Carbonate-Enhanced Transformation of Ferrihydrite to Hematite

pH-dependent transport of tetracycline in saturated porous media: Single and combined effects of surfactants and iron oxide colloids

Herein, sodium dodecyl sulfate (SDS) and rhamnolipid (Rha) were employed to investigate their influences on TC mobility and ferrihydrite colloid-mediated transport of TC at variable pH values (5.0-9.0). In the binary system, surfactants suppressed TC transport because of surfactants' bridging effects; similarly, ferrihydrite colloids also restrained TC mobility stemming from the colloid-associated TC retention. Interestingly, the degree of the inhibitory effects of colloids/surfactants increased with decreasing pH values. Surprisingly, the mutual influences of surfactants and colloids on TC movement displayed a strong pH dependence. Concretely, surfactants strengthened the repressive impacts of ferrihydrite colloids on TC mobility at pH 5.0 caused by the enhanced TC deposition on colloids attached to sand surfaces through the linking effects of surfactants. Nevertheless, at pH 7.0, adding surfactants reduced the repressive effects due to increased TC-colloid mobility and enhanced electrostatic repulsion. Unexpectedly, colloids accelerated the transport of TC with surfactants at pH 9.0 owing to colloids acting as TC carriers, the enhanced TC2-/TC- species mobility, and competitive retention. Notably, SDS exhibited a greater effect on individual TC mobility or colloid-mediated TC transport than Rha at a certain pH, which was related to the different surfactant-binding abilities of sand grains/ferrihydrite colloids.

The fate of cadmium during the hydrolysis and solid-state transformation of iron

The ubiquitous hydrolysis of Fe(III) ions in nature leads to the formation of ferrihydrite (Fh), which then undergoes solid-state transformation into crystalline mineral phases. However, the impact of this process on the geochemical fate of cadmium (Cd) in aquatic environments is not yet fully understood. This investigation systematically examined the partitioning mechanisms of Cd during the aforementioned processes through extraction techniques and multiple characterization methodologies. The experimental results demonstrated that Fe(III) ions first undergo the diffusion-limited aggregation (DLA) stage, forming loosely bound clusters with high Cd co-precipitation capacity. Upon transitioning to the reaction-limited aggregation (RLA) stage, Fe clusters exhibit increased structural complexity, wherein the expansion of radius of gyration predominantly governs Cd retention. As colloidal Fh develops, surface adsorption becomes the predominant mechanism for Cd sequestration. Throughout solid-state transformation in systems containing 1 mol% Cd, dehydroxylation processes induced acidification, facilitating the progressive liberation of Cd. Conversely, in systems with 10 mol% Cd, significant Cd incorporation induces distortion in the crystalline lattice structure, promoting system alkalinization and, consequently, enhancing Cd immobilization. pH gradient solid-state transformation experiments demonstrated that when initial pH values were below 8, final pH measurements were significantly lower than initial values, and vice versa. pH conditions potentially regulate the speciation of Fe within Fh, ultimately exerting substantial influence on Cd partitioning behavior. In summary, Fh formation beneficially stabilizes Cd, while its subsequent solid-state transformation triggers Cd redistribution.

Sunlight-Driven Transformation of Ferrihydrite via Ligand-to-Metal Charge Transfer: The Critical Factors and Arsenic Repartitioning

Ferrihydrite, a poorly ordered metastable iron oxide, is closely associated with dissolved organic matter (DOM) in soils and sediments. Although sunlight-induced photoreductive dissolution of ferrihydrite via ligand-to-metal charge transfer (LMCT) has been extensively studied, its potential impacts on mineralogical transformation and environmental behaviors of coexisting contaminants remain largely unknown. Here, we systematically investigated the effects of environmental parameters (e.g., solution pH, pO2 level, arsenic speciation, and content) on ferrihydrite transformation with the representative DOM-oxalate under simulated solar irradiation. Results showed that the oxalate-mediated LMCT process synchronously initiated Fe(II) production and proton consumption, the latter of which facilitated interfacial electron transfer and atom exchange (IET-AEFh-Fe2+) processes among ferrihydrite and newly formed Fe(II). At pH 5.0-8.0, ferrihydrite was prone to transform into goethite due to sufficient Fe(II) (approximately 80-2700 μM) from LMCToxa and enough affinity of Fe(II) with mineral to trigger IET-AEFh-Fe2+, while it only underwent reductive dissolution at pH 3.0-5.0 or kept a quasi-steady state over pH 8.0. Increasing the pO2 level and arsenic content hampered the recrystallization of ferrihydrite by reducing Fe(II) duration or altering the surface property of ferrihydrite, whereas the presence of As(III/V) also led to the formation of lepidocrocite with As(V) being more prominent. Additionally, chemical extraction and As K-edge EXAFS spectroscopy revealed that As was consecutively incorporated into the structures of goethite and lepidocrocite in the form of As(V) regardless of primary As speciation. These findings shed novel insights into low-crystalline iron oxide transformation and element migration driven by sunlight in natural environments.

