2-line Ferrihydrite Research Articles

Seasonal Variations in Ochreous Precipitates and Drainage Waters in the Grantcharitsa Tungsten Deposit, Western Rhodopes, Bulgaria

Seasonal variations of drainage waters and ochreous products of their discharge from the closed abandoned old gallery at the Grantcharitsa scheelite deposit (Bulgaria) were studied by field and laboratory methods for the period 2019–2023. The drainage is generated under anoxic conditions and is inherently diluted (EC = 100–202 µS/cm) with S (6–12 mg/L), Si (6–22 mg/L), Na (6–10 mg/L), Fe (0.2–3.3 mg/L), and W (0.19–3.5 µg/L), at a pH 4.4–6.5 and temperature 7–11.5 °C, with dissolved oxygen DO (2.1–7.7 mg/L). The concentrations of Fe and W and the pH of the water are variable and reach their maximum values during the dry (autumn) season. It was found that such parameters as pH, Eh, DO, Fe and W content change dramatically at a distance of up to 3 m from the water outlet; the values of pH, DO and Eh are sharply increased with a simultaneous nearly 5–6-times reduction in iron and tungsten content. The decrease in the contents of these elements is associated with the precipitation of ochreous material consisting of nanoscale ferrihydrite with an intermediate structural ordering between 2-line and 6-line ferrihydrite (major phase), hematite, goethite, quartz, montmorillonite and magnetite. The formation of ferrihydrite occurs as a result of abiotic and biotic processes with the participation of iron-oxidizing bacteria. Besides Fe2O3 (55.5–64.0 wt.%), the ochreous sediment contains SiO2 (12.0–16.4 wt.%), SO3 (1.3–2.4 wt.%), Al2O3 (3.1–6.8 wt.%) and WO3 (0.07–0.11 wt.%). It has been shown that drainage waters and ochreous sediments do not inherently have a negative impact on the environment. The environmental problem arises with intense snowmelt and heavy rainfall, as a result of which the accumulated sediments are washed away and carried in the form of suspensions into the water systems. It is suggested that by providing atmospheric oxygen access to the closed gallery (via local boreholes), it is possible to stop the generation of iron-enriched drainage.

Speciation Controls the Kinetics of Iron Hydroxide Precipitation and Transformation at Alkaline pH.

The formation of energetically favorable and metastable mineral phases within the Fe-H2O system controls the long-term mobility of iron complexes in natural aquifers and other environmentally and industrially relevant systems. The fundamental mechanism controlling the formation of these phases has remained enigmatic. We develop a general partial equilibrium model, leveraging recent synchrotron-based data on the time evolution of solid Fe(III) hydroxides along with aqueous complexes. We combine thermodynamic considerations and particle-morphology-dependent kinetic rate equations under full consideration of the aqueous phase in disequilibrium with one or more of the forming minerals. The new model predicts the rate of amorphous 2-line ferrihydrite precipitation, dissolution, and overall transformation to crystalline goethite. It is found that the precipitation of goethite (i) occurs from solution and (ii) is limited by the comparatively slow dissolution of the first forming amorphous phase 2-line ferrihydrite. A generalized transformation mechanism further illustrates that differences in the kinetics of Fe(III) precipitation are controlled by the coordination environment of the predominant Fe(III) hydrolysis product. The framework allows modeling of other iron(bearing) phases across a broad range of aqueous phase compositions.

The long-term transformation of FeIII-AsV coprecipitates at room temperature under oxic conditions: New insights for the fate and the speciation of As

The long-term stability of FeIII-AsV coprecipitates, a typically hydrometallurgical or naturally produced As-bearing wastes in tailings or in other environments, is critical to evaluating the As risk caused by them. A wide pH range, different Fe/As molar ratios, reaction media, and neutralization reagents were considered in order to find the mechanisms controlling the fate of As during the 1640 days of transformation at 25 °C. The results indicated that at pH 4 and 12, As continuously released from the solid phase. The components and their proportions determined the fate of As at pH 4. However, at pH 12, crystalline calcium carbonates (CCA) formed due to the CO2 in the air and this combined with the adsorption capacity of As on the 2-line ferrihydrite controlling the fate of As. If pH changed to 8 and 10, yukonite formed after the release of As. The CCA also appeared in the presence of Ca. Therefore, these two processes controlled the fate of As at this pH range. These findings are important for understanding and predicting the transport of As under various environmental conditions. The technology chosen for As remediation in soils and As removal from waste waters will also be benefit from these results.

