Effect of humic acid on uranium(VI) retention and transport through quartz columns with varying pH and anion type

Migration of depleted uranium from a corroded penetrator in soil vadose zone in Bosnia and Herzegovina

Depleted uranium (DU) from corroded armor penetrators can migrate through the soil vadose zone and cause environmental problems, yet studies on such migration at former theatres of war are scarce. Here, we investigated vertical DU migration in a soil profile due to a penetrator (3-8 cm beneath the soil surface) corroded over 7 years in Bosnia and Herzegovina. The highest concentration of DU was ∼45,300 mg/kg at 6-10 cm, with the concentration decreasing markedly with increasing depth. The majority of the DU accumulated within the top 20 cm and the DU front reached ∼42 cm beneath the penetrator. In addition, particles with varying U concentrations (3-65 wt%) were observed at 0-15 cm, with U primarily co-located with O, Si, Al, maghemite, and hematite. Particularly, metaschoepite was identified at 6-10 cm. Finally, X-ray absorption spectroscopy analysis found U was hexavalent in the soil profile. These findings suggest that the downward migration of DU was likely present as a soluble form adsorbed on clay minerals and Fe oxides. Overall, we show that the rate of DU migration within the vadose zone is comparatively slow, although if the penetrator is left in the soil for decades, it could pose a serious long-term risk. ENVIRONMENTAL IMPLICATIONS: Over 90 % of the depleted uranium (DU) penetrators fired in previous conflicts missed their armored targets and were left in the soil to corrode. The corroded penetrators can not only contaminate soil but also pose a risk to groundwater. The present study examined the migration of DU in a soil profile that included a DU penetrator that had been corroding for over 7 years. Studying the dynamics of DU migration is essential to develop effective remediation strategies to mitigate long-term environmental risks and safeguard ecosystems and human health from DU contamination.

Efficient uranium sequestration ability and mechanism of live and inactivated strain of Streptomyces sp. HX-1 isolated from uranium wastewater

Prokaryotes are effective biosorbents for the recovery of uranium and other heavy metals. However, the potential mechanism of uranium bioaccumulation by filamentous strain (actinobacteria) remains unclear. This study demonstrates the potential for and mechanism of uranium bioaccumulation by living (L-SS) and inactivated (I-SS) Streptomyces sp. HX-1 isolated from uranium mine waste streams. Uranium accumulation experiments showed that L-SS and I-SS had efficient uranium adsorption potentials, with removal rates of 92.93 and 97.42%, respectively. Kinetic and equilibrium data indicated that the bioaccumulation process was consistent with the pseudo-second-order kinetic, Langmuir, and Sips isotherm models. FTIR indicated that the main functional groups of L-SS and I-SS binding uranium were uranyl, carboxyl, and phosphate groups. Moreover, the results of XRD, XPS, SEM-EDS, and TEM-EDS analyses revealed for the first time that L-SS has biomineralization and bioreduction capacity against uranium. L-SS mineralize U(VI) into NH4UO2PO4 and [Formula: see text] through the metabolic activity of biological enzymes (phosphatases). In summary, Streptomyces sp. HX-1 is a novel and efficient uranium-fixing biosorbent for the treatment of uranium-contaminated wastewater.

"One-can" strategy for the synthesis of hydrothermal biochar modified with phosphate groups and efficient removal of uranium(VI)

Significant selectivity, reasonable surface modification and increased structural porosity were three key factors to improve the competitiveness of biochar in the adsorption field. In this study, a hydrothermal bamboo-derived biochar modified with phosphate groups (HPBC) was synthesized using "one-can" strategy. BET showed that this method could effectively increase the specific surface area (137.32 m2 g-1) and simulation of wastewater experiments indicated HPBC had an excellent selectivity for U(VI) (70.35%), which was conducive to removal of U(VI) in real and complex environments. The accurate matchings of pseudo-second-order kinetic model, thermodynamic model and Langmuir isotherm showed that at 298 K, pH = 4.0, the adsorption process dominated by chemical complexation and monolayer adsorption was spontaneous, endothermic and disordered. Saturated adsorption capacity of HPBC could reach 781.02 mg g-1 within 2 h. The introduction of phosphoric acid and citric acid by "one-can" method not only provided abundant -PO4 to assist adsorption, but also activated oxygen-containing groups on the surface of the bamboo matrix. Results showed that adsorption mechanism of U(VI) by HPBC included electrostatic action and chemical complexation involving P-O, PO and ample oxygen-containing functional groups. Therefore, HPBC with high phosphorus content, outstanding adsorption performance, excellent regeneration, remarkable selectivity and green value provided a novel solution for the field of radioactive wastewater treatment.

