Oxidative Dissolution Research Articles

In This Issue

Dissolution of redox-active metal oxides plays a fundamental role in the environment and technological applications. However, dissolution mechanisms remain poorly understood at nanoscale, particularly the interplay between acidic and reductive ...Iron-based redox-active minerals are ubiquitous in soils, sediments, and aquatic systems. Their dissolution is of great importance for microbial impacts on carbon cycling and the biogeochemistry of the lithosphere and hydrosphere. Despite its widespread ...

Sustainable porous biochar coated MnO2 composites as the cathode in aqueous Zn/Mn batteries

Sustainable development is the theme of the current era, and the reutilization of resources and clean energy are the keys to achieve it. Zn/Mn batteries had become a research hotspot due to its advantages of safety and efficiency, but the poor conductivity and the dissolution of manganese oxides restrict the further development of the batteries. Herein, we put forward a strategy to form a uniform graphitized carbon layer with porous structure on the MnO2 by a simple hydrothermal reaction from pistachio shell, which immobilizes MnO2 and effectively inhibits its dissolution into the electrolyte. The electrochemical results shows that the specific capacity of biochar composites cathode reaches 292.9 mA h g−1 after 50 stable cycles at a current density of 100 mA g−1, which is three times higher than that of the non-coated MnO2 cathode (92.5 mA h g−1). This unique porous material with high conductivity provides some new ideas for the modification of Mn-based zinc ion batteries and the sustainable reuse of biological resources.

Dissolution of Actinide Oxides in Carbonate Solutions

Dissolution of Actinide Oxides in Carbonate Solutions

Corrosion of Metals During Use in Arthroplasty.

Arthroplasty implants can undergo corrosion at the modular components, trunnion, and hinges, owing to implant material makeup, micromotion, and interaction with body fluid. In this review, various mechanisms of corrosion in arthroplasty were explored with suggestions on means of improvement. We identified 10 methods including pitting, crevice, mechanically assisted crevice corrosion, fretting, fretting initiated crevice corrosion, mechanically assisted taper corrosion, galvanic corrosion, stress/tension, fatigue corrosion, and inflammatory cell induced corrosion. The position of implants on the galvanic series, and their ability to maintain passivation contribute to their longevity in service. Due to the relative motion of arthroplastic components, bio-tribocorrosion may disrupt passive oxide films, and pitting is initiated at interfaces. Thus, corrosion in arthroplasty as an electrochemical phenomenon mainly starts on one spot and progresses in 3 steps: (1) the oxidative dissolution of metal from implant surfaces into the aqueous active environment, releasing cations, (2) the attraction of electrons to the opposite charge created at another point of the implant surface, producing current flow, and (3) the formation of oxides of metal and metal hydroxides deposited as rust at the surface of the implant. Recent innovations in material manufacturing continue to improve the efficiency of arthroplasty; however, the component parts remain susceptible to bio-tribocorrosion. Thus, a complete eradication of corrosion in arthroplasty would require futuristic materials with improvement in recent materials and designs, derived from knowledge of existing retrieved implants, and strategies to provide overall surface finishes that protect against bio-tribocorrosion.