pH induced incongruent-dissolution impacts Al-ferrihydrite transformations and As mobilization

This study investigated the role of Al and As fate during the transformation process of ferrihydrite influenced by different pH values under oxic conditions. The results indicate that the Al doping greatly enhanced the transformation of ferrihydrite (Fh) to Al-substituted goethite at all acidic or alkaline pH values under oxic conditions by promoting the incongruent dissolution and reprecipitation reactions of Al-substituted ferrihydrite (AlFh). Under acidic conditions, the preferential dissolution of structural Fe (4.73 mg/L) from AlFh occurs, whereas under alkaline conditions, the preferential dissolution of structural Al (1.25 mg/L) takes place. In contrast, under neutral conditions, the low solubility of Fh and AlFh induces the significant particle assembly, with Fe/Al minerals primarily transforming into goethite through oriented aggregation. As predominantly remains in an adsorbed state at all pH values during the transformation of Fh and AlFh, with the highest proportion of adsorbed As (86.9-96.7%) observed under neutral conditions. During the aging process, the adsorbed As gradually transforms into non-extractable As, and the changes in As speciation within Fe/Al minerals are closely coupled with the transformation of AlFh and Fh. Under alkaline and acidic conditions, the proportion of non-extractable As in the transformation products of Fh and AlFh increases by 14.02-19.72% and 12.27-16.28%, respectively, while under neutral conditions, it increases only by 12-13.02%. Therefore, regulating soil pH can partially modify As speciation and mitigate its environmental impact by altering the mineral transformation process. The results of this study facilitate better understanding of the role of Al substitution in the transformation of Fh and the cycling of As in the environment.

Molecular Insights into the Synergistic Inhibition of Microplastics-Derived Dissolved Organic Matter and Anions on the Transformation of Ferrihydrite

Ferrihydrite (Fh), as a ubiquitous iron (oxyhydr)oxide, plays an essential role in nutrient cycling and pollutant transformation due to its high surface area and diversified reaction sites. In the natural environment, Fh transformation could be easily influenced by coexisting components (particularly dissolved organic matter (DOM) and anions). As a new and important carbon source, microplastic-derived DOM (MP-DOM) directly or indirectly affects the morphology and fate of Fh, but limited knowledge exists about the combined effect of MP-DOM and anions on Fh transformation. Herein, this study elucidates the joint effects of polystyrene DOM (PS-DOM) and anions (such as Cl-, SO42-, and PO43-) on Fh transformation. Single anions (especially PO43-) were shown to inhibit the transformation of Fh to hematite (Hm) by hindering the dissolution and recrystallization of Fe(III). However, the inhibitory effect was strongly enhanced when PS-DOM and anions coexisted, which is attributed to their synergetic effects on inhibiting dissolution/recrystallization by occupying more active sites and hindering electron transfer. Furthermore, Fh transformation was predominantly controlled by PS-DOM, especially those containing high-unsaturation, high-oxidation-state, and O-rich phenolic compounds. These findings provide a new perspective on the significance of considering the joint effects of DOM and anions in evaluating the transformation of iron minerals.

Effects of surfactants on the adsorption of norfloxacin onto ferrihydrite: comparison between anionic and cationic surfactants

There is little information on how widespread surfactants affect the adsorption of norfloxacin (NOR) onto iron oxide minerals. In order to elucidate the effects of various surfactants on the adsorption characteristics of NOR onto typical iron oxides, we have explored the different influences of sodium dodecylbenzene sulfonate (SDBS), an anionic surfactant, and didodecyldimethylammonium bromide (DDAB), a cationic surfactant, on the interactions between NOR and ferrihydrite under different solution chemistry conditions. Interestingly, SDBS facilitated NOR adsorption, whereas DDAB inhibited NOR adsorption. The adsorption-enhancement effect of SDBS was ascribed to the enhanced electrostatic attraction, the interactions between the adsorbed SDBS on ferrihydrite surfaces and NOR molecules, and the bridging effect of SDBS between NOR and iron oxide. In comparison, the adsorption-inhibition effect of DDAB owning to the adsorption site competitive adsorption between NOR and DDAB for the effective sites as well as the steric hindrance between NOR-DDAB complexes and the adsorbed DDAB on ferrihydrite surfaces. Additionally, the magnitude of the effects of surfactants on NOR adsorption declined with increasing pH values from 5.0 to 9.0, which was related to the amounts of surfactant binding to ferrihydrite surfaces. Moreover, when the background electrolyte was Ca2+, the enhanced effect of SDBS on NOR adsorption was caused by the formation of NOR-Ca2+-SDBS complexes. The inhibitory effect of DDAB was due to the DDAB coating on ferrihydrite, which undermined the cation-bridging effect. Together, the findings from this work emphasize the essential roles of widely existing surfactants in controlling the environmental fate of quinolone antibiotics.