Effects of gamma(γ)-irradiation on the physicochemical properties and bioavailability of iron oxyhydroxides coprecipitated with varying concentrations of Na-alginate

This study investigated the impacts of γ-radiation at a final dose of 25 kGy on the physicochemical properties of a range of wet iron oxyhydroxides (2-line ferrihydrite, lepidocrocite, and goethite) synthesized in the presence of varying concentrations of the polysaccharide Na-alginate (starting solutions containing C/Fe ratios of 0, 0.5, 1.0, and 1.5). The degree of impact to the minerals was examined by chemical extractions, and various analytical techniques including XRD, FTIR-ATR, Mössbauer spectroscopy, and N2 adsorption-desorption isotherms, as well as by assessing their bioavailability towards the model Fe(III) reducing bacteria Shewanella putrefaciens CN32. Across all the coprecipitates studied, it was found that γ-irradiation led to a substantial dissolution of Fe and the concomitant release of Fe(II) and alginate into solution. Despite this observation, the bulk mineralogy and crystallinity of the studied iron oxyhydroxides, as determined by XRD and FTIR-ATR, did not appear to change. However, analyses via Mössbauer spectroscopy (77 and 5 K) revealed that the crystallinity of the 2-line ferrihydrites increased post-irradiation. Moreover, among the minerals studied, the specific surface area and porosity decreased for only the post-irradiated 2-line ferrihydrite coprecipitates with a C/Fe ratio of 0.5 or 1.0. The bioreduction rates of the studied minerals and their irradiated counterparts did not significantly differ, whereas the extent of bioreduction of post-irradiated 2-line ferrihydrite coprecipitates (C/Fe 0.5, 1.0, and 1.5) exhibited a significant increase of up to 28%. In contrast, the extent of Fe reduction for select post-irradiated lepidocrocite (C/Fe 0.5) and goethite (C/Fe 1.5) coprecipitates was slightly higher than determined for their corresponding native controls. The observed differences in bioavailability between the native and irradiated coprecipitates were attributed to irradiation induced alteration of particle aggregation and coagulation as determined by particle size analyses and visual observations. In summary, the findings suggest that even a low total dose of 25 kGy, γ-radiation can lead to significant physicochemical changes in coprecipitates with relatively low organic matter content. Therefore, future research designed to investigate organic matter-Fe composite systems in natural samples should be cognizant of the potential effects of sterilization through γ-irradiation. These effects could potentially result in inaccurate over- or under- estimations of the bioavailability of Fe or organic matter, as well as the possible adsorption capacity of organic matter-Fe coprecipitates for contaminants.

Preparation and Characterization of Ferrihydrite: Application in Arsenic Removal from Aqueous Solutions

Arsenic pollution is a public health hazard in Burkina Faso due to its impact on human health and water resources. To mitigate this pollution, ferrihydrite material has been synthesized and characterized to be used as adsorbent for arsenic removal in aqueous solutions. This study aimed to contribute to improve of access to clean drinking water by removing arsenic from water using ferrihydrite. Arsenic species such as As(III) and As(V) were removed through batch adsorption. Experiments were carried out in batch mode using arsenic aqueous solutions. The characterization of ferrihydrite using Scanning Electron Microscopy (SEM) coupled with Energy Dispersive Spectroscopy (EDX), X-ray Diffraction (XRD), Infrared (IR), and Brunauer Emmett Teller (BET) showed that an amorphous and microporous 2-line ferrihydrite. The total specific surface area and pH at point of zero charge (pHpzc) were 184.518 m²/g and 9.41, respectively. The optimal adsorbent doses were 4 g/L for As (V) and 8 g/L for As (III). The optimum pH range for the adsorption of As (V) and As (III) was between 2 and 10, The maximum adsorption capacity was 15.07 mg/g for As(V) and 13.01 mg/g for As(III) with increasing concentration between 2 and 16 mg/L. Equilibrium time for As (V) and As (III) on ferrihydrite was found to be 720 min and 960 min, respectively. The adsorption of As(V) and As(III) was consistent with the Langmuir monolayer model on ferrihydrite. Arsenic adsorption was occurred according to spontaneous chemical reaction. Arsenic removal was occurred on a monolayer following the pseudo-second order kinetic.

