Quantifying Microbially Mediated Kinetics of Ferrihydrite Transformation and Arsenic Reduction: Role of the Arsenate-Reducing Gene Expression Pattern

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.

Interface Visualization of Bio-material Interaction Via Cryo-AEM Using the Biosynthesis of Iron-Based Nanoparticles as a Model

Although interaction between organisms and nonorganisms is vital in environmental processes, it is difficult to characterize at nanoscale resolution. Biosynthesis incorporates intracellular and extracellular processes involving crucial interfacial functions and electron and substance transfer processes, especially on the inorganic-organic interface. This work chooses the biosynthesis of iron-based nanoparticles (nFe) as a model for biomaterial interaction and employs Cryo-AEM (i.e., S/TEM, EELS, and EDS analysis based on sample preparation with cryo-transfer holder system), combined with CV, Raman, XPS, and FTIR to reveal the inorganic-organic interface process. The inorganic-organic interactions in the biosynthesis of iron-based nanoparticles by Shewanella oneidensis MR-1 (M-nFe) were characterized by changes in electron cloud density, and the corresponding chemical shifts of Fe and C EELS edges confirm that M-nFe acquires electrons from MR-1 on the interface. Capturing intact filamentous-like, slightly curved, and bundled structure provides solid evidence of a "circuit channel" for electron transfer between organic and inorganic interface. CV results also confirm that adding M-nFe can enhance electron transfer from MR-1 to ferric ions. A mechanism for the synthesis of M-nFe with MR-1 based on intracellular and extracellular conditions under facultative anaerobic was visualized, providing a protocol for investigating the organic-inorganic interface.

A mechanistic approach to arsenic adsorption and immobilization in aqueous solution, groundwater, and contaminated paddy soil using pine-cone magnetic biochar

Arsenic (As) poisoning in groundwater and rice paddy soil has increased globally, impacting human health and food security. There is an urgent need to deal with As-contaminated groundwater and soil. Biochar can be a useful remedy for toxic contaminants. This study explains the synthesis of pinecone-magnetic biochar (PC-MBC) by engineering the pinecone-pristine biochar with iron salts (FeCl3.6H2O and FeSO4.7H2O) to investigate its effects on As(V) adsorption and immobilization in water and soil, respectively. The results indicated that PC-MBC can remediate As(V)-contaminated water, with an adsorption capacity of 12.14 mg g-1 in water. Isotherm and kinetic modeling showed that the adsorption mechanism involved multilayer, monolayer, and diffusional processes, with chemisorption operating as the primary interface between As(V) and biochar. Post-adsorption analysis of PC-MBC, using FTIR and XRD, further revealed chemical fixing and outer-sphere complexation between As(V) and Fe, O, NH, and OH as the main reasons for As(V) adsorption onto PC-MBC. Recycling of PC-MBC also had excellent adsorption even after several regeneration cycles. Similarly, PC-MBC successfully immobilized As in paddy soil. Single and sequential extraction results showed the transformation of mobile forms of As to a more stable form, confirmed by non-destructive analysis using SEM, EDX, and elemental dot mapping. Thus, Fe-modified pine-cone biochar could be a suitable and cheap adsorbent for As-contaminated water and soil.

Manganese promotes stability of natural arsenic sinks in a groundwater system with arsenic-immobilization minerals: Natural remediation mechanism and environmental implications