Silicate impedes arsenic release and oxidation from ferrihydrite

Silicate fertilization is a common farming practice and an effective method to mitigate arsenic (As) pollution in paddy. Investigating the interaction between silicate and ferrihydrite on As retention is key to comprehensively understand the mechanism of As sequestration by silicate fertilization. Our results indicated that the transformation of ferrihydrite into goethite and hematite was inversely proportional to Si/Fe ratios. The added silicate impeded the decrease of solution pH from neutral to acidity, and imposed strong inhibitory effect on goethite formation. The aqueous As in silicate-free system was 3.43 times higher than that with Si/Fe ratio at 0.33, but similar results were not observed in those with high-level As pollution due to the inhibitory effect of As on ferrihydrite transformation. Solid characterization showed that silicate was monomerically adsorbed to ferrihydrite through Si-O-Fe bond, which impeded the reductive dissolution, Fe atom exchange, internal atomic rearrangement and dehydration of ferrihydrite. As(III) oxidation weakened in silicate-coprecipitated ferrihydrite due to the lack of Fe(II) catalysis stem from ferrihydrite dissolution. This work demonstrated that As release could be effectively impeded through the inhibitory effect of silicate on ferrihydrite transformation, thereby providing new insights into the understanding of As accumulation reduction in rice by silicate fertilization.

Identification of key factors and mechanism determining arsenic mobilization in paddy soil-porewater-rice system

Arsenic (As) mobilization in paddy fields poses significant health risks, necessitating a thorough understanding of the controlling factors and mechanisms to safeguard human health. We conducted a comprehensive investigation of the soil-porewater-rice system throughout the rice life cycle, focusing on monitoring arsenic distribution and porewater characteristics in typical paddy field plots. Soil pH ranged from 4.79 to 7.98, while porewater pH was weakly alkaline, varying from 7.2 to 7.47. Total arsenic content in paddy soils ranged from 6.8 to 17.2 mg/kg, with arsenic concentrations in porewater during rice growth ranging from 2.97 to 14.85 μg/L. Specifically, arsenite concentrations in porewater ranged from 0.48 to 7.91 μg/L, and arsenate concentrations ranged from 0.73 to 5.83 μg/L. Through principal component analysis (PCA) and analysis of redox factors, we identified that arsenic concentration in porewater is predominantly influenced by the interplay of reduction and desorption processes, contributing 43.5 % collectively. Specifically, the reductive dissolution of iron oxides associated with organic carbon accounted for 23.3 % of arsenic concentration dynamics in porewater. Additionally, arsenic release from the soil followed a sequence starting with nitrate reduction, followed by ferric ion reduction, and subsequently sulfate reduction. Our findings provide valuable insights into the mechanisms governing arsenic mobilization within the paddy soil-porewater-rice system. These insights could inform strategies for irrigation management aimed at mitigating arsenic toxicity and associated health risks.

Time-varying contributions of FeII and FeIII to AsV immobilization under anoxic/oxic conditions: The impacts of biochar and dissolved organic carbon

