Fe Oxides Research Articles

Influencing Factors and Prediction Models of Mercury Phytoavailability and Transference in a Soil–Lettuce System under Chinese Agricultural Soils

Mercury (Hg) is a highly toxic contaminant posing serious ecological and human health risks. This study investigates the Hg transfer characteristics and prediction models in a soil–lettuce system, employing bioconcentration factors (BCF), path analysis (PA), and Freundlich-type functions. A pot experiment was conducted in a greenhouse, where lettuce was grown in a range of Chinese agricultural soils (n = 21) and deliberately spiked with Hg using Hg(NO3)2 solution. The results indicated that lettuce grown in Hg-spiked acidic soils (pH < 6.5) accumulated total Hg (THg) levels up to 14.01 µg kg−1, surpassing the safe consumption limit of 10 µg kg−1. The BCF for lettuce THg was less than 1.0, suggesting a low transfer of Hg from soil to lettuce. Notably, BCF values were significantly higher in acidic soils (0.02) compared to alkaline soils (0.005). Path analysis accounted for 82% of the variation in lettuce THg content, identifying soil THg, pH, and amorphous (Amo) Al and Fe oxides as primary direct factors. Additionally, soil-available Hg (AvHg), exchangeable Hg (ExHg), clay, and organic matter (OM) were significant indirect factors affecting lettuce THg content. To validate the findings of the path analysis, an extended Freundlich-type equation was developed using stepwise multiple linear regression (SMLR). This model exhibited high predictive accuracy (R2 = 0.82, p ≤ 0.001), with soil pH, THg, and amorphous Al and Fe oxides being the key variables for predicting Hg transfer in the soil–lettuce system. The insights from this study can guide the management of safe lettuce production in Hg-contaminated soils, ensuring the mitigation of Hg exposure through agricultural produce.

Siderite and vivianite as energy sources for the extreme acidophilic bacterium Acidithiobacillus ferrooxidans in the context of mars habitability

Past and present habitability of Mars have been intensely studied in the context of the search for signals of life. Despite the harsh conditions observed today on the planet, some ancient Mars environments could have harbored specific characteristics able to mitigate several challenges for the development of microbial life. In such environments, Fe2+ minerals like siderite (already identified on Mars), and vivianite (proposed, but not confirmed) could sustain a chemolithoautotrophic community. In this study, we investigate the ability of the acidophilic iron-oxidizing chemolithoautotrophic bacterium Acidithiobacillus ferrooxidans to use these minerals as its sole energy source. A. ferrooxidans was grown in media containing siderite or vivianite under different conditions and compared to abiotic controls. Our experiments demonstrated that this microorganism was able to grow, obtaining its energy from the oxidation of Fe2+ that came from the solubilization of these minerals under low pH. Additionally, in sealed flasks without CO2, A. ferrooxidans was able to fix carbon directly from the carbonate ion released from siderite for biomass production, indicating that it could be able to colonize subsurface environments with little or no contact with an atmosphere. These previously unexplored abilities broaden our knowledge on the variety of minerals able to sustain life. In the context of astrobiology, this expands the list of geomicrobiological processes that should be taken into account when considering the habitability of environments beyond Earth, and opens for investigation the possible biological traces left on these substrates as biosignatures.

Enhanced Arsenate Immobilization by Kaolinite via Heterogeneous Pathways during Ferrous Iron Oxidation.

Clay minerals are ubiquitous in subsurface environments and have long been recognized as having a limited or negligible impact on the fate of arsenic (As) due to their negatively charged surfaces. Here, we demonstrate the significant role of kaolinite (Kln), a pervasive clay mineral, in enhancing As(V) immobilization during ferrous iron (Fe(II)) oxidation at near-neutral pH. Our results showed that Fe(II) oxidation alone was not capable of immobilizing As(V) at relatively low Fe/As molar ratios (≤2) due to the generation of Fe(III)-As(V) nanocolloids that could still migrate easily as truly dissolved As did. In the presence of kaolinite, dissolved As(V) was significantly immobilized on the kaolinite surfaces via forming Kln-Fe(III)-As(V) ternary precipitates, which had large sizes (at micrometer levels) to reduce the As mobility. The kaolinite-induced heterogeneous pathways for As(V) immobilization involved Fe(II) adsorption, heterogeneous oxidation of adsorbed Fe(II), and finally heterogeneous nucleation/precipitation of Fe(III)-As(V) phases on the edge surfaces of kaolinite. The surface precipitates were mixtures of amorphous basic Fe(III)-arsenate and As-rich hydrous ferric oxide. Our findings provide new insights into the role of clay minerals in As transformation, which is significant for the fate of As in natural and engineered systems.