Arsenic (As)-immobilizing iron (Fe)-manganese (Mn) minerals (AFMM) represent potential As sinks in As-enriched groundwater environments. The process and mechanisms governing As bio-leaching from AFMM through interaction with reducing bacteria, however, remain poorly delineated. This study examined the transformation and release of As from AFMM with varying Mn/Fe molar ratios (0:1, 1:5, 1:3, and 1:1) in the presence of As(V)-reducing bacteria specifically Shewanella putrefaciens CN32. Notably, strain CN32 significantly facilitated the bio-reduction of As(V), Fe(III), and Mn(IV) in AFMM. In systems with Mn/Fe molar ratios of 1:5, 1:3, and 1:1, As bio-reduction decreased by 28%, 34%, and 47%, respectively, compared to the system with a 0:1 ratio. This Mn-induced inhibition of Fe/As bio-reduction was linked to several concurrent factors: preferential Mn bio-reduction, reoxidation of resultant Fe(II)/As(III) due to Mn components, and As adsorption onto emergent Fe precipitates. Both the reductive dissolution of AFMM and the bio-reduction of As(V) predominantly controlled As bio-release. Structural equation models indicated that reducing bacteria destabilize natural As sinks more through As reduction than through Mn(II) release, Fe reduction, or Fe(II) release. Systems with Mn/Fe molar ratios of 1:5, 1:3, and 1:1 showed a decrease in As bio-release by 24%, 41%, and 59%, respectively, relative to the 0:1 system. The observed suppression of As bioleaching was ascribed to both the inhibition of As/Fe bio-reduction by Mn components and the immobilization of As by freshly generated Fe precipitates. These insights into the constraining effect of Mn on the biotransformation and bioleaching of As from AFMM are crucial for grasping the long-term stability of natural As sinks in groundwater, and enhance strategies for in-situ As stabilization in As-afflicted aquifers through Nature-Based Solutions.

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.

Dynamic coupling of ferrihydrite transformation and associated arsenic desorption/redistribution mediated by sulfate-reducing bacteria

Sulfate-reducing bacteria play an important role in the geochemistry of iron (oxyhydr)oxide and arsenic (As) in natural environments; however, the associated reaction processes are yet to be fully understood. In this study, batch experiments coupled with geochemical, spectroscopic, microscopic, and thermodynamic analyses were conducted to investigate the dynamic coupling of ferrihydrite transformation and the associated As desorption/redistribution mediated by Desulfovibrio vulgaris (D. vulgaris). The results indicated that D. vulgaris could induce ferrihydrite transformation via S2--driven and direct reduction processes. In the absence of SO42-, D. vulgaris directly reduced ferrihydrite, and As desorption and re-sorption occurred simultaneously during the partial transformation of ferrihydrite to magnetite. The increase in SO42- loading promoted the S2--driven reduction of ferrihydrite and accelerated the subsequent mineralogical transformation. In the low and medium SO42- treatments, ferrihydrite was completely transformed to a mixture of magnetite and mackinawite, which increased the fraction of As in the residual phase and stabilized As. In the high SO42- treatment, although the replacement of ferrihydrite by only mackinawite also increased the fraction of As in the residual phase, 22.1% of the total As was released into the solution due to the poor adsorption affinity of As to mackinawite and the conversion of As5+ to As3+. The mechanisms of ferrihydrite reduction, mineralogy transformation, and As mobilization and redistribution mediated by sulfate-reducing bacteria are closely related to the surrounding SO42- loadings. These results advance our understanding of the biogeochemical behavior of Fe, S, and As, and are helpful for the risk assessment and remediation of As contamination.

The key roles of Fe oxyhydroxides and humic substances during the transformation of exogenous arsenic in a redox-alternating acidic paddy soil

Arsenic (As) from mine wastewater is a significant source for acidic paddy soil pollution, and its mobility can be influenced by alternating redox conditions. However, mechanistic and quantitative insights into the biogeochemical cycles of exogenous As in paddy soil are still lacking. Herein, the variations of As species in paddy soil spiking with As(III) or As(V) were investigated in the process of 40 d of flooding followed 20 d of drainage. During flooding process, available As was immobilized in paddy soil spiking As(III) and the immobilized As was activated in paddy soil spiking As(V) owing to deprotonation. The contributions of Fe oxyhydroxides and humic substances (HS) to As immobilization in paddy soil spiking As(III) were 80.16% and 18.64%, respectively. Whereas the contributions of Fe oxyhydroxides and HS to As activation in paddy soil spiking As(V) were 47.9% and 52.1%, respectively. After entering drainage, available As was mainly immobilized by Fe oxyhydroxides and HS and adsorbed As(III) was oxidized. The contribution of Fe oxyhydroxides to As fixation in paddy soil spiking As(III) and As(V) was 88.82% and 90.26%, respectively, and of HS to As fixation in paddy soil spiking As(III) and As(V) was 11.12% and 8.95%, respectively. Based on the model fitting results, the activation of Fe oxyhydroxides and HS bound As followed with available As(V) reduction were key processes during flooding. This may be because the dispersion of soil particles and release of soil colloids activated the adsorbed As. Immobilization of available As(III) by amorphous Fe oxyhydroxides followed with adsorbed As(III) oxidation were key processes during drainage. This may be ascribe to the occurrence of coprecipitation and As(III) oxidation mediated by reactive oxygen species from Fe(II) oxidation. The results are beneficial for a deeper understanding of As species transformation at the interface of paddy soil-water as well as an estimation pathway for the impacts of key biogeochemical cycles on exogenous As species under a redox-alternating condition.

