A critical review of uranium contamination in groundwater: Treatment and sludge disposal

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.

U(VI) mitigation via forward osmosis: Elucidation of retention mechanisms and co-ion effects

Uranium (U) is a chemical and radioactive toxic contaminant affecting many groundwater systems. The focus of this study was to evaluate the suitability of forward osmosis (FO) for uranium rejection from contaminated groundwater under field-relevant conditions. Laboratory experiments with aqueous solution containing uranium were performed with FO membrane to understand the uranium rejection mechanism under varied pH, draw solution concentration, and presence of co-ions. Further, experiments were performed with U-contaminated field groundwater. Results of the hydrogeochemcial modelling using PHREEQC indicated that the rejection mechanism of uranium was highly dependent on aqueous speciation. Uranium rejection was maximum at alkaline pH with ca. 99% rejection due to charge-based interactions between membrane and dominant uranyl complexes. The results of the co-ion study indicated that nitrate and phosphate ions decrease uranium rejection. Whereas, bicarbonates, calcium, and magnesium ions concentrated uranium in feed solution. Further, the uranium adsorption onto the membrane surface primarily depended on pH of the aqueous solution with maximum adsorption at pH 5.5. Our results show that the World Health Organization's drinking water guideline value of 30 μgL-1 for U could be achieved via FO process in field groundwater containing low dissolved solids.

Hydroxyapatite derived from eggshell embedded on functionalized g-C3N4 for synergistic extraction of U(VI) from aqueous solution

In this paper, we report hydroxyapatite derived from egg-shell biowaste embedded on diglycolamic acid functionalized graphitic carbon nitride nanocomposite (abbreviated as HAp@D-gCN). The compositional and morphological characteristics of HAp@D-gCN were evaluated using scanning electron microscope, X-ray diffraction, BET, FTIR techniques and surface charge using zeta potential measurement. The sorption of U(VI) species on HAp@D-gCN was investigated through batch studies as a function of pH, contact time, initial U(VI) concentration, adsorbent dosage and ionic strength. The adsorption of U(VI) onto HAp@D-gCN was confirmed by FTIR, XRD and EDS elemental mapping. Adsorption kinetics follow pseudo second order model and it attains equilibrium within 20 min. Adsorption isotherm data correlates well with Langmuir isotherm model with a maximum sorption capacity of 993.6 mg of U(VI) per gram of HAp@D-gCN at 298K. U(VI) can be leached from the loaded adsorbent using 0.01 M Na2CO3 as desorbing agent and its sorption capacity remains unaffected even after 4 adsorption-desorption cycles. Hence, the present study reveals that HAp@D-gCN nanocomposite could serve as an environmental friendly material with potential application in environmental remediation.

Effects of uranium mining on the rhizospheric bacterial communities of three local plants on the Qinghai-Tibet Plateau

In this study, we used 16S high-throughput sequencing to investigate the effects of uranium mining on the rhizospheric bacterial communities and functions of three local plant species, namely, Artemisia frigida, Acorus tatarionwii Schott., and Salix oritrepha Schneid. The results showed that uranium mining significantly reduced the diversity of rhizospheric bacteria in the three local plant species, including the Shannon index and Simpson index (P < 0.05). Interestingly, we found that Sphingomonas and Pseudotrichobacter were enriched in the rhizosphere soil of the three local plants from uranium mining areas, indicating their important ecological role. The three plants were enriched in various dominant rhizospheric bacterial populations in the uranium mining area, including Vicinamidobacteriaceae, Nocardioides, and Gaiella, which may be related to the unique microecological environment of the plant rhizosphere. The rhizospheric bacterial community of A. tatarionwii plants from tailings and open-pit mines also showed a certain degree of differentiation, indicating that uranium mining is the main factor driving the differentiation of plant rhizosphere soil communities on the plateau. Functional prediction revealed that rhizospheric bacteria from different plants have developed different functions to cope with stress caused by uranium mining activities, including enhancing the translational antagonist Rof, the translation initiation factor 2B subunit, etc. This study explores for the first time the impact of plateau uranium mining activities on the rhizosphere microecology of local plants, promoting the establishment of effective soil microecological health monitoring indicators, and providing a reference for further soil pollution remediation in plateau uranium mining areas.

Uranium adsorption efficiency of diglycolamic acid functionalized graphitic carbon nitride adsorbent: Kinetic, isotherm, and thermodynamic studies

This study proposes the use of diglycolamic acid-functionalized graphitic carbon nitride (HDGA-gCN) as an adsorbent for uranium removal. Our experiments showed that at pH 6.0, HDGA-gCN had a high adsorption capacity of 263.2 mg g-1 and achieved equilibrium in 30 min. The adsorption isotherm was well-fitted by the Langmuir model, and the adsorption kinetics followed a pseudo-second-order equation. U(VI) adsorption on HDGA-gCN is due to electrostatic interactions between the amine, diglycolamic acid, and uranium species. The thermodynamic parameters indicate that adsorption is spontaneous and exothermic. The loaded U(VI) can be desorbed using 0.1 M Na2CO3, and HDGA-gCN exhibited an exceptional adsorption percentage for U(VI) compared to other coexisting ions. HDGA-gCN had faster kinetics, adsorption capacity, and reusability, making it suitable for U(VI) remediation.