The redistribution process of As(Ⅲ) and Fe(Ⅱ) caused by As/Fe ratio, organic matter, and co-existing ions: Co-precipitation and co-oxidation

The contamination of arsenic (As) in aqueous environments has drawn widespread attention, and iron compounds may largely alter the migration ability of As. However, the stability of As(III) in Fe-As system with the intervention of organic matter (OM) remains unclear. Herein, we had explored the co-precipitation and co-oxidation processes of As-Fe system by using batch experiments combined with Fourier Transform Infrared Spectroscopy (FTIR) and X-ray Photoelectron Spectroscopy (XPS) in this research. The precipitation quantity of As(III) increased (28.85–92.41 %) when the As/Fe ratio decreased, and increased (24.20–64.20 %) with pH increased. The main active substance for oxidizing As(III) was H2O2, which was produced in the As-Fe system. FTIR and XPS revealed that As(III) was first oxidized in neutral, and then absorbed and enteredthe interior of Fe(OH)3 colloids. But under alkaline conditions, As(III) was adsorbed by Fe (Oxyhydr) oxides firstly, and then oxidized. The intervention of OM would inhibit the redistribution process of As(III) in aqueous environments. Functional groups and unsaturation of the carbon chain were the dominant factors that affected the precipitation and oxidation processes of As(III), respectively. Co-existing ions (especially PO43-) also signally affected the precipitation quantity of As(Ⅲ) in the system and, when coexisting with OM, could exacerbate this process. The influence of co-existing ions on the redistributive process of As(III) in the As-Fe system with/without OM were as follows: PO43- > SO42- > mixed ions > SiO32-. Moreover, high concentration of OM and PO43- might lead to morphological alterations of As, acting as a threat to aqueous environments. In summary, the present findings were to further understand and appreciate the changes of As toxicity in the aqueous environments. Particularly, the coexistence of OM and As can potentially increase the risk to drinking water safety.

Effect of Nb addition on the steam oxidation behavior and kinetic of FeCrAl alloys

The effects of Nb on the steam oxidation kinetic behavior of FeCrAl alloy were studied. Nb addition significantly decreased the weight gain at 1200 °C. Nb addition also promoted the formation of Al2O3 and suppressed the growth of Fe and Cr oxides, resulting in the thinner oxide film in the heating stage. Also, the oxide film is much easier to wrinkle at 1200 °C. In addition, pre-formed Nb2O5 promotes the formation of discontinuous granular Al2O3 in oxide films and substrates.

Magnetite enhances As immobilization during nitrate reduction and Fe(II) oxidation by Acidovorax sp. strain BoFeN1

Arsenic (As) cycling in groundwater is commonly coupled to the biogeochemical cycling of iron (Fe) and the associated transformation of Fe minerals present. Numerous laboratory studies suggested that Fe minerals can act as nucleation sites for further crystal growth and as catalysts for abiotic Fe(II) oxidation. In view of the widespread existence of magnetite in anoxic environments where As is often dissolved, we firstly exploited magnetite to enhance As immobilization during nitrate-reducing Fe(II) oxidation (NRFO) induced by Acidovorax sp. strain BoFeN1, a mixotrophic nitrate-reducing Fe(II)-oxidizing bacterium that can oxidize Fe(II) through both enzymatic and abiotic pathways. Subsequently, we investigated how magnetite affects NRFO and As immobilization. Results demonstrated a significant increase in As(III) removal efficiency from 75.4 % to 97.2 % with magnetite, attributed to the higher amount of NRFO and As(III) oxidation promoted by magnetite. It was found that magnetite stimulated the production of extracellular polymeric substances (EPS), which could decrease the diffusion of nitrate in the periplasm of bacteria and shield them against encrustation, resulting in a more rapid reduction of nitrate in the system with magnetite than that without magnetite. Meanwhile, Fe(II) was almost completely oxidized in the presence of magnetite during the whole 72 h experiment, while in the absence of magnetite, 47.7 % of Fe(II) remained, indicating that magnetite could obviously accelerate the chemical oxidation of Fe(II) with nitrite (the intermediates of nitrate bioreduction). Furthermore, the formation of labile Fe(III), an intermediate product of electron transfer between Fe(II) and magnetite, was reasonably deduced to be vital for anoxic As(III) oxidation. Additionally, the XPS analysis of the solid phase confirmed the oxidation of 43.8 % of As(III) to As(V). This study helps to understand the biogeochemical cycling of Fe and As in the environment, and provides a cost-effective and environmentally friendly option for in situ remediation of As-contaminated groundwater.