Reductive Sequestration of Cr(VI) and Immobilization of C during the Microbially Mediated Transformation of Ferrihydrite-Cr(VI)-Fulvic Acid Coprecipitates

Cr(VI) detoxification and organic matter (OM) stabilization are usually influenced by the biological transformation of iron (Fe) minerals; however, the underlying mechanisms of metal-reducing bacteria on the coupled kinetics of Fe minerals, Cr, and OM remain unclear. Here, the reductive sequestration of Cr(VI) and immobilization of fulvic acid (FA) during the microbially mediated phase transformation of ferrihydrite with varying Cr/Fe ratios were investigated. No phase transformation occurred until Cr(VI) was completely reduced, and the ferrihydrite transformation rate decreased as the Cr/Fe ratio increased. Microscopic analysis was uncovered, which revealed that the resulting Cr(III) was incorporated into the lattice structure of magnetite and goethite, whereas OM was mainly adsorbed on goethite and magnetite surfaces and located within pore spaces. Fine line scan profiles showed that OM adsorbed on the Fe mineral surface had a lower oxidation state than that within nanopores, and C adsorbed on the magnetite surface had the highest oxidation state. During reductive transformation, the immobilization of FA by Fe minerals was predominantly via surface complexation, and OM with highly aromatic and unsaturated structures and low H/C ratios was easily adsorbed by Fe minerals or decomposed by bacteria, whereas Cr/Fe ratios had little effect on the binding of Fe minerals and OM and the variations in OM components. Owing to the inhibition of crystalline Fe minerals and nanopore formation in the presence of Cr, Cr sequestration and C immobilization can be synchronously favored at low Cr/Fe ratios. These findings provide a profound theoretical basis for Cr detoxification and synchronous sequestration of Cr and C in anoxic soils and sediments.

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.

Hematite-mediated Mn(II) abiotic oxidation under oxic conditions: pH effect and mineralization

Interactions between manganese (Mn) and iron (Fe) are widespread processes in soils and sediments, however, the abiotic transformation mechanisms are not fully understood. Herein, Mn(II) oxidation on hematite were investigated at various pH under oxic condition. Mn(II) oxidation rates increased from 3 × 10^-4 to 8 × 10^-2 h^-1 as pH increased from 7.0 to 9.0, whereas hematite enhanced Mn(II) oxidation rates to 1 h^-1. During oxidation process, high pH could promote the oxidation of Mn(II) into Mn minerals, resulting in the rapid consumption of the newly-formed H⁺, and high pH facilitated Mn(II) adsorption and oxidation by altering Mn(II) reactivity and speciation. Only granule-like hausmannite was found on the hematite surface at pH 7.0, whereas hausmannite particles and feitknechtite and manganite nanowires were formed at pH from 7.5 to 9.0. Moreover, a co-shell structured nanowire composed of manganite and feitknechtite was observed owing to autocatalytic reactions. Specifically, electron transfers between Mn(II) and O₂ occurred on the surface or through bulk phase of hematite, and direct electron transfers in the O₂-Mn(II) complex and indirect electron transfers in the O₂-Fe(II/III)-Mn(II) complex may both have contribution to the overall reactions. The findings provide a comprehensive interpretation of Fe-Mn interaction and have implications for the formation of soil Fe-Mn oxyhydroxides with unique properties in controlling element cycling.

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.

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.

