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

Sequestration of Labile Organic Matter by Secondary Fe Minerals from Chemodenitrification: Insight into Mineral Protection Mechanisms

Labile organic matter (OM) immobilized by secondary iron (Fe) minerals from chemodenitrification may be an effective way to immobilize organic carbon (OC). However, the underlying mechanisms of coupled chemodenitrification and OC sequestration are poorly understood. Here, OM immobilization by secondary Fe minerals from chemodenitrification was investigated at different C/Fe ratios. Kinetics of Fe(II) oxidation and nitrite reduction rates decreased with increasing C/Fe ratios. Despite efficient sequestration, the immobilization efficiency of OM by secondary minerals varied with the C/Fe ratios. Higher C/Fe ratios were conducive to the formation of ferrihydrite and lepidocrocite, with defects and nanopores. Three contributions, including inner-core Fe-O and edge- and corner-shared Fe-Fe interactions, constituted the local coordination environment of mineral-organic composites. Microscopic analysis at the molecular scale uncovered that labile OM was more likely to combine with secondary minerals with poor crystallinity to enhance its stability, and OM distributed within nanopores and defects had a higher oxidation state. After chemodenitrification, high molecular weight substances and substances high in unsaturation or O/C ratios including phenols, polycyclic aromatics, and carboxylic compounds exhibited a stronger affinity to Fe minerals in the treatments with lower C/Fe ratios. Collectively, labile OM immobilization can occur during chemodenitrification. The findings on OM sequestration coupled with chemodenitrification have significant implications for understanding the long-term cycling of Fe, C, and N, providing a potential strategy for OM immobilization in anoxic soils and sediments.

Fe(II)-catalyzed phase transformation of Cd(II)-bearing ferrihydrite-kaolinite associations under anoxic conditions: New insights to role of kaolinite and fate of Cd(II)

Cadmium-bearing ferrihydrite-kaolinite associations (Cd-associations) are commonly found in cadmium-contaminated paddy soils in tropical and subtropical regions. In the presence of anaerobic conditions caused by flooding, the creation of Fe(II) can facilitate the transformation of ferrihydrite into secondary Fe (hydr)oxides, resulting in the redistribution of Cd. However, the role of kaolinite in iron oxides transformation and changes in Cd chemical species have largely not been determined. In this study, Cd-associations were prepared for reaction with Fe(II) under anoxic conditions. The results obtained from powder XRD and EXAFS indicated that the presence of kaolinite association noticeably hastened the transformation of ferrihydrite into crystalline goethite. Specific surface area and electrochemical analyses revealed that smaller particle sizes and higher reactivity of ferrihydrite within Cd-associations collaboratively contribute to the acceleration. Chemical analyses demonstrated a significant negative correlation between ferrihydrite-Fe and aqueous-Cd, and a significant positive correlation between crystalline-Fe and residual-Cd. HRTEM analyses indicated that a portion of the Cd was incorporated into the crystal lattices of lepidocrocite and goethite, with the majority of Cd being sequestered within goethite lattice. These findings provide new insights into the roles of clay minerals in the geochemical cycling of Fe and Cd in paddy soils under anoxic conditions.

Effect of soil organic matter-mediated electron transfer on heavy metal remediation: Current status and perspectives

Soil contamination by heavy metals poses major risks to human health and the environment. Given the current status of heavy metal pollution, many remediation techniques have been tested at laboratory and contaminated sites. The effects of soil organic matter-mediated electron transfer on heavy metal remediation have not been adequately studied, and the key mechanisms underlying this process have not yet been elucidated. In this review, microbial extracellular electron transfer pathways, organic matter electron transfer for heavy metal reduction, and the factors affecting these processes were discussed to enhance our understanding of heavy metal pollution. It was found that microbial extracellular electrons delivered by electron shuttles have the longest distance among the three electron transfer pathways, and the application of exogenous electron shuttles lays the foundation for efficient and persistent remediation of heavy metals. The organic matter-mediated electron transfer process, wherein organic matter acts as an electron shuttle, promotes the conversion of high valence state metal ions, such as Cr(VI), Hg(II), and U(VI), into less toxic and morphologically stable forms, which inhibits their mobility and bioavailability. Soil type, organic matter structural and content, heavy metal concentrations, and environmental factors (e.g., pH, redox potential, oxygen conditions, and temperature) all influence organic matter-mediated electron transfer processes and bioremediation of heavy metals. Organic matter can more effectively mediate electron transfer for heavy metal remediation under anaerobic conditions, as well as when the heavy metal content is low and the redox potential is suitable under fluvo-aquic/paddy soil conditions. Organic matter with high aromaticity, quinone groups, and phenol groups has a stronger electron transfer ability. This review provides new insights into the control and management of soil contamination and heavy metal remediation technologies.