Providing high stability to suppress metal dissolution in LiMn0.5Fe0.5PO4 cathode materials by Zn doping

LiMnxFe1-xPO4 (LMFP) is a promising cathode material for Li-ion batteries owing to its higher energy density than that of LiFePO4. However, the Jahn-Teller distortion caused by Mn3+ and oxidation of Fe2+ leading electrolyte decomposition lowers its cycling performance and structure stability. To address these problems, this study proposes an optimized Zn-doping content to enhance the electrochemical performance of LMFP. Results indicate that doping with 1.0 % Zn (LMFP-Zn 1.0 %) achieves the optimal cycling performance. Especially, LMFP-Zn 1.0 % exhibits 92.6 % capacity retention after 100 cycles at 2C. The HR-TEM and XPS analyses confirm that this level of doping suppresses metal dissolution and structure destruction effectively after cycling at high C-rates, resulted in improving the electrochemical performance and stability of LMFP cathode materials.

Metal(loid)s in urban soil from historical municipal solid waste landfill: Geochemistry, source apportionment, bioaccessibility testing and human health risks

Landfills, especially those poorly managed, can negatively affect the environment and human beings through chemical contamination of soils and waters. This study investigates the soils of a historical municipal solid waste (MSW) landfill situated in the heart of a residential zone in the capital of Slovakia, Bratislava, with an emphasis on metal (loid) contamination and its consequences. Regardless of the depth, many of the soils exhibited high metal (loid) concentrations, mainly Cd, Cu, Pb, Sb, Sn and Zn (up to 24, 2620, 2420, 134, 811 and 6220 mg/kg, respectively), classifying them as extremely contaminated based on the geo-accumulation index (Igeo >5). The stable lead isotopic ratios of the landfill topsoil varied widely (1.1679–1.2074 for 206Pb/207Pb and 2.0573–2.1111 for 208Pb/206Pb) and indicated that Pb contained a natural component and an anthropogenic component, likely municipal solid waste incineration (MSWI) ash and construction waste. Oral bioaccessibility of metal (loid)s in the topsoil was variable with Cd (73.2–106%) and Fe (0.98–2.10%) being the most and least bioaccessible, respectively. The variation of metal (loid) bioaccessibility among the soils could be explained by differences in their geochemical fractionation as shown by positive correlations of bioaccessibility values with the first two fractions of BCR (Community Bureau of Reference) sequential extraction for As, Cd, Mn, Ni, Pb, Sn and Zn. The results of geochemical fractionation coupled with the mineralogical characterisation of topsoil showed that the reservoir of bioaccessible metal (loid)s was calcite and Fe (hydr)oxides. Based on aqua regia metal (loid) concentrations, a non-carcinogenic risk was demonstrated for children (HI = 1.59) but no risk taking into account their bioaccessible concentrations (HI = 0.65). This study emphasises the need for detailed research of the geochemistry of wastes deposited in urban soils to assess the potentially hazardous sources and determine the actual bioaccessibility and human health risks of the accumulated metal (loid)s.