Storage and Distribution of Organic Carbon and Nutrients in Acidic Soils Developed on Sulfidic Sediments: The Roles of Reactive Iron and Macropores.

In a boreal acidic sulfate-rich subsoil (pH 3-4) developing on sulfidic and organic-rich sediments over the past 70 years, extensive brownish-to-yellowish layers have formed on macropores. Our data reveal that these layers ("macropore surfaces") are strongly enriched in 1 M HCl-extractable reactive iron (2-7% dry weight), largely bound to schwertmannite and 2-line ferrihydrite. These reactive iron phases trap large pools of labile organic matter (OM) and HCl-extractable phosphorus, possibly derived from the cultivated layer. Within soil aggregates, the OM is of a different nature from that on the macropore surfaces but similar to that in the underlying sulfidic sediments (C-horizon). This provides evidence that the sedimentary OM in the bulk subsoil has been largely preserved without significant decomposition and/or fractionation, likely due to physiochemical stabilization by the reactive iron phases that also existed abundantly within the aggregates. These findings not only highlight the important yet underappreciated roles of iron oxyhydroxysulfates in OM/nutrient storage and distribution in acidic sulfate-rich and other similar environments but also suggest that boreal acidic sulfate-rich subsoils and other similar soil systems (existing widely on coastal plains worldwide and being increasingly formed in thawing permafrost) may act as global sinks for OM and nutrients in the short run.

Radiation-chemical synthesis and characterization of ferrihydrite from iron (III) nitrate

In this study, 2-line ferrihydrite (2L Fh) nanoparticles (NPles)were synthesized by radiation-chemical method from an alcoholic solution of iron (III) nitrate for the first time. The X-ray diffraction analysis confirmed that the synthetic powder exhibited the characteristic pattern of 2L Fh NPles. DSC-TG analysis conducted in air atmosphere further verified the formation of 2L Fh. SEM analysis showed the presence of mesoporous plate-like structures in the 2L Fh powder, consisting of aggregates of NPs with an average size of approximately 20 nm. The absence of impurity peaks on the X-ray diffractograms and energy dispersion spectra (EDX) confirmed the chemical purity of the produced 2L Fh NPles. Additionally, the XPS method detected the presence of nitrogen and carbon adsorbed to the developed surface of the 2L Fh plates. 2L Fh NPles, when dried in air at a temperature of 50 °C, rapidly dissolved in water. 2L Fh NPles alcohol suspensions were stabilized using surfactants polyethylenimine (PEI) and acetylacetone (AcAs). 2L Fh NPles showed good photocatalytic properties when irradiated with ultraviolet light of methyl violet (MV) dye. These 2L Fh NPles, synthesized using an environmentally friendly radiation-chemical method, have immense potential for applications in biomedicine and photocatalysis.

Nanocrystalline iron hydroxide lignocellulose filters for arsenate remediation.

Harmful levels of environmental contaminants, such as arsenic (As), persist readily in the environment, threatening safe drinking water supplies in many parts of the world. In this paper, we present a straightforward and cost-effective filtration technology for the removal of arsenate from potable water. Biocomposite filters comprised of nanocrystalline iron oxides or oxyhydroxides mineralized within lignocellulose scaffolds constitute a promising low cost, low-tech avenue for the removal of these contaminants. Two types of iron oxide mineral phases, 2-line ferrihydrite (Fh) and magnetite (Mt), were synthesized within highly porous balsa wood using an environmentally benign modification process and studied in view of their effective removal of As from contaminated water. The mineral deposition pattern, minerology, as well as crystallinity, were assessed using scanning electron microscopy, transmission electron microscopy, micro-computed X-ray tomography, confocal Raman microscopy, infrared spectroscopy, and X-ray powder diffraction. Our results indicate a preferential distribution of the Fh mineral phase within the micro-porous cell wall and radial parenchyma cells of rays, while Mt is formed primarily at the cell wall/lumen interface of vessels and fibers. Water samples of known As concentrations were subjected to composite filters in batch incubation and gravity-driven flow-through adsorption tests. Eluents were analyzed using microwave plasma optical emission spectroscopy (MP-AES) and inductively coupled plasma mass spectrometry (ICP-MS). By subjecting the filters to a flow of contaminated water, the time for As uptake was reduced to minutes rather than hours, while immobilizing the same amount of As. The retention of As within the composite filter was further confirmed through energy-dispersive X-ray mappings. Apart from addressing dangerously high levels of arsenate in potable water, these versatile iron oxide lignocellulosic filters harbor tremendous potential for addressing current and emerging environmental contaminants that are known to adsorb on iron oxide mineral phases, such as phosphate, polycyclic aromatic hydrocarbons or heavy metals.