Clay minerals inhibit the release of Cd(II) during the phase transformation of Cd(II)-ferrihydrite coprecipitates

Clay minerals and iron (hydr)oxides are important geosorbents in controlling the migration of heavy metal cations in the environment. Despite the widespread occurrence of clay minerals/iron (hydr)oxides composites, their complex mutual effects on the fate of heavy metal cations are not well recognized. In this work, we investigated the effect of clay minerals on the redistribution of Cd(II) during the phase transformation of ferrihydrite containing coprecipitated Cd(II) (Cd-Fh). Three systems were considered: i.e., Cd-Fh, Cd-Fh/kaolinite composite, and Cd-Fh/montmorillonite composite. Our results showed that the transformation of Fh into goethite and hematite caused the release of Cd(II), while the presence of kaolinite and montmorillonite inhibited the phase transformation of Fh and the release of Cd(II), with montmorillonite being more effective in these process. Multiple factors contributed to the reduced release of Cd(II), including the retarded transformation of Fh, the buffering of solution pH, and the re-adsorption of the released Cd(II). Our findings show that clay minerals have multiple effects in reducing the release of heavy metal cations from Fh during its transformation process, which sheds new light on understanding the critical roles of nanominerals in modulating the migration and bioavailability of heavy metal cations in the environment.

Enhanced microbial degradation mediated by pyrogenic carbon toward p-nitrophenol: Role of carbon structures and iron minerals

Pyrogenic carbon (PC) including black carbons and engineered carbons can mediate the extracellular electron transfer to facilitate the biogeochemical reaction with organic pollutants. Yet, the role of carbon structures and iron minerals on PC-mediated microbial degradation is still lacking of understanding. Herein, we studied the electrochemical properties of PCs produced from varied feedstock with regards to the mediated degradation of p-nitrophenol (PNP) by Shewanella putrefaciens CN32 in anoxic system. Mediated degradation by PCs was enhanced by facilitating extracellular electron transfer through oxygenated group and graphitic structure. Graphitic crystallites improved the electron-accepting capacity (as suggested by ID/IG and EAC) and diminished the electrochemical impedance (as suggested by Rct), contributing to PNP degradation under the anoxic system. Furthermore, more interfacial adsorption was conducive to the mediated reduction by the graphitic structure on PCs of high-temperature. In the presence of iron minerals, both hematite and goethite significantly facilitated PC-mediated degradation, which could be ascribed to the enhancement of the electron-donating capacity of microorganism and the accumulation of the reductive-state PCs by the interaction with generated Fe(II). This work paves a feasible way to the technical design on the remediation of phenolic contaminants by PC-mediated microbial degradation in environment.

Organic Matter Counteracts the Enhancement of Cr(III) Extractability during the Fe(II)-Catalyzed Ferrihydrite Transformation: A Nanoscale- and Molecular-Level Investigation

Phase transformation of ferrihydrite to more stable Fe (oxyhydr)oxides, catalyzed by iron(II) [Fe(II)], significantly influences the mobility of heavy metals [e.g., chromium (Cr)] associated with ferrihydrite. However, the impact of organic matter (OM) on the behavior of Cr(III) in the Fe(II)-catalyzed transformation of ferrihydrite and the underlying mechanisms are unclear. Here, the Fe(II)-catalyzed transformation of the coprecipitates of Fe(III), Cr(III), or rice straw-derived OM was studied at the nanoscale and molecular levels using Fe and Cr K-edge X-ray absorption spectroscopy and spherical aberration corrected scanning transmission electron microscopy (Cs-STEM). Batch extraction results suggested that the OM counteracted the enhancement of Cr(III) extractability during the Fe(II)-catalyzed transformation. Cs-STEM and XAS analysis suggested that Cr(III) could be incorporated into the goethite formed by Fe(II)-catalyzed ferrihydrite transformation, which, however, was inhibited by the OM. Furthermore, Cs-STEM analysis also provided direct nanoscale level evidence that residual ferrihydrite could re-immobilize the released Cr(III) during the Fe(II)-catalyzed transformation process. These results highlighted that the decreased extractability of Cr(III) mainly resulted from the inhibition of OM on the Fe(II)-catalyzed transformation of ferrihydrite to secondary Fe (oxyhydr)oxides, which facilitates insightful understanding and prediction of the geochemical cycling of Cr in soils with active redox dynamics.