Crystal phase conversion of amorphous Fe−Mn binary oxides promote the sequestration and redistribution of arsenic, cadmium, and lead in the soil: Comparison with crystalline form

Iron and manganese oxides participate in nearly all geochemical processes in soils. The crystal phase conversion of the Fe−Mn oxides is a crucial process that controls the transport and the fate of metal(loid)s (such as Pb, Cd, and As). However, this conversion and its contribution in immobilization are seldom reported. In this study, the phase transition process of amorphous Fe−Mn oxides has been investigated using TEM, HE−XRD and PDF analyses. The role of heavy metal atoms in the Fe−Mn oxides lattice has been revealed through atomic−scale structural characterization, including bond lengths and coordination numbers. In detail, Mn oxides and Fe oxides exhibit synergistic effects. Mn oxides oxidize As(III) to As(V). Meanwhile, the adsorbed As, Cd, and Pb irons were redistributed and incorporated into the lattice of the Fe−Mn oxides during the crystal phase conversion of the Fe oxides. During this process, the proportion of residual fraction of As, Cd, and Pb increased by 30 %, 25 %, and 9 %, respectively, after 28 d. 100 %, 41.8 %, and 60.7 % of decarbonate−extractable As, DTPA−extractable Cd and Pb could be immobilized, respectively. This study provides insight into the contribution of Fe−Mn oxides in controlling the cycling of metals and offers an advanced strategy in remediation of As, Cd, and Pb in contaminated soil.

Experimental insights and modeling innovations: Deciphering Fe(VI) oxidation in imidazole ionic liquids through QSAR and RFR

In this investigation, we conducted a detailed analysis of the oxidation of 16 imidazole ionic liquid variants by Fe(VI) under uniform experimental setups, thereby securing a dataset of second-order reaction rate constants (kobs). This methodology ensures superior data consistency and comparability over traditional methods that amalgamate disparate data from varied studies. Utilizing 16 chemical structural parameters obtained via Density Functional Theory (DFT) as descriptors, we developed a Quantitative Structure Activity Relationship (QSAR) model. Through rigorous correlation analysis, Principal Component Analysis (PCA), Multiple Linear Regression (MLR), and Applicability Domain (AD) evaluation, we identified a pronounced negative correlation between the molecular orbital gap energy (Egap) and kobs. MLR analysis further underscored Egap as a pivotal predictive variable, with its lower values indicating heightened oxidative reactivity towards Fe(VI) in the ionic liquids, leading the QSAR model to achieve a predictive accuracy of 0.95. Furthermore, we integrated an advanced machine learning approach - Random Forest Regression (RFR), which adeptly highlighted the critical factors influencing the oxidation efficiency of imidazole ionic liquids by Fe(VI) through elaborate decision trees, feature importance assessment, Recursive Feature Elimination (RFE), and cross-validation strategies. The RFR model demonstrated a remarkable predictive performance of 0.98. Both QSAR and RFR models pinpointed Egap as a key descriptor significantly affecting oxidation efficiency, with the RFR model presenting lower root mean square errors, establishing it as a more reliable predictive tool. The application of the RFR model in this study significantly improved the model's stability and the intuitive display of key influencing factors, introducing promising advanced analytical tools to the field of environmental chemistry.

A modified dithionite reduction method for the quantification of iron-bound organic carbon in marine sediments

Iron-bound organic carbon (Fe-OC) plays a significant role in the global marine carbon cycle, facilitating the long-term accumulation and storage of organic carbon. However, the conventional method for quantifying Fe-OC, known as the citrate-bicarbonate-dithionite (CBD) method, has limitations in terms of its efficiency in extracting both Fe-OC and iron (Fe). Another CBD method, commonly used for Fe speciation analysis in paleoenvironmental reconstructions, has shown efficient and selective extraction of Fe oxides; however, its effectiveness in extracting Fe-OC remains uncertain. To evaluate the efficiency of the two CBD methods, synthetic samples spiked with organic acid/environmental organic matter‑iron (hydr)oxide coprecipitates and a marine sediment sample were subjected to extraction. The CBD approach for Fe speciation analysis at pH 4.8 on spiked samples demonstrated higher efficiency compared to the conventional CBD method, but not for samples of marine sediment. This result suggests that the synthetic Fe-OC in the spiked samples did not accurately represent the realistic state of Fe-OC in marine sediments. We found that the pH of the leaching solution during the CBD extraction significantly influenced the efficiency of the Fe-OC and Fe extraction. Our tests at pH 4.8, 5.5, 6.0, 6.5, and 7.0 revealed that lower pH enhanced the extraction of Fe, but hindered the release of Fe-OC. Our study revealed that the new CBD (pH 6.5) method resulted in approximately 30% higher yields of Fe-OC and Fe compared to the conventional CBD method, despite of significant variability in the Fe-OC data. The modified CBD (pH 6.5) method uses lower concentration of extraction reagent and reaction temperature, both of which is considered beneficial. Therefore, we suggest that the new CBD method (pH 6.5) is at least of similar efficiency as the conventional method, and can be employed alongside the established method to facilitate the evaluation of the role of Fe-OC in the marine carbon cycle.