Transformation of 2-Line Ferrihydrite to Goethite at Alkaline pH.

The transformation of 2-line ferrihydrite to goethite from supersaturated solutions at alkaline pH ≥ 13.0 was studied using a combination of benchtop and advanced synchrotron techniques such as X-ray diffraction, thermogravimetric analysis, and X-ray absorption spectroscopy. In comparison to the transformation rates at acidic to mildly alkaline environments, the half-life, t1/2, of 2-line ferrihydrite reduces from several months at pH = 2.0, and approximately 15 days at pH = 10.0, to just under 5 h at pH = 14.0. The calculated-first order rate constants of transformation, k, increase exponentially with respect to the pH and follow the progression log10 k = log10 k0 + a·pH3. Simultaneous monitoring of the aqueous Fe(III) concentration via inductively coupled plasma optical emission spectroscopy demonstrates that (i) goethite likely precipitates from solution and (ii) its formation is rate-limited by the comparatively slow redissolution of 2-line ferrihydrite. The analysis presented can be used to estimate the transformation rate of naturally occurring 2-line ferrihydrite in aqueous electrolytes characteristic to mine and radioactive waste tailings as well as the formation of corrosion products in cementitious pore solutions.

Molecular-scale insight into selenium isotope fractionation caused by adsorption on Fe (oxyhydr)oxides

Selenium (Se) isotopes have recently emerged as a potential proxy for tracing biogeochemical processes and reconstructing the evolution of global Se cycle in the oceans. However, little is known about Se isotope fractionation mechanism during the sequestration of dissolved Se oxyanions by marine ferromanganese oxides that are mainly composed of Fe and Mn (oxyhydr)oxides. Here we elucidate the molecular mechanism governing equilibrium Se isotope fractionation during adsorption on 2-line ferrihydrite, by combining isotope ratio measurements, extended X-ray absorption fine structure (EXAFS) analyses, and high level quantum chemical calculations. Results show that Se isotopes can be fractionated 0.89‰ (Δ82/76Seaqueous-adsorbed) with enrichment of lighter isotopes in the solid phase during the adsorption of Se(IV) on 2-line ferrihydrite, which is primarily driven by the formation of bidentate-binuclear inner-sphere complexes. By contrast, little or no Se isotope fractionation (<0.2‰) was observed during Se(VI) adsorption due to the outer-sphere complexation. In combination with previous results, our findings would provide molecular-scale insights into Se isotope compositions in marine ferromanganese oxides and lead to an improved understanding of Se biogeochemical cycle in the ocean. Our study also has an implication for the systematical understanding of mechanisms governing isotope fractionations of other metal oxyanions like Mo, which highlights the controls of their proton dissociation constants, electronic configurations, and radius ratios. This would improve our understanding for isotope signatures of metals in the dissolved species and the adsorbed sinks in modern and ancient environments.

Insights into the physiological and genomic characterization of three bacterial isolates from a highly alkaline, terrestrial serpentinizing system.

