbial Fe reduction in the surrounding soil. In contrast, 57Fe-ferrihydrite in mineral-soil mixes transformed to a highly disordered mixed-valence Fe(II)-Fe(III) phase, suggesting hindered transformation to crystalline Fe minerals. The 57Fe-lepidocrocite transformed to goethite and small amounts of the highly disordered Fe phase. The extent of reductive dissolution of minerals in 57Fe-mineral-soil mixes during anoxic periods increased with every redox cycle, while ferrihydrite and lepidocrocite precipitated during oxic periods. The results demonstrate that the soil matrix strongly impacts Fe oxyhydroxide transformations when minerals are in close spatial association or direct contact with other soil components. This can lead to highly disordered and reactive Fe phases from ferrihydrite rather than crystalline mineral products and promoted goethite formation from lepidocrocite.

Compositional transformation of Ni2+ and Fe0 during the removal of Ni2+ by nanoscale zero-valent iron and the implications to groundwater remediation

The use of nanoscale zero-valent iron (nZVI) to remove heavy metal ions like Ni2+ from groundwater has been extensively studied; however, the compositional transformation of the Ni2+ and Fe0 during the removal is not clearly comprehensible. This study provides an insight into the componential, structural, and morphological transformations of Ni2+ and Fe0 at a solid-liquid interface using various characterization devices. The underlying mechanism of transformation was investigated along with the toxicity/impact of the transformed products on the groundwater ecosystem. The results indicated that Fe0 is transformed into lath-like lepidocrocite (γ-FeOOH), twin-crystal goethite (α-FeOOH), and spherical magnetite (Fe3O4), while Ni2+ is converted into Fe0.7Ni0.3 alloy and Fe-Ni composite (trevorite - NiFe2O4) with a fold-fan morphology. The Fe0 transformation mechanism includes the redox of Fe0 with Ni2+, H2O, and dissolved oxygen, the combination of Fe2+ and OH- produced by Fe0 corrosion to amorphous ferrihydrite, and the further mineralogical transformation to Fe oxides with the aid of Fe2+ adsorbed on ferrihydrite. The conversion of Ni2+ is accomplished by reduction by Fe0 and surface coordination with Fe oxides. Compared with Ni2+ and Fe0, the toxicity and bioavailability of the transformed products are significantly reduced, hence conducive to the application of zero-valent iron technology in groundwater remediation.

Arsenic transformation and redistribution in groundwater induced by the complex geochemical cycling of iron and sulfur

Iron (hydr)oxides are effective sorbents of arsenic that undergo reductive dissolution when exposed to dissolved sulfide, which significantly impacts the movement and repartition of arsenic in groundwater. This study investigated the sulfidation of As-bearing ferrihydrite and its consequences on arsenic repartitioning as well as formation and transformation of secondary minerals induced by sulfide in batch experiments. The sulfidation of As(III) and As(V) adsorbed on ferrihydrite shows very different results. In the As(V) system, sulfidation resulted in the production of significant amounts of elemental sulfur (S0) and Fe2+, and Fe2+ and sulfide combine to form mackinawite. Subsequently, Fe2+ adsorbed and catalyzed the conversion of residual ferrihydrite to lepidocrocite. However, in the As(III) system, As(III) was protonated in the presence of sulfide to produce thioarsenate, which accounted for 87.9 % of the total aqueous arsenic concentration. The formation of thioarsenate also consumed the S0 produced by the sulfidation, resulting in no detectable S0 during solid phase characterization. The adsorption of thioarsenate on iron minerals notably affected the surface charge density of ferrihydrite, hindering the further formation of secondary minerals. Studies on the influence of thiolation on As-Fe-S system are of great significance for understanding the migration and redistribution of arsenic in groundwater systems under sulfur-rich conditions.

Linear Free Energy Relationship for Predicting the Rate Constants of Munition Compound Reduction by the Fe(II)-Hematite and Fe(II)-Goethite Redox Couples

Abiotic reduction by iron minerals is arguably the most important fate process for munition compounds (MCs) in subsurface environments. No model currently exists that can predict the abiotic reduction rates of structurally diverse MCs by iron (oxyhydr)oxides. We performed batch experiments to measure the rate constants for the reduction of three classes of MCs (poly-nitroaromatics, nitramines, and azoles) by hematite or goethite in the presence of aqueous Fe2+. The surface area-normalized reduction rate constant (kSA) depended on the aqueous-phase one-electron reduction potential (EH1) of the MC and the thermodynamic state (i.e., pe and pH) of the iron oxide-Feaq2+ system. A linear free energy relationship (LFER), similar to that reported previously for nitrobenzene, successfully captures all MC reduction rate constants that span 6 orders of magnitude: log ( k S A ) = ( 1.12 ± 0.04 ) [ 0.53 E H 1 59 m V - ( p H + p e ) ] + ( 5.52 ± 0.23 ) . The finding that the rate constants of all the different classes of MCs can be described by a single LFER suggests that these structurally diverse nitro compounds are reduced by iron oxide-Feaq2+ couples through a common mechanism up to the rate-limiting step. Multiple mechanistic implications of the results are discussed. This study expands the applicability of the LFER model for predicting the reduction rates of legacy and emerging MCs and potentially other nitro compounds.

Electron Transfer, Atom Exchange, and Transformation of Iron Minerals in Soils: The Influence of Soil Organic Matter

Despite substantial experimental evidence of electron transfer, atom exchange, and mineralogical transformation during the reaction of Fe(II)_aq with synthetic Fe(III) minerals, these processes are rarely investigated in natural soils. Here, we used an enriched Fe isotope approach and Mössbauer spectroscopy to evaluate how soil organic matter (OM) influences Fe(II)/Fe(III) electron transfer and atom exchange in surface soils collected from Luquillo and Calhoun Experimental Forests and how this reaction might affect Fe mineral composition. Following the reaction of ⁵⁷Fe-enriched Fe(II)_aq with soils for 33 days, Mössbauer spectra demonstrated marked electron transfer between sorbed Fe(II) and the underlying Fe(III) oxides in soils. Comparing the untreated and OM-removed soils indicates that soil OM largely attenuated Fe(II)/Fe(III) electron transfer in goethite, whereas electron transfer to ferrihydrite was unaffected. Soil OM also reduced the extent of Fe atom exchange. Following reaction with Fe(II)_aq for 33 days, no measurable mineralogical changes were found for the Calhoun soils enriched with high-crystallinity goethite, while Fe(II) did drive an increase in Fe oxide crystallinity in OM-removed LCZO soils having low-crystallinity ferrihydrite and goethite. However, the presence of soil OM largely inhibited Fe(II)-catalyzed increases in Fe mineral crystallinity in the LCZO soil. Fe atom exchange appears to be commonplace in soils exposed to anoxic conditions, but its resulting Fe(II)-induced recrystallization and mineral transformation depend strongly on soil OM content and the existing soil Fe phases.