Effect of manganese oxides on arsenic speciation and mobilization in different arsenic-adsorbed iron-minerals under microbially-reducing conditions

The oxidation and immobilization of arsenic (As) by manganese oxides have been shown to reduce As toxicity and bioavailability under abiotic conditions. In this study, we investigate the impact of manganese oxide (δ-MnO₂) on the fate of different Fe-minerals-adsorbed As in the presence of As(V)-reducing bacteria Bacillus sp. JQ. Results showed that in the absence of δ-MnO₂, As release in goethite was much higher than in ferrihydrite and hematite during microbial reduction. Adding 3.1 mM Mn reduced As release by 0.3%, 46.3%, and 6.7% in the ferrihydrite, goethite, and hematite groups, respectively. However, aqueous As was dominated by As(III) in the end, because the oxidation effect of δ-MnO₂ was limited and short-lived. Additionally, the fraction of solid-phase As(V) increased by 9.8% in ferrihydrite, 39.4% in goethite, and 7.4% in hematite in the high-Mn treatments, indicating that δ-MnO₂ had the most significant oxidation and immobilization effect on goethite-adsorbed As. This was achieved because goethite particles were evenly distributed on δ-MnO₂ surface, which supported As(III) oxidation by δ-MnO₂; while ferrihydrite strongly aggregated, which hindered the oxidation of As(III). Our study shows that As-oxidation and immobilization by manganese oxides cannot easily be assessed without considering the mineral composition and microbial conditions of soils.

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.

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.

Encapsulins from Ca. Brocadia fulgida: An effective tool to enhance the tolerance of engineered bacteria (pET-28a-cEnc) to Zn2

Zn²⁺ is largely discharged from many industries and poses a severe threat to the environment, making its remediation crucial. Encapsulins, proteinaceous nano-compartments, may protect cells against environmental stresses by sequestering toxic substances. To determine whether hemerythrin-containing encapsulins (cEnc) from anammox bacteria Ca. Brocadia fulgida can help cells deal with toxic substances such as Zn²⁺, we transferred cEnc into E.coli by molecular biology technologies for massive expression and then cultured them in media with increasing Zn²⁺ levels. The engineered bacteria (with cEnc) grew better and entered the apoptosis phase later, while wild bacteria showed poor survival. Furthermore, tandem mass tag-based quantitative proteomic analysis was used to reveal the underlying regulatory mechanism by which the genetically-engineered bacteria (with cEnc) adapted to Zn²⁺ stress. When Zn²⁺ was sequestered in cEnc as a transition, the engineered bacteria presented a complex network of regulatory systems against Zn²⁺-induced cytotoxicity, including functions related to ribosomes, sulfur metabolism, flagellar assembly, DNA repair, protein synthesis, and Zn²⁺ efflux. Our findings offer an effective and promising stress control strategy to enhance the Zn²⁺ tolerance of bacteria for Zn²⁺ remediation and provide a new application for encapsulins.

Kinetic modeling of As release from contaminated soils: Consideration of particle size and co-contamination of Cu

Prediction on the release kinetics of metalloids from soils is challenging due to the physio-chemical heterogeneity of soil and the varying binding abilities of metalloid contaminants on soil. In this study, the kinetics of As(V), together with Cu(II), release from two typical field contaminated soils were investigated by the stirred-flow experiments. We formulated the quantitative models to describe the release kinetics of As(V) from the contaminated soils with consideration of varying soil particle size and presence of Cu(II). The results showed that the release kinetics of As(V) and Cu(II) from different particle size fractions and at different reaction pH was well described by the model. The models also indicated that the bidentate binding sites on goethite were the major contributor for As(V) release, while soil organic matter (SOM) mainly controlled the Cu(II) release. Finer particle size fractions had more significant contributions to As(V) and Cu(II) release due to higher concentrations of reactive metal(loid)s and more reactive adsorbents. Moreover, the models also showed applicability for predicting metal(loid) release from the bulk soils by considering the contribution of each soil particle size fraction, and the kinetic behaviors of two individual contaminants, As(V) and Cu(II), can be modeled independently. Our results provided a modeling framework to predict the release kinetics of metal(loid)s from soils co-contaminated with different cation and anion pollutants with consideration on the effects of physical and chemical heterogeneity of soils.