Renewable biomass-based aerogels: from structural design to functional regulation

Global population growth and industrialization have exacerbated the nonrenewable energy crises and environmental issues, thereby stimulating an enormous demand for producing environmentally friendly materials. Typically, biomass-based aerogels (BAs), which are mainly composed of biomass materials, show great application prospects in various fields because of their exceptional properties such as biocompatibility, degradability, and renewability. To improve the performance of BAs to meet the usage requirements of different scenarios, a large number of innovative works in the past few decades have emphasized the importance of micro-structural design in regulating macroscopic functions. Inspired by the ubiquitous random or regularly arranged structures of materials in nature ranging from micro to meso and macro scales, constructing different microstructures often corresponds to completely different functions even with similar biomolecular compositions. This review focuses on the preparation process, design concepts, regulation methods, and the synergistic combination of chemical compositions and microstructures of BAs with different porous structures from the perspective of gel skeleton and pore structure. It not only comprehensively introduces the effect of various microstructures on the physical properties of BAs, but also analyzes their potential applications in the corresponding fields of thermal management, water treatment, atmospheric water harvesting, CO2 absorption, energy storage and conversion, electromagnetic interference (EMI) shielding, biological applications, etc. Finally, we provide our perspectives regarding the challenges and future opportunities of BAs. Overall, our goal is to provide researchers with a thorough understanding of the relationship between the microstructures and properties of BAs, supported by a comprehensive analysis of the available data.

The permeability evolution mechanism of ore-bearing strata during acid in-situ leaching of uranium: A case study of Bayanwula uranium mine in Inner Mongolia of China

Uranium mainly comes from ISL of sandstone-type uranium deposits in China. The change of porosity and permeability caused by blockage of ore-bearing strata is one of the most serious problems in acid ISL of uranium. In this paper, the groundwater tracer test was carried out before and 1 year after ISL to explore the pore and permeability evolution characteristics of the ore-bearing layer during ISL. The test results showed that the leaching solution migrated along two seepage channels and the water-bearing medium was isotropic. After 1 year of ISL, the flow rate of the leaching solution decreased obviously. However, the flow rate of the leaching solution in slower channel decreased more than that in the faster channel in all directions, which was caused by the more adequate chemical reactions between the leaching solution and the minerals of the ore-bearing layer and the more corresponding precipitation in the slower channel. In addition, the flow rate along the direction of groundwater flow decreased less than that in the direction of vertical groundwater flow. This was closely related to the transformation of aquifer medium by hydrodynamic field. Initial stage of ISL, the occurrence of plugging is closely related to the precipitation-dissolution process of iron and aluminum minerals under the change of pH, which is accompanied by the continuous precipitation of gypsum.

Reproducible responses of geochemical and microbial successional patterns in the subsurface to carbon source amendment

Carbon amendments designed to remediate environmental contamination lead to substantial perturbations when injected into the subsurface. For the remediation of uranium contamination, carbon amendments promote reducing conditions to allow microorganisms to reduce uranium to an insoluble, less mobile state. However, the reproducibility of these amendments and underlying microbial community assembly mechanisms have rarely been investigated in the field. In this study, two injections of emulsified vegetable oil were performed in 2009 and 2017 to immobilize uranium in the groundwater at Oak Ridge, TN, USA. Our objectives were to determine whether and how the injections resulted in similar abiotic and biotic responses and their underlying community assembly mechanisms. Both injections caused similar geochemical and microbial succession. Uranium, nitrate, and sulfate concentrations in the groundwater dropped following the injection, and specific microbial taxa responded at roughly the same time points in both injections, including Geobacter, Desulfovibrio, and members of the phylum Comamonadaceae, all of which are well established in uranium, nitrate, and sulfate reduction. Both injections induced a transition from relatively stochastic to more deterministic assembly of microbial taxonomic and phylogenetic community structures based on 16S rRNA gene analysis. We conclude that geochemical and microbial successions after biostimulation are reproducible, likely owing to the selection of similar phylogenetic groups in response to EVO injection.

Fabrication of cellulose-collagen based biosorbent as eco-friendly scavengers for uranyl ions

The aim of the present research is to fabricate a biosorbent using agricultural waste for removal of uranium from contaminated water i.e. "waste to wealth" approach. Cellulose extracted from wheat straw was mercerized and a novel semi-interpenetrating polymer network (semi-IPN) was fabricated through graft copolymerization of polyvinyl alcohol onto hybrid mercerized cellulose + collagen backbone. Response surface methodology was used for optimization of different reaction parameters as a function of % grafting (195.1 %) was carried out. Semi-IPN was found to possess higher thermal stability. Adsorption results revealed that the optimum parameters for the elimination of uranium using semi-IPN were: adsorbent dose = 0.15 g, pH = 6.0, contact time = 120 min and initial U (VI) concentration = 100 μg/L. The pseudo-second-order kinetic model gave the best description of the adsorption equilibrium data as the calculated qe value is nearest to the experimental qe for the different initial U(VI) concentrations. Adsorption experiments followed Langmuir isotherm with R2 = 0.999. Furthermore, recyclability and reusability studies showed that the adsorption efficiency of semi-IPN was 82 % after 5 cycles indicating the superior recycling execution of fabricated biosorbent. Thus, the fabricated ecofriendly device can be used effectively for the removal of uranium from contaminated wastewater sources.