Controlling product selectivity in oxidative desulfurization using an electrodeposited iron oxide film

Sulfur-containing compounds must be removed from raw fuel oils before use and recently, there has been an effort to identify and optimize a more energy efficient method of oil processing. One promising route is electrochemical oxidative desulfurization (ODS), and in this work, we investigate an electrodeposited iron oxide film (FeO_x(OH)_y) as a working electrode to catalyze the oxidation of dibenzothiophene (DBT). The FeO_x(OH)_y film displays unexpected selectivity for the DBT sulfoxide (DBTO)-departing from the catalytic behavior of gold, which favors the dimerization of DBT. In addition, we observe a morphological change within our FeO_x(OH)_y film from γ-FeOOH to γ-Fe₂O₃. This change provides insight to the activity of each structure for ODS as the rate of oxidation increases after the incorporation of γ-Fe₂O₃. Our experimental observations are corroborated with DFT calculations, which suggest that the adsorption energy of DBT on Au is significantly greater than on the FeO_x(OH)_y, favoring the formation of dimeric and oligomeric products. Calculations also demonstrate that DBT binds preferably in a monodentate configuration but that oxidation occurs via DBT bound via a bidentate configuration. Monodentate binding on γ-FeOOH is significantly stronger than binding on γ-Fe₂O, resulting in easier conversation to bidentate binding on γ-Fe₂O₃.

A New Approach for Investigating Iron Mineral Transformations in Soils and Sediments Using 57Fe-Labeled Minerals and 57Fe Mössbauer Spectroscopy

Iron minerals in soils and sediments play important roles in many biogeochemical processes and therefore influence the cycling of major and trace elements and the fate of pollutants in the environment. However, the kinetics and pathways of Fe mineral recrystallization and transformation processes under environmentally relevant conditions are still elusive. Here, we present a novel approach enabling us to follow the transformations of Fe minerals added to soils or sediments in close spatial association with complex solid matrices including other minerals, organic matter, and microorganisms. Minerals enriched with the stable isotope ⁵⁷Fe are mixed with soil or sediment, and changes in Fe speciation are subsequently studied by ⁵⁷Fe Mössbauer spectroscopy, which exclusively detects ⁵⁷Fe. In this study, ⁵⁷Fe-labeled ferrihydrite was synthesized, mixed with four soils differing in chemical and physical properties, and incubated for 12+ weeks under anoxic conditions. Our results reveal that the formation of crystalline Fe(III)(oxyhydr)oxides such as lepidocrocite and goethite was strongly suppressed, and instead formation of a green rust-like phase was observed in all soils. These results contrast those from Fe(II)-catalyzed ferrihydrite transformation experiments, where formation of lepidocrocite, goethite, and/or magnetite often occurs. The presented approach allows control over the composition and crystallinity of the initial Fe mineral, and it can be easily adapted to other experimental setups or Fe minerals. It thus offers great potential for future investigations of Fe mineral transformations in situ under environmentally relevant conditions, in both the laboratory and the field.

Cooperative effect of slow-release ferrous and phosphate for simultaneous stabilization of As, Cd and Pb in soil

The different chemical behavior of anionic As and cationic Cd and Pb makes the simultaneous stabilization of soils contaminated with arsenic (As), cadmium (Cd), and lead (Pb) challenging. The use of soluble, insoluble phosphate materials and iron compounds cannot simultaneously stabilize As, Cd, and Pb in soil effectively due to the easy re-activation of heavy metals and poor migration. Herein, we propose a new strategy of "cooperatively stabilizing Cd, Pb, and As with slow-release ferrous and phosphate". To very this theory, we developed ferrous and phosphate slow-release materials to simultaneously stabilize As, Cd, and Pb in soil. The stabilization efficiency of water-soluble As, Cd and Pb reached 99% within 7d, and the stabilization efficiencies of NaHCO₃-extractable As, DTPA-extractable Cd and Pb reached 92.60%, 57.79% and 62.81%, respectively. The chemical speciation analysis revealed that soil As, Cd and Pb were transformed into more stable states with the reaction time. The proportion of residual fraction of As, Cd, and Pb increased from 58.01% to 93.82%, 25.69 to 47.86%, 5.58 to 48.54% after 56 d, respectively. Using ferrihydrite as a representative soil component, the beneficial interactions of phosphate and slow-release ferrous material in stabilizing Pb, Cd, and As were demonstrated. The slow-release ferrous and phosphate material reacted with As and Cd/Pb to form stable ferrous arsenic and Cd/Pb phosphate. Furthermore, the slow-release phosphate converted the adsorbed As into dissolved As, then the dissolved As reacted with released ferrous to form a more stable form. Concurrently, As, Cd and Pb were structurally incorporated into the crystalline iron oxides during the ferrous ions-catalyzed transformation of amorphous iron (hydrogen) oxides. The results demonstrates that the use of slow-release ferrous and phosphate materials can aid in the simultaneous stabilization of As, Cd, and Pb in soil.

Fe(II)-Catalyzed Transformation of Ferrihydrite with Different Degrees of Crystallinity

Natural occurring ferrihydrite (Fh) nanoparticles have varying degrees of crystallinity, but how Fh crystallinity affects its transformation behavior remains elusive. Here, we investigated the Fe(II)-catalyzed transformation of Fh with different degrees of crystallinity (i.e., Fh-2h, Fh-12h, and Fh-85C). X-ray diffraction patterns of Fh-2h, Fh-12h, and Fh-85C exhibited two, five, and six diffraction peaks, respectively, indicating the order of crystallinity: Fh-2h < Fh-12h < Fh-85C. Fh with the lower crystallinity has a higher redox potential, corresponding to the faster Fe(II)-Fh interfacial electron transfer and Fe(III)_labile production. With the increase of initial Fe(II) concentration ([Fe(II)_aq]_int.) from 0.2 to 5.0 mM, the transformation pathways of Fh-2h and Fh-12h change from Fh → lepidocrocite (Lp) → goethite (Gt) to Fh → Gt, but that of Fh-85C switches from Fh → Gt to Fh → magnetite (Mt). The changes are rationalized using a computational model that quantitatively describes the relationship between the free energies of formation for starting Fh and nucleation barriers of competing product phases. Gt particles from the Fh-2h transformation exhibit a broader width distribution than those from Fh-12h and Fh-85C. Uncommon hexagonal Mt nanoplates are formed from the Fh-85C transformation at [Fe(II)_aq]_int.= 5.0 mM. The findings are crucial to comprehensively understand the environmental behavior of Fh and other associated elements.