Engineering black carbon (e.g. biochar) has been widely found in natural environments due to natural processes and extensive applications in engineering systems, and could influence the geochemical processes of coexisting arsenic (AsV) and FeII, especially when they are exposed to oxic conditions. Here, we studied time-varying kinetics and efficiencies of AsV immobilization by solid-phase FeII (FeIIsolid) and FeIII (FeIIIsolid) in FeII-AsV-biochar systems under both anoxic and oxic conditions at pH 7.0, with focuses on the effects of biochar surface and biochar-derived dissolved organic carbon (DOC). Under anoxic conditions, FeII could rapidly immobilize AsV via co-adsorption onto biochar surfaces, which also serves as the dominant pathway of AsV immobilization at the initial stage of reaction (0-5 min) under oxic conditions at high biochar concentrations. Subsequently, with increasing biochar concentrations, FeIIIsolid precipitation from aqueous FeII (FeIIaq) oxidation (5-60 min) starts to play an important role in AsV immobilization but in decreased efficiencies of AsV immobilization per unit iron. In the following stage (60-300 min), FeIIsolid oxidation is suppressed and leads to AsV release into solutions at >1.0 g·L-1 biochar. The decreasing efficiency of AsV immobilization over time is attributed to the gradual release of DOC into solution from biochar particles, which significantly inhibit AsV immobilization when FeIIIsolid is generated from FeIIsolid oxidation in the vicinity of biochar surfaces. Specifically, 4.06 mg·L of biochar-derived DOC can completely inhibit the immobilization of AsV in the 100 μM FeII system under oxic conditions. The findings are crucial to comprehensively understand and predict the behavior of FeII and AsV with coexisting engineering black carbon in natural environments.

The Influence of Seawater on Fe(II)-Catalyzed Ferrihydrite Transformation and Its Subsequent Consequences for C Dynamics

Short-range-ordered minerals like ferrihydrite often bind substantial organic carbon (OC), which can be altered if the minerals transform. Such mineral transformations can be catalyzed by aqueous Fe(II) (Fe(II)aq) in redox-dynamic environments like coastal wetlands, which are inundated with seawater during storm surges or tidal events associated with sea-level rise. Yet, it is unknown how seawater salinity will impact Fe(II)-catalyzed ferrihydrite transformation or the fate of bound OC. We reacted ferrihydrite with Fe(II)aq under anoxic conditions in the absence and presence of dissolved organic matter (DOM). We compared treatments with no salts (DI water), NaCl-KCl salts, and artificial seawater mixes (containing Ca and Mg ions) with or without SO42-/HCO3-. Both XRD and Mössbauer showed that NaCl-KCl favored lepidocrocite formation, whereas Ca2+/Mg2+/SO42-/HCO3- ions in seawater overrode the effects of NaCl-KCl and facilitated goethite formation. We found that the highly unsaturated and phenolic compounds (HuPh) of DOM selectively bound to Fe minerals, promoting nanogoethite formation in seawater treatments. Regardless of salt presence, only 5-9% of Fe-bound OC was released during ferrihydrite transformation, enriching HuPh relative to aliphatics in solution. This study offers new insights into the occurrence of (nano)goethite and the role of Fe minerals in OC protection in coastal wetlands.

Hazardous effects of nickel ferrite nanoparticles and nickel chloride in early life stages of the freshwater snail Biomphalaria glabrata (Say, 1818)

Nickel ferrite nanoparticles (NiF NPs) have growing applications in biomedical and nanomedicine fields. However, knowledge concerning their ecotoxicity during the early developmental stages of invertebrates, such as gastropods, remains scarce. Thus, the current study aimed to evaluate whether NiF NPs and nickel chloride (NiCl2) induce toxic effects on embryos and newly hatched snails of freshwater species Biomphalaria glabrata (Say, 1818). NiF NPs were synthesized and characterized by multiple techniques, and their ecotoxicity was assessed by Biomphalaria embryotoxicity test (BET) during 144 h of exposure and an acute toxicity test (96 h) using newly hatched snails. NiF NPs induced mortality, developmental delay, reduced hatching rate, and promoted morphological changes in B. glabrata. Also, NiF NPs induced higher toxicity in embryos than in newly hatched B. glabrata. Overall, results showed that the early developmental stages of gastropods are a target group for nanoparticle toxicity in freshwater ecosystems.

Iron (hydr)oxides-induced activation of sulfite for contaminants degradation: The critical role of structural Fe(III)

Iron-based sulfite (S(IV)) activation has emerged as a novel strategy to generate sulfate radicals (SO4•-) for contaminants degradation. However, numerous studies focused on dissolved iron-induced homogeneous activation processes while the potential of structural Fe(III) remains unclear. In this study, five iron (hydr)oxide soil minerals (FeOx) including ferrihydrite, schwertmannite, lepidocrocite, goethite and hematite, were successfully employed as sources of structural Fe(III) for S(IV) activation. Results showed that the catalytical ability of structural Fe(III) primarily depended on the crystallinity of FeOx instead of their specific surface area and particle size, with ferrihydrite and schwertmannite being the most active. Furthermore, in-situ ATR-FTIR spectroscopy and 2D-COS analysis revealed that HSO3- was initially adsorbed on FeO6 octahedrons of FeOx via monodentate inner-sphere complexation, ultimately oxidized into SO42- which was then re-adsorbed via outer-sphere complexation. During this process, strong oxidizing SO4•- and •OH were formed for pollutants degradation, confirmed by radical quenching experiments and electron spin resonance. Moreover, FeOx/S(IV) system exhibited superior applicability with respect to recycling test, real waters and twenty-six pollutants degradation. Eventually, plausible degradation pathways of three typical pollutants were proposed. This study highlights the feasibility of structural Fe(III)-containing soil minerals for S(IV) activation in wastewater treatment.