Investigation of Potential Drivers of Elevated Uranium Prevalence in Indian Groundwaters with a Unified Speciation Model

Elevated uranium (U) (>WHO limit of 30 μg L^-1) in Indian groundwaters is primarily considered geogenic, but the specific mineralogical sources and mechanisms for U mobilization are poorly understood. In this contribution, statistical and geochemical analyses of well-constrained metadata of Indian groundwater quality (n = 342 of 8543) were performed to identify key parameters and processes that influence U concentrations. For geochemical predictions, a unified speciation model was developed from a carefully compiled and updated thermodynamic database of inorganic, organic (Stockholm Humic model), and surface complexation reactions and associated constants. Critical U contamination was found at shallow depths (<100 m) within the Indo-Gangetic plain, as determined by bivariate nonparametric Kendall's Tau_b and probability-based association tests. Analysis of aquifer redox states, multivariate hierarchical clusters, and principal components indicated that U contamination was predominant not just in oxic but mixed (oxic-anoxic) aquifers under high Fe, Mn, and SO₄ concentrations, presumably due to U release from dissolution of Fe/Mn oxides or Fe sulfides and silicate weathering. Most groundwaters were undersaturated with respect to relevant U-bearing solids despite being supersaturated with respect to atmospheric CO₂ (average pCO₂ of reported dissolved inorganic carbonate (DIC) data = 10^-1.57 atm). Yet, dissolved U did not appear to be mass limited, as predicted solubilities from reported sediment concentrations of U were ∼3 orders of magnitude higher. Integration of surface complexation models of U on typical aquifer adsorbents, ferrihydrite, goethite, and manganese dioxide, was necessary to explain dissolved U concentrations. Uranium contamination probabilities with increasing dissolved Ca and Mn exhibited minima at equilibrium solubilities of calcite [∼50 mg L^-1] and rhodochrosite [∼0.14 mg L^-1], respectively, at an average groundwater pH of ∼7.5. A potential indirect control of such U-free carbonate solids on U mobilization was suggested. For locations (n = 37) where dissolved organic carbon was also reported, organic complexes of U contributed negligibly to dominant U speciation at the groundwater pH. Overall, the unified model suggested competitive dissolution-precipitation and adsorption-desorption controls on U speciation. The model provides a quantitative framework that can be extended to understand dominant mobilization mechanisms of geogenic U in aquifers worldwide after suitable modifications to the relevant aquifer parameters.

Modeling Uranium Transport in Rough-Walled Fractures with Stress-Dependent Non-Darcy Fluid Flow

The reactive-transportation of radioactive elements in fractured rock mass is critical to the storage of radioactive elements. To describe the reactive-transportation and distribution morphology of a uranium-containing solution, a stress-dependent reactive transport model was developed, and the simulator of FLAC3D-CFD was employed. The uranium transport experiment subjected to the variation of confining stress of 5–19 MPa and hydraulic pressure of 0.5–3.5 MPa was conducted in fractured rock mass. The results show that the uranium-containing solution transport and distribution is significantly dependent on the evolution of the connected channel in rough-walled fracture, which is significantly influenced by the confining stress and hydraulic pressure. In more detail, the increase of confining stress resulted in the anisotropic of seepage channel in aperture, and corresponding turbulence flow and uranium retention were presented at the fracture aperture of 2–5 μm. As the increase of hydraulic pressure, flow regime evolved from the inertial flow to vortex flow, and the transformation region is 16 MPa confining stress and 1.5 MPa hydraulic pressure. The evolution of loading paths also dominates the flow and solute transport, and high seepage speed and strong solute transport were presented at the k = 1 (ratio of vertical stress loading to horizontal stress unloading), and a laminar flow and weak solute transport were presented at k = 0.