Electron exchange capacity of pyrogenic dissolved organic matter (pyDOM): complementarity of square-wave voltammetry in DMSO and mediated chronoamperometry in water

Pyrogenic dissolved organic matter (pyDOM) is derived from black carbon, which is important in the global carbon cycle and other biogeochemical redox processes. The electron-exchange capacity (EEC) of pyDOM has been characterized in water using mediated chronoamperometry (MCA), which gives precise results under specific operational conditions, but the broader significance of these EECs is less clear. In this study, we described a novel but complementary electrochemical approach to quantify EECs of pyDOM without mediation using square-wave voltammetry (SWV) in dimethyl sulfoxide (DMSO). Using both the SWV and MCA methods, we determined EECs for 10 pyDOMs, 6 natural organic matter (NOM) samples, and 2 model quinones. The two methods gave similar EECs for model quinones, but SWV gave larger EECs than MCA for NOM and pyDOM (by several-fold and 1-2 orders of magnitude, respectively). The differences in the EECs obtained by SWV and MCA likely are due to multiple factors, including the potential range of electrons sampled, kinetics of electron transfer from (macro)molecular structures, and coupling of electron and proton transfer steps. Comparison of the results obtained by these two methods should provide new insights into important environmental processes such as carbon-cycling, wildfire recovery, and contaminant mitigation using carbon-based amendments.

Mechanistic insights into the detoxification of Cr(VI) and immobilization of Cr and C during the biotransformation of ferrihydrite-polygalacturonic acid-Cr coprecipitates

Coupled reactions among chromium (Cr), organic matter (OM), and iron (Fe) minerals play significant roles in Cr and carbon (C) cycling in Cr-contaminated soils. Although the inhibitory effects of Cr or polysaccharides acid (PGA) on ferrihydrite transformation have been widely studied, mechanistic insights into detoxification of Cr(VI) and immobilization of Cr and C during the microbially mediated reductive transformation of ferrihydrite remain unclear. In this study, underlying sequestration mechanisms of Cr and C during dissimilatory Fe reduction at various Cr/Fe ratios were investigated. Solid-phase analysis showed that reductive transformation rates of ferrihydrite were impeded by high Cr/Fe ratio and more magnetite was found at low Cr loadings. Microscopic analysis showed that formed Cr(III) was immobilized by magnetite and goethite through isomorphous substitution, whereas PGA was adsorbed on the crystalline Fe mineral surface. Spectroscopic results uncovered that binding of Fe minerals and PGA was achieved by surface complexation of structural Fe with carboxyl functional groups, and that the adhesion order of PGA functional groups and Fe minerals was influenced by the Cr/Fe ratios. These findings have significant implications for remediating Cr contaminants, realizing C fixation, and developing a quantitative model for Cr and C cycling by coupling reductive transformation in Cr-contaminated environments.

Photoinduced transformation of ferrihydrite in the presence of aqueous sulfite and its influence on the repartitioning of Cd

The photoinduced transformation of ferrihydrite is an important process that can predict the geochemical cycle of Fe in anoxic environments as well as the fate of trace elements bonded to Fe minerals. We report that the photooxidation of sulfite by UV irradiation produces hydrated electrons (super-reductants), which significantly promote ferrihydrite reduction to Fe(II), and SO₃^•- (a moderate oxidant), enabling its further oxidation to more crystalline Fe(III) products. The experimental results show that the concentration of sulfite was key in influencing the rate and extent of surface-bound Fe(II) formation, which ultimately determined the distribution of individual products. For example, fitting of the Mössbauer spectroscopy data revealed that the relative abundances of mineral species after 8 h of treatment in the UV/sulfite systems were 41.9% lepidocrocite and 58.1% ferrihydrite at 2 mM SO₃^2-; 41.8% goethite, 28.2% lepidocrocite, and 29.1% ferrihydrite at 5 mM SO₃^2-; and 100% goethite at 10 mM SO₃^2-. The combined results of the chemical speciation analysis and the Cd K-edge EXAFS characterization provided compelling evidence that Cd was firmly incorporated into the structure of newly formed minerals, particularly at high sulfite concentrations. These findings provide an understanding of the role of UV/sulfite in facilitating ferrihydrite transformation and promoting Cd stabilization in oxygen-deficit soils and aquatic environments.

Redox Potentials of Magnetite Suspensions under Reducing Conditions

Predicting the redox behavior of magnetite in reducing soils and sediments is challenging because there is neither agreement among measured potentials nor consensus on which Fe(III) | Fe(II) equilibria are most relevant. Here, we measured open-circuit potentials of stoichiometric magnetite equilibrated over a range of solution conditions. Notably, electron transfer mediators were not necessary to reach equilibrium. For conditions where ferrous hydroxide precipitation was limited, Nernstian behavior was observed with an E_H vs pH slope of -179 ± 4 mV and an E_H vs Fe(II)_aq slope of -54 ± 4 mV. Our estimated E_H^o of 857 ± 8 mV closely matches a maghemite|aqueous Fe(II) E_H^o of 855 mV, suggesting that it plays a dominant role in poising the solution potential and that it's theoretical Nernst equation of E_H[mV] = 855 - 177 pH - 59 log [Fe²⁺] may be useful in predicting magnetite redox behavior under these conditions. At higher pH values and without added Fe(II), a distinct shift in potentials was observed, indicating that the dominant Fe(III)|Fe(II) couple(s) poising the potential changed. Our findings, coupled with previous Mössbauer spectroscopy and kinetic data, provide compelling evidence that the maghemite/Fe(II)_aq couple accurately predicts the redox behavior of stoichiometric magnetite suspensions in the presence of aqueous Fe(II) between pH values of 6.5 and 8.5.

Oxyanion Surface Complexes Control the Kinetics and Pathway of Ferrihydrite Transformation to Goethite and Hematite

The rate and pathway of ferrihydrite (Fh) transformation at oxic conditions to more stable products is controlled largely by temperature, pH, and the presence of other ions in the system such as nitrate (NO₃^-), sulfate (SO₄^2-), and arsenate (AsO₄^3-). Although the mechanism of Fh transformation and oxyanion complexation have been separately studied, the effect of surface complex type and strength on the rate and pathway remains only partly understood. We have developed a kinetic model that describes the effects of surface complex type and strength on Fh transformation to goethite (Gt) and hematite (Hm). Two sets of oxyanion-adsorbed Fh samples were prepared, nonbuffered and buffered, aged at 70 ± 1.5 °C, and then characterized using synchrotron X-ray scattering methods and wet chemical analysis. Kinetic modeling showed a significant decrease in the rate of Fh transformation for oxyanion surface complexes dominated by strong inner-sphere (SO₄^2- and AsO₄^3-) versus weak outer-sphere (NO₃^-) bonding and the control. The results also showed that the Fh transformation pathway is influenced by the type of surface complex such that with increasing strength of bonding, a smaller fraction of Gt forms compared with Hm. These findings are important for understanding and predicting the role of Fh in controlling the transport and fate of metal and metalloid oxyanions in natural and applied systems.