Oxidative compensation mechanism of Fe-S synergetic inhibition of Cd activity in paddy field during flooding and drainage

It is known that the transformation of Fe and S forms in soil affects the migration and activity of Cd, but the coordinated regulation of Cd activity by Fe and S under different redox conditions is still unclear. Here, Diffusive gradients in thin films (DGT), an in-situ monitoring technique, is used to explore the difference of the regulation of Cd activity in paddy fields with ferrihydrite (FH) and ferrihydrite coprecipitated by sulfate (FH-S) under the flooding and drainage conditions. The addition of FH-S and FH significantly reduced the activity of Cd (Dissolved, Exchanged, and CDGT-Cd). Compared with pure FH, the adsorption extent of Cd in FH was enhanced by increasing concentrations of SO42- (i.e., S/Fe ratio), which is attributed to the decrease in the crystallinity of FH by sulfate. During soil flooding, the addition of FH-S promoted the production of metal sulfide (CdS and FeS/FeS2). The activity of Cd increased after drainage, while the FH-S treatment groups delayed the release of Cd. After 30 days of drainage, the concentration of Cd in FH-S treatment groups decreased by 28.9-44.1 % compared with the control group. The fresh FeS/FeS2 is not the main adsorbent for fixing Cd, and due to the existence of oxidation compensation mechanism, the preferential oxidation of FeS/FeS2 delays the release of Cd in the drainage stage. Our study shed new light on the mechanism of Fe-S synergistic regulation of Cd and remediation of Cd-contaminated soils.

Formation of ferrihydrite induced by low pe+pH in paddy soil reduces Cd uptake by rice: Evidence from Cd isotope fractionation

Ferrihydrite (Fh) is an important iron mineral in paddy soil and is prone to phase transition during dynamic redox condition, which affects Cd distribution and induces Cd isotope fractionation across soil to rice. Here, we conducted rice culture experiments with or not Fh application under different irrigation regimes to study the relationship between Fe species and Cd availability, as well as the isotope ratio of Cd in different Cd pools in paddy soil-rice system. Fh addition under continuous flooding (FL) with the decrease of pe + pH from 9.36 to 3.44 promoted the formation of amorphous Fe oxides as increased by 120.1% and facilitated Cd immobilization along with the increase of Fe/Mn oxides bound Cd by 25.3%, compared with continuous drying (DY) treatment. The isotopically heavy Cd were preferentially enriched from soil to extractable Cd (Δ^114/110Cd_{extractable Cd-soil} = 0.39-0.62‰) and from soil to grain (Δ^114/110Cd_grain-soil = 0.40-0.66‰) particularly at low pe + pH and with Fh addition, while light Cd were enriched in Fe/Mn oxides (Δ^114/110Cd_{Fe/Mn oxides bound Cd-extractable Cd} = -0.65 ∼ -0.14‰). Besides, the expression of transporters involved in Cd transport in rice like OsNRAMP1, OsNRAMP1, OsHMA3, OsHMA2 and OsLCT1 were suppressed under low pe + pH condition. These findings indicated that low pe + pH facilitated Cd stabilization by the existence of more amorphous iron oxides, which induced the enrichment of heavy Cd isotope in liquid phase and light in Fe/Mn (oxy)hydroxides, respectively.

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.

Effects of Fe oxides and their redox cycling on Cd activity in paddy soils: A review

Cadmium (Cd) contamination of soils is a global problem, particularly in paddy soils. Fe oxides, as a key fraction of paddy soils, can significantly affect the environmental behavior of Cd, which is controlled by complicated environmental factors. Therefore, it is necessary to systematically collect and generalize relevant knowledge, which can provide more insight into the migration mechanism of Cd and a theoretical basis for future remediation of Cd contaminated paddy soils. This paper summarized that (1) Fe oxides influence Cd activity through adsorption, complexation, and coprecipitation during transformation; (2) compared with the flooded period, the activity of Cd during the drainage period is stronger in paddy soils, and the affinity of different Fe components for Cd was distinct; (3) Fe plaque reduced Cd activity but was associated with plant Fe²⁺ nutritional status; (4) the physicochemical properties of paddy soils have the greatest impact on the interaction between Fe oxides and Cd, especially with pH and water fluctuations.