Highly efficient immobilization of environmental uranium contamination with Pseudomonas stutzeri by biosorption, biomineralization, and bioreduction

Uranium is a heavy metal with both chemotoxicity and radiotoxicity. Due to the increasing consumption of uranium, the remediation of uranium contamination and recovery of uranium from non-conventional approach is highly needed. Microorganism exhibits high potential for immobilization of uranium. This study for the first time isolated a marine Pseudomonas stutzeri strain MRU-UE1 with high uranium immobilization capacity of 308.72 mg/g, which is attributed to the synergetic mechanisms of biosorption, biomineralization, and bioreduction. The uranium is found to be immobilized in forms of tetragonal chernikovite (H₂(UO₂)₂(PO₄)₂·8H₂O) by biomineralization and CaU(PO₄)₂ by bioreduction under aerobic environment, which is rarely observed and would broaden the application of this strain in aerobic condition. The protein, phosphate group, and carboxyl group are found to be essential for the biosorption of uranium. In response to the stress of uranium, the strain produces inorganic phosphate group, which transformed soluble uranyl ion to insoluble uranium-containing precipitates, and poly-β-hydroxybutyrate (PHB), which is observed for the first time during the interaction between microorganism and uranium. In summary, P. stutzeri strain MRU-UE1 would be a promising alternative for environmental uranium contamination remediation and uranium extraction from seawater.

From Adsorption to Precipitation of U(VI): What is the Role of pH and Natural Organic Matter?

We investigated interfacial reactions of U(VI) in the presence of Suwannee River natural organic matter (NOM) at acidic and neutral pH. Laboratory batch experiments show that the adsorption and precipitation of U(VI) in the presence of NOM occur at pH 2 and pH 4, while the aqueous complexation of U by dissolved organic matter is favored at pH 7, preventing its precipitation. Spectroscopic analyses indicate that U(VI) is mainly adsorbed to the particulate organic matter at pH 4. However, U(VI)-bearing ultrafine to nanocrystalline solids were identified at pH 4 by electron microscopy. This study shows the promotion of U(VI) precipitation by NOM at low pH which may be relevant to the formation of mineralized deposits, radioactive waste repositories, wetlands, and other U- and organic-rich environmental systems.

Potentiality of living Bacillus pumilus SWU7-1 in biosorption of strontium radionuclide

Bacillus pumilus SWU7-1 was isolated from strontium ion (Sr(II))-uncontaminated soil, its biosorption potential was evaluated, and the effect of γ-ray radiation treatment on its biosorption was discussed. Domesticated under Sr(II) stress promoted the biosorption ability of B. pumilus to Sr(II), and the biosorption efficiency increased from 46.09% to 94.69%. At a lower initial concentration, the living bacteria had the ability to resist the biosorption of Sr(II). The optimal initial concentration range was 54-130 mg/L. The biosorption profile was better matched by Langmuir than Freundlich model, showing that the biosorption process of Sr(II) by the experimental strain was closer to the surface adsorption. According to Langmuir model, the maximum biosorption capacity of B. pumilus on Sr (II) was 299.4 mg/g. During the bacterial growth in the biosorption process, the changes in biosorption capacity and efficiency can be divided into two phases, and a pseudo-second-order model is followed in each phase. There was no significant difference in the biosorption efficiency of bacteria with different culture time after γ-ray radiation, and all of them were above 90%, which showed that B. pumilus had significant radiation resistance under experimental conditions. This study emphasized the potential application of B. pumilus in the treatment of radioactive Sr(II) pollution by biosorption.