The terrestrial serpentinite-hosted ecosystem known as "The Cedars" is home to a diverse microbial community persisting under highly alkaline (pH ~ 12) and reducing (Eh < -550 mV) conditions. This extreme environment presents particular difficulties for microbial life, and efforts to isolate microorganisms from The Cedars over the past decade have remained challenging. Herein, we report the initial physiological assessment and/or full genomic characterization of three isolates: Paenibacillus sp. Cedars ('Paeni-Cedars'), Alishewanella sp. BS5-314 ('Ali-BS5-314'), and Anaerobacillus sp. CMMVII ('Anaero-CMMVII'). Paeni-Cedars is a Gram-positive, rod-shaped, mesophilic facultative anaerobe that grows between pH 7-10 (minimum pH tested was 7), temperatures 20-40°C, and 0-3% NaCl concentration. The addition of 10-20 mM CaCl2 enhanced growth, and iron reduction was observed in the following order, 2-line ferrihydrite > magnetite > serpentinite ~ chromite ~ hematite. Genome analysis identified genes for flavin-mediated iron reduction and synthesis of a bacillibactin-like, catechol-type siderophore. Ali-BS5-314 is a Gram-negative, rod-shaped, mesophilic, facultative anaerobic alkaliphile that grows between pH 10-12 and temperatures 10-40°C, with limited growth observed 1-5% NaCl. Nitrate is used as a terminal electron acceptor under anaerobic conditions, which was corroborated by genome analysis. The Ali-BS5-314 genome also includes genes for benzoate-like compound metabolism. Anaero-CMMVII remained difficult to cultivate for physiological studies; however, growth was observed between pH 9-12, with the addition of 0.01-1% yeast extract. Anaero-CMMVII is a probable oxygen-tolerant anaerobic alkaliphile with hydrogenotrophic respiration coupled with nitrate reduction, as determined by genome analysis. Based on single-copy genes, ANI, AAI and dDDH analyses, Paeni-Cedars and Ali-BS5-314 are related to other species (P. glucanolyticus and A. aestuarii, respectively), and Anaero-CMMVII represents a new species. The characterization of these three isolates demonstrate the range of ecophysiological adaptations and metabolisms present in serpentinite-hosted ecosystems, including mineral reduction, alkaliphily, and siderophore production.

Chelated heavy metals removal by in-situ formed Fe(II) and Fe(III) iron (oxy)hydroxides: Mechanism and performance

For more than a century, coagulation-precipitation using Fe(II) and Fe(III) are ubiquitous in wastewater treatment, however, a systematic understanding of the underlying mechanism and their performance toward chelated heavy metals (e.g. CuII-EDTA) is still lacking. In this study, we therefore investigate the efficiency of Fe(II) and Fe(III) salts coagulation-precipitation, the properties (coagulation and crystallization behavior) of in-situ formed iron (oxy)hydroxides and explore the reaction mechanism using a suite of characterization techniques. Our results reveal that no matter Fe(II) and Fe(III) exhibited an exceptional removal performance for Cu(II) in terms of kinetics and iron dosage along with pH. Impressively, under optimal condition, over 95 % of chelated Cu(II) was removed within 10 min from Fe(II) while about 85 % from Fe(III) dosing. The highly reductive structural Fe(II) existing on mixed-valent iron bearing mineral results in reductive decomplexation of CuII-EDTA in the Fe(II) coagulation-precipitation process in compare with replacement decomplexation in the Fe(III) coagulation-precipitation process, and then released free metal ions (Cu(I) and Cu(II)) can be removed by adsorption and coprecipitation. In addition, the in-situ formed Fe(III) (oxy)hydroxides were amorphous 2-line ferrihydrite with stable flocs and better sedimentation behavior than crystalline lepidocrocite from Fe(II) dosing, and these factors are crucial to the subsequent retention of heavy metals. The elucidation of this study has highlighted the great potential of using in-situ formed iron (oxy)hydroxides for cost-effective treatment of chelated heavy metal-bearing industrial wastewaters.

An investigation of the internal morphology of asbestos ferruginous bodies: constraining their role in the onset of malignant mesothelioma

BackgroundAsbestos is a fibrous mineral that was widely used in the past. However, asbestos inhalation is associated with an aggressive type of cancer known as malignant mesothelioma (MM). After inhalation, an iron-rich coat forms around the asbestos fibres, together the coat and fibre are termed an “asbestos ferruginous body” (AFB). AFBs are the main features associated with asbestos-induced MM. Whilst several studies have investigated the external morphology of AFBs, none have characterised the internal morphology. Here, cross-sections of multiple AFBs from two smokers and two non-smokers are compared to investigate the effects of smoking on the onset and growth of AFBs. Morphological and chemical observations of AFBs were undertaken by transmission electron microscopy, energy dispersive x-ray spectroscopy and selected area diffraction.ResultsThe AFBs of all patients were composed of concentric layers of 2-line or 6-line ferrihydrite, with small spherical features being observed on the outside of the AFBs and within the cross-sections. The spherical components are of a similar size to Fe-rich inclusions found within macrophages from mice injected with asbestos fibres in a previous study. As such, the spherical components composing the AFBs may result from the deposition of Fe-rich inclusions during frustrated phagocytosis. The AFBs were also variable in terms of their Fe, P and Ca abundances, with some layers recording higher Fe concentrations (dense layers), whilst others lower Fe concentrations (porous layers). Furthermore, smokers were found to have smaller and overall denser AFBs than non-smokers.ConclusionsThe AFBs of smokers and non-smokers show differences in their morphology, indicating they grew in lung environments that experienced disparate conditions. Both the asbestos fibres of smokers and non-smokers were likely subjected to frustrated phagocytosis and accreted mucopolysaccharides, resulting in Fe accumulation and AFB formation. However, smokers’ AFBs experienced a more uniform Fe-supply within the lung environment compared to non-smokers, likely due to Fe complexation from cigarette smoke, yielding denser, smaller and more Fe-rich AFBs. Moreover, the lack of any non-ferrihydrite Fe phases in the AFBs may indicate that the ferritin shell was intact, and that ROS may not be the main driver for the onset of MM.