Nitrobenzene reduction promoted by the integration of carbon nanotubes and Geobacter sulfurreducens

Electron shuttles (ES) can mediate long-distance electron transfer between extracellular respiratory bacteria (ERB) and the surroundings. However, the effects of graphite structure in ES on the extracellular electron transfer (EET) process remain ambiguous. This work investigated the function of graphite structure in the process of nitrobenzene (NB) degradation by Geobacter sulfurreducens PCA, in which highly aromatic carbon nanotubes (CNTs) was studied as a typical ES. The results showed that the addition of 1.5 g L^-1 of CNTs improved the NB biodegradation up to 81.2%, plus 18.8% NB loss due to the adsorption property of CNTs, achieving complete removal of 200 μM NB within 9 h. The amendment of CNTs greatly increased the EET rate, indicating that graphite structure exhibited excellent electron shuttle performance. Furthermore, Raman spectrum proved that CNTs obtained better graphite structure after 90 h of cultivation with strain PCA, resulting in higher electrochemical performance. Also, CNTs was perceived as the "Contaminant Reservoir", which alleviated the toxic effect of NB and shortened the distance of EET process. Overall, this work focused on the effects of material graphite structure on the EET process, which enriched the understanding of the interaction between CNTs and ERB, and these results might promote their application in the in-situ bioremediation of nitroaromatic-polluted environment.

Pyrogenic Carbon Improves Cd Retention during Microbial Transformation of Ferrihydrite under Varying Redox Conditions

Fe(III) (oxyhydr)oxides are ubiquitous in paddy soils and play a key role in Cd retention. Recent studies report that pyrogenic carbon (PC) may largely affect the microbial transformation processes of Fe(III) (oxyhydr)oxides, yet the impact of PC on the fate of Fe(III) (oxyhydr)oxide-associated Cd during redox fluctuations remains unclear. Here, we investigated the effects of PC on Cd retention during microbial (Shewanella oneidensis MR-1) transformation of Cd(II)-bearing ferrihydrite under varying redox conditions. The results showed that in the absence of PC, microbial reduction of ferrihydrite resulted in Cd release under anoxic conditions and Fe(II) oxidation by oxygen resulted in Cd retention under subsequent oxic conditions. The presence of PC facilitated microbial ferrihydrite reductive dissolution under anoxic conditions, promoted Fe(II) oxidative precipitation under oxic conditions, and inhibited Cd release under both anoxic and oxic conditions. The presence of PC and frequent shifts in redox conditions (i.e., redox cycling) inhibited the transformation of ferrihydrite to highly crystalline goethite and magnetite that exhibited less Cd adsorption. As a result, PC enhanced Cd retention by 41-59% and 55-77% after the redox shift and redox cycling, respectively, while in the absence of PC, Cd retention decreased by 5% after the redox shift and increased by 11% after redox cycling. Sequential extraction analysis revealed that 63-78% of Cd was associated with Fe minerals, while 3-12% of Cd was bound to PC, indicating that PC promoted Cd retention mainly through inhibiting ferrihydrite transformation. Our results demonstrate the great impacts of PC on improving Cd retention under dynamic redox conditions, which is essential for applying PC in remediating Cd-contaminated paddy soils.

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.

Anaerobic syntrophic system composed of phosphate solubilizing bacteria and dissimilatory iron reducing bacteria induces cadmium immobilization via secondary mineralization

Secondary mineralization is a promising method for remediating cadmium (Cd) pollution in sediments, but the poor stability of Cd-containing secondary minerals is a bottleneck that limits the development of this approach. The existence of phosphate can enhance the formation of stable secondary minerals and points a new direction for Cd immobilization. In this research, a novel syntrophic system composed of phosphate solubilizing bacteria (PSB) and dissimilatory iron reducing bacteria (DIRB) was established and the effect and mechanism of Cd immobilization in the system were also explored. The results showed that under the conditions of DIRB:PSB (V:V)= 3:1, syntrophic bacteria dosage of 5% and glucose dosage of 5 g/L, Cd incorporated in the secondary minerals could account for about 60% of the total Cd. In the pH range of 5-9, alkaline environment was conducive to the immobilization of Cd and the percentage of combined Cd was up to 58%, while the combined Cd in secondary minerals decreased from 62% to 56% with the increase of initial Cd concentration from 0.1 to 0.3 mmol/L. In addition, XRD, XPS, Mössbauer and other characterization results showed that secondary minerals, such as Cd exchange hydroxyapatite (Cd-HAP) and kryzhanovskite (Fe₃(PO₄)₂(OH)₃) were formed in this new system. The established syntrophic system of PSB and DIRB is thus a prospective bioremediation technology for Cd immobilization in sediments and can avoid the potential risk might be caused by the addition of phosphorus-containing materials.