Effects of carbonate on ferrihydrite transformation in alkaline media

Alkaline media widely exist in natural and engineered systems such as semiarid/arid areas, radioactive waste sites, and mine tailings. In these settings, the commonly occurring iron (oxyhydr)oxides differed in their ability to influence the fate of nutrients and contaminants. Due to the substantially increased atmospheric carbon dioxide (CO2) concentration, carbonate stands to increase in these media. However, how increasing carbonate affects the transformation of poorly crystalline iron (oxyhydr)oxides (e.g., two-line ferrihydrite) under alkaline conditions still remains unclear. Here, kinetics of ferrihydrite transformation were evaluated at pH ∼10 as a function of [carbonate] = 0-286 mM using synchrotron-based X-ray and vibrational spectroscopic techniques. The results showed that carbonate slowed down ferrihydrite transformation slightly and suppressed goethite formation, but promoted hematite formation regardless of its concentration. At low carbonate concentration (11.42 mM), the effect of carbonate on product formation was obvious due to the weak inner-sphere complex; however, at high carbonate concentration (80-286 mM), the effect was retarded because of the adsorption equilibrium of carbonate as well as the initial carbonate adsorption followed by desorption. Moreover, carbonate modified the morphology of hematite from rhombic to ellipsoidal to honeycomb and goethite from rod-like to needle-like to spindle-like due to the inner-sphere adsorption-desorption of carbonate and adsorption of hydroxyl ions on reactive sites of iron (oxyhydr)oxides in alkaline media. The results suggest that the concurrently increasing carbonate with enhanced atmospheric CO2 could control the transformation and occurrence of iron (oxyhydr)oxides in natural and engineered environments and have important implications for the biogeochemical cycles of iron and carbon.

Artificial Humic Acid Mediated Carbon-Iron Coupling to Promote Carbon Sequestration

Fe (hydr)oxides have a substantial impact on the structure and stability of soil organic carbon (SOC) pools and also drive organic carbon turnover processes via reduction-oxidation reactions. Currently, many studies have paid much attention to organic matter-Fe mineral-microbial interactions on SOC turnover, while there is few research on how exogenous carbon addition abiotically regulates the intrinsic mechanisms of Fe-mediated organic carbon conversion. The study investigated the coupling process of artificial humic acid (A-HA) and Fe(hydr)oxide, the mechanism of inner-sphere ligands, and the capacity for carbon sequestration using transmission electron microscopy, thermogravimetric, x-ray photoelectron spectroscopy, and wet-chemical disposal. Furthermore, spherical aberration-corrected scanning transmission electron microscopy-electron energy loss spectroscopy and Mössbauer spectra have been carried out to demonstrate the spatial heterogeneity of A-HA/Fe (hydr)oxides and reveal the relationship between the increase in Fe-phase crystallinity and redox sensitivity and the accumulation of organic carbon. Additionally, the dynamics of soil structures on a microscale, distribution of carbon-iron microdomains, and the cementing-gluing effect can be observed in the constructing nonliving anthropogenic soils, confirming that the formation of stable aggregates is an effective approach to achieving organic carbon indirect protection. We propose that exogenous organic carbon inputs, specifically A-HA, could exert a substantial but hitherto unexplored effect on the geochemistry of iron-carbon turnover and sequestration in anoxic water/solid soils and sediments.