Humic acid-nanoceria composite as a sustainable adsorbent for simultaneous removal of uranium(VI), chromium(VI), and fluoride ions from aqueous solutions

In this article, the multifunctional behavior of novel, efficient, and cost-effective humic acid-coated nanoceria (HA@CeO2 NPs) was utilized for the sorptive removal of U(VI), Cr(VI), and F- ions at different conditions. The production cost of HA@CeO2 was $19.28/kg and was well characterized by DLS, FESEM, HRTEM, FTIR, XRD, XPS, and TGA. Batch adsorption study for U(VI) (at pH ~ 8), Cr(VI) (at pH ~ 1), and F- (at pH ~ 2) revealed that the maximum percentage of sorption was > 80% for all the cases. From the contact time experiment, it was concluded that pseudo-second-order kinetics followed, and hence, the process should be a chemisorption. The adsorption study revealed that U(VI) and Cr(VI) followed the Freundlich isotherm, whereas F- followed the Langmuir isotherm. Maximum adsorption capacity for F- was 96 mg g-1. Experiments in real water suggest that adsorption is decreased in Kaljani River water (~ 12% for Cr(VI) and ~ 11% for F-) and Kochbihar Lake water (25.04% for Cr(VI) and 20.5% for F-) because of competing ion effect. Mechanism was well established by the kinetic study as well as XPS analysis. Because of high adsorption efficiency, HA@CeO2 NPs can be used for the removal of other harmful water contaminants to make healthy aquatic life as well as purified drinking water.

Redox-active phytic acid-based self-assembled hybrid material for enhanced uranium adsorption from highly acidic solution

Achieving efficient uranium adsorption from highly acidic wastewater is still considered challenging. Here, an inorganic-organic hybridized self-assembly material (rPFE-10) with redox activity was constructed by phytic acid (PA), ethylenediamine (EDA), and Fe(II) via a facile one-pot route, and further applied for U(VI) removal. In the static adsorption experiment, rPFE-10 achieved the maximum U(VI) adsorption capacity of 717.1 mg/g at the optimal pH of 3.5. It also performed preeminently in a highly acidic condition of pH = 1.0, with the highest adsorption capacity of 551.2 mg/g and an equilibrium time of 30 min. Moreover, rPFE-10 exhibited a pH-responsive adsorption selectivity for U(VI) and An-Ln (S(U(VI)) and S(An-Ln)), which increased to 69 % and 94 % respectively as pH decreased from 3.0 to 1.0. Additionally, the spectral analysis revealed a reconstruction mechanism induced by multiple synergistic adsorption, in which U(VI) exchange with EDA+/2+ and Fe2+/3+ and earned suitable coordination geometry and ligand environment to coordinate with PA (mainly P-OH), while partial U(VI) is reduced by Fe(II) in framework. This work not only highlights the facile strategy for enhanced U(VI) retention in highly acidic solution, but expands the potential application of supramolecular self-assembly material in treatment of nuclear wastewater.

A review of remediation technologies for uranium-contaminated water

Uranium is a naturally existing radioactive element present in the Earth's crust. It exhibits lithophilic characteristics, indicating its tendency to be located near the surface of the Earth and tightly bound to oxygen. It is ecotoxic, hence the need for its removal from the aqueous environment. This paper focuses on the variety of water treatment processes for the removal of uranium from water and this includes physical (membrane separation, adsorption and electrocoagulation), chemical (ion exchange, photocatalysis and persulfate reduction), and biological (bio-reduction and biosorption) approaches. It was observed that membrane filtration and ion exchange are the most popular and promising processes for this application. Membrane processes have high throughput but with the challenge of high power requirements and fouling. Besides high pH sensitivity, ion exchange does not have any major challenges related to its application. Several other unique observations were derived from this review. Chitosan/Chlorella pyrenoidosa composite adsorbent bearing phosphate ligand, hydroxyapatite aerogel and MXene/graphene oxide composite has shown super-adsorbent performance (>1000 mg/g uptake capacity) for uranium. Ultrafiltration (UF) membranes, reverse osmosis (RO) membranes and electrocoagulation have been observed not to go below 97% uranium removal/conversion efficiency for most cases reported in the literature. Heat persulfate reduction has been explored quite recently and shown to achieve as high as 86% uranium reduction efficiency. We anticipate that future studies would explore hybrid processes (which are any combinations of multiple conventional techniques) to solve various aspects of the process design and performance challenges.

Uranium removal in groundwater by Priestia sp. isolated from uranium-contaminated mining soil

Priestia sp. WW1 was isolated from a uranium-contaminated mining soil and identified. The uranium removal characteristics and mechanism of Priestia sp. WW1 were investigated. The results showed that the removal efficiency of uranium decreased with the increase of initial uranium concentration. When the uranium initial concentration was 5 mg/L, the uranium removal efficiency achieved 92.1%. The increase of temperature could promote the uranium removal. Carbon source could affect the removal rate of uranium, which was the fastest when the methanol was used as carbon source. The solution pH had significant effect on the uranium removal efficiency, which reached the maximum under solution pH 5.0. The experimental results and FTIR as well as XPS demonstrated that Priestia sp. WW1 could remove uranium via both adsorption and reduction. The common chloride ions, sulfate ions, Mn(II) and Cu(II) enhanced the uranium removal, while Fe(III) depressed the uranium removal. The Priestia sp. WW1 could effectively remove the uranium in the actual mining groundwater, and the increase of initial biomass could improve the removal efficiency of uranium in the actual mining groundwater. This study provided a promising bacterium for uranium remediation in the groundwater.