Biochar inhibited hydrogen radical-induced Cd bioavailability in a paddy soil

Herein, hydrogen (H·) radical was observed as a new pathway to produce hydroxyl (OH·) radicals that promoted cadmium sulfide (CdS) dissolution and thus Cd solubility in paddy soils. In soil incubation experiments, the bioavailable Cd concentrations in flooded paddy soils were increased by 8.44 % as the soil was aerated for 3d. For the first time, the H· radical was observed in aerated soil sludge. The association of CdS dissolution with free radicals was thereafter confirmed in an electrolysis experiment. Both H· and OH· radicals in electrolyzed water were confirmed by the electron paramagnetic resonance analysis. In the system with CdS, water electrolysis increased soluble Cd2+ concentration by 60.92 times, which was compromised by 43.2 % when the radical scavenger was introduced. This confirmed the free radicals can lead to oxidative dissolution of CdS. The H· radical was generated in systems with fulvic acid or catechol irradiated by ultraviolet lights, indicating soil organic carbon could be an important precursor for H· and OH· radicals. Biochar application decreased soil DTPA-Cd by 22–56 % invoking mechanisms besides adsorption. First, biochar quenched radicals and reduced CdS dissolution by 23.6 % in electrolyzed water in which -C-OH of biochar was oxidized to CO. Second, biochar boosted Fe/S-reducing bacteria and thus compromised CdS dissolution, as affirmed by a reversal correlation between soil available Fe2+ and DTPA-Cd concentrations. A similar phenomenon occurred in Shewanella oneidensis MR-1-inoculated soils. This study provided new insights into the bioavailability of Cd and offered feasible measures to remediate Cd-contaminated paddy soils with biochars.

New Insight into Enhancing Organic-Rich Shale Gas Recovery: Shut-in Performance Increased through Oxidative Fluids

Oxidizing stimulation of organic-rich shale reservoirs, as a supplement of hydraulic fracturing, was proposed to enhance shale gas recovery. Previous publications revealed that the interaction between organic-rich shale and oxidative fluids causes the components’ dissolution, which induces lots of pores and microfractures, resulting in rock microfracturing without confined pressure and associated increments of the matrix permeability, and improving unpropped fracture conductivity. However, the enhancement of shale gas recovery with oxidative fluids still lacks an implementation clue targeted for specific engineering problems. In recent years, water–rock interaction inducing microfractures indicates a positive effect of retained fracturing fluid on the stimulation after the fracturing operation, which sheds light in the enhancement of shale gas production by shut-in. The objectives of this study are to provide a new perspective whereby the shut-in performance to enhance shale gas recovery could be increased by the injection of oxidative fluids into the formation during the fracturing operation. Firstly, the mechanisms of shut-in performance increased by oxidative dissolution, which illustrate the increment of the density of fracture networks, the improvement of fracture network conductivity, and the promotion of gas desorption and diffusivity, are demonstrated. Then, the feasibility of using oxidative fluids to increase shut-in performance, which follows the geological and engineering characteristics of organic-rich shale reservoirs, is evaluated. Finally, according to the analysis of production performance for two typical types of shale gas wells, in which one is a low gas production and a high fracturing fluid recovery (LGP-HFR) and the other is a high gas production and a low fracturing fluid recovery (HGP-LFR), a shut-in strategy with oxidative fluids to enhance shale gas recovery is developed. This indicates that the injection of oxidative fluids during the fracturing operation may become a promising and cost-effective approach to enhance shale gas recovery.

Biochar Promotes Arsenopyrite Weathering in Simulated Alkaline Soils: Electrochemical Mechanism and Environmental Implications.

Oxidation dissolution of arsenopyrite (FeAsS) is one of the important sources of arsenic contamination in soil and groundwater. Biochar, a commonly used soil amendment and environmental remediation agent, is widespread in ecosystems, where it participates in and influences the redox-active geochemical processes of sulfide minerals associated with arsenic and iron. This study investigated the critical role of biochar on the oxidation process of arsenopyrite in simulated alkaline soil solutions by a combination of electrochemical techniques, immersion tests, and solid characterizations. Polarization curves indicated that the elevated temperature (5-45 °C) and biochar concentration (0-1.2 g·L-1) accelerated arsenopyrite oxidation. This is further confirmed by electrochemical impedance spectroscopy, which showed that biochar substantially reduced the charge transfer resistance in the double layer, resulting in smaller activation energy (Ea = 37.38-29.56 kJ·mol-1) and activation enthalpy (ΔH* = 34.91-27.09 kJ·mol-1). These observations are likely attributed to the abundance of aromatic and quinoid groups in biochar, which could reduce Fe(III) and As(V) as well as adsorb or complex with Fe(III). This hinders the formation of passivation films consisting of iron arsenate and iron (oxyhydr)oxide. Further observation found that the presence of biochar exacerbates acidic drainage and arsenic contamination in areas containing arsenopyrite. This study highlighted the possible negative impact of biochar on soil and water, suggesting that the different physicochemical properties of biochar produced from different feedstock and under different pyrolysis conditions should be taken into account before large-scale applications to prevent potential risks to ecology and agriculture.