Reduction capacity of humic acid and its association with the evolution of redox structures during composting

The reducing capacity (RC) of compost-derived humic acid (HA) is related to the type and number of redox-active functional moieties in its structure and has a considerable environmental influence on its geochemical redox cycle. Composting treatment can affect the redox-active fractions of organic substances through microbial transformation and degradation. However, the relationship between the RC of compost-derived HA and its fluorescence component and infrared spectra remains unclear. In this study, we assessed the response of the organic reducing capacity (ORC) and inorganic reducing capacity (IRC) of compost-derived HA to the stabilization of organic solid waste materials by analyzing the redox-active functional groups of HA extracted at different composting times. The results demonstrated that the RC of compost-derived HA continuously increased during composting because of the formation of fulvic- and humic-like fluorescent components, which consist of amide, phenolic hydroxyl, quinone, and aromatic groups. Adsorption occurred between HA and FeCit by aliphatic and out-of-plane aromatic CH, which released free hydrogen and increased the Fe-binding site; consequently, ORC was obviously higher than IRC. The results of this study could provide an understanding of the transformation of the fluorescent substances and functional groups that affect redox properties during composting; therefore, this study has considerable significance for exploring the application of compost products.

Sunlight-Induced Interfacial Electron Transfer of Ferrihydrite under Oxic Conditions: Mineral Transformation and Redox Active Species Production

Fe(II)-catalyzed ferrihydrite transformation under anoxic conditions has been intensively studied, while such mechanisms are insufficient to be applied in oxic environments with depleted Fe(II). Here, we investigated expanded pathways of sunlight-driven ferrihydrite transformation in the presence of dissolved oxygen, without initial addition of dissolved Fe(II). We found that sunlight significantly facilitated the transformation of ferrihydrite to goethite compared to that under dark conditions. Redox active species (hole-electron pairs, reactive radicals, and Fe(II)) were produced from the ferrihydrite interface via the photoinduced electron transfer processes. Experiments with systematically varied wet chemistry conditions probed the relative contributions of three pathways for the production of hydroxyl radicals: (1) oxidation of water (5.0%); (2) reduction of dissolved oxygen (40.9%); and (3) photolysis of Fe(III)-hydroxyl complexes (54.1%). Results also showed superoxide radicals as the main oxidant for Fe(II) reoxidation under acidic conditions, thus promoting the ferrihydrite transformation. The presence of inorganic ions (chloride, sulfate, and nitrate) did not only affect the hydrolysis and precipitation of Fe(III) but also the generation of radicals via photoinduced charge transfer reactions. The involvement of redox active species and the accompanying mineral transformations would exert a profound effect on the fate of multivalent elements and organic contaminants in aquatic environments.

Coexisting Goethite Promotes Fe(II)-Catalyzed Transformation of Ferrihydrite to Goethite

In redox-affected soil environments, electron transfer between aqueous Fe(II) and solid-phase Fe(III) catalyzes mineral transformation and recrystallization processes. While these processes have been studied extensively as independent systems, the coexistence of iron minerals is common in nature. Yet it remains unclear how coexisting goethite influences ferrihydrite transformation. Here, we reacted ferrihydrite and goethite mixtures with Fe(II) for 24 h. Our results demonstrate that with more goethite initially present in the mixture more ferrihydrite turned into goethite. We further used stable Fe isotopes to label different Fe pools and probed ferrihydrite transformation in the presence of goethite using 57Fe Mössbauer spectroscopy and changes in the isotopic composition of solid and aqueous phases. When ferrihydrite alone underwent Fe(II)-catalyzed transformation, Fe atoms initially in the aqueous phase mostly formed lepidocrocite, while those from ferrihydrite mostly formed goethite. When goethite was initially present, more goethite was formed from atoms initially in the aqueous phase, and nanogoethite formed from atoms initially in ferrihydrite. Our results suggest that coexisting goethite promotes formation of more goethite via Fe(II)-goethite electron transfer and template-directed nucleation and growth. We further hypothesize that electron transfer onto goethite followed by electron hopping onto ferrihydrite is another possible pathway to goethite formation. Our findings demonstrate that mineral transformation is strongly influenced by the composition of soil solid phases.

Novel Insights into the Impact of Nano-Biochar on Composition and Structural Transformation of Mineral/Nano-Biochar Heteroaggregates in the Presence of Root Exudates

Multiple lines of existing evidence indicate that natural organic matter (NOM) could protect poorly crystalline Fe(III) (oxyhydr)oxides from Fe(II)-catalyzed mineral transformation. Conversely, we find that nano-sized biochar (nano-BC), a pyrogenic form of NOM, promotes the phase transformation of ferrihydrite (Fh) in nano-BC/Fh heteroaggregates in the presence of aqueous Fe(II) and rice root exudates. The nano-BC/Fh heteroaggregates are composed of a core-shell like structure where the inner-layered nano-BC is more compacted and plays the dominant role in accelerating the phase transformation of Fh relative to that in the outer sphere. The extent of phase transformation is more regulated by the reversible redox reactions between quinone and hydroquinone in nano-BC than the electron transfer via its condensed aromatic structures. Furthermore, the reductive organic acids in root exudates contribute to the mineral transformation of nano-BC/Fh associations by donating electrons to Fe(III) through nano-BC. Our results suggest that heteroaggregates between nano-BC and Fe minerals are subjected to partial dissociation during their co-transport, and the stably attached nano-BC is favorable to the phase transformation of poorly crystalline Fe minerals (e.g., Fh), which might have profound implications on biogeochemical cycles of carbon and Fe in the prevailing redox environments.

Cu Nanoparticle-Decorated Boron-Carbon-Nitrogen Nanosheets for Electrochemical Determination of Chloramphenicol

In the present work, irregular Cu nanoparticle-decorated boron-carbon-nitrogen (Cu-BCN) nanosheets were successfully synthesized. A Cu-BCN dispersion was deposited on a bare glassy carbon electrode (GCE) to prepare an electrochemical sensor (Cu-BCN/GCE) for the detection of chloramphenicol (CAP) in the environment. Cu-BCN was characterized using high-resolution scanning transmission electron microscopy (HRSTEM), scanning electron microscopy (SEM), X-ray diffraction (XRD) analysis, and X-ray photoelectron spectroscopy (XPS). The performance of the Cu-BCN/GCE was studied using electrochemical impedance spectroscopy (EIS), and its advantages were proven by electrode comparison. Differential pulse voltammetry (DPV) was used to optimize the experimental conditions, including the amount of Cu-BCN deposited, enrichment potential, deposition time, and pH of the electrolyte. A linear relationship between the CAP concentration and current response was obtained under the optimized experimental conditions, with a wide linear range and a limit of detection (LOD) of 2.41 nmol/L. Cu-BCN/GCE exhibited high stability, reproducibility, and repeatability. In the presence of various organic and inorganic species, the influence of the Cu-BCN-based sensor on the current response of CAP was less than 5%. Notably, the prepared sensor exhibited excellent performance in real-water samples, with satisfactory recovery.