Mobilization of As, Fe, and Mn from Contaminated Sediment in Aerobic and Anaerobic Conditions: Chemical or Microbiological Triggers?

We integrated aqueous chemistry, spectroscopy, and microbiology techniques to identify chemical and microbial processes affecting the release of arsenic (As), iron (Fe), and manganese (Mn) from contaminated sediments exposed to aerobic and anaerobic conditions. The sediments were collected from Cheyenne River Sioux Tribal lands in South Dakota, which has dealt with mining legacy for several decades. The range of concentrations of total As measured from contaminated sediments was 96 to 259 mg kg^-1, which co-occurs with Fe (21 000-22 005 mg kg^-1) and Mn (682-703 mg kg^-1). The transition from aerobic to anaerobic redox conditions yielded the highest microbial diversity, and the release of the highest concentrations of As, Fe, and Mn in batch experiments reacted with an exogenous electron donor (glucose). The reduction of As was confirmed by XANES analyses when transitioning from aerobic to anaerobic conditions. In contrast, the releases of As, Fe and Mn after a reaction with phosphate was at least 1 order of magnitude lower compared with experiments amended with glucose. Our results indicate that mine waste sediments amended with an exogenous electron donor trigger microbial reductive dissolution caused by anaerobic respiration. These dissolution processes can affect metal mobilization in systems transitioning from aerobic to anaerobic conditions in redox gradients. Our results are relevant for natural systems, for surface and groundwater exchange, or other systems in which metal cycling is influenced by chemical and biological processes.

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.

Flooding-drainage regulate the availability and mobility process of Fe, Mn, Cd, and As at paddy soil

Speciation changes in Fe and Mn during the soil flooding-drainage process strongly affect the Cd and As bioavailability in paddy soils. However, owing to a lack of in-situ dynamic monitoring technology, the regularity and mechanism of synergetic changes in Fe, Mn, Cd, and As in paddy soils have not been sufficiently studied. Diffusive gradients in thin films (DGT) were used to investigate the dissolution/transformation process of FeMn oxides and their effects on the bioavailability of Cd and As in three contaminated paddy fields that underwent incubated flooding for 40 d followed by a 20 d oxidation period. In-situ monitoring showed that the labile Cd concentrations decreased rapidly upon flooding but bioavailability of As increased significantly, with As and Cd concentrations largely depending upon Fe (II) content. We discovered that the transformation pathway of Iron Oxide-LDH (Fe^II-Fe^III)-Goethite was the key process in reducing the activity of soil Cd. A higher Mn/Fe ratio and lower organic matter content delayed the Fe reduction process, which subsequently delayed Cd immobilization. Mobilization of Cd upon soil drainage was caused by a decrease in soil pH resulting in the release of Cd from secondary minerals.

Significance of Shewanella Species for the Phytoavailability and Toxicity of Arsenic-A Review

Simple Summary

The availability of some toxic heavy metals, such as arsenic (As), is related to increased human and natural activities. This type of metal availability in the environment is associated with various health and environmental issues. Such problems may arise due to direct contact with or consumption of plant products containing this metal in some of their parts. A microbial approach that employs a group of bacteria (Shewanella species) is proposed to reduce the negative consequences of the availability of this metal (As) in the environment. This innovative strategy can reduce As mobility, its spread, and uptake by plants in the environment. The benefits of this approach include its low cost and the possibility of not exposing other components of the environment to unfavourable consequences.

Abstract

The distribution of arsenic continues due to natural and anthropogenic activities, with varying degrees of impact on plants, animals, and the entire ecosystem. Interactions between iron (Fe) oxides, bacteria, and arsenic are significantly linked to changes in the mobility, toxicity, and availability of arsenic species in aquatic and terrestrial habitats. As a result of these changes, toxic As species become available, posing a range of threats to the entire ecosystem. This review elaborates on arsenic toxicity, the mechanisms of its bioavailability, and selected remediation strategies. The article further describes how the detoxification and methylation mechanisms used by Shewanella species could serve as a potential tool for decreasing phytoavailable As and lessening its contamination in the environment. If taken into account, this approach will provide a globally sustainable and cost-effective strategy for As remediation and more information to the literature on the unique role of this bacterial species in As remediation as opposed to conventional perception of its role as a mobiliser of As.