Exploring H2-effects on radiation-induced oxidative dissolution of UO2-based spent nuclear fuel using numerical simulations

Using a recently developed approach for numerical simulation of radiation-induced oxidative dissolution of spent nuclear fuel, we have explored the impact of three possible contributions to the inhibiting effect of molecular hydrogen. The three contributions are (1) effect on oxidant production in irradiated water, (2) reduction of oxidized uranium catalyzed by noble metal inclusions (fission products) and (3) reaction with surface-bound hydroxyl radicals preventing the oxidation of uranium. The simulations show that the first contribution is of fairly small importance while the second contribution can result in complete inhibition of the oxidative dissolution. This is well in line with previous work. Interestingly, the simulations imply that the third contribution, the reaction between H2 and the surface-bound hydroxyl radical formed upon reaction between the radiolysis product H2O2 and UO2, can account for the inhibition observed in systems where noble metal inclusions are not present. This is discussed in view of previously published experimental data.

Investigating the effect of mono di carboxylic acids as hydrogen bond donor on solvation of copper in choline chloride-based deep eutectic solvents

Deep eutectic solvents (DESs) are green alternatives to highly acidic solvents or ionic liquids (ILs). The flexible complexation properties of eutectic solvents open their scope as a prospective dissolution medium for metal extraction. The present study investigates the range of monocarboxylic acids-choline chloride (Monocarboxy A-ChCl) and dicarboxylic acid-choline chloride (Dicarboxy A-ChCl) DESs as solvent media for copper oxide dissolution with a discussion on physical properties and their effect on dissolution. Thermal stability, freezing point, Kamlet Taft parameter, density, and viscosity of Carboxy A-ChCl DESs were experimentally determined.The density of Carboxy A-ChCl DESs follows the trend: FA-ChCl < Acetic A-ChCl < OA-ChCl < MA-ChCl < CA-ChCl. The viscosity of Monocarboxy A-ChCl was observed to be lesser than Dicaboxy A-ChCl DESs. The polarity of Monocarboxy A-ChCl was observed to be in the range of 1.2–0.8, while Dicarboxy A-ChCl, polarity was greater than 1. Further, the effect of hydrogen bond donors (HBD), hydrogen bond acceptors (HBA), KFT parameters, viscosity, fukui function of HBD, reaction time and temperature affecting the dissolution of Cu are elaborated in detail. Most dependant to least dependent parameters with respect to dissolution is recognized and some crucial conclusions were put together. Formic acid: Choline chloride (FA-ChCl) DES indicated the highest copper dissolution in the range > 106 ppm. Copper dissolution from ionic CuO to low polar Monocarboxy A-ChCl DESs was higher than in high polar Dicarboxy A-ChCl DESs. DFT calculation reveals that Formic Acid (FA) has the highest fukui functions compared to other carboxylic acids, which is directly reflected in the higher solubility of copper oxides. Additionally, the effect of reaction time and temperature on CuO dissolution in best-performing FA-ChCl DES was studied, 75 °C, 27 h, 100 °C, 18 h were chosen as the optimum time and temperature for maximum dissolution of Cu. The results cover important parameters of Carboxy A-ChCl DES, which could be helpful in metal extraction applications.