Electroactive Fe-biochar for redox-related remediation of arsenic and chromium: Distinct redox nature with varying iron/carbon speciation

Electroactive Fe-biochar has attracted significant attention for As(III)/Cr(VI) immobilization through redox reactions, and its performance essentially lies in the regulation of various Fe/C moieties for desired redox performance. Here, a series of Fe-biochar with distinct Fe/C speciation were rationally produced via two-step pyrolysis of iron minerals and biomass waste at 400-850 °C (BCX-Fe-Y, X and Y represented the first- and second-step pyrolysis temperature, respectively). The redox transformation of Cr(VI) and As(III) by Fe-biochar was evaluated in simulated wastewater under oxic or anoxic conditions. Results showed that more effective Cr(VI) reduction could be achieved by BCX-Fe-400, while a higher amount of As (III) was oxidized by BCX-Fe-850 under the anoxic environment. Besides, BCX-Fe-400 could generate more reactive oxygen species (e.g.,^•OH) by reducing the O₂, which enhanced the redox-related transformation of pollutants under the oxic situation. The evolving redox performance of Fe-biochar was governed by the transition of the redox state from reductive to oxidative related to the Fe/C speciation. The small-sized amorphous/low-crystalline ferrous minerals contributed to a higher electron-donating capacity (0.43-1.28 mmol g^-1) of BCX-Fe-400. In contrast, the oxidative surface oxygen-functionalities (i.e., carboxyl and quinoid) on BCX-Fe-850 endowed a stronger electron-accepting capacity (0.71-1.39 mmol g^-1). Moreover, the graphitic crystallites with edge-type defects and porous structure facilitated the electron transfer, leading to a higher electron efficiency of BCX-Fe-850. Overall, we unveiled the roles of both Fe and C speciation in maneuvering the redox reactivity of Fe-biochar, which can advance our rational design of electroactive Fe-biochar for redox-related environmental remediation.

Stabilization of Ferrihydrite and Lepidocrocite by Silicate during Fe(II)-Catalyzed Mineral Transformation: Impact on Particle Morphology and Silicate Distribution

Interactions between aqueous ferrous iron (Fe(II)) and secondary Fe oxyhydroxides catalyze mineral recrystallization and/or transformation processes in anoxic soils and sediments, where oxyanions, such as silicate, are abundant. However, the effect and the fate of silicate during Fe mineral recrystallization and transformation are not entirely understood and especially remain unclear for lepidocrocite. In this study, we reacted (Si-)ferrihydrite (Si/Fe = 0, 0.05, and 0.18) and (Si-)lepidocrocite (Si/Fe = 0 and 0.08) with isotopically labeled ⁵⁷Fe(II) (Fe(II)/Fe(III) = 0.02 and 0.2) at pH 7 for up to 4 weeks. We followed Fe mineral transformations with X-ray diffraction and tracked Fe atom exchange by measuring aqueous and solid phase Fe isotope fractions. Our results show that the extent of ferrihydrite transformation in the presence of Fe(II) was strongly influenced by the solid phase Si/Fe ratio, while increasing the Fe(II)/Fe(III) ratio (from 0.02 to 0.2) had only a minor effect. The presence of silicate increased the thickness of newly formed lepidocrocite crystallites, and elemental distribution maps of Fe(II)-reacted Si-ferrihydrites revealed that much more Si was associated with the remaining ferrihydrite than with the newly formed lepidocrocite. Pure lepidocrocite underwent recrystallization in the low Fe(II) treatment and transformed to magnetite at the high Fe(II)/Fe(III) ratio. Adsorbed silicate inactivated the lepidocrocite surfaces, which strongly reduced Fe atom exchange and inhibited mineral transformation. Collectively, the results of this study demonstrate that Fe(II)-catalyzed Si-ferrihydrite transformation leads to the redistribution of silicate in the solid phase and the formation of thicker lepidocrocite platelets, while lepidocrocite transformation can be completely inhibited by adsorbed silicate. Therefore, silicate is an important factor to include when considering Fe mineral dynamics in soils under reducing conditions.

Understanding the Importance of Labile Fe(III) during Fe(II)-Catalyzed Transformation of Metastable Iron Oxyhydroxides

Transformation of metastable Fe(III) oxyhydroxides is a prominent process in natural environments and can be significantly accelerated by the coexisting aqueous Fe(II) (Fe(II)_aq). Recent evidence points to the solution mass transfer of labile Fe(III) (Fe(III)_labile) as the primary intermediate species of general importance. However, a mechanistic aspect that remains unclear is the dependence of phase outcomes on the identity of the metastable Fe(III) oxyhydroxide precursor. Here, we compared the coupled evolution of Fe(II) species, solid phases, and Fe(III)_labile throughout the Fe(II)-catalyzed transformation of lepidocrocite (Lp) versus ferrihydrite (Fh) at equal Fe(III) mass loadings with 0.2-1.0 mM Fe(II)_aq at pH = 7.0. Similar to Fh, the conversion of Lp to product phases occurs by a dissolution-reprecipitation mechanism mediated by Fe(III)_labile that seeds the nucleation of products. Though for Fh we observed a transformation to goethite (Gt), accompanied by the transient emergence and decline of Lp, for initial Lp we observed magnetite (Mt) as the main product. A linear correlation between the formation rate of Mt and the effective supersaturation in terms of Fe(III)_labile concentration shows that Fe(II)-induced transformation of Lp into Mt is governed by the classical nucleation theory. When Lp is replaced by equimolar Gt, Mt formation is suppressed by opening a lower barrier pathway to Gt by heterogeneous nucleation and growth on the added Gt seeds. The collective findings add to the mechanistic understanding of factors governing phase selections that impact iron bioavailability, system redox potential, and the fate and transport of coupled elements.

Enhanced arsenic removal using lateritic bauxite modified by heating and blending

The use of bauxite to remove arsenic from wastewater has been confirmed to be effective, but its removal efficiency in literature is not up to 90%, hence the need to devise a more effective method to remove arsenic from wastewater. In this study, a novel material was prepared by thermally modifying and blending yellow and red lateritic bauxite to form thermally modified and blended yellow and red lateritic bauxite (TYB + TRB). The adsorption isotherm, morphology and chemical composition of the novel material were determined by the Langmuir and Freundlich models, Scanning Electron Microscopy (SEM) and X-ray Fluorescence (XRF), respectively. Application of the novel material in arsenic removal resulted in >97% removal efficiency within 60 min. The arsenic adsorption by TYB + TRB conformed to the Freundlich model. The SEM image depicted a compacted earth material after use of the TYB + TRB to remove arsenic from the wastewater. The XRF results also showed a drastic reduction in the chemical composition of the novel bauxite except Ti, thus suggesting the occurrence of multiple mechanisms during the arsenic removal. This study demonstrated the potential of TYB + TRB to be developed and used as the most suitable material for wastewater treatment.