Iron-based metal organic framework (Fe-MOF)-doped sesame stalk biochar enhances antibiotic degradation: Important role of free radicals and comparison of multiple degradation processes

A novel iron-based metal organic framework-doped sesame stalk biochar (Fe-MOF@SBC) composite was successfully synthesized using sonication and used for the degradation of sulfamethoxazole (SMX), norfloxacin (NOR), erythromycin (ERY) and tetracycline (TC) in aqueous solution. The characterization results indicated that Fe oxide was successfully loaded on the biochar. The Fe-MOF@SBC composite presented a porous structure enriched in micropores and mesopores. Adsorption mechanisms, including pore filling, π–π electron donor–acceptor (EDA) interactions, hydrophobic interactions, hydrogen bonding and electrostatic interactions, and the catalytic effect of the Fe-MOF@SBC composite facilitated antibiotic degradation. Specifically, single-layer and multilayer adsorption occurred, and abundant oxygen-containing functional groups were involved in the degradation of NOR, which might be one reason for the high degradation rate of NOR by the Fe-MOF@SBC composite. Scavenging experiments combined with electron paramagnetic resonance (EPR) analysis indicated that free radicals such as •OH, O2−• and 1O2 played vital roles in the degradation of SMX, NOR, ERY and TC. Furthermore, the proposed degradation pathways and possible intermediates of SMX, NOR, ERY and TC were identified, which indicated that the adsorption properties and free radical (•OH, O2−• and 1O2) activity of the Fe-MOF@SBC composite strongly promoted the removal of SMX, NOR, ERY and TC.

Role of manganese oxides and oxyhydroxides in phosphate sequestration in natural aquatic environments

Excessive inputs of phosphorus (P) to aquatic environments can lead to eutrophication. Adsorption of phosphate on Fe oxides is one of the main process that can limit P availability. Oxidized forms of Mn have also been suggested to play a role in P trapping. However, although a considerable number of studies have shown the relationships between the geochemistry of Fe and that of phosphates, few studies have attempted to show the direct links between Mn and P. In the present study, we studied the adsorption of phosphate on synthetized Mn(III) and Mn(IV) oxides placed under natural conditions. The aim was to compare the role of Fe and Mn oxides in phosphate adsorption. Two muddy sediments were collected in a river bed at the edge of a large agricultural area. A sandy sediment was collected downstream. A muddy and a sandy sample were taken in a coastal environment. The experiments on phosphate adsorption by sediment and Mn oxides were carried out in slurries containing in situ waters spiked with 10 μM phosphate. Control experiments without Mn-oxide addition showed that the natural sediments tested still had the capacity to adsorb phosphate, in particular due to the presence of reactive Fe(III) oxides, extractable by an ascorbate solution. The addition of Mn(III) and Mn(IV) oxides in much larger quantities than the initial quantity of Fe oxides had little impact on the rate of phosphate adsorption. For both Mn(III) and Mn(IV) oxides, the Mn/P ratio between added particulate Mn and adsorbed P was very high, with values between 260 and 1000. The Fe/P ratio of the iron oxides contained in the slurries was between 6 and 20. On average, the P adsorption capacity of the Fe oxides was 50 times greater than that of the Mn oxides. Manganese oxides are generally less abundant than iron oxides in natural environments. Mn oxides therefore play a minor role in P sequestration compared with Fe oxides. Even if there are environments where Mn oxides can concentrate, the reduction of Mn oxides and subsequent liberation of adsorbed P does not represent a major risk for eutrophication of aquatic ecosystems.