Oxidative dissolution of Cr(OH)3 and mixed Fe-Cr(III) phases by aqueous Mn(III)-pyrophosphate complex

The key role of manganese (Mn) in the biogeochemical cycle of trace elements has been of great interest in recent years. Nevertheless, the redox properties of aqueous Mn(III) have been studied to a lesser extent. Mn(III) is not stable in solution by itself. However, when complexed with inorganic ligands, it has shown potential to oxidize and reduce trace elements. In the present study, we are exploring the redox characteristics of the complex Mn(III)-Pyrophosphate (Mn(III)-PP). This complex is stable over a wide range of pH values but requires the ratio of Mn:PP to be less than 1:6. Specifically, the redox reaction of chromium (Cr(III)) and Mn(III)-PP is investigated. A solid, Cr(OH)3, is used as a source of Cr(III). For this reaction, environmentally relevant parameters, such as pH, ionic strength, ratio Mn(III)/Cr(III), and excess of ligand, were assessed. Results showed that Mn(III) can effectively oxidize Cr(III) to Cr(VI), taking about 15 days for the reaction to complete. This reaction occurs only under acidic conditions (pH 4), and with a low excess of Pyrophosphate. The initial Mn(III) concentration decreases as the Cr(VI) is produced, and Cr(VI) can be adsorbed back into the Cr(OH)3 surface, limiting the mobility of this toxic species. Despite this adsorption, significant amounts of Cr(VI) are release in the aqueous phase. This study shows the importance of a mobile species (Mn-PP complex) in the oxidation of Cr(III) and the release of Cr(VI) to the environment.

Developing the Ascorbic Acid Test: A Candidate Standard Tool for Characterizing the Intrinsic Reactivity of Metallic Iron for Water Remediation

Granular metallic iron (gFe0) materials have been widely used for eliminating a wide range of pollutants from aqueous solutions over the past three decades. However, the intrinsic reactivity of gFe0 is rarely evaluated and existing methods for such evaluations have not been standardized. The aim of the present study was to develop a simple spectrophotometric method to characterize the intrinsic reactivity of gFe0 based on the extent of iron dissolution in an ascorbic acid (AA—0.002 M or 2 mM) solution. A modification of the ethylenediaminetetraacetic acid method (EDTA method) is suggested for this purpose. Being an excellent chelating agent for FeII and a reducing agent for FeIII, AA sustains the oxidative dissolution of Fe0 and the reductive dissolution of FeIII oxides from gFe0 specimens. In other words, Fe0 dissolution to FeII ions is promoted while the further oxidation to FeIII ions is blocked. Thus, unlike the EDTA method that promotes Fe0 oxidation to FeIII ions, the AA method promotes only the formation of FeII species, despite the presence of dissolved O2. The AA test is more accurate than the EDTA test and is considerably less expensive. Eight selected gFe0 specimens (ZVI1 through ZVI8) with established diversity in intrinsic reactivity were tested in parallel batch experiments (for 6 days) and three of these specimens (ZVI1, ZVI3, ZVI5) were further tested for iron leaching in column experiments (for 150 days). Results confirmed the better suitability (e.g., accuracy in assessing Fe0 dissolution) of the AA test relative to the EDTA test as a powerful screening tool to select materials for various field applications. Thus, the AA test should be routinely used to characterize and rationalize the selection of gFe0 in individual studies.

Impact of Minor Alloy Components on the Electrocapillarity and Electrochemistry of Liquid Metal Fractals

AbstractExploring and controlling surface tension‐driven phenomena in liquid metals may lead to unprecedented possibilities for next‐generation microfluidics, electronics, catalysis, and materials synthesis. In pursuit of these goals, the impact of minor constituents within liquid alloys is largely overlooked. Herein, it is showed that the presence of a fraction of solute metals such as tin, bismuth, and zinc in liquid gallium can significantly influence their electrocapillarity and electrochemistry. The instability‐driven fractal formation of liquid alloy droplets is investigated with different solutes and reveals the formation of distinctive non‐branched droplets, unstable fractals, and stable fractal modes under controlled voltage and alkaline solution conditions. In their individually unique fractal morphology diagrams, different liquid alloys demonstrate significantly shifted voltage thresholds in transition between the three fractal modes, depending on the choice of the solute metal. Surface tension measurements, cycle voltammetry and surface compositional characterizations provide strong evidence that the minor alloy components drastically alter the surface tension, surface electrochemical oxidation, and oxide dissolution processes that govern the droplet deformation and instability dynamics. The findings that minor components are able to regulate liquid alloys’ surface tensions, surface element distributions and electrochemical activities offer great promises for harnessing the tunability and functionality of liquid metals.