Health risk assessment of uranium intake from private residential drinking groundwater facilities based on geological characteristics across the Republic of Korea

Groundwater contributes to an average of 8 % of the total water source capacity in the Republic of Korea. Hence, private residential households in rural areas in Korea are still using groundwater for drinking without any regular water quality inspection. This can increase the risk of exposure to natural radionuclides like uranium through drinking groundwater. This study investigated the uranium level in drinking groundwater all over the country by analyzing 11,451 samples from private residential drinking groundwater facilities and compared the exposure amount and its associated carcinogenic and non-carcinogenic risk based on the geological characteristics of the aquifer. Results yield that although the average hazard quotient (HQ) and excess cancer risk (ECR) of exposure to natural uranium through drinking groundwater were respectively below 1 and 1 × 10-6 and do not indicate a potential health hazard, significantly high HQ and ECR up to respectively 70 and 4 × 10-4 in samples where the aquifer is the Jurassic granite observed. Accordingly, regular water quality investigation and onsite treatment methods are required to provide healthy drinking water in such areas.

The bioreduction of U(VI) and Pu(IV): Experimental and thermodynamic studies

The experimental and thermodynamic bioreduction of U(VI)aq and Pu(IV)am was studied in order to more accurately predict their transport velocities in groundwater and assess the contamination risks to the associated environments. The results obtained in this study emphasize the impact of carbonate-calcium and humic acids at 7.1 and anoxic solutions on the rate and extent of U(VI)aq and Pu(IV)am bioreduction by Shewanella putrefaciens. We found that the bioreduction rate of U(VI)aq became slow in the presence of NaHCO3/CaCl2. The more negative standard redox potentials of the ternary complexes of U(VI)-Ca2+-CO32- accounted for the decreased rate of bioreduction, e.g., [Formula: see text]  = -0.6797 V ≪ [Formula: see text]  = 0.3862 V. The bioreduction of Pu(IV)am seemed feasible, while humic acids accepted the adequate extracellular electrons secreted by S. putrefaciens, and the redox potential of Eh(HAox/HAred) was lower than Eh(PuO2(am)/Pu3+), e.g., Eh(HAox/HAred) ≦ Eh(PuO2(am)/Pu3+) if humic acids accepted ≧ 7.952 × 10-7 mol of electrons. The standard redox potentials, Eho(PuO2(am)/Pu3+) = 0.9295 V ≫ [Formula: see text]  = -0.6797 V, cannot explain the reduction extent of Pu(IV)am (8.9%), which is notably smaller than that of U(VI)aq (74.9%). In fact, the redox potential of Pu(IV)am was distinctly negative under the experimental conditions of trace-level Pu(IV)am (∼2.8 × 10-9 mol/L Pu(IV) if Pu(IV)am was completely dissolved), e.g., Eh(PuO2(am)/Pu3+) = -0.1590 V (α(Pu3+) = 10-10 mol/L, pH = 7.1). Therefore, the chemical factor of Pu3+ activity, leading to a rapid drop in Eh(PuO2(am)/Pu3+) at trace-level Pu(IV)am, was responsible for the relatively small reduction extent of Pu(IV)am.

Characterization of a uranium-tolerant green microalga of the genus Coelastrella with high potential for the remediation of metal-polluted waters

Uranium (U) contamination of terrestrial and aquatic ecosystems poses a significant threat to the environment and human health due to the chemotoxicity of this actinide. The characterization of organisms that tolerate and accumulate U is crucial to decipher the mechanisms evolved to cope with the radionuclide and to propose new effective strategies for the bioremediation of U-contaminated environments. Here, we isolated a unicellular green microalga of the genus Coelastrella from U-contaminated wastewater. We showed that Coelastrella sp. PCV is much more tolerant to U than Chlamydomonas reinhardtii and Chlorella vulgaris. Coelastrella sp. PCV is able to accumulate U very rapidly and then gradually release it into the medium, behaving as an excluder to limit the toxic effects of U. The ability of Coelastrella sp. PCV to accumulate U is remarkably high, with up to 240 mg of tightly bound U per g of dry biomass. Coelastrella sp. PCV is able to grow and maintain high photosynthesis in natural metal-contaminated waters from a wetland near a reclaimed U mine. In a single one-week growth cycle, Coelastrella sp. PCV is able to capture 25-55 % of the U from the contaminated waters and shows lipid droplet accumulation. Coelastrella sp. PCV is a very promising microalga for the remediation of polluted waters with valorization of algal biomass that accumulates lipids.