Humic acid and fulvic acid facilitate the formation of vivianite and the transformation of cadmium via microbially-mediated iron reduction

The effects of humic acids (HA) and fulvic acids (FA) on the fate of Cd in anaerobic environment upon microbial reduction of Cd-bearing ferrihydrite (Fh) with Geobacter metallireducens were investigated. The results showed that HA and FA could promote the reductive dissolution of Fh and the formation of vivianite. After incubation of 38 d, vivianite accounted for 47.19%, 59.22%, and 48.53% of total Fe in biological control batch (BCK), HA and FA batches (C/Fe molar ratio of 1.0), respectively, by Mössbauer spectroscopy analysis. In terms of Cd, HA and FA could promote the release of adsorbed Cd during the initial bioreduction process, but reassuringly, after 38 d the dissolved Cd with HA and FA addition batches were 0.58-0.91 and 0.99-1.08 times of the BCK, respectively. The proportions of residual Cd in HA batches were higher than FA and BCK batches, indicating that HA was better than FA in immobilizing Cd. This might be because the quinone groups in HA could act as electron shuttle. This study showed that HA facilitated the transformation of vivianite better than FA, and Cd can be stabilized by resorption or co-precipitation with vivianite, providing a theoretical support for the translocation of Cd in sediment-water interface.

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.

A new DGT technique based on nano-sized Mg2Al layered double hydroxides with DTPA for sampling of eight anionic and cationic metals

In this work, a new resin gel incorporated with layered double hydroxide nanoparticles modified with diethylenetriaminepentaacetic acid is developed for application in diffusive gradients in thin-film devices (abbreviated as LDHs DGT) to monitor eight anions and cations (such as Fe, Mn, Co, Ni, Cu, Cd, Pb, and As) in natural waters and soils. The accumulated anions and cations were quantitatively recovered by one-step elution using 0.5 mol·L^-1 HNO₃ with an optimized elution time of 30 min. The performance of the LDHs DGT was independent of solution pH (5-8) and ionic strengths (5-100 mmol·L^-1). The capacities of the LDHs DGT for Mn(II), Fe(II), Co(II), Ni(II), Cu(II), As(V), Cd(II), and Pb(II) individually are determined to be 202.9, 363.6, 246.9, 88.8, 99.5, 75.3, 159.8, and 671.7 μg·cm^-2. During the field deployments in a nature river, LDHs DGT measured concentrations of cations and anions were almost like those measured by the traditional sampling method (except Fe(II), Cd(II), and Co(II)). In addition, bioavailable Cd measured by LDHs DGT correlated well with Cd in rice grains (R² = 0.55), indicating that LDHs DGT is a reliable tool for assessing the risk of Cd.

Humic acid controls cadmium stabilization during Fe(II)-induced lepidocrocite transformation

Abiotic reduction of iron (oxyhydr)oxides by aqueous Fe(II) is one of the key processes affecting the Fe cycle in soil. Lepidocrocite (Lep) occurs naturally in anaerobic, clayey, non-calcareous soils in cooler and temperate regions; however, little is known about the impacts of co-precipitated humic acid (HA) on Fe(II)-induced Lep transformation and its consequences for heavy metal immobilization. In this study, the Fe(II)-induced phase transformation of Lep-HA co-precipitates was analyzed as a function of the C/Fe ratio, and its implications for subsequent Cd(II) concentration dynamic in dissolved and solid form was further investigated. The results revealed that secondary Fe(II)-bearing magnetite commonly formed during the Fe(II)-induced transformation of Lep, which further changed the mobility and distribution of Cd(II). The co-precipitated HA resulted in a decrease in the Fe solid phase transformation as the C/Fe ratios increased. Magnetite was found to be a secondary mineral in the 0.3C/Fe ratio Lep-HA co-precipitate, while only Lep was observed at a C/Fe ratio of 1.2 using X-ray diffraction (XRD) and Mössbauer spectroscopy. Based on XRD, scanning electron microscopy (SEM), Mössbauer, X-ray photoelectron spectroscopy (XPS), and Fourier transform infrared spectroscopy (FTIR) results, newly formed magnetite may immobilize Cd(II) through surface complexes, incorporation, or structural substitution. The presence of HA was beneficial for binding Cd(II) and affected the mineralogical transformation of Lep into magnetite, which further induced the distribution of Cd(II) into the newly formed secondary minerals. These results provide insights into the behavior of Cd(II) in response to reaction between humic matter and iron (oxyhydr)oxides in anaerobic environments.