Direct and Indirect Electron Transfer Routes of Chromium(VI) Reduction with Different Crystalline Ferric Oxyhydroxides in the Presence of Pyrogenic Carbon

Electron transfer mediated by iron minerals is considered as a critical redox step for the dynamics of pollutants in soil. Herein, we explored the reduction process of Cr(VI) with different crystalline ferric oxyhydroxides in the presence of pyrogenic carbon (biochar). Both low- and high-crystallinity ferric oxyhydroxides induced Cr(VI) immobilization mainly via the sorption process, with a limited reduction process. However, the Cr(VI) reduction immobilization was inspired by the copresence of biochar. Low-crystallinity ferric oxyhydroxide had an intense chemical combination with biochar and strong sorption for Cr(VI) via inner-sphere complexation, leading to the indirect electron transfer route for Cr(VI) reduction, that is, the electron first transferred from biochar to iron mineral through C-O-Fe binding and then to Cr(VI) with Fe(III)/Fe(II) transformation on ferric oxyhydroxides. With increasing crystallinity of ferric oxyhydroxides, the direct electron transfer between biochar and Cr(VI) became the main electron transfer avenue for Cr(VI) reduction. The indirect electron transfer was suppressed in the high-crystallinity ferric oxyhydroxides due to less sorption of Cr(VI), limited combination with biochar, and higher iron stability. This study demonstrates that electron transfer mechanisms involving iron minerals change with the mineral crystallization process, which would affect the geochemical process of contaminants with pyrogenic carbon.

Enhanced Microbial Ferrihydrite Reduction by Pyrogenic Carbon: Impact of Graphitic Structures

Electron-shuttling agents such as pyrogenic carbon (PC) can mediate long-distance electron transfer and play numerous key roles in aquatic and soil biogeochemical processes. The electron-shuttling capacity of PC relies on both the surface oxygen-containing functional groups and bulk graphitic structures. Although the impacts of oxygen-containing functional groups on the electron-shuttling performance of PC are well studied, there remains insufficient understanding on the function of graphitic structures. Here, we studied the functions of PC in mediating microbial (Shewanella oneidensis MR-1) reduction of ferrihydrite, a classic and geochemically important soil redox process. The results show that PC enhanced microbial ferrihydrite reduction by 20-115% and the reduction rates increased with PC pyrolysis temperature increasing from 500 to 900 °C. For PC prepared at low temperature (500-600 °C), the electron-shuttling capacity of PC is mainly attributed to its oxygen-containing functional groups, as indicated by a 50-60% decline in the ferrihydrite reduction rate when PC was reduced under a H₂ atmosphere to remove surface oxygen-containing functional groups. In stark contrast, for PC prepared at higher temperature (700-900 °C), the formation of PC graphitic structures was enhanced, as suggested by the higher electrical conductivity; accordingly, the graphitic structure exhibits greater importance in shuttling electrons, as demonstrated by a minor decline (10-18%) in the ferrihydrite reduction rate after H₂ treatment of PC. This study provides new insights into the nonlinear and combined role of graphitic structures and oxygen-containing functional groups of PC in mediating electron transfer, where the pyrolysis temperature of PC acts as a key factor in determining the electron-shuttling pathways.

Organic Matter from Redoximorphic Soils Accelerates and Sustains Microbial Fe(III) Reduction

Microbial reduction of Fe(III) minerals is a prominent process in redoximorphic soils and is strongly affected by organic matter (OM). We herein determined the rate and extent of microbial reduction of ferrihydrite (Fh) with either adsorbed or coprecipitated OM by Geobacter sulfurreducens. We focused on OM-mediated effects on electron uptake and alterations in Fh crystallinity. The OM was obtained from anoxic soil columns (effluent OM, efOM) and included-unlike water-extractable OM-compounds released by microbial activity under anoxic conditions. We found that organic molecules in efOM had generally no or only very low electron-accepting capacity and were incorporated into the Fh aggregates when coprecipitated with Fh. Compared to OM-free Fh, adsorption of efOM to Fh decelerated the microbial Fe(III) reduction by passivating the Fh surface toward electron uptake. In contrast, coprecipitation of Fh with efOM accelerated the microbial reduction, likely because efOM disrupted the Fh structure, as noted by Mössbauer spectroscopy. Additionally, the adsorbed and coprecipitated efOM resulted in a more sustained Fe(III) reduction, potentially because efOM could have effectively scavenged biogenic Fe(II) and prevented the passivation of the Fh surface by the adsorbed Fe(II). Fe(III)-OM coprecipitates forming at anoxic-oxic interfaces are thus likely readily reducible by Fe(III)-reducing bacteria in redoximorphic soils.

Compositional evolution of nanoscale zero valent iron and 2,4-dichlorophenol during dechlorination by attapulgite supported Fe/Ni nanoparticles

Transformation of chloro-organic compounds by nFe(0) has been studied extensively, but limited study exists on the transformation and fate of nFe(0) during the dechlorination of chloro-organics even though such knowledge is important in predicting its surface chemistry, particularly, toxicity in the environment. In this study, the nFe(0) core became hollowed, collapsed and gradually corroded into poorly crystallized ferrihydrite (Fe₅O₃(OH)₉) at the pristine reaction time, which later gave rise to lath-like lepidocrocite (γ-FeOOH), acicular goethite (α-FeOOH) and cubic magnetite (Fe₃O₄) by the end of the reaction time (120 min). Also, dechlorination of 2,4-DCP into 2-CP, 4-CP and phenol was achieved within 120 min. The rapid dechlorination of 2,4-DCP and transformation of nFe(0) could not be achieved significantly without doping Ni on nFe(0) and supporting on attapulgite. The schematic representation of the transformation and compositional evolution of nFe(0) in A-nFe/Ni was proposed. These findings are critical in understanding the compositional evolution and the fate of nFe(0) upon reaction with chloro-organics and can provide guidance for more efficient uses of the nFe(0) reactivity towards the target contaminants in groundwater remediation.

Effects of oxalate and citrate on the behavior and redistribution of Cr(VI) during ferrihydrite-Cr(VI) co-precipitates transformation

Understanding the influence of organic matters on the fate of Cr(VI) during ferrihydrite-Cr(VI) (Fh-Cr) co-precipitates transformation helps to study the retention of Cr(VI) by iron oxides in the environment. In this paper, Fh-Cr was prepared by co-precipitation and the redistribution of Cr(VI) in the oxalate or citrate system during the transformation of Fh-Cr was studied. X-ray diffraction, Fourier transform infrared spectroscopy, Transmission electron microscopy, and X-ray photoelectron spectroscopy were used to characterize Fh-Cr for aging 7 days at 70 °C. Results showed that both oxalate and citrate could hinder the release of Cr(VI) from Fh-Cr and abate the harm of Cr(VI). Oxalate improved the transformation from Fh-Cr to hematite and promoted Cr(VI) to be enfolded into the secondary minerals to further immobilize Cr at initial pH of 5.0 and 7.0, while citrate evidently reduced the release of Cr(VI) through stabilizing Fh-Cr at initial pH of 9.0. Besides, reduction of Cr(VI) by oxalate and citrate was through forming the surface complexes that promoted electron transfer from oxalate or citrate to Cr(VI), which can effectively abate the harm of Cr(VI). The findings of this study can promote understanding of the influences of organic matters on Cr(VI) immobilization during transformation of iron oxides in nature.