Novel Insights into Sb(III) Oxidation and Immobilization during Ferrous Iron Oxygenation: The Overlooked Roles of Singlet Oxygen and Fe (oxyhydr)oxides Formation.

Reactive oxygen species (ROS) produced from the oxygenation of reactive Fe(II) species significantly affect the transformation of metalloids such as Sb at anoxic-oxic redox interfaces. However, the main ROS involved in Sb(III) oxidation and Fe (oxyhydr)oxides formation during co-oxidation of Sb(III) and Fe(II) are still poorly understood. Herein, this study comprehensively investigated the Sb(III) oxidation and immobilization process and mechanism during Fe(II) oxygenation. The results indicated that Sb(III) was oxidized to Sb(V) by the ROS produced in the aqueous and solid phases and then immobilized by formed Fe (oxyhydr)oxides via adsorption and coprecipitation. In addition, chemical analysis and extended X-ray absorption fine structure (EXAFS) characterization demonstrated that Sb(V) could be incorporated into the lattice structure of Fe (oxyhydr)oxides via isomorphous substitution, which greatly inhibited the formation of lepidocrocite (γ-FeOOH) and decreased its crystallinity. Notably, goethite (α-FeOOH) formation was favored at pH 6 due to the greater amount of incorporated Sb(V). Moreover, singlet oxygen (1O2) was identified as the dominant ROS responsible for Sb(III) oxidation, followed by surface-adsorbed ·OHads, ·OH, and Fe(IV). Our findings highlight the overlooked roles of 1O2 and Fe (oxyhydr)oxide formation in Sb(III) oxidation and immobilization during Fe(II) oxygenation and shed light on understanding the geochemical cycling of Sb coupled with Fe in redox-fluctuating environments.

Alteration of soil pH induced by submerging/drainage and application of peanut straw biochar and its impact on Cd(II) availability in an acidic soil to indica-japonica rice varieties

The effects of soil pH variations induced by submergence/drainage and biochar application on soil cadmium (Cd) availability to different rice (Oryza sativa L.) varieties are not well understood. This study aims to investigate the possible reasons for available Cd(II) reduction in paddy soil as influenced by biochar and to determine Cd(II) absorption and translocation rates in different parts of various rice varieties. A pot experiment in a greenhouse using four japonica and four indica rice varieties was conducted in Cd(II) contaminated paddy soil with peanut straw biochar. The results indicated that the submerging led to an increase in soil pH due to the consumption of protons (H+) by the reduction reactions of iron/manganese (Fe/Mn) oxides and sulfate (SO42−) and thus the decrease in soil available Cd(II) contents. However, the drainage decreased soil pH due to the release of protons during the oxidation of Fe2+, Mn2+, and S2− and thus the increase in soil available Cd(II) contents. Application of the biochar increased soil pH during soil submerging and inhibited the decline in soil pH during soil drainage, and thus decreased soil available Cd(II) contents under both submerging and drainage conditions. The indica rice varieties absorbed more Cd(II) in their roots and accumulated higher amounts of Cd(II) in their shoots and grains than the japonica rice varieties. The Cd(II) sensitive varieties exhibited a greater absorption and translocation rate of Cd(II) compared to the tolerant varieties of both indica and japonica rice. Biochar inhibited the absorption and accumulation of Cd(II) in the rice varieties, which ultimately lowered the Cd(II) contents in rice grains below the national food safety limit (0.2 mg kg−1). Overall, planting japonica rice varieties in Cd(II) polluted paddy soils combined with the use of biochar can effectively reduce Cd(II) content in rice grains which protects human health against Cd(II) toxicity.