Mechanism of mercapto-modified palygorskite in reducing soil Cd activity

Mercapto-modified palygorskite (MP) is an efficient novel amendment with superior ability to decrease soil Cd bioavailability, but the unclear immobilization mechanism has become the bottleneck of its performance improvement and precise application. In order to clarify the Cd reducing mechanism of MP, long-term and short-term soil incubation with three types of soils (paddy soil, alluvial soil and yellow mountain soil) and sorption verification experiments were conducted to investigate the dynamic process of soil labile Cd impacted by MP and the synergetic effects on labile Fe, Mn, S and dissolved organic carbon via in-situ diffusive gradients in thin-films and soil solution sampling techniques. MP with four dosages rapidly and continuously decreased soil labile Cd contents by 14.50 % ∼ 89.16 % in long-term incubation, meanwhile low-dosage MP reduced soil labile Fe and Mn contents, but high-dosage MP increased their contents. With MP dosages increased, the effects of Fe-Mn oxides on soil labile Cd content gradually weakened. MP effectively promoted the reduction of Fe adsorbed by clay minerals and enhanced their ability to adsorb Cd. Short-term incubation showed that MP could decline soil labile Cd by 7.17 % ∼ 44.74 %, especially at the dosage 0.4 %. MP was a reduction catalyst to facilitate Fe reduction, which profited for clay minerals adsorbing Cd. The sorption experiments indicated that 0.30 % MP could adsorb 73.34 % Cd²⁺, promote the release of Fe²⁺ from the soil, and stimulate the ability of clay minerals to adsorb Cd. The results revealed that MP decreased soil labile Cd content within 2 d, and MP made soil Cd activity change out of the influence of soil Fe/Mn redox system. The mechanism will be beneficial for the large-scale application of MP in safe utilization of Cd contaminated soil.

Effect of humic acid on bioreduction of facet-dependent hematite by Shewanella putrefaciens CN-32

Interfacial reactions between iron (Fe) (hydr)oxide surfaces and the activity of bacteria during dissimilatory Fe reduction affect extracellular electron transfer. The presence of organic matter (OM) and exposed facets of Fe (hydr)oxides influence this process. However, the underlying interfacial mechanism of facet-dependent hematite and its toxicity toward microbes during bioreduction in the presence of OM remains unknown. Herein, humic acid (HA), as typical OM, was selected to investigate its effect on the bioreduction of hematite {100} and {001}. When HA concentration was increased from 0 to 500 mg L^-1, the bioreduction rates increased from 0.02 h^-1 to 0.04 h^-1 for hematite {100} and from 0.026 h^-1 to 0.05 h^-1 for hematite {001}. Since hematite {001} owned lower resistance than hematite {100} irrespective of the HA concentration, and hematite {100} was less favorable for reduction. Microscopy-based analysis showed that more hematite {001} nanoparticles adhered to the cell surface and were bound more closely to the bacteria. Moreover, less cell damage was observed in the HA-hematite {001} treatments. As the reaction progressed, some bacterial cells died or were inactivated; confocal laser scanning microscopy showed that bacterial survival was higher in the HA-hematite {001} treatments than in the HA-hematite {100} treatments after bioreduction. Spectroscopic analysis revealed that facet-dependent binding was primarily realized by surface complexation of carboxyl functional groups with structural Fe atoms, and that the binding order of HA functional groups and hematite was affected by the exposed facets. The exposed facets of hematite could influence the electrochemical properties and activity of bacteria, as well as the binding of bacteria and Fe oxides in the presence of OM, thereby governing the extracellular electron transfer and concomitant bioreduction of Fe (hydr)oxides. These results provide new insights into the interfacial reactions between OM and facet-dependent Fe oxides in anoxic, OM-rich soil and sediment environments.

Multiple Effects of Humic Components on Microbially Mediated Iron Redox Processes and Production of Hydroxyl Radicals

Microbially mediated iron redox processes are of great significance in the biogeochemical cycles of elements, which are often coupled with soil organic matter (SOM) in the environment. Although the influences of SOM fractions on individual reduction or oxidation processes have been studied extensively, a comprehensive understanding is still lacking. Here, using ferrihydrite, Shewanella oneidensis MR-1, and operationally defined SOM components including fulvic acid (FA), humic acid (HA), and humin (HM) extracted from black soil and peat, we explored the SOM-mediated microbial iron reduction and hydroxyl radical (^•OH) production processes. The results showed that the addition of SOM inhibited the transformation of ferrihydrite to highly crystalline iron oxides. Although FA and HA increased Fe(II) production over four times on average due to complexation and their high electron exchange capacities, HA inhibited 30-43% of the ^•OH yield, while FA had no significant influence on it. Superoxide (O₂^•-) was the predominant intermediate in ^•OH production in the FA-containing system, while one- and two-electron transfer processes were concurrent in HA- and HM-containing systems. These findings provide deep insights into the multiple mechanisms of SOM in regulating microbially mediated iron redox processes and ^•OH production.