Divergent repartitioning of antimony and arsenic during jarosite transformation: A comparative study under aerobic and anaerobic conditions

Jarosite is the host mineral of Sb(V) and As(V) in mining environments. However, the repartitioning of Sb and As during its transformation is poorly understood. Additionally, the mutual effect between the redistribution behavior of As and Sb during jarosite conversion remains unclear. Here, we investigated the transformation of Sb(V)-, As(V)- and Sb(V)-As(V)-jarosite at pH 5.5 under aerobic and anaerobic conditions without a reductant. The results indicated that co-precipitated Sb(V) promotes jarosite dissolution, and the final products were mainly goethite and hematite. In contrast, the co-precipitated As(V) retarded jarosite dissolution and altered the transformation pathway, mainly forming lepidocrocite, which might be attributed to the formation of As-Fe complexes on the jarosite surface. The inhibiting or promoting effect increased with the increase in co-precipitated As or Sb concentration. In the treatment with Sb(V)-As(V)-jarosite, the inhibition effect of co-precipitated As(V) on mineral dissolution was predominant, but the end-products were mainly goethite and hematite. Compared with the aerobic system, the dissolution and transformation of jarosite in treatments in the anaerobic system occurred faster, although without a reductant, which was possibly associated with the reduced CO2 content in the reaction solutions after degassing. In all treatments, the release of Sb(aq) and As(aq) into the solution was negligible during jarosite transformation. The transformation processes drove As into the surface-bound exchangeable and poorly crystalline phases, while Sb was typically redistributed in the poorly crystalline phase. During the transformation of Sb(V)-As(V)-jarosite, the co-existence of As significantly increased the proportion of Sb distributed on the solid surface and in the poorly crystalline phase. These findings are valuable for predicting the long-term fate of Sb and As in mining environments.

Remediation of arsenic-contaminated calcareous agricultural soils by iron-oxidizing bacteria combined with organic fertilizer

In arid soil with low-iron and high-calcium carbonate contents, the fate of arsenic (As) is mainly controlled by the contents of calcium and organic matter in the soil. However, there is still a lack of knowledge about their interaction and that effect on their absorption by maize. The purpose of this study was to explore the long-term immobilization and repair mechanism of in situ As-contaminated farmland. We designed three treatments: iron-oxidizing bacteria (FeOB), organic fertilizer, FeOB and organic fertilizer added in combination. After 140-day field farmland remediation trial, the results showed that the FeOB can effectively immobilize the water-soluble As (F_S1) in soil, and the organic fertilizer promoted the remediation of FeOB. In addition, the content of As in maize grains was reduced after treatment by FeOB and organic fertilizer. The XRD and XPS analysis of the topsoil showed that the combined treatment of FeOB and organic fertilizer promoted the formation of calcium arsenate mineral with low solubility and high stability; As(III) would gradually transform into As(V). The biological iron (hydr)oxide can increase the contents of Fe and As in the rhizosphere and form iron plaques on the surface of the roots by SEM-EDS analysis of maize root. Collectively, these results clarify the main biogeochemical ways to control the fate of As in calcareous soils with low-iron and low-organic matter contents and provide a basis for in situ remediation of As.

Production of reactive oxygen species from oxygenation of Fe(II)-carbonate complexes: The critical roles of carbonate

Hydroxyl radicals (•OH) production upon the oxygenation of reduced iron minerals at the oxic/anoxic interface has been well recognized. However, little is known in the influencing environmental factors and the involved mechanisms. In this study, much more •OH could be efficiently produced from oxygenation of Fe(II) with 20-200 mM carbonate. Both carbonate concentration and anoxic reaction time play a critical role in •OH production. High carbonate facilitates the formation of Fe(II)_{high reactivity}, i.e., surface-adsorbed and structural Fe(II) with low crystalline that is reactive toward O₂ reaction for •OH production, while long anoxic reaction time enables the transfer from Fe(II)_{high reactivity} to Fe(II)_{low reactivity}, i.e., Fe(II) at interior sites with high crystalline, that is hardly oxidized by O₂. Furthermore, the degradation pathway of p-nitrophenol (PNP) is highly dependent on the carbonate concentration that low carbonate facilitates •OH oxidation of PNP (80.2%) while high carbonate enhanced O₂^•- reduction of PNP (48.7%). Besides, carbonate also influences the structural evolution of Fe mineral during oxygenation by retarding its hydrolysis and following transformation. Our finding sheds new light on understanding the important role of oxyanions such as carbonate in iron redox cycles and directing contaminant attenuation in subsurface environment.