Evaluation of coal mine drainage and associated precipitates for radium and rare earth element concentrations

Coal mine drainage (CMD) and associated metal-rich precipitates have recently been proposed as unconventional sources of rare earth elements (REEs). However, the potential occurrence of radium (Ra), a known carcinogen, with the REE-bearing phases has not been investigated. We hypothesized that Ra may occur in solids that are precipitated from CMD as a “radiobarite” solid solution ((Ba,Sr,Ra)SO4) and/or adsorbed with hydrous metal oxides. REEs have been documented to sorb or co-precipitate with iron (Fe), manganese (Mn), and aluminum (Al) oxyhydroxide in CMD solids. Likewise, Ra has been documented to sorb to hydrous Fe and Mn oxides especially where sulfate (SO4) and/or barium (Ba) concentrations are insufficient to precipitate radiobarite. Thus, we conducted the first-ever survey of Ra concentrations in corresponding CMD water and solid samples in the United States. Samples were analyzed from 4 untreated and 9 treated CMD sites in both the bituminous and anthracite coal regions of Pennsylvania across a range of pH and SO4 concentrations. The dissolved Ra in CMD was relatively low (<0.5 Bq/L), consistent with radiobarite solubility; however, CMD solids were largely composed of amorphous Fe, Al, and Mn oxyhydroxide and silicate minerals. Ra was associated with Mn-enriched CMD solids, upwards of 875 Bq/kg. Total REE + yttrium (Y) content in the CMD solids was enriched upwards of 3600 mg/kg and was significantly correlated with Al content. These preliminary results suggest that REE extraction may target Al-rich solids to avoid Ra in Mn-rich solids.

FePO4.2H2O recovery from acidic phosphate-rich waste streams

Phosphorous not only needs to be removed to prevent eutrophication of wastewater effluent receiving surface water bodies, but it also has to be recovered as a scarce finite reserve. Phosphorus chemical precipitation as NH4MgPO4·6H2O, Ca3(PO4)2, or Fe3(PO4)2 ·8H2O is the most common method of phosphorus recovery from phosphorus-rich streams. These minerals ideally form under neutral to alkaline pH conditions, making acidic streams problematic for their formation due to the need for pH adjustments. This study proposes FePO4 .2H2O (strengite-like compounds) recovery from acidic streams due to its simplicity and high efficiency, while also avoiding the need for pH-adjusting chemicals. The effect of initial pH, temperature, Fe (III) dosing rates, and Fe (II) dosage under different oxidation conditions (pO2 = 0.2, 1, 1.5 bar, different H2O2 dosing rates) on phosphorus recovery percentage and product settleability were evaluated in this study. The precipitates formed were analyzed using optical microscopy, SEM, XRD, SQUID, Raman, and ICP. Experiments showed that Fe (III) dosing achieved phosphorus recovery of over 95 % at an initial pH of 3 or higher, and the product exhibited poor settleability in all initial pH (1.5-5), and temperature (20–80 °C) tests. On the other hand, Fe (II) dosage instead of Fe (III) resulted in good product settleability but varying phosphorus recovery percentages depending on the oxidation conditions. The novelty of the study lies in revealing that the Fe (II) oxidation rate serves as a crucial process-design parameter, significantly enhancing product settleability without the requirement of carrier materials or crystallizers. The study proposes a novel strategy with controlled Fe2+-H2O2 dosing, identifying an Fe (II) oxidation rate of 4.7 × 10−4 mol/l/min as the optimal rate for achieving over 95 % total phosphorus recovery, along with excellent settleability with a volumetric index equal to only 8 ml/gP.

Identification of solid phase speciation of trace elements in dust dry deposition using focused ion beam and selected area electron diffraction