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.

Effects of Calcium on Arsenate Adsorption and Arsenate/Iron Bioreduction of Ferrihydrite in Stimulated Groundwater

The reduction and transformation of arsenic-bearing ferrihydrite by arsenate-iron reducing bacteria is one of the main sources of arsenic enrichment in groundwater. During this process the coexistence cations may have a considerable effect. However, the ionic radius of calcium is larger than that of iron and shows a low affinity for ferrihydrite, and the effect of coexisting calcium on the migration and release of arsenic in arsenic-bearing ferrihydrite remains unclear. This study mainly explored the influence of adsorbed Ca²⁺ on strain JH012-1-mediated migration and release of arsenate in a simulated groundwater environment, in which 3 mM ferrihydrite and pH 7.5. Ca²⁺ were pre-absorbed on As(V)-containing ferrihydrite with a As:Fe ratio of 0.2. Solid samples were analyzed by X-ray diffraction (XRD), scanning electron microscopic (SEM), Fourier transform infrared spectroscopy (FTIR), and X-ray photoelectron spectroscopy (XPS). The results show that calcium and arsenate can synergistically adsorb on ferrihydrite due to the electrostatic interactions, and the adsorbed Ca²⁺ mainly exists on the surface through the outer-sphere complex. Adsorbed Ca²⁺ entering the stimulated groundwater was easily disturbed and led to an extra release of 3.5 mg/L arsenic in the early stage. Moreover, adsorbed Ca²⁺ inhibited biogenic ferrous ions from accumulating on ferrihydrite. As a result, only 12.30% Fe(II) existed in the solid phase, whereas 29.35% existed without Ca²⁺ adsorption. Thus, the generation of parasymplesite was inhibited, which is not conducive to the immobilization of arsenic in groundwater.

Elucidating the redox-driven dynamic interactions between arsenic and iron-impregnated biochar in a paddy soil using geochemical and spectroscopic techniques

Iron (Fe)-modified biochar, a renewable amendment that synthetizes the functions of biochar and Fe materials, demonstrates a potential to remediate arsenic (As)-contaminated soils. However, the effectiveness of Fe-based biochar to immobilize As in paddy soils under varying redox conditions (Eh) has not been quantified. We tested the capability of the raw (RBC) and Fe-impregnated (FeBC) biochars to immobilize As in a paddy soil under various Eh conditions (from -400 to +300 mV) using a biogeochemical microcosm system. In the control, As was mobilized (686.2-1535.8 μg L^-1) under reducing conditions and immobilized (61.6-71.1 μg L^-1) under oxidizing conditions. Application of FeBC immobilized As at Eh < 0 mV by 32.6%-81.1%, compared to the control, because of the transformation of As-bound Fe (hydro)oxides (e.g., ferrihydrite) and the formation of complexes (e.g., ternary As-Fe-DOC). Application of RBC immobilized As at Eh < -100 mV by 16.0%-41.3%, compared to the control, due to its porous structure and oxygen-containing functional groups. Mobilized As at Eh > +200 mV was caused by the increase of pH after RBC application. Amendment of the Fe-modified biochar can be a suitable approach for alleviating the environmental risk of As under reducing conditions in paddy soils.