Influence on Uranium(VI) migration in soil by iron and manganese salts of humic acid: Mechanism and behavior

Soil contains large amounts of humic acid (HA), iron ions and manganese ions, all of which affect U(VI) migration in the soil. HA interacts with iron and manganese ions to form HA salts (called HA-Fe and HA-Mn in this paper); however, the effects of HA-Fe and HA-Mn on the migration of U(VI) is not fully understood. In this study, HA-Fe and HA-Mn were compounded by HA interactions with ferric chloride hexahydrate and manganese chloride tetrahydrate, respectively. The influence of HA, HA-Fe and HA-Mn on U(VI) immobilization and migration was investigated by bath adsorption experiments and adsorption-desorption experiments using soil columns. The results showed that the presence of HA, HA-Fe and HA-Mn retarded the migration of U(VI) in soil. Supported by X-ray photoelectron spectroscopy (XPS) and BCR sequential extraction analyses, a plausible explanation for the retardation was that HA-Fe and HA-Mn could reduce hexavalent uranium to stable tetravalent uranium and increase the specific gravity of Fe/Mn oxide-bound uranium and organic/sulfide-bound uranium, which made it difficult for them to longitudinally migrate in soil. Scanning electron microscopy (SEM), Fourier transform infrared (FTIR) spectroscopy, X-ray diffraction (XRD), and surface area and pore size analyses indicated that the complex formed between the hydroxyl, amino and carboxyl groups of HA-Fe and U(VI) increased the crystallinity of HA-Fe. The reaction between U(VI) and the hydroxyl, amino, aldehyde, keto and chlorine-containing groups of HA-Mn had no effect on the crystallinity of HA-Mn. Notably, the column desorption experiment found that the U(VI) immobilized in the soil remigrated under the effect of rain leaching, and acid rain promoted uranium remigration better than neutral rain. The findings provide some guidance for the decommissioning disposal of uranium contaminated site and it's risk assessments.

Transport of uranium(VI) in red soil in South China: influence of initial pH and carbonate concentration

Uranium-contaminated wastewater associated with uranium (U) mining and processing inevitably releases into soil environment. In order to assess the risk of U wastewater contamination to groundwater through percolation, U adsorption and transport behavior in a typical red soil in South China was investigated through batch adsorption and column experiments, and initial pH and carbonate concentration were considered of the high-sulfate background electrolyte solution. Results demonstrated that U adsorption isotherms followed the Freundlich model. The adsorption of U to red soil significantly decreased with the decrease of the initial pH from 7 to 3 in the absence of carbonate, protonation-deprotonation reactions controlled the adsorption capacity, and lnC_s had a linear relationship with the equilibrium pH (pH_eq). In the presence of carbonate, the adsorption was much greater than that in the absence of carbonate owing to the pH_eq values buffered by carbonate, but the adsorption decreased with the increase of the carbonate concentration from 3.5 to 6.5 mM. Additionally, the breakthrough curves (BTCs) obtained by column experiments showed that large numbers of H⁺ and CO₃^2- competed with the U species for adsorption sites, which resulted in BTC overshoot (C/C₀ > 1). Numerical simulation results indicated that the BTCs at initial pH 4 and 5 could be well simulated by two-site chemical non-equilibrium model (CNEM), whereas the BTCs of varying initial carbonate concentrations were suitable for one-site CNEM. The fractions of equilibrium adsorption sites (f) seemed to correlate with the fractions of positively charged complexes of U species in solution. The values of partition coefficients (k_d^') were lower than those measured in batch adsorption experiments, but they had the same variation trend. The values of first-order rate coefficient (ω) for all BTCs were low, representing a relatively slow equilibrium between U in the liquid and solid phases. In conclusion, the mobility of U in the red soil increased with the decrease of the initial pH and with the increase of the initial carbonate concentrations.