BackgroundFew studies have been conducted to identify solid phase speciation of trace elements within the interior of airborne particles, although the adverse effects of ambient particles are closely related to the speciation of toxic elements and therefore the identification of chemical binding sites and solid phase associations is essential for assessing the impacts of trace elements in ambient air on environmental quality and human health. ObjectiveA combined use of focused ion beam scanning electron microscopy (FIB-SEM) and selected area electron diffraction (SAED) patterns from transmission electron microscopy (TEM) was applied with an energy dispersive X-ray spectroscopy (EDS) to investigate the solid phase speciation of trace elements within the interior of dust dry deposition obtained from an urban area. ResultsThe study results using the high-resolution microscopic techniques for the characterization of nano- and micron-sized particles revealed lead chromate for Pb and Cr6+, barite for Ba, rutile for Ti, and calcite, gypsum, and quicklime for Ca within the carbon matrix, implying their traffic-related sources. Fe oxides were observed for Fe, Mn, Ni, and Cr3+, with cerianite and Fe-La alloys for rare earth elements (REEs). ConclusionsThree types of anthropogenic sources were inferred based on the morphology and texture of the solid phases, as well as their primary usage: traffic road marking paints for Pb, Cr6+, Ba, and Ti, cement materials for Ca, and industrial activities for Fe, Mn, Ni, Cr3+, and REEs. The presence of submicron-sized toxic metal-bearing particles in dust dry deposition indicated that the toxic metals (i.e., Pb, Cr6+) can penetrate into the respiratory system along with the nanoparticles. The study results demonstrate that the solid phase speciation of trace elements detected using FIB-SEM and TEM-SAED can be effectively utilized for the identification of sources and the assessment of chemical toxicity of atmospheric dust.

Green and high-yield recovery of phosphorus from municipal wastewater for LiFePO4 batteries

The rapidly growing demand for lithium iron phosphate (LiFePO4) as the cathode material of lithium–ion batteries (LIBs) has aggravated the scarcity of phosphorus (P) reserves on Earth. This study introduces an environmentally friendly and economical method of P recovery from municipal wastewater, providing the P source for LiFePO4 cathodes. The novel approach utilizes the sludge of Fe-coagulant-based chemical P removal (CPR) in wastewater treatment. After a sintering treatment with acid washing, the CPR sludge, enriched with P and Fe, transforms into purified P–Fe oxides (Fe2.1P1.0O5.6). These oxides can substitute up to 35% of the FePO4 reagent as precursor, producing a carbon-coated LiFePO4 (LiFePO4/C) cathode with a specific discharge capacity of 114.9 mA·h·g−1 at 0.1 C (C represents current density, 1 C equaling 170 mA·g−1) (C), and cycle stability of 99.2% after 100 cycles. The enhanced cycle performance of the as-prepared LiFePO4/C cathode may be attributed to the incorporations of impurities (such as Ca2+ and Na+) from sludge, with improved stability of crystal structure. Unlike conventional P-fertilizers, this P recovery technology converts 100% of P in CPR sludge into the production of value-added LiFePO4/C cathodes. The recovered P from municipal wastewater can meet up to 35% of the P demand in the Chinese LIBs industry, offering a cost-effective solution for addressing the pressing challenges of P scarcity.

Elucidating microbial iron corrosion mechanisms with a hydrogenase-deficient strain of Desulfovibrio vulgaris.

Sulfate-reducing microorganisms extensively contribute to the corrosion of ferrous metal infrastructure. There is substantial debate over their corrosion mechanisms. We investigated Fe0 corrosion with Desulfovibrio vulgaris, the sulfate reducer most often employed in corrosion studies. Cultures were grown with both lactate and Fe0 as potential electron donors to replicate the common environmental condition in which organic substrates help fuel the growth of corrosive microbes. Fe0 was corroded in cultures of a D. vulgaris hydrogenase-deficient mutant with the 1:1 correspondence between Fe0 loss and H2 accumulation expected for Fe0 oxidation coupled to H+ reduction to H2. This result and the extent of sulfate reduction indicated that D. vulgaris was not capable of direct Fe0-to-microbe electron transfer even though it was provided with a supplementary energy source in the presence of abundant ferrous sulfide. Corrosion in the hydrogenase-deficient mutant cultures was greater than in sterile controls, demonstrating that H2 removal was not necessary for the enhanced corrosion observed in the presence of microbes. The parental H2-consuming strain corroded more Fe0 than the mutant strain, which could be attributed to H2 oxidation coupled to sulfate reduction, producing sulfide that further stimulated Fe0 oxidation. The results suggest that H2 consumption is not necessary for microbially enhanced corrosion, but H2 oxidation can indirectly promote corrosion by increasing sulfide generation from sulfate reduction. The finding that D. vulgaris was incapable of direct electron uptake from Fe0 reaffirms that direct metal-to-microbe electron transfer has yet to be rigorously described in sulfate-reducing microbes.