Pig carcass-derived biochar caused contradictory effects on arsenic mobilization in a contaminated paddy soil under fluctuating controlled redox conditions

Contamination of paddy soils by arsenic (As) is of great concern for human health and the environment. The impact of animal-derived biochar on As mobilization under fluctuating redox conditions in paddy soils has not been studied. Consequently, we investigated the effects of pig carcass-derived biochar (PB) on As (im)mobilization in a contaminated paddy soil under controlled redox potential (Eh) using a biogeochemical microcosm-setup. The addition of PB decreased the concentration of dissolved As at Eh = +100 and +200 mV by 38.7% and 35.4%, respectively (compared to the control), because of the co-precipitation of As with Fe-Mn oxides and the complexation between As and aromatic organic molecules. However, under reducing conditions (Eh = -300 mV), PB increased the dissolved As by 13.5% through promoting reduction and decomposition of As-bearing Fe minerals (e.g., ferrihydrite-As, Fe-humic-As). Under oxidizing conditions (Eh = +250 mV), PB increased the dissolved As by 317.6%, due to the associated increase of pH. We conclude that As mobilization in PB-treated paddy soils is highly affected by Eh. PB can be used to reduce the environmental risk of As under moderately reducing conditions, but it may increase the risk under highly reducing and oxidizing conditions in paddy soils.

Arsenic biotransformation and mobilization: the role of bacterial strains and other environmental variables

Over several decades, arsenic (As) toxicity in the biosphere has affected different flora, fauna, and other environmental components. The majority of these problems are linked with As mobilization due to bacterial dissolution of As-bearing minerals and its transformation in other reservoirs such as soil, sediments, and ground water. Understanding the process, mechanism, and various bacterial species involved in these processes under the influence of some ecological variables greatly contributes to a better understanding of the fate and implications of As mobilization into the environments. This article summarizes the process, role, and various types of bacterial species involved in the transformation and mobilization of As. Furthermore, insight into how Fe(II) oxidation and resistance mechanisms such as methylation and detoxification against the toxic effect of As(III) was highlighted as a potential immobilization and remediation strategy in As-contaminated sites. Furthermore, the significance and comparative advantages of some useful analytical tools used in the evaluation, speciation, and analysis of As are discussed and how their in situ and ex situ applications support assessing As contamination in both laboratory and field settings. Nevertheless, additional research involving advanced molecular techniques is required to elaborate on the contribution of these bacterial consortia as a potential agronomic tool for reducing As availability, particularly in natural circumstances. Graphical abstract. Courtesy of conceptual model: Aminu Darma.

Highly Sensitive and Stable Determination of As(III) under Near-Neutral Conditions: Benefit from the Synergetic Catalysis of Pt Single Atoms and Active S Atoms over Pt1/MoS2

Designing new catalysts with high activity and stability is crucial for the effective analysis of environmental pollutants under mild conditions. Here, we developed a superior catalyst of Pt single atoms anchored on MoS₂ (Pt₁/MoS₂) to catalyze the determination of As(III). A detection sensitivity of 3.31 μA ppb^-1 was obtained in acetate buffer solution at pH 6.0, which is the highest compared with those obtained by other Pt-based nanomaterials currently reported. Pt₁/MoS₂ exhibited excellent electrochemical stability during the detection process of As(III), even in the coexistence of Cu(II), Pb(II), and Hg(II). X-ray absorption fine structure spectroscopy and theoretical calculations revealed that Pt single atoms were stably fixed by four S atoms and activated the adjacent S atoms. Then, Pt and S atoms synergistically interacted with O and As atoms, respectively, and transferred some electrons to H₃AsO₃, which change the rate-determining step of H₃AsO₃ reduction and reduce reaction energy barriers, thereby promoting rapid and efficient accumulation for As(0). Compared with Pt nanoparticles, the weaker interaction between arsenic species and Pt₁/MoS₂ enabled the effortless regeneration and cyclic utilization of active centers, which is more favorable for the oxidation of As(0). This work provides inspiration for developing highly efficient sensing platforms from the perspective of atomic-level catalysis and affords references to explore the detection mechanism of such contaminants.

Metal and metal(loids) removal efficiency using genetically engineered microbes: Applications and challenges

The environment is being polluted in different many with metal and metalloid pollution, mostly due to anthropogenic activity, which is directly affecting human and environmental health. Metals and metalloids are highly toxic at low concentrations and contribute primarily to the survival equilibrium of activities in the environment. However, because of non-degradable, they persist in nature and these metal and metalloids bioaccumulate in the food chain. Genetically engineered microorganisms (GEMs) mediated techniques for the removal of metals and metalloids are considered an environmentally safe and economically feasible strategy. Various forms of GEMs, including fungi, algae, and bacteria have been produced by recombinant DNA and RNA technologies, which have been used to eliminate metal and metalloids compounds from the polluted areas. Besides, GEMs have the potentiality to produce enzymes and other metabolites that are capable of tolerating metals stress and detoxify the pollutants. Thus, the aim of this review is to discuss the use of GEMs as advanced tools to produce metabolites, signaling molecules, proteins through genetic expression during metal and metalloids interaction, which help in the breakdown of persistent pollutants in the environment.