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.

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.

Mechanistic and modeling insights into the immobilization of Cd and organic carbon during abiotic transformation of ferrihydrite induced by Fe(II)

Iron (Fe) oxides and fulvic acid (FA) are the key components affecting the fate of cadmium (Cd) in soil. The presence of FA influences Fe mineral transformation, and FA may complicate phase transformation and dynamic behavior of Cd. How varying Fe minerals and FA affect Cd immobilization during the ferrihydrite transformation induced by various Fe(II) concentrations, however, is still lack of quantitative understanding. In this study, we built a model for Cd species quantification during phase transformation based on mechanistic insights obtained from batch experiments. Spectroscopic analysis showed that Fe(II) concentrations affected secondary Fe minerals formation under the condition of co-existence of Cd and FA, and ultimately changed the distribution of Cd and FA. Microscopic analysis revealed that besides surface adsorption, part of Cd was sequestrated by magnetite, whereas FA was able to diffuse into lepidocrocite defects. The model revealed that adsorbed Cd was mainly controlled by FA and ferrihydrite, and direct complexation of Cd by FA had a strong impact on the continuous change in Cd at lower Fe(II) concentration. The results contribute to an in-depth understanding of the mobility of Cd in the environment and provide a method for quantifying the dynamic behavior of heavy metals in multi-reactant systems.

Tetracycline-Induced Release and Oxidation of As(III) Coupled with Concomitant Ferrihydrite Transformation

Cocontamination with tetracycline (TC) and arsenic (As) is very common in paddy fields. However, the process and underlying mechanism of arsenite (As(III)) transformation on iron mineral surfaces in the presence of antibiotic contaminants remain unclear. In this study, the release and oxidation of As(III) on ferrihydrite (Fh) surfaces and Fh transformation in the presence of TC under both aerobic and anaerobic conditions were investigated. Our results indicated that the TC-induced reductive dissolution of Fh (Fe(II) release) and TC competitive adsorption significantly promote the release of As, especially under anaerobic conditions. The release of As was increased with increasing TC concentration, whereas it decreased with increasing pH. Interestingly, under both aerobic and anaerobic conditions, the addition of TC enhanced the oxidation of As(III) by Fh and induced the partial transformation of Fh to lepidocrocite. Under aerobic conditions, the adsorbed Fe(II) activated the production of reactive oxygen species (·OH and ¹O₂) from dissolved O₂, with Fe(IV) being responsible for As(III) oxidation. Under anaerobic conditions, the abundant oxygen vacancies of Fh affected the oxidation of As(III) during Fh recrystallization. Thus, this study provided new insights into the role of TC on the migration and transformation of As coupled with Fe in soils.

Diel Fluctuation of Extracellular Reactive Oxygen Species Production in the Rhizosphere of Rice

Reactive oxygen species (ROS) are ubiquitous on earth and drive numerous redox-centered biogeochemical processes. The rhizosphere of wetland plants is a highly dynamic interface for the exchange of oxygen and electrons, presenting the basis of the precedent for ROS production, yet whether extracellular ROS are produced in the rhizosphere remains unknown. Here, we designed a microfluidic chip setup to detect in-situ ROS productions in the rhizosphere of rice with spatial and temporal resolutions. Fluorescence imaging clearly displayed the hot spots of ROS generation in the rhizosphere. The formation concentration of the hydroxyl radical (^•OH, a representative ROS, 10^-6 M) was comparable to those by the classical photochemical route (10^-6-10^-7 M) in aquatic systems, therefore highlighting the rhizosphere as an unrecognized hotspot for ROS production. Moreover, the rhizosphere ROS production exhibits diel fluctuation, which simultaneously fluctuated with dissolved oxygen, redox potential, and pH, all driven by radial oxygen loss near the root in the daytime. The production and diel fluctuation of ROS were confirmed in the rhizosphere of rice root incubated in natural soils. We demonstrated that the extracellular ROS production was triggered by the interplay between root-released oxygen and microbial respiration released extracellular electrons, while iron mineral and organic matter might play key roles in storing and shuttling electrons. Our results highlight the rhizosphere as a widespread but previously unappreciated hotspot for ROS production, which may affect pollutant redox dynamics and biogeochemical processes in soils.