Kinetic and equilibrium of U(VI) biosorption onto the resistant bacterium Bacillus amyloliquefaciens

This study evaluated U(VI) biosorption properties by the resistant bacterium, Bacillus amyloliquefaciens, which was isolated from the soils with residual radionuclides. The effect of biosorption factors (uptake time, pH, ionic concentration, biosorbent dosage and temperature) on U(VI) removal was determined by batch experiments. The uptake processes were characterized by using SEM, FTIR, and XPS. The experimental data of U(VI) biosorption were fitted by the pseudo-second-order. The maximum uptake capacity was 179.5 mg/g at pH 6.0 by Langmuir model. The thermodynamic results: ΔG^о, ΔH^о and ΔS^о for uptake processes were calculated as -6.359 kJ/mol, 14.20 kJ/mol and 67.19 J/mol/K, respectively. The results showed that the biosorption of Bacillus amyloliquefaciens will be an ideal method to remove radionuclides.

Organic Functional Group Chemistry in Mineralized Deposits Containing U(IV) and U(VI) from the Jackpile Mine in New Mexico

We investigated the functional group chemistry of natural organic matter (NOM) associated with both U(IV) and U(VI) in solids from mineralized deposits exposed to oxidizing conditions from the Jackpile Mine, Laguna Pueblo, NM. The uranium (U) content in unreacted samples was 0.44-2.6% by weight determined by X-ray fluorescence. In spite of prolonged exposure to ambient oxidizing conditions, ≈49% of U(IV) and ≈51% of U(VI) were identified on U L_III edge extended X-ray absorption fine structure spectra. Loss on ignition and thermogravimetric analyses identified from 13% to 44% of NOM in the samples. Carbonyl, phenolic, and carboxylic functional groups in the unreacted samples were identified by fitting of high-resolution X-ray photoelectron spectroscopy (XPS) C 1s and O 1s spectra. Peaks corresponding to phenolic and carbonyl functional groups had intensities higher than those corresponding to carboxylic groups in samples from the supernatant from batch extractions conducted at pH 13, 7, and 2. U(IV) and U(VI) species were detected in the supernatant after batch extractions conducted under oxidizing conditions by fitting of high-resolution XPS U 4f spectra. The outcomes from this study highlight the importance of the influence of pH on the organic functional group chemistry and U speciation in mineralized deposits.

Competition/Cooperation between Humic Acid and Graphene Oxide in Uranyl Adsorption Implicated by Molecular Dynamics Simulations

Molecular dynamics (MD) simulations were performed to investigate the influence of curvature and backbone rigidity of an oxygenated surface, here graphene oxide (GO), on its adsorption of uranyl in collaboration with humic acid (HA). The planar curvature of GO was found to be beneficial in impeding the folding of HA. This, together with its rigidity that helps stabilize the extended conformation of HA, offered rich binding sites to interact with uranyl with only marginal loss of binding strength. According to our simulations, the interaction between uranyl and GO was mainly driven by electrostatic interactions. The presence of HA not only provided multiple sites to compete/cooperate with GO for adsorption of free uranyl but also interacted with GO acting as a "bridge" to connect uranyl and GO. The potential of mean force (PMF) profiles implied that HA significantly enhanced the interaction strength between uranyl and GO and stabilized the uranyl-GO complex. Meanwhile, GO could reduce the diffusion coefficients of uranyl and HA and retard their migrations in aqueous solution. This work provides theoretical hints on the GO-based remediation strategies for the sites contaminated by uranium or other heavy metal ions and oxygenated organic pollutants.