Uranium recovery from weakly acidic wastewater using recyclable γ-Fe2O3@meso-SiO2

U(VI)-containing acidic wastewater produced from uranium mining sites is an environmental hazard. Highly efficient capture of U(VI) from such wastewater is of great significance. In this study, a mesoporous core-shell material (i.e. γ-Fe2O3@meso-SiO2) with magnetically and vertically oriented channels was rationally designed through a surfactant-templating method. Batch experiment results showed that the material had an efficiency level of >99.7% in removing U(VI) and a saturated adsorption capacity of approximately 41.40 mg/g, with its adsorption reaching equilibrium in 15 min. The U(VI) adsorption efficiency of the material remained above 90% in a solution with competing ions and in acidic radioactive wastewater, indicating its ability to selectively adsorb U(VI). The material exhibited high adsorption efficiency and desorption efficiency in five cycles of desorption and regeneration experiments. According to the results, the mechanism through which γ-Fe2O3@meso-SiO2 adsorbs U(VI) was dominated by chemical complexation and electrostatic attraction between these two substances. Therefore, γ-Fe2O3@meso-SiO2 is not only beneficial to control the environmental migration of uranium, but also has good selective adsorption and repeated regeneration performance when used to recover U(VI) from weakly acidic wastewater in uranium mining.

Crystal structures of three uranyl-acetate-bipyridine complexes crystallized from hydraulic fracking fluid

Hydraulic fracking exposes shale plays to acidic hydraulic fracking fluid (HFF), releasing toxic uranium (U) along with the desired oil and gas. With no existing methods to ensure U remains sequestered in the shale, this study sought to add organic ligands to HFF to explore potential U retention in shale plays. To test this possibility, incubations were set up in which uranyl acetate and one organic bipyridine ligand (either 2,2'-, 2,3'-, 2,4'-, or 4,4'-bipyridine) were added to pristine HFF as the crystallization medium. After several months and complete evaporation of all volatiles, bulk yellow crystalline material was obtained from the incubations, three of which yielded crystals suitable for single-crystal analysis, resulting in two novel structures and a high-quality structure of a previously described compound. The UO2VI acetate complexes bis(acetato-κ2O,O')(2,2'-bipyridine-κ2N,N')dioxidouranium(VI), [U(C2H3O2)2O2(C10H8N2)2] or [2,2'-bipyridine]UVIO2(CH3CO2)2, (I), and bis(acetato-κ2O,O')(2,4'-bipyridine-κN1')dioxidouranium(VI), [U(C2H3O2)2O2(C10H8N2)2] or [2,4'-bipyridine]2UVIO2(CH3CO2)2, (III), contain eight-coordinate UVI in a pseudo-hexagonal bipyramidal coordination geometry and are molecular, packing via weak C-H...O/N interactions, whereas catena-poly[bis(2,3'-bipyridinium) [di-μ-acetato-μ3-hydroxido-μ-hydroxido-di-μ3-oxido-hexaoxidotriuranium(VI)]-2,3'-bipyridine-water (1/1/1)], (C10H9N2)2[U3(C2H3O2)2O8(OH)2]·C10H8N2·H2O or {[2,3'-bipyridinium]2[2,3'-bipyridine][(UVIO2)3(O)2(OH)2(CH3CO2)2·H2O]}n, (II), forms an ionic one-dimensional polymer with seven-coordinate pentagonal bipyramidal UVI centers and hydrogen-bonding interactions within each chain. The formation of these crystals could indicate the potential for bipyridine to bind with U in shale during fracking, which will be explored in a future study via ICP-MS (inductively coupled plasma mass spectrometry) analyses of U concentration in HFF/bipyridine/shale incubations. The variation seen here between the molecular structures may indicate variance in the ability of bipyridine isomers to form complexes with U, which could impact their ability to retain U within shale in the context of fracking.

Effects of uranium mining on soil bacterial communities and functions in the Qinghai-Tibet plateau

The microecological effects of plateau uranium mining are still unknown. In this study, we used 16S rRNA high-throughput sequencing to analyze the impact of plateau uranium mining on the microbial diversity and community structure of tailings soil, tunnel soil, and soil at different depths in an open pit. The results showed that uranium mining significantly reduced soil microbial community richness and diversity indicators, including Chao1, Pielou evenness, and Shannon index (P < 0.05). Uranium mining activities significantly reduced the abundance of RB41, Vicinamidactaceae, and Nitrospira (P < 0.05). Interestingly, the abundance of Thiobacillus, Sphingomonas, and Sulfuriferula significantly increased in the soil samples from various environments and depths during uranium mining (P < 0.05). Beta diversity analysis found that uranium mining resulted in the differentiation of soil microbial communities. Functional enrichment analysis found that uranium mining resulted in the functional enrichment of DNA binding response regulator, DNA helicase, methyl-accepting chemotaxis protein, and Helicase conserved C-terminal domain, whereas cell wall synthesis, nonspecific serine/threonine protein kinase, RNA polymerase sigma-70 factor, and ATP binding cassette transporter were significantly affected by uranium mining (P < 0.05). In addition, we also found that different uranium mining environments and soil depths enriched diverse microbial populations and functions to cope with the environmental pressures that were elicited by uranium mining, including Gaiella, Gemmatimonas, Lysobacter, Pseudomonas, signal transformation histidine kinase, DNA-directed DNA polymerase, and iron complex outer membrane receptor protein functions (P < 0.05). The results have enhanced our understanding of the impact of uranium mining on plateau soil microecological stability and the mechanism of microbial response to uranium mining activities for the first time and aided us in screening microbial strains that can promote the environmental remediation of uranium mining in plateaus.