Ferrous iron enhances arsenic sorption and oxidation by non-stoichiometric magnetite and maghemite

Arsenic-contaminated waters affect millions of people on a daily basis. Because the toxicity of As is dependent on the redox state, understanding As biogeochemistry, particularly in reducing environments, is critical to addressing the environmental risk that As poses. Sorption of As to Fe(III)-(oxyhydr)oxides is an important mechanism for As removal from solution under anoxic conditions. However, dissolved ferrous Fe (Fe(II)) also occurs under anoxic conditions, and the impact that Fe(II)-catalyzed recrystallization of crystalline Fe minerals has on As sorption mechanisms is not clear. Our research investigates the potential for non-stoichiometric magnetite, a commonly occurring mixed-valence Fe oxide in anoxic aquifers, to adsorb and/or incorporate inorganic As species during Fe(II)-catalyzed recrystallization at neutral pH, with particular focus on the impact of mineral stoichiometry (Fe(II):Fe(III) = 0.23 and 0.0) and varying Fe(II) concentrations. By following aqueous As concentrations and speciation over time coupled with As K-edge X-ray absorption spectroscopy, our results demonstrate that the presence of Fe(II) substantially enhanced As removal from solution. In addition, we highlight a Fe(II)-induced mechanism through which highly mobile, toxic As(III) species are oxidized on the mineral surface to form As(V). Furthermore, the presence of Fe(II) promotes the structural incorporation of As(V) into the non-stoichiometric magnetite and maghemite structures. These results highlight the potential of Fe(II)-reacted non-stoichiometric magnetite or maghemite as pathways for long-term As sequestration in anoxic environments.

Utilization of electrochemical treatment and surface reconstruction to achieve long lasting catalyst for NOx removal

The development of catalysts has seen tremendous growth recently but most strategies only report utilization of catalysts for a few initial cycles without taking into account the influence of oxygen poisoning. Here, the magnetic Fe₃O₄@EDTA-Fe (MEFe, having a core Fe₃O₄ particle with EDTA-Fe coating) was investigated as a model catalyst for long-term recycling for the removal of nitrogen oxide (NO_x) from NO/O₂ mixture, followed by N₂O recovery. The concentration of oxygen in the flue gas was found to have a strong impact on NO_x absorption and catalytic response. To circumvent the oxygen poisoning, the MEFe was subjected to electrochemical treatment in the presence of neutral red (N.R.) and NO removal efficiency was ∼95 % noted. Furthermore, the surface of the catalyst degraded significantly (p < 0.05) after 6-7 repetitive cycling due to surface catalytic reactions, surface poisoning, oxidation of metallic species as well as residual stresses. The MEFe surface was reconstructed after 7 cycles using EDTA solution and Fe source to achieve similar surface coating as the fresh MEFe catalyst. The reconstructed MEFe exhibited similar NO_x absorption capability as the fresh MEFe and the reconstruction loop was repeated several times to achieve long term cycling, which make the catalyst cost-effective. Hence, it is proposed that a successful regeneration process can be employed for promising, sustainable and long-lasting catalytic treatment of air pollutants.

Fe(II)-catalyzed transformation of Fe (oxyhydr)oxides across organic matter fractions in organically amended soils

The Fe(II)-catalyzed transformation of ferrihydrite into highly crystalline forms may represent an important pathway for soil organic matter (SOM) destabilization under moderately reducing conditions. However, the link between redox-driven changes in soil Fe mineral composition and crystallinity and SOM chemical properties in the field remains elusive. We evaluated abiotic Fe(II)-catalyzed mineralogical transformation of Fe (oxyhydr)oxides in bulk soils and two particle-size SOM fractions, namely the fine silt plus clay (<20 μm, FSi + Cl) and fine sand (50-200 μm, FSa) fractions of an agricultural soil unamended or amended with biochar, compost, or the combination of both. After spiking with Fe(II) and incubating for 7 days under anoxic and sterile conditions at neutral pH, the FSa fractions (Fe(II):Fe (III) molar ratios ≈ 3.3) showed more significant ferrihydrite transformations with respect to FSi + Cl fractions (Fe(II):Fe (III) molar ratios ≈ 0.7), with the consequent production of well-ordered Fe oxides in most soils, particularly those unamended or amended with biochar alone. Nonetheless, poorly crystalline ferrihydrite still constituted about 45% of the FSi + Cl fractions of amended soils after reaction with Fe(II), which confirms that the higher SOM and clay mineral content in this fraction may possibly inhibit atom exchange between aqueous Fe(II) and the solid phase. Building on our knowledge of abiotic Fe(II)-catalyzed mineralogical changes, the suppression of ferrihydrite transformation in FSi + Cl fractions in amended soils could ultimately lead to a slower turnover of ferrihydrite, possibly preserving the carbon sequestration potential associated with this mineral. Conversely, in both bulk soils and FSa fractions, the extent to which mineral transformation occur seemed to be contingent on the quality of the amendment used.

Effect of Natural Organic Matter on the Fate of Cadmium During Microbial Ferrihydrite Reduction

Natural organic matter (NOM) is known to affect the microbial reduction and transformation of ferrihydrite, but its implication toward cadmium (Cd) associated with ferrihydrite is not well-known. Here, we investigated how Cd is redistributed when ferrihydrite undergoes microbial reduction in the presence of NOM. Incubation with Geobacter sulfurreducens showed that both the rate and the extent of reduction of Cd-loaded ferrihydrite were enhanced by increasing concentrations of NOM (i.e., C/Fe ratio). Without NOM, only 3-4% of Fe(III) was reduced, but around 61% of preadsorbed Cd was released into solution due to ferrihydrite transformation to lepidocrocite. At high C/Fe ratio (1.6), more than 35% of Fe(III) was reduced, as NOM can facilitate bioreduction by working as an electron shuttle and decreased aggregate size, but only a negligible amount of Cd was released into solution, thus decreasing Cd toxicity and prolonging microbial Fe(III) reduction. No ferrihydrite transformation was observed at high C/Fe ratios using Mössbauer spectroscopy and X-ray diffraction, and X-ray absorption spectroscopy indicated the proportion of Cd-OM bond increased after microbial reduction. This study shows that the presence of NOM leads to less mobilization of Cd under reducing condition possibly by inhibiting ferrihydrite transformation and recapturing Cd through Cd-OM bond.

Magnetic Nanoparticles as MRI Contrast Agents

Iron oxide nanoparticles (IONPs) have emerged as a promising alternative to conventional contrast agents (CAs) for magnetic resonance imaging (MRI). They have been extensively investigated as CAs due to their high biocompatibility and excellent magnetic properties. Furthermore, the ease of functionalization of their surfaces with different types of ligands (antibodies, peptides, sugars, etc.) opens up the possibility of carrying out molecular MRI. Thus, IONPs functionalized with epithelial growth factor receptor antibodies, short peptides, like RGD, or aptamers, among others, have been proposed for the diagnosis of various types of cancer, including breast, stomach, colon, kidney, liver or brain cancer. In addition to cancer diagnosis, different types of IONPs have been developed for other applications, such as the detection of brain inflammation or the early diagnosis of thrombosis. This review addresses key aspects in the development of IONPs for MRI applications, namely, synthesis of the inorganic core, functionalization processes to make IONPs biocompatible and also to target them to specific tissues or cells, and finally in vivo studies in animal models, with special emphasis on tumor models.