Effect of Staphylococcus epidermidis on U(VI) sequestration by Al-goethite

Effect of Staphylococcus epidermidis (S. epidermidis) on U(VI) sequestration by Al-goethite were conducted under different geologic conditions. The batch experiments showed that S. epidermidis significantly enhanced the adsorption rates of U(VI) at pH < 9.0 due to the additional metal binding sites. The maximum adsorption capacities of U(VI) on Al-goethite and Al-goethite +S. epidermidis at pH 4.0 and 310 K were calculated from Langmuir equation to be 13.16 and 47.86 mg/g, respectively. The decreased adsorption of U(VI) on Al-goethite+ S. epidermidis at high carbonate and pH conditions were primarily driven by the electrostatic repulsion between negatively charged U(VI)-carbonate complexes and the negatively charged adsorbents. According to XPS analysis, the adsorbed U(VI) can be reduced to U(IV) by S. epidermidis, whereas inhibited reduction of U(VI) on Al-goethite + S. epidermidis at high pH could be attributed to the complexation of structural Fe(III) with the oxygen-containing functional groups of S. epidermidis. FT-IR analysis further demonstrated that the bonding of structural Fe(III) with functional groups (e.g., carboxyl and phosphate groups) of S. epidermidis. These results herein are important to understand the fate and transport of U(VI) on the mineral-bacteria ternary systems in the near-surface environment.

Accumulation of U(VI) on the Pantoea sp. TW18 isolated from radionuclide-contaminated soils

Pantoea sp. TW18 isolated from radionuclide-contaminated soils was used for the bioremediation of radionuclides pollution. Accumulation mechanism of U(VI) on Pantoea sp. TW18 was investigated by batch experiments and characterization techniques. The batch experiments revealed that Pantoea sp. TW18 rapidly reached accumulation equilibrium at approximately 4 h with a high accumulation capacity (79.87 mg g^-1 at pH 4.1 and T = 310 K) for U(VI). The accumulation data of U(VI) onto Pantoea sp. TW18 can be satisfactorily fitted by pseudo-second-order model. The accumulation of U(VI) on Pantoea sp. TW18 was affected by pH levels, not independent of ionic strength. Analysis of the FT-IR and XPS spectra demonstrated that accumulated U(VI) ions were primarily bound to nitrogen- and oxygen-containing functional groups (i.e., carboxyl, amide and phosphoryl groups) on the Pantoea sp. TW18 surface. This study showed that Pantoea sp. TW18 can be considered as a promising sorbent for remediation of radionuclides in environmental cleanup.

Accumulation of 152+154Eu(III) by Aspergillus sydowii and Trichoderma harzianum

Radionuclides-resistant filamentous fungi were isolated from radionuclides' contaminated soils. Effects of contact time, mycelia dosage, pH, ionic strength and thiol compounds on ^152+154Eu(III) accumulation on two kinds of filamentous fungi (Aspergillus sydowii and Trichoderma harzianum, denoted as A. sydowii and T. harzianum, respectively) were investigated by batch techniques. The maximum tolerance to Eu(III) concentration of A. sydowii and T. harzianum reached 3000 mg/L and 3500 mg/L, and the Eu(III) accumulation on A. sydowii and T. harzianum can be fitted better with the pseudo-second-order kinetic model, respectively. Filamentous fungi were characterized by FT-IR and acid base titrations, and morphological structures of mycelia changed obviously under Eu(III) stress by SEM and TEM analysis. The results suggested that filamentous fungi could play an important role in the migration and transformation of radionuclides in the environment.

Dried powder of corn stalk as a potential biosorbent for the removal of iodate from aqueous solution

Removal of IO₃^- from environmental samples with low-cost methods and materials is very useful approach for especially large-scale applications. Corn stalk is highly abundant agriculture residual, which is employed as useful biosorbent in many studies. In the present work, dried powder of corn stalk is applied for the removal of IO₃^- under various conditions. The results indicate that the K_d is 49.73 ml g^-1 under general conditions (m/V = 8 g L^-1, t = 5 day, equilibrium pH = 7 ± 0.3, T = 298 K and C₀ = 15 mg L^-1). The sorption kinetics follows the pseudo-second-order equation, and the isotherm is well described by the Langmuir model. The sorption reaction was non-spontaneous and endothermic. Hydroxyl and carbonyl groups of the corn stalk contribute to IO₃^- sorption by ion-exchange, electrostatic attraction and redox reactions. Spectroscopic analyses and the effect of equilibrium pH prove that corn stalk was not only removed IO₃^- from aqueous solution but also reduced IO₃^- into I₂ and I^-. These results demonstrate that corn stalk is a promising biosorbent for the environmental remediation of radioactive iodine pollution.