Siderophore-Mediated Mobilization of Manganese Limits Iron Solubility in Mixed Mineral Systems.

Recent laboratory and field studies show the need to consider the formation of aqueous Mn(III)-siderophore complexes in manganese (Mn) and iron (Fe) geochemical cycling, a shift from the historical view that aqueous Mn(III) species are unstable and thus unimportant. In this study, we quantified Mn and Fe mobilization by desferrioxamine B (DFOB), a terrestrial bacterial siderophore, in single (Mn or Fe) and mixed (Mn and Fe) mineral systems. We selected manganite (γ-MnOOH), δ-MnO2, lepidocrocite (γ-FeOOH), and 2-line ferrihydrite (Fe2O3·0.5H2O) as relevant mineral phases. We found that DFOB mobilized Mn(III) as Mn(III)-DFOB complexes to varying extents from both Mn(III,IV) oxyhydroxides but reduction of Mn(IV) to Mn(III) was required for the mobilization of Mn(III) from δ-MnO2. The initial rates of Mn(III)-DFOB mobilization from manganite and δ-MnO2 were not affected by the presence of lepidocrocite but decreased by a factor of 5 and 10 for manganite and δ-MnO2, respectively, in the presence of 2-line ferrihydrite. Additionally, the decomposition of Mn(III)-DFOB complexes through Mn-for-Fe ligand exchange and/or ligand oxidation led to Mn(II) mobilization and Mn(III) precipitation in the mixed-mineral systems (∼10% (mol Mn/mol Fe)). As a result, the concentration of Fe(III) mobilized as Fe(III)-DFOB decreased by up to 50% and 80% in the presence of manganite and δ-MnO2, respectively, compared to the single mineral systems. Our results demonstrate that siderophores, through their complexation of Mn(III), reduction of Mn(III,IV), and mobilization of Mn(II), can redistribute Mn to other soil minerals and limit the bioavailability of Fe in natural systems.

Biochar as an enhancer of the stability, mesoporous structure and oxytetracycline adsorption capacity of ferrihydrite: Role of the silicon component

The char component of biochar can act as an electron shuttle and redox agent to accelerate the transformation of ferrihydrite, but how the silicon component of biochar affects ferrihydrite transformation and pollutant removal remains unclear. In this paper, infrared spectroscopy, electron microscopy, transformation experiments and batch sorption experiments were conducted to examine a 2-line ferrihydrite formed by alkaline precipitation of Fe3+ on a rice straw-derived biochar. Fe-O-Si bonds were developed between the precipitated ferrihydrite particles and biochar silicon component, increasing mesopore volume (for mesopores with diameters of 10–100 nm) and surface area of ferrihydrite as the Fe-O-Si formation probably alleviated the aggregation of ferrihydrite particles. The Fe-O-Si bonding-contributed interactions blocked the transformation to goethite for ferrihydrite precipitated on biochar in a 30-day ageing and a 5-day Fe2+ catalysis ageing. Moreover, there was an increase of oxytetracycline adsorption capacity onto ferrihydrite-loaded biochar, which reached amazingly 3460 mg/g at the maximum, due to the Fe-O-Si bonding-contributed increase of surface area and oxytetracycline coordination sites. Ferrihydrite-loaded biochar as a soil amendment enhanced oxytetracycline adsorption and reduced the bacterial toxicity of dissolved oxytetracycline better than ferrihydrite did. These results provide new perspectives for the role of biochar (especially its silicon component) as an iron-based material carrier and a soil additive in the environmental effects of iron (hydr) oxides in water and soil.