A Combined X-ray Absorption and Mössbauer Spectroscopy Study on Fe Valence and Secondary Mineralogy in Granitoid Fracture Networks: Implications for Geological Disposal of Spent Nuclear Fuels

Underground repository in crystalline bedrock is a widely accepted solution for long-term disposal of spent nuclear fuels. During future deglaciations, meltwater will intrude via bedrock fractures to the depths of future repositories where O₂ left in the meltwater could corrode metal canisters and enhance the migration of redox-sensitive radionuclides. Since glacial meltwater is poor in reduced phases, the quantity and (bio)accessibility of minerogenic Fe(II) in bedrock fractures determine to what extent O₂ in future meltwater can be consumed. Here, we determined Fe valence and mineralogy in secondary mineral assemblages sampled throughout the upper kilometer of fractured crystalline bedrock at two sites on the Baltic Shield, using X-ray absorption and Mössbauer spectroscopic techniques that were found to deliver matching results. The data point to extensive O₂-consuming capacity of the bedrock fractures, because Fe(II)-rich phyllosilicates were abundant and secondary pyrite was dispersed deep into the bedrock with no overall increase in Fe(II) concentrations and Fe(II)/Fe(III) proportions with depth. The results imply that repeated Pleistocene deglaciations did not cause a measurable decrease in the Fe(II) pool. In surficial fractures, largely opened during glacial unloading, ferrihydrite and illite have formed abundantly via oxidative transformation of Fe(II)-rich phyllosilicates and recently exposed primary biotite/hornblende.

Three-dimensional transfer of Cr(VI) co-precipitated with ferrihydrite containing silicate and its redistribution and retention during aging

Understanding the redistribution and retention of chromium(VI) (Cr(VI)) co-precipitated with silicate-containing ferrihydrite during aging is essential to the stabilization and immobilization of Cr(VI) in nature. In this work, Cr(VI) was removed by co-precipitated with silicate-containing ferrihydrite with various Si/Fe ratios at different precipitating pH. The co-precipitates were characterized by X-ray diffraction, transmission electron microscopy, Fourier transform infrared spectroscopy, and X-ray photoelectron spectroscopy before and after aging for 9 days at 80 °C. Results showed that silicate not only competed with Cr(VI) adsorbed on ferrihydrite surface by forming inner-sphere complexes, but also inhibited ferrihydrite transforming into more stable and compact iron phases. Ferrihydrite only transformed to hematite at pH₀ 5.0, and converted to hematite and goethite at pH₀ 7.5 and 10.0. Cr(VI) was initially removed by silicate-containing ferrihydrite with the removal efficiencies > 99.64% at initial pH of 5.0, and it was obviously incorporated into hematite and goethite during the transformation of ferrihydrite. The transformation products influenced the redistribution of Cr(VI), which was beneficial to the retention of Cr(VI) inside the co-precipitates, but not conducive to the adsorption for Cr(VI). The findings that Cr(VI) was removed by a common and metastable precursor of silicate-containing ferrihydrite can promote understanding of three-dimensional transfer and behavior of Cr(VI) during the transformation of silicate-containing ferrihydrite.

Ferrihydrite Growth and Transformation in the Presence of Ferrous Iron and Model Organic Ligands

Ferrihydrite (Fh) is a poorly crystalline Fe(III)-oxyhydroxide found in abundance in soils and sediments. With a high specific surface area and sorption capacity at circumneutral pH, ferrihydrite is an important player in the biogeochemical cycling of nutrients and trace elements in redox-dynamic environments. Under reducing conditions, exposure to Fe(II) induces mineral transformations in ferrihydrite; the extent and trajectory of which may be greatly influenced by organic matter (OM). However, natural OM is heterogeneous and comprises a range of molecular weights (MWs) and varied functional group compositions. To date, the impact that the chemical composition of the associated OM has on Fe(II)-catalyzed mineral transformations is not clear. To address this knowledge gap, we coprecipitated ferrihydrite with model organic ligands selected to cover a range of MWs (25 000-50 000 vs <200 Da) as well as carboxyl content (polygalacturonic acid (PGA) > citric acid (CA) > galacturonic acid (GA)). Coprecipitates (C:Fe ≈ 0.6) were reacted with 1 mM ⁵⁷Fe(II) for 1 week at pH 7, with time-resolved solid-phase analysis (via X-ray diffraction, X-ray absorption spectroscopy, and electron microscopy) revealing that all ligands inhibited Fe(II)-catalyzed ferrihydrite mineral transformations and the formation of crystalline secondary mineral phases compared to a pure ferrihydrite. For carboxyl-rich coprecipitates (Fh-PGA and Fh-CA), mineral transformations were less inhibited than in the carboxyl poor Fh-GA, and a crystalline lepidocrocite "shell" was formed surrounding the residual ferrihydrite core. However, Fe isotope analysis revealed that all coprecipitates underwent near complete atom exchange. Collectively, our results highlight that ferrihydrite is indeed an active mineral phase in redox-dynamic environments, but that its stability under reducing conditions, and thus capacity for nutrient and trace element retention, depends on the chemical characteristic of the associated OM, specifically OM-induced changes in the particle surface charge and the distribution of organic functional groups.

Decreases in Iron Oxide Reducibility during Microbial Reductive Dissolution and Transformation of Ferrihydrite

Ferrous iron formed during microbial ferric iron reduction induces phase transformations of poorly crystalline into more crystalline and thermodynamically more stable iron (oxyhydr)oxides. Yet, characterizing the resulting decreases in the reactivity of the remaining oxide ferric iron toward reduction (i.e., its reducibility) has been challenging. Here, we used the reduction of six-line ferrihydrite by Shewanella oneidensis MR-1 as a model system to demonstrate that mediated electrochemical reduction (MER) allows directly following decreases in oxide ferric iron reducibility during the transformation of ferrihydrite into goethite and magnetite which we characterized by X-ray diffraction analysis and transmission electron microscopy imaging. Ferrihydrite was fully reducible in MER at both pH_MER of 5.0 and 7.5. Decreases in iron oxide reducibility associated with ferrihydrite transformation into magnetite were accessible at both pH_MER because the formed magnetite was not reducible under either of these conditions. Conversely, decreases in iron oxide reducibility associated with goethite formation were apparent only at the highest tested pH_MER of 7.5 and thus the thermodynamically least favorable conditions for iron oxide reductive dissolution. The unique capability to adjust the thermodynamic boundary conditions in MER to the specific reducibilities of individual iron (oxyhydr)oxides makes this electrochemical approach broadly applicable for studying changes in iron oxide reducibility in heterogeneous environmental samples such as soils and sediments.