Effect of phosphate on ferrihydrite transformation and the associated arsenic behavior mediated by sulfate-reducing bacterium

Although PO₄^3- is commonly found in association with iron (oxyhydr)oxide, the effect of PO₄^3- on ferrihydrite reduction, mineralogical transformation, and associated As behavior in sulfate-reducing bacteria (SRB)-rich environments remains unclear. In this study, batch experiments, together with geochemical, mineralogical, and biological analyses, were conducted to elucidate these processes. The results showed that SRB can reduce ferrihydrite via direct and indirect processes, and PO₄^3- promoted ferrihydrite reduction by supporting SRB growth at low and medium PO₄^3- loadings. However, at high loadings, PO₄^3- stabilized the ferrihydrite. PO₄^3- shifted the transformation of ferrihydrite from magnetite and mackinawite to vivianite, which scavenges As effectively by incorporating As into its particle. In systems with 0.5 mM SO₄^2-, PO₄^3- exerted a weak effect on As mobilization. However, in systems with 10 mM SO₄^2-, substantial amounts of As were released into the solution, and PO₄^3- impacted As behavior strongly. Low PO₄^3- loadings increased the mobilization of As because of the competitive adsorption of PO₄^3- on mackinawite. Medium and high PO₄^3- loadings were beneficial for As immobilization because of the substitution of mackinawite by vivianite. These findings have important implications for understanding the biogeochemistry of iron (oxyhydr)oxide and As behavior in SRB-containing sediments.

Iron (hydr)oxide formation in Andosols under extreme climate conditions

Redox-driven biogeochemical cycling of iron plays an integral role in the complex process network of ecosystems, such as carbon cycling, the fate of nutrients and greenhouse gas emissions. We investigate Fe-(hydr)oxide (trans)formation pathways from rhyolitic tephra in acidic topsoils of South Patagonian Andosols to evaluate the ecological relevance of terrestrial iron cycling for this sensitive fjord ecosystem. Using bulk geochemical analyses combined with micrometer-scale-measurements on individual soil aggregates and tephra pumice, we document biotic and abiotic pathways of Fe released from the glassy tephra matrix and titanomagnetite phenocrysts. During successive redox cycles that are controlled by frequent hydrological perturbations under hyper-humid climate, (trans)formations of ferrihydrite-organic matter coprecipitates, maghemite and hematite are closely linked to tephra weathering and organic matter turnover. These Fe-(hydr)oxides nucleate after glass dissolution and complexation with organic ligands, through maghemitization or dissolution-(re)crystallization processes from metastable precursors. Ultimately, hematite represents the most thermodynamically stable Fe-(hydr)oxide formed under these conditions and physically accumulates at redox interfaces, whereas the ferrihydrite coprecipitates represent a so far underappreciated terrestrial source of bio-available iron for fjord bioproductivity. The insights into Fe-(hydr)oxide (trans)formation in Andosols have implications for a better understanding of biogeochemical cycling of iron in this unique Patagonian fjord ecosystem.

Mechanistic insights into the interfacial adsorption behaviors of Cr(VI) on ferrihydrite: Effects of pH and naturally coexisting anions in the environment

Interfacial interaction of hexavalent chromium (Cr[VI]) with ferrihydrite (Fh) plays a key role in the behavior of Cr(VI) in the environment. In this study, H2PO4-, SO42-, NO3-, Cl-, and HCO3- were chosen as coexisting anions to explore their inhibition of the capacity of Fh to adsorb Cr(VI). We employed X-ray diffraction, scanning electron microscopy, attenuated total reflection Fourier transform infrared spectroscopy, and X-ray photoelectron spectroscopy to thoroughly characterize Fh reaction products before and after adsorption of Cr(VI). The results clearly revealed that pH has a marked effect on the extent of Cr(VI) adsorption onto Fh, and this process is also highly dependent on the types of anions present. H2PO4- exhibited the most evident inhibition of Cr(VI) adsorption, even at low concentrations. Similarly, the inhibition of Cr(VI) adsorption by HCO3- increased markedly with increasing pH. In contrast, SO42- only slightly competed with Cr(VI) for reactive Fh surface sites. The anions Cl- and NO3- exhibited almost no inhibitory effect on Cr(VI) adsorption. The differential order of adsorptive affinity of all six anions for Fh was as follows: H2PO4- > HCO3- > SO42- ≈ HCrO4- > NO3- ≈ Cl-. Based on these results, we further provide mechanistic insights into the complexities of Cr(VI) adsorption/desorption behaviors on Fh surfaces. Using Fh as a geosorbent, these interfacial properties could be exploited to mediate the immobilization and release of chromate from and/or into contaminated environments such as aquifers.