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.

Assessing environmental fate of hexavalent chromium as influenced by fractionation of ferrihydrite with dissolved organic matter

The dynamic interactions among iron (Fe) oxides, dissolved organic matter (DOM) and toxic trace metals play crucial roles in risk assessment and environmental remediation. Although the inhibitory effects of DOM on the iron oxides transformation process have been studied previously, there is still a lack of mechanistic and quantitative understanding on the kinetics of Cr(VI) and ferrihydrite transformation in the present of DOM. In this study, we investigated the fractionation process of DOM on ferrihydrite and its influence on the fate of Cr(VI) and transformation of ferrihydrite. The result of three-dimension excitation emission matrix (3D-EEM), Q-Exactive LC-MS/MS, X-ray diffraction (XRD) and high-resolution transmission electron microscopy (HR-TEM) indicated that fulvic acid-like compounds of DOM were the mainly fractionated compounds on the surface of ferrihydrite, which further inhibited the transformation of ferrihydrite. Besides, bracewellite (CrO(OH)) generated as an accompanied mineral during the transformation of ferrihydrite in the present of Cr(VI). Based on the DFT theoretical calculation, we concluded that Cr(VI) mainly in the form of HCr O₄^- was more inclined to be adsorbed on iron-oxide tetrahedron by inner-sphere monodentate mononuclear configurations. The findings on the dynamic coupling among Fe oxide transformation and Cr(VI) sequestration under the effect of DOM provided the basis for accurately predicting the fate of trace elements and iron mineral.

Immobilization and redistribution process of As(V) during As(V)-bearing ferrihydrite reduction by Geobacter sulfurreducens under the influence of TiO2 nanoparticles

The redistribution process of arsenate (As(V)) and the variation in As(V) content in different locations must be clarified to ensure low mobility of As(V) during microbial ferrihydrite reduction. In this study, we investigated As(V) immobilization and redistribution processes when ferrihydrite was incubated with Geobacter sulfurreducens in the presence of titanium dioxide (TiO₂) nanoparticles. Our study results showed that, As(V) in the aqueous phase and ferrihydrite were redistributed on light minerals (goethite), heavy minerals (ferrihydrite and magnetite), and extracellular polymeric substances (EPS) induced by G. sulfurreducens during ferrihydrite reduction. Interestingly, we found that As(V) in the form of arsenate ion (AsO₄^3-) was adsorbed by the functional groups of the EPS, while the formed Fe^II₃(As^VO₄)₂ was wrapped in the network structure of EPS. Moreover, the addition of TiO₂ nanoparticles did not promote but delayed the entire ferrihydrite reduction, As(V) immobilization and redistribution processes. Furthermore, changes in the aqueous arsenic and iron concentrations are closely related to the formation time of secondary minerals. Our study findings provide new insights into the As(V) immobilization process mediated by G. sulfurreducens under anaerobic conditions.

Insights into the underlying mechanisms of stability working for As(III) removal by Fe-Mn binary oxide as a highly efficient adsorbent

Fe-Mn binary oxide has received increasing interest in treating As(III)-containing polluted groundwater due to its low cost and environmental friendliness. Although the stability of Fe-Mn binary oxide is as important as its adsorption ability, little is known about whether and why Fe-Mn binary oxide is stable during As(III) removal. In this study, five successive cycles were conducted to evaluate the stability of Fe-Mn binary oxide for As(III) removal. As(III) oxidation/adsorption kinetics and the speciation distribution of the released Fe and Mn elements within single Fe oxide, Mn oxide, and Fe-Mn binary oxide were investigated by using characterization techniques of TEM-EDS mapping, selected area electron diffraction (SAED), and XPS combined with a binary component reactor, where Fe and Mn oxides were separated by a semipermeable membrane. The results revealed that Fe-Mn binary oxide could maintain excellent stability, although As(III) oxidation/adsorption behavior was coupled with the release of Fe and Mn ions from its surface. The great stability of Fe-Mn binary oxide for As(III) removal was attributed to the rapid return of aqueous Fe(II) and Mn(II) to the solid surface, which subsequently formed new mineral phases mediated by Fe and Mn oxides, thus considerably decreasing the loss of released Mn(II) and Fe(II).