Fulvic Acid-Mediated Interfacial Reactions on Exposed Hematite Facets during Dissimilatory Iron Reduction

Although the dual role of natural organic matter (NOM) as an electron shuttle and an electron donor for dissimilatory iron (Fe) reduction has been extensively investigated, the underlying interfacial interactions between various exposed facets and NOM are poorly understood. In this study, fulvic acid (FA), as typical NOM, was used and its effect on the dissimilatory reduction of hematite {001} and {100} by Shewanella putrefaciens CN-32 was investigated. FA accelerates the bioreduction rates of hematite {001} and {100}, where the rate of hematite {100} is lower than that of hematite {001}. Secondary Fe minerals were not observed, but the HR-TEM images reveal significant defects. The ATR-FTIR results demonstrate that facet-dependent binding mainly occurs via surface complexation between the surface iron atoms and carboxyl groups of NOM. The spectroscopic and mass spectrometry analyses suggest that organic compounds with large molecular weight, highly aromatic and unsaturated structures, and lower H/C ratios are easily adsorbed on Fe oxides or decomposed by bacteria in FA-hematite {001} treatment after iron reduction. Due to the metabolic processes of cells, a significant number of compounds with higher H/C and medium O/C ratios appear. The Tafel curves show that hematite {100} possessed higher resistance (4.1-2.6 Ω) than hematite {001} (3.5-2.2 Ω) at FA concentrations ranging from 0 to 500 mg L^-1, indicating that hematite {100} is less conductive during the electron transfer from reduced FA or cells to Fe oxides than hematite {001}. Overall, the discrepancy in the iron bioreduction of two exposed facets is attributed to both the different electrochemical activities of the Fe oxides and the different impacts on the properties and composition of OM. Our findings shed light on the molecular mechanisms of mutual interactions between FA and Fe oxides with various facets.

Hematite facet-mediated microbial dissimilatory iron reduction and production of reactive oxygen species during aerobic oxidation

Microbial dissimilatory iron reduction and aerobic oxidation affect the biogeochemical cycles of many elements. Although the processes have been widely studied, the underlying mechanisms, and especially how the surface structures of iron oxides affect these redox processes, are poorly understood. Therefore, {001} facet-dominated hematite nanoplates (HNP) and {100} facet-dominated hematite nanorods (HNR) were used to explore the effects of surface structure on the microbial dissimilatory iron reduction and aerobic oxidation processes. During the reduction stage, the production of total Fe(II) normalized by specific surface area (SSA) was higher for HNP than HNR due to steric effects and the ligand-bound conformation of the connection between iron on different exposed facets and electron donors from microorganisms. However, during the aerobic oxidation stage, both the SSA- and Fe(II)-normalized reactive oxygen species (ROS), including hydrogen peroxide (H₂O₂) and hydroxyl radical (•OH), were higher for HNR than HNP. Theoretical calculation results showed that the {100} facets exhibited a lower activation energy barrier for oxygen reduction reaction than {001} facets, supporting the experimental observation that {100} facet-dominated HNR had a higher ROS production efficiency than {001} facet-dominated HNP. These results indicated that surface characteristics not only mediated the microbial reduction of Fe(III) but also affected the aerobic oxidation of microbially reduced Fe(II). Accessibility of electron donors to surface iron atom determined the reduction of Fe(III), and activation energy barrier for oxygen reduction by surface Fe(II) dominated the ROS production during the redox processes. This study advances our understanding of the mechanisms through which ROS are produced by iron (oxyhydr)oxides during microbial dissimilatory iron reduction and aerobic oxidation processes.