Transformation, Absorption and Toxicological Mechanisms of Silver Nanoparticles in the Gastrointestinal Tract Following Oral Exposure.

Oral exposure is known as the primary way for silver nanoparticles (AgNPs), which are commonly used as food additives or antibacterial agents in commercial products, to enter the human body. Although the health risk of AgNPs has been a concern and extensively researched over the past few decades, there are still numerous knowledge gaps that need to be filled to disclose what AgNPs experience in the gastrointestinal tract (GIT) and how they cause oral toxicity. In order to gain more insight into the fate of AgNPs in the GIT, the main gastrointestinal transformation of AgNPs, including aggregation/disaggregation, oxidative dissolution, chlorination, sulfuration, and corona formation, is first described. Second, the intestinal absorption of AgNPs is presented to show how AgNPs interact with epithelial cells and cross the intestinal barrier. Then, more importantly, we make an overview of the mechanisms underlying the oral toxicity of AgNPs in light of recent advances as well as the factors affecting the nano-bio interactions in the GIT, which have rarely been thoroughly elaborated in published literature. At last, we emphatically discuss the issues that need to be addressed in the future to answer the question "How does oral exposure to AgNPs cause detrimental effects on the human body?".

Chelating agents for the removal of calcareous deposits from archaeological ceramic materials. Compositional evaluation after immersion and physical gel application methods

The removal of calcareous deposits from archaeological ceramics is a very normal conservation-restoration treatment. Among the products used, chelating agents are quite common, including ethylenediaminetetraacetic acid salts (EDTA) (Berducou in La Conservation en archéologie: méthodes et pratique de la conservation-restauration des vestiges archéologiques, Masson, Paris, 1990; Buys and Oakley The conservation and restoration of ceramics, Butterworth-Heinemann, Oxford, 1993; Crisci et al. in Appl Phys A Mater Sci Process, 2010. https://doi.org/10.1155/2009/893528). Nevertheless, some studies have proved that they can cause damages on the ceramic pieces, regarding changes in their composition, such as dissolution of calcareous components and metallic oxides leaching (Gibson in Stud Conserv, 1971. https://doi.org/10.1155/2009/893528; Fernández and Seva Sautuola in Rev del Inst Prehist y Arqueol 9:471–982, 2003). As a consequence, their artistic values might also change. In spite of that, these products are nowadays still in use, meaning that the treatments might be changing the information that archaeological ceramics carry. However, from the 80 s onwards a more secure alternative to direct application methods based on thickening agents was developed. With the aim of analysing the degradation mechanisms that may take place after the cleaning treatments’ application, ceramic specimens with artificial calcareous deposits (Sáenz-Martínez et al. in Eur Phys J Plus 136:798, 2021) were treated with a low-concentrated solution of EDTA tetrasodium salt applied by immersion and thickened with xanthan gum powder (Vanzan® NF-C). Finally, the products from the cleaning treatments were neutralised, respectively, by immersion and by rinsing with deionized water. The composition of the ceramic samples was established before the growth of calcareous deposits and after the treatments, in order to determine their effectiveness and safety. According to the results, EDTA salt treatments, both by immersion and thickened, were effective regarding the removal of the calcareous deposits and did not modify the elemental and mineralogical original composition of the specimens (XRF, XRPD, TG-DSC). In addition, no gel residues were detected by FTIR-ATR.

Nanoporous Gold: From Structure Evolution to Functional Properties in Catalysis and Electrochemistry.