Statistically and visually analyzing the latest advancements and future trends of uranium removal

Uranium contamination and remediation is a very important environmental research area. Removing radioactive and toxic uranium from contaminated media requires fundamental knowledge of targets and materials. To explore the-State-of-the-Art in uranium contamination control, we employed a statistical tool called CiteSpace to visualize and statistically analyze 4203 peer-reviewed papers on uranium treatment published between 2008 and 2022. The primary content presentations of visual analysis were co-authorships, co-citations, keyword co-occurrence analysis with cluster analysis, which could offer purposeful information of research hots and trends in the field of uranium removal. The statistical analysis results indicated that studies on uranium removal have focused on adsorption of uranium from aqueous solution. From 2008 to 2022, biochar and biological treatment were firstly used to sequester uranium, then adsorption for uranium removal dominates with adsorbents of graphene oxide, primary nanofiber magnetic polymers and metal-organic frameworks (MOFs). In recent years, photocatalysts and metal-organic frameworks are expected to be two of the most popular research topics. In addition, we further highlighted the characteristics and applications of MOFs and GOs in uranium removal. Overall, a statistical review was proposed to visualize and summarize the knowledge and research trends regarding uranium treatment.

Enhancing Uranium Removal with a Titanium-Incorporated Zirconium-Based Metal-Organic Framework

The urgent need to efficiently and rapidly decontaminate uranium contamination in aquatic environments underscores its significance for ecological preservation and environmental restoration. Herein, a series of titanium-doped zirconium-based metal-organic frameworks were meticulously synthesized through a stepwise process. The resultant hybrid bimetallic materials, denoted as NU-Zr-n%Ti, exhibited remarkable efficiency in eliminating uranium (U (VI)) from aqueous solution. Batch experiments were executed to comprehensively assess the adsorption capabilities of NU-Zr-n%Ti. Notably, the hybrid materials exhibited a substantial increase in adsorption capacity for U (VI) compared to the parent NU-1000 framework. Remarkably, the optimized NU-Zr-15%Ti displayed a noteworthy adsorption capacity (∼118 mg g-1) along with exceptionally rapid kinetics at pH 4.0, surpassing that of pristine NU-1000 by a factor of 10. This heightened selectivity for U (VI) persisted even when diverse ions exist. The dominant mechanisms driving this high adsorption capacity were identified as the robust electrostatic attraction between the negatively charged surface of NU-Zr-15%Ti and positively charged U (VI) species as well as surface complexation. Consequently, NU-Zr-15%Ti emerges as a promising contender for addressing uranium-laden wastewater treatment and disposal due to its favorable sequestration performance.

Coupling flow and electric fields to simulate migration and remediation of uranium in groundwater remediated by electroosmosis and a permeable reactive bio-barrier

Combined remediation technologies are increasingly being considered to uranium contaminated groundwater, such as the joint utilize of permeable reactive bio-barrier (Bio-PRB) and electrokinetic remediation (EKR). While the assessment of uranium plume evolution in the combined remediation system (CRS) have often been impeded by insufficient understanding of multi-physical field superposition. Therefore, advanced knowledge in multi-physical field coupling in groundwater flow will be crucial to the practical application of these techniques. A two-dimensional multi-physical field coupling model was constructed for predicting the uranium degradation in CRS. The study demonstrates that the coupling model is able to predict the uranium plume evolution and rapidly evaluate the performance of CRS components. The results show that field electric direction and flow field strength are the key factors that affect the retardation and remediation performance of CRS. The reverse electric field direction significantly affected the contact reaction time of uranium in the system. The uranium residence time in the reverse electric field was 3.8 d, which was significantly greater than the original electric field (2.0 d). Depending on the voltage, the reverse electric field direction was 16%-36% more efficient than the original direction. The strength of the flow field was about two orders of magnitude higher than that of the electric field, so the groundwater flow rate dominated remediation efficiency. Reducing the flow rate by 1/2 could improve the performance of the system by approximately 66%. In addition, the coupling model can be utilized to design standard CRS for real site of uranium contaminated groundwater. To meet the optimal performance, the direction of the electric field should be set opposite to the flow field. This work has successfully used a coupling model to predict uranium contaminant-plume evolution in CRS and estimate the performance of each component.

Reduction and adsorption of uranium(VI) from aqueous solutions using nanoscale zero-valent manganese

Nano zero-valent manganese (nZVMn) is theoretically expected to exhibit high reducibility and adsorption capacity, yet its feasibility, performance, and mechanism for reducing and adsorbing hexavalent uranium (U(VI)) from wastewater remain unclear. In this study, nZVMn was prepared via borohydride reduction, and its behaviors about reduction and adsorption of U(VI), as well as the underlying mechanism, were investigated. Results indicated that nZVMn exhibited a maximum U(VI) adsorption capacity of 625.3 mg/g at a pH of 6 and an adsorbent dosage of 1 g/L, and the co-existing ions (K⁺, Na⁺, Mg²⁺, Cd²⁺, Pb²⁺, Tl⁺, Cl^-) at studied range had little interference on U(VI) adsorption. Furthermore, nZVMn effectively removed U(VI) from rare-earth ore leachate at a dosage of 1.5 g/L, resulting in a U(VI) concentration of lower than 0.017 mg/L in the effluent. Comparative tests demonstrated the superiority of nZVMn over other manganese oxides (Mn₂O₃ and Mn₃O₄). Characterization analyses, including X-ray diffraction and depth profiling X-ray photoelectron spectroscopy, combined with density functional theory calculation revealed that the reaction mechanism of U(VI) using nZVMn involved reduction, surface complexation, hydrolysis precipitation, and electrostatic attraction. This study provides a new alternative for efficient removal of U(VI) from wastewater and improves the understanding of the interaction between nZVMn and U(VI).