Iron–organic carbon associations stimulate carbon accumulation in paddy soils by decreasing soil organic carbon priming

Iron-bound organic carbon (Fe-OC) complexes are important for stabilizing soil organic carbon (SOC) against biodegradation. However, it is unclear how the stabilization of OC and its release from Fe minerals subsequently affect the priming effect on SOC mineralization. To address the knowledge gap, we incubated typical paddy soil for 60 days by adding 2-line ferrihydrite (2LFh) or 6-line Fh (6LFh)-bound glucose, each with both high and low amounts of glucose, under anaerobic conditions. Approximately 21% more CO2 was derived from 2LFh-bound glucose than from 6LFh-bound glucose. Glucose addition alone stimulated SOC mineralization and caused a positive priming effect (0.27% of SOC). In contrast, 2LFh- and 6LFh-bound glucose inhibited SOC mineralization to both CO2 and CH4 and subsequently induced a negative priming effect, ranging from −0.33% to −0.55% SOC. Compared to 2LFh-bound glucose, 6LFh-bound glucose induced a lower priming effect on CO2 emissions (2-fold lower), which was attributed to the lower Fe-reduction rate of 6LFh and OC released. In addition, the available nutrients adsorbed by 6LFh were more difficult to release than those by 2LFh, which aggravated microbial nutrient limitation, and further decreased microbial activity. The priming effect for CH4 emissions was directly proportional to the glucose level loaded. The Fe reduction rates were higher in Fh-bound high amount of glucose than that in the Fh-bound low amount of glucose, which subsequently provided more available C sources for methanogens. Thus, Fe minerals have a high capacity for SOC accumulation, as they prevent bound OC from mineralization and decrease native SOC priming. Moreover, the protection of SOC by Fe minerals depended on its crystalline structure and the amount of OC loading. Our results show that promoting the transformation from weakly crystalline Fe oxides to more crystalline forms would increase SOC accumulation and stability over the complete rice-growing period.

Human Skin-Cell-Based Sensor for Environmental Arsenic Detection and for Creating Social Awareness

Arsenic (As) toxicity is a significant threat to global public health. Moreover, the lacks of social awareness and understanding of the impact of As in affected communities are also of concern. Therefore, subppm level detection of As in environmental waters and associated public awareness are crucial for remediation programs. We developed a sustainable As sensing methodology by merging the fundamental concepts of As cytotoxicity with an alternative approach for selectivity. A cellular platform was prepared on an electrospun scaffold using As-sensitive keratinocyte cells. Arsenic-induced reactive oxygen species (ROS) were quantified using a fluorimetric probe, 2′,7′-dichlorofluorescin diacetate, commonly used to detect oxidative stress within cells. Experiments were conducted with a mixture of arsenite and arsenate, the predominant forms of As present in natural conditions, in a 1:1 ratio. We also quantified unknown As concentrations in real water samples. The selectivity to As was achieved by exposing the contaminated water composed of several ions to an As adsorbing material, namely, confined metastable 2-line ferrihydrite (CM2LF). An adsorption–desorption protocol enabled As extraction in field conditions. The ROS resulting from cells’ responses to the As extract were used as the signature of As concentration. The sensor could precisely quantify even 5 ppb of As in tap water, and the theoretical limit of detection (LOD) was 2.7 ppb. A sustainable device using the cellular platform is proposed for As detection in field conditions that can also be used for social awareness, demonstrating the impact of As on human biology in affected regions.

Microbial community characteristics of cadmium speciation transformation in soil after iron-based materials application

Although numerous studies on the stabilization of heavy metals in contaminated soil with iron-based materials have been conducted, the role of microorganisms in iron species and Cd speciation transformation has not been fully elucidated. Here, 2-line ferrihydrite (MGE) and goethite (GE) were prepared and used to stabilize Cd in contaminated soil. Based on the microbial community characteristics in the early, middle, and later stages, the role of microorganisms in Cd speciation transformation was explored. The exchangeable Cd decreased by 10.73 %–51.31 %, while the iron‑manganese oxide-bound speciation, and the residual speciation increased after MGE/GE application. The relative abundance of Proteobacteria, and Sphingomonas were obviously increased, and Trichoderma and Mortierella were decreased in the MGE group. The outcome in the GE group was the opposite. The complexity of the bacterial and fungal community network was increased in the MGE and GE groups. The lignin in MGE could stimulate the growth of iron-reducing bacteria (FeRB) and promote the reduction of iron minerals to Fe2+. The Fe2+ in the soil was oxidized and hydrolysed, which might lead to an increase in the soil pH. During the process of iron species transformation and iron redox mediated by FeRB, Cd was transformed to more stable speciation.