Nanoporous gold (NPG) is characterized by a bicontinuous network of nanometer-sized metallic struts and interconnected pores formed spontaneously by oxidative dissolution of the less noble element from gold alloys. The resulting material exhibits decent catalytic activity for low-temperature, aerobic total as well as partial oxidation reactions, the oxidative coupling of methanol to methyl formate being the prototypical example. This review not only provides a critical discussion of ways to tune the morphology and composition of this material and its implication for catalysis and electrocatalysis, but will also exemplarily review the current mechanistic understanding of the partial oxidation of methanol using information from quantum chemical studies, model studies on single-crystal surfaces, gas phase catalysis, aerobic liquid phase oxidation, and electrocatalysis. In this respect, a particular focus will be on mechanistic aspects not well understood, yet. Apart from the mechanistic aspects of catalysis, best practice examples with respect to material preparation and characterization will be discussed. These can improve the reproducibility of the materials property such as the catalytic activity and selectivity as well as the scope of reactions being identified as the main challenges for a broader application of NPG in target-oriented organic synthesis.

Molecular characteristics of natural organic matter in the groundwater system with geogenic iodine contamination in the Datong Basin, Northern China

Natural organic matter (NOM) plays an important role in the iodine mobilization in the groundwater system. In this study, the groundwater and sediments from iodine affected aquifers in the Datong Basin were collected to perform chemistry analysis and molecular characteristics of NOM by Fourier transform ion cyclotron resonance mass spectrometer (FT-ICR-MS). Total iodine concentrations in groundwater and sediments ranged from 1.97 to 926.1 μg/L and 0.001–2.86 μg/g, respectively. A positive correlation was observed between DOC/NOM and groundwater/sediment iodine. FT-ICR-MS results showed that the DOM in the high-iodine groundwater system is characterized by less aliphatic and more aromatic compounds with higher NOSC, indicating the features of more unsaturated larger molecule structures and more bioavailability. Aromatic compounds could be the main carriers of sediment iodine and were easily absorbed on amorphous iron oxides to form the NOM-Fe-I complex. More aliphatic compounds, especially those containing N/S, experienced a higher degree of biodegradation, which further mediated the reductive dissolution of amorphous iron oxides and the transformation of iodine species, thereby causing the release of iodine into groundwater. The findings of this study provide some new insights into the mechanisms of high-iodine groundwater.

CO2 Release Driven by the Combination of Sulfide Oxidation and Carbonate Dissolution in the Upper Changjiang River: Effect of Erosion and Lithology on Chemical Weathering

AbstractThe positive link between weathering and orogeny suggests that the drawdown of atmospheric carbon dioxide (CO2) over the Cenozoic should be closely related to the enhanced silicate weathering caused by Cenozoic orogeny, especially the uplift of the Tibetan plateau. However, the uplift of orogens could also exhume carbonate and sulfide minerals, the coupling weathering of which can act as a source for atmospheric CO2 at the long‐term timescale and counteract the CO2 consumption by silicate weathering. In this study, the solute concentrations and isotopic ratios in the upper Changjiang River are determined to investigate the co‐weathering of silicate, carbonate, and sulfide minerals and its impact on the long‐term carbon cycle. An inverse mixing model incorporating elemental and isotopic ratios is established to quantitatively identify the sources of cations and weathering acids. The inversion of our samples reveals that carbonate weathering dominates the chemical composition of river water, even with the presence of secondary calcite precipitation. The sulfide oxidation widely occurs in the study area and the bulk of generated sulfuric acid is buffered by carbonate minerals. The erosion is decoupled from sulfide oxidation and chemical weathering, likely due to lithology and secondary calcite precipitation. Overall, we find that the CO2 release from sulfide oxidation coupled with carbonate dissolution exceeds the CO2 drawdown from silicate weathering in the Changjiang River basin, indicating the weathering will act as a source for atmospheric CO2 at the timescale between marine carbonate compensation (105 years) and sulfate reduction (&lt;10 Ma).