Humic acids restrict the transformation and the stabilization of Cd by iron (hydr)oxides

Iron (hydr)oxides and their association with organic matter significantly affect the mobility of heavy metals in natural soils and sediments. However, the behavior of cadmium (Cd) during crystalline iron (hydr)oxide formation in the presence of humic acid (HA) is still unknown. In this study, the speciation of Cd in iron (hydr)oxide-HA coprecipitates were studied by extraction, surface complexation model (SCM) calculation and characterization of the composites during the aging. The results showed that aging promoted the stabilization of ~30-50% of the added Cd ions with minerals in the binary iron (hydr)oxide systems. The reduction of Cd occurred earlier than hematite formation, indicating that the aggregation of amorphous iron (hydr)oxide led to the initial immobilization of Cd. The presence of HA restricted the crystallization of iron (hydr)oxide by the formation of tight mineral nanoparticle-HA aggregates, while there were negligible changes in the speciation of Cd and Fe during aging at high HA concentrations. Therefore, HA promoted the adsorption of Cd onto amorphous iron (hydr)oxide but limited the partition of Cd to mineral aggregates. The knowledge about the role of HA in iron (hydr)oxide transformation and Cd speciation is of great significance for the prediction of heavy metal behavior in soils and sediments.

Autotrophic Fe-Driven Biological Nitrogen Removal Technologies for Sustainable Wastewater Treatment

Fe-driven biological nitrogen removal (FeBNR) has become one of the main technologies in water pollution remediation due to its economy, safety and mild reaction conditions. This paper systematically summarizes abiotic and biotic reactions in the Fe and N cycles, including nitrate/nitrite-dependent anaerobic Fe(II) oxidation (NDAFO) and anaerobic ammonium oxidation coupled with Fe(III) reduction (Feammox). The biodiversity of iron-oxidizing microorganisms for nitrate/nitrite reduction and iron-reducing microorganisms for ammonium oxidation are reviewed. The effects of environmental factors, e.g., pH, redox potential, Fe species, extracellular electron shuttles and natural organic matter, on the FeBNR reaction rate are analyzed. Current application advances in natural and artificial wastewater treatment are introduced with some typical experimental and application cases. Autotrophic FeBNR can treat low-C/N wastewater and greatly benefit the sustainable development of environmentally friendly biotechnologies for advanced nitrogen control.

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.

Rapid measurement of waterborne bacterial viability based on difunctional gold nanoprobe

Rapid measurement of waterborne bacterial viability is crucial for ensuring the safety of public health. Herein, we proposed a colorimetric assay for rapid measurement of waterborne bacterial viability based on a difunctional gold nanoprobe (dGNP). This versatile dGNP is composed of bacteria recognizing parts and signal indicating parts, and can generate color signals while recognizing bacterial suspensions of different viabilities. This dGNP-based colorimetric assay has a fast response and can be accomplished within 10 min. Moreover, the proposed colorimetric method is able to measure bacterial viability between 0% and 100%. The method can also measure the viability of other bacteria including Staphylococcus aureus, Shewanella oneidensis, and Escherichia coli O157H7. Furthermore, the proposed method has acceptable recovery (95.5–104.5%) in measuring bacteria-spiked real samples. This study offers a simple and effective method for the rapid measurement of bacterial viability and therefore should have application potential in medical diagnosis, food safety, and environmental monitoring.

A colorimetric method is proposed to measure waterborne bacterial viability by using a difunctional gold nanoprobe that can generate color signals while recognizing bacterial suspensions of different viabilities.

Extracellular polymeric substances from Shewanella oneidensis MR-1 biofilms mediate the transformation of Ferrihydrite

Extracellular polymeric substances (EPS) of dissimilatory iron-reducing bacteria (DIRB) such as Shewanella oneidensis MR-1 play a crucial role in the biotransformation of iron-containing minerals, but the mechanism has not been fully deciphered. Herein, abiotic and biotic transformation of ferrihydrite (Fh) were compared to clarify the contributions of MR-1, EPS-free MR-1 (MR-1-EPS), loosely bound EPS (LB-EPS), and tightly bound EPS (TB-EPS). The results of abiotic Fh transformation indicated that EPS did not block the Fh surfaces and thus has an insignificant effect on the adsorbed Fe(II)-Fh interaction. The complexation of the Fe(III) intermediate (Fe(III)_active) with EPS, especially LB-EPS, however, inhibited the nucleation of secondary Fe minerals and changed the crystallization pathway. For biotic Fh transformation, on the other hand, EPS had dual effects that accelerated Fh bioreduction due to the enhanced extracellular electron transfer (EET) and constrained the following Fh mineralization by cutting of the chain reactions leading to mineral crystallization. Our finding also suggested that the effects of EPS on Fh biotransformation largely depend on the chemical properties of EPS, especially the polar functional groups such as carboxyl and phosphate, because of their important abilities for the cell attachment and Fe(II)/Fe(III) binding.