Bio-functionalized magnetic nanoparticles for cost-effective adsorption of U(vi): experimental and theoretical investigation

U(vi) removal using cost-effective (production cost: $14.03 per kg), biocompatible, and superparamagnetic Cinnamomum tamala (CT) leaf extract-coated magnetite nanoparticles (CT@MNPs or CT@Fe₃O₄ nanoparticles) from water resources was studied. From pH-dependent experiments, the maximum adsorption efficiency was found to be at pH 8. Isotherm and kinetic studies were performed and found to follow Langmuir isotherm and pseudo-second order kinetics, respectively. The maximum adsorption capacity of CT@MNPs was calculated to be 45.5 mg of U(vi) per g of nanoparticles (NPs). Recyclability studies suggest that over 94% sorption was retained even after four consecutive cycles. The sorption mechanism was explained by the point of the zero-charge experiment and the XPS measurement. Additionally, calculations using density functional theory (DFT) were carried out to support the experimental findings.

Groundwater Quality and Health: Making the Invisible Visible

Linking groundwater quality to health will make the invisible groundwater visible, but there are knowledge gaps to understand the linkage which requires cross-disciplinary convergent research. The substances in groundwater that are critical to health can be classified into five types according to the sources and characteristics: geogenic substances, biogenic elements, anthropogenic contaminants, emerging contaminants, and pathogens. The most intriguing questions are related to quantitative assessment of human health and ecological risks of exposure to the critical substances via natural or induced artificial groundwater discharge: What is the list of critical substances released from discharging groundwater, and what are the pathways of the receptors' exposure to the critical substances? How to quantify the flux of critical substances during groundwater discharge? What procedures can we follow to assess human health and ecological risks of groundwater discharge? Answering these questions is fundamental for humans to deal with the challenges of water security and health risks related to groundwater quality. This perspective provides recent progresses, knowledge gaps, and future trends in understanding the linkage between groundwater quality and health.

Reduction of natural radioactivity in groundwater with different salinity through adsorption of uranium and radium in filter materials

In many small communities in the Mediterranean area, groundwater is usually the only water body available. Depending mainly on the surrounding geology, their concentration of naturally occurring radionuclides may pose a radiological hazard. Removal of uranium and radium from drinking water is the best way to avoid it, i.e., reverse osmosis (RO), but consuming a lot of energy. Thus, two modified drinking water treatment plants (DWTPs) using zeolites coated with manganese dioxide as adsorbent material were analyzed as an alternative to RO. Groundwater salinity can negatively affect this process. Radium removal decreased as water salinity increased; but it had a major impact on uranium, rendering the adsorption effectless in one DWTP. Waste management and how to avoid it from becoming radioactive are of major concern. Radium and uranium were associated to the reducible fraction in the filter material and also to the carbonate fraction in the case of uranium. Regeneration of the filter material using KCl solutions was able to remove 81% and 63% of uranium and radium, respectively.

A strategy for bioremediation of nuclear contaminants in the environment

Radionuclides released from nuclear contamination harm the environment and human health. Nuclear pollution spread over large areas and the costs associated with decontamination is high. Traditional remediation methods include both chemical and physical, however, these are expensive and unsuitable for large-scale restoration. Bioremediation is the use of plants or microorganisms to remove pollutants from the environment having a lower cost and can be upscaled to eliminate contamination from soil, water and air. It is a cheap, efficient, ecologically, and friendly restoration technology. Here we review the sources of radionuclides, bioremediation methods, mechanisms of plant resistance to radionuclides and the effects on the efficiency of biological adsorption. Uptake of radionuclides by plants can be facilitated by the addition of appropriate chemical accelerators and agronomic management, such as citric acid and intercropping. Future research should accelerate the use of genetic engineering and breeding techniques to screen high-enrichment plants. In addition, field experiments should be carried out to ensure that this technology can be applied to the remediation of nuclear contaminated sites as soon as possible.

A cradle to cradle approach towards remediation of uranium from water using carbonized arecanut husk fiber

Sustainable materials for remediation of pollutants from water is the need of the hour. In this study two carbonaceous adsorbents prepared through hydrothermal carbonisation and pyrolysis from arecanut husk fiber, an agricultural waste material were used for the adsorption of uranium from water. Batch adsorption data as interpreted using the Langmuir model showed adsorption capacities of 250 mg g-1 and 200 mg g-1 respectively at pH 6 for the hydrochar (AHFC) and the pyrochar (AHFT) exceeding that reported for most of the unmodified biochars. The adsorption followed pseudo-second order kinetics and was exothermic in nature. The high selectivity and excellent removal efficiencies on application to environmental ground water samples and good regeneration capacity make these sorbents promising eco-friendly materials for uranium remediation from water.