Suppression of Vanadium Oxide Dissolution via Cation Metathesis within a Coordination Supramolecular Network for Durable Aqueous Zn-V2 O5 Batteries.

Aqueous zinc metal batteries (ZMBs) are a promising sustainable technology for large-scale energy storage applications. However, the water is often associated with problematic parasitic reactions on both anode and cathode, leading to the low durability and reliability of ZMBs. Here, a multifunctional separator for the Zn-V2 O5 batteries by growing the coordination supramolecular network (CSN:Zn-MBA, MBA = 2-mercaptobenzoic acid) on the conventional non-woven fabrics (NWF) is developed. CSN tends to form a stronger coordination bond as a softer cation, enabling a thermodynamically preferred Zn2+ to VO2 + substitution in the network, leading to the formation of VO2 -MBA interface, that strongly obstructs the VO2 (OH)2 - penetration but simultaneously allows Zn2+ transfer. Moreover, Zn-MBA molecules can adsorb the OTF- and distribute the interfacial Zn2+ homogeneous, which facilitate a dendrite-free Zn deposition. The Zn-V2 O5 cells with Zn-MBA@NWF separator realize high capacity of 567 mAh g-1 at 0.2 A g-1 , and excellent cyclability over 2000 cycles with capacity retention of 82.2% at 5 A g-1 . This work combines the original advantages of the template and new function of metals via cation metathesis within a CSN, provides a new strategy for inhibiting vanadium oxide dissolution.

Oxidative dissolution of (U,Ce)O2 materials in aqueous solutions containing H2O2

Homogeneous and heterogeneous U1-xCexO2 (with 0≤ x≤ 0.25) materials were prepared via wet and dry chemistry routes, respectively before being submitted to dynamic leaching experiments. The feeding solution containing 0.20 mmol.L−1 H2O2 was kept under air and renewed to guarantee the stability of H2O2 during the experiment. Normalized alteration rates were determined from U concentration in the leachates. For homogeneous (U,Ce)O2 materials, the dissolution rate was divided by a factor of 3 when increasing the Ce content from 0.08 to 0.25. Surface characterizations revealed that studtite precipitated all over UO2 pellet surface and only on the UO2 grains of heterogeneous U0.92Ce0.08O2 samples. The behaviour of this heterogeneous material was similar to that observed for (U,Pu)O2 in the same conditions, which revealed the reliability of cerium as a plutonium analogue.

Increasing soil Mn abundance promotes the dissolution and oxidation of Cr(III) and increases the accumulation of Cr in rice grains

Hexavalent chromium (Cr(VI)) is more readily taken up by plants than trivalent chromium (Cr(III)) due to its similar chemical structure to phosphate and sulfate. In paddy soils, Cr(VI) of natural origin are mainly produced from Cr(III) oxidized by O2 and Mn(III/IV) oxides, which are affected by rice radial oxygen loss (ROL) and Mn(II)-oxidizing microorganisms (MOM). However, little is known about the effect of ROL and Mn abundance on rice Cr uptake. Here, we investigated the effects on Cr(VI) generation and the subsequent Cr uptake and accumulation with the involvement of two rice cultivars with distinct ROL capacities by increasing soil Mn abundance. Results showed that Mn(II) addition to the soil led to more Cr(III) being released into the pore water, and the dissolved Cr(III) was oxidized to Cr(VI) by ROL and biogenic Mn(III/IV) oxides. The concentration of Cr(VI) in soil and pore water increased linearly with the addition of Mn(II) doses. Mn(II) addition promoted the root-to-shoot translocation and grain accumulation of Cr derived mainly from newly generated Cr(VI) in the soil. These results emphasize that rice ROL and MOM promote the oxidative dissolution of Cr(III) at a high level of soil Mn, resulting in more Cr accumulation in rice grains and increasing dietary Cr exposure risks.