Impacts of wet-dry alternations on cadmium and zinc immobilisation in soil remediated with iron oxides

Chemical immobilisation is extensively used for in-situ remediation of heavy metals contaminated soil. Immobilised heavy metals could be reactivated by multiple factors such as pH, moisture, temperature, rainfall, etc., among which rainfall is very important, especially acid rain in southern China. Wet-dry alternations were used to simulate the leaching of metals by rainwater. The variation of cadmium (Cd) and zinc (Zn) speciation distribution in soil immobilised with iron oxides (goethite (GE) and 2-line ferrihydrite (GLS)) was investigated. The impacts of wet-dry alternations on the properties of the soil and amendments were also assessed. In the soil without amendments (OS) and amended with GE (GS), the stable fractions were reactivated and transformed into labile fractions under wet-dry alternations. In the soil amended with GLS (LS), the exchangeable and carbonate-bound Cd decreased while the soluble, Fe–Mn oxide bound and organic bound Cd increased. The carbonate-bound Zn was transformed into the Fe–Mn oxide-bound Zn. Transformation from the amorphous iron oxide into crystalline iron oxide in GS and LS were 4.9% and 5.3%. The Pearson correlation analysis showed that the soil pH and the iron-oxide speciation were strongly correlated with Cd/Zn fractions in the soil. The specific surface area, pore volume and adsorption capacity of the iron oxides decreased by 9.26%, 38.89% and 62–73% (for GE), 1.88%, 22.22% and 26–55% (for GLS). The altered soil properties and morphological differences between the two iron oxides under wet-dry alternations were important reasons for Cd/Zn reactivation.

Repartitioning of co-precipitated Mo(VI) during Fe(II) and S(-II) driven ferrihydrite transformation

Molybdenum is an essential element for most biological systems. Biosynthesis of Mo-enzymes that support global biogeochemical cycles of N, C, and S depends upon bioavailable molybdate (MoO42−). Interactions with Fe(III) (oxyhydr)oxides can limit Mo bioavailability in aerobic environments, while redox-driven mineral transformations can enhance Mo sequestration. Here, we examine Mo repartitioning during Fe(II) and S(-II) driven ferrihydrite transformation under anaerobic conditions. We reacted Mo(VI) co-precipitated 2-line ferrihydrite with Fe(II)(aq) or S(-II)(aq) and monitored geochemical and mineralogical changes over time. Inductively coupled plasma–mass spectrometry (ICP–MS) and synchrotron powder X-ray diffraction (XRD) results revealed rapid Fe(II)(aq) and S(-II)(aq) uptake and limited Mo release during extensive ferrihydrite transformation to goethite [α-FeOOH] and lepidocrocite [γ-FeOOH] in the Fe(II) and S(-II) experiments. Transmission electron microscopy–selected area electron diffraction (TEM-SAED), coupled with Mo LIII-edge and K-edge X-ray absorption near edge structure (XANES) spectroscopy, revealed partial Mo(VI) reduction and precipitation of kamiokite [Fe2MoIV3O8] and sidwillite [MoVIO3‧2H2O] in both experiments. Extended X-ray absorption fine structure (EXAFS) spectroscopy at the Mo K-edge revealed MoO, MoFe and MoMo bonding consistent with kamiokite precipitation in both Fe(II) and S(-II) experiments, and the absence of MoS bonding in the S(-II) experiments. Similar Mo(VI) repartitioning pathways during Fe(II) and S(-II) driven ferrihydrite transformation suggests that: (i) Fe(II) served as the electron donor for ferrihydrite transformation in both experiments, with Fe(II)(aq) likely produced in situ via ferrihydrite sulfidation in the S(-II) experiments; and (ii) co-precipitation inhibited Mo release and limited subsequent interactions S(-II)(aq) during ferrihydrite transformation. Overall, our findings indicate that initial association with FeOx phases can strongly influence Mo sequestration pathways in anaerobic environments.