Graphene Synthesis Techniques and Environmental Applications

Graphene is fundamentally a two-dimensional material with extraordinary optical, thermal, mechanical, and electrical characteristics. It has a versatile surface chemistry and large surface area. It is a carbon nanomaterial, which comprises sp² hybridized carbon atoms placed in a hexagonal lattice with one-atom thickness, giving it a two-dimensional structure. A large number of synthesis techniques including epitaxial growth, liquid phase exfoliation, electrochemical exfoliation, mechanical exfoliation, and chemical vapor deposition are used for the synthesis of graphene. Graphene prepared using different techniques can have a number of benefits and deficiencies depending on its application. This study provides a summary of graphene preparation techniques and critically assesses the use of graphene, its derivates, and composites in environmental applications. These applications include the use of graphene as membrane material for the detoxication and purification of water, active material for gas sensing, heavy metal ions detection, and CO₂ conversion. Furthermore, a trend analysis of both synthesis techniques and environmental applications of graphene has been performed by extracting and analyzing Scopus data from the past ten years. Finally, conclusions and outlook are provided to address the residual challenges related to the synthesis of the material and its use for environmental applications.

Biosorption of uranium by immobilized Nostoc sp. and Scenedesmus sp.: kinetic and equilibrium modeling

Different activities related to uranium mining and nuclear industry may have a negative impact on the environment. Bioremediation of nuclear pollutants using microorganisms is an effective, safe, and economic method. The present study compared the uranium biosorption efficiency of two immobilized algae: Nostoc sp. (cyanophyte) and Scenedesmus sp. (chlorophyte). Effects of metal concentration, contact time, pH, and biosorbent dosage were also studied. The maximum biosorption capacity (60%) by Nostoc sp. was obtained at 300 mg/l uranium solution, 60 min, pH 4.5, and 4.2 g/l algal dosage, whereas Scenedesmus sp. maximally absorbed uranium (65 %) at 150 mg/l uranium solution, 40 min, pH 4.5, and 5.6 g/l of algal dosage. The interaction of metal ions as Na2SO4, FeCl3, CuCl2, NiCl2, CoCl2, CdCl2, and AlCl3 did not support the uranium biosorption by algae. The obtained data was adapted to the linearized form of the Langmuir isotherm model. The experimental qmax values were 130 and 75 mg/g for Nostoc sp. and Scenedesmus sp., respectively. Moreover, the pseudo-second-order kinetic model was more applicable, as the calculated parameters were close to the experimental data. The biosorbents were also characterized by Fourier-transform infrared spectroscopy (ATR-FTIR), energy-dispersive X-ray spectroscopy (EDX), and scanning electron microscopy (SEM) analyses. The results suggest the applicability of algae, in their immobilized form, for recovery and biosorption of uranium from aqueous solution.

Surface ion-imprinted brewer's spent grain with low template loading for selective uranyl ions adsorption from simulated wastewater

Efficient removal of uranyl ions from wastewater requires excellent selectivity of the adsorbents. Herein, we report a new strategy using a high monomer/template molar ratio of 500:1 to prepare surface ion-imprinted brewer's spent grain (IIP-BSG) for selective U(VI) removal using binary functional monomers (2-hydroxyethyl methacrylate and diethyl vinylphosphonate) with high site accessibility and easy template removal. IIP-BSG exhibits a maximum U(VI) adsorption capacity of 165.7 mg/g, a high selectivity toward U(VI) in the presence of an excess amount of Eu(III) (Eu/U molar ratio = 20), a good tolerance of salinity, and a high reusability. In addition, mechanism studies have revealed electrostatic interaction and a coordination of uranyl ions by carboxyl and phosphoryl groups, the predominant contribution of high-energy (specific) sites during selective adsorption, and internal mass transfer as the rate-controlling step of U(VI) adsorption. Furthermore, IIP-BSG shows great potentials to separate U(VI) from lanthanides in simulated nuclear wastewater (pH₀ = 3.5) and selectively concentrate U(VI) from simulated mine water (pH₀ = 7.1). This study proves that the ion-imprinting effect can be achieved using a very low template amount with reduced production cost and secondary pollution, which benefits large-scale promotion of the ion-imprinted materials for selective uranyl ions removal.

Development, hotspots and trend directions of groundwater salinization research in both coastal and inland areas: a bibliometric and visualization analysis from 1970 to 2021

As a global concern, the issue of groundwater salinization refers to the phenomenon of an increase in the overall chemical content over background levels in the groundwater. It involves a long-term process that could degrade groundwater quality and restrict its availability for drinking, irrigation and industry. For the effective protection and further research of groundwater resources, policy strongly depends on understanding the development, hotspots and trend directions of groundwater salinization research, which involves the degree, sources and processes of global groundwater salinization. However, such a comprehensive and systematic analysis has not been performed, and it is difficult to have a deeper understanding of groundwater salinization. The purpose of this paper is to analyze the knowledge structure, hot topics and trends in the field of groundwater salinization based on 6651 Web of Science (WoS) publications combined with CiteSpace for in-depth bibliometric and visual analysis. The results showed that 292 institutions in 125 countries have published articles in this field from 1970 to 2021. The USA was one of the most prolific contributors, with the largest number of publications and active institutions. Cooperation among authors has become frequent in recent years, and they tend to cooperate in groups. According to the analysis of co-occurrence keywords and co-cited articles, "water resources", "sea level rise" and "variable density flow" were identified as three hot topics. A keyword burst analysis revealed the emerging trends of concerns about global climate change and the sustainable utilization of water resources. In addition, the possible opportunities and challenges were explored that may be faced in groundwater salinization research. The outcomes of this study are significant for future research on groundwater management and pollution control.