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The photoelectrocatalytic reduction of soluble U(VI) to relatively insoluble U(IV) is a promising method for uranium wastewater treatment. However, the strong reliance on sacrificial agents and an inert atmosphere due to the rapid charge reorganization remains a great challenge. The construction of an inner electrical field at a heterojunction interface accompanied by an external electric field provides a promising strategy for enhancing charge transfer. Herein, a photoelectrode was proposed by in-situ coating nano-TiO 2 cone arrays (NTCA) on a titanium mesh, and used for uranium removal from complex effluents under air conditions without sacrificial agents. The enhanced spatial charge separation driven by the Schottky junction built-in electric field and an external bias voltage, coupled with rapid fluid diffusion, granted the photoelectrode an outstanding photoelectrocatalytic uranium removal performance with photocurrent densities of up to 3 mA cm−2 at −0.55 V Ag/AgCl. The impressive uranium removal performance was maintained in a wide range of chemical conditions, as well as in real seawater and industrial wastewater. Theoretical calculation revealed that uranyl ions were confined on the NTCA surface as [UO 2 (H 2 O) 5 ]2+, further reduced to UO 2. This work demonstrated a viable route for uranium separation from aqueous solutions with inexhaustible solar energy and minimal electrical energy. [Display omitted] ● Efficient photoelectrocatalytic removal of uranium was achieved from wide wastewater conditions. ● The nanocone arrays with Schottky junction enabled the enhanced charge separation. ● Synergy of internal and external electric fields achieved a photocurrent density up to 3 mA/cm−2 at −0.55 V Ag/AgCl. ● Photoelectrocatalytic reduction is a viable route for practical treatment of uranium wastewater. [ABSTRACT FROM AUTHOR]

Accurate prediction of the colloid-driven transport of radionuclides in porous media is critical for the long-term safety assessment of radioactive waste disposal repository. However, the co-transport and corelease process of radionuclides with colloids have not been well documented, the intrinsic mechanisms for colloids-driven retention/transport of radionuclides are still pending for further discussion. Thus the controlling factors and governing mechanisms of co-transport and co-release behavior of Eu(III) with bentonite colloids (BC) were discussed and quantified by combining laboratory-scale column experiments, colloid filtration theory and advection dispersion equation model. The results showed that the role of colloids in facilitating or retarding the Eu(III) transport in porous media varied with cations concentration, pH, and humic acid (HA). The transport of Eu(III) was facilitated by the dispersed colloids under the low ionic strength and high pH conditions, while was impeded by the aggregated colloids cluster. The enhancement of Eu(III) transport was not monotonically risen with the increase of colloids concentration, the most optimized colloids concentration in facilitating Eu(III) transport was approximately 150 mg L−1. HA showed significant promotion on both Eu(III) and colloid transport because of not only its strong Eu(III) complexion ability but also the increased dispersion of HA-coated colloid particles. The HA and BC displayed a synergistic effect on Eu(III) transport, the co-transport occurred by forming the ternary BC-HA-Eu(III) hybrid. The transport patterns could be simulated well with a two-site model that used the advection dispersion equation by reflecting the blocking effect. The retarded Eu(III) on the stationary phase was released and remobilized by the introduction of colloids, or by a transient reduction in cation concentration. The findings are essential for predicting the geological fate and the migration risk of radionuclides in the repository environment. [Display omitted] • Colloid-driven transport of Eu(III) in porous media is related to colloid stability. • A optimal colloids concentration exists in facilitating Eu(III) transport. • HA and BC show synergistic effect on Eu(III) transport by forming ternary hybrid. • The retarded colloids and Eu(III) is released as water chemistry condition varies. [ABSTRACT FROM AUTHOR]

Abstract: The sorption of Th(IV) on attapulgite was studied as a function of pH, ionic strength, temperature, attapulgite contents and Th(IV) concentrations under ambient conditions using a batch technique. The results indicated that sorption of Th(IV) on attapulgite was strongly affected by pH values, and weakly dependent on ionic strength. Sorption of Th(IV) was dominated by surface complexation, although ion exchange also contributed to this sorption. Sorption of Th(IV) increased with increasing temperature of the system. Enthalpy (ΔH 0), entropy (ΔS 0) and Gibbs free energy (ΔG 0) were calculated from the temperature-dependent sorption data; the results indicated that the sorption of Th(IV) on attapulgite was a spontaneous process. Sorption–desorption hysteresis indicated that the sorption of Th(IV) was irreversible, and that the Th(VI) adsorbed on attapulgite was difficult to be desorbed from solid to liquid phases. [Copyright &y& Elsevier]

Although nanoscale zero-valent iron (nZVI) has been intensively investigated for the remediation of uranium contaminated water, few efforts have been dedicated to U(VI) removal and recovery by nZVI from strongly alkaline solutions (e.g., pH 10.7) in the presence of carbonate. This work demonstrates that U(VI) can be removed by nZVI from strongly alkaline solutions in the anaerobic atmosphere. Results show that reductive precipitation is responsible for U(VI) removal with an effective reduction efficiency of nZVI up to 95.5 ± 3.3% (6.09 ± 0.21 g of U/g of Fe). In the aerobic atmosphere, however, the efficiency is no more than 8.3% (0.53 g of U/ g of Fe) due to relatively rapid corrosion reactions of nZVI as well as the re-oxidation of U(IV). The difference in the efficiency originates not only from the variation in the kinetics of corrosion reactions, but also from U(VI) redox and coordinate chemistry. Under the aerobic condition, uranium removal reaches the maximum at 1 h of reaction and then decreases, because uranium delivery from solid phase occurs due to successive processes of U(IV) oxidation, UO 2 2+ complexation with carbonate, and [UO 2 (CO 3) 3 ]4- desorption from surfaces. The uranium delivery limits the application of nZVI for uranium removal under the aerobic condition, but provides a candidate approach for the uranium recovery from the spent nZVI. A recovery of 100% can be obtained by sodium carbonate solution in the presence of O 2. Both the oxidation and desorption processes control the uranium recovery rate. This study not only offers new insight into the mechanisms of interactions between U(VI) and nZVI especially in strongly alkaline media with high concentration of carbonate, but also provides a reference approach to the issue of U(VI) removal and recover from the wastewater generated in the nuclear industry. [Display omitted] • nZVI can remove U(VI) from wastewater at pH 10.7 in the anaerobic atmosphere. • Electron efficiency for U(VI) removal is 95.5 ± 3.3% in the anaerobic atmosphere. • U(VI) removal capacity is up to 6.09 ± 0.21 g of U/g of Fe in the absence of O 2. • U recovery from the spent nZVI is controlled by U(IV) oxidation & U(VI) desorption. • U removal is via reductive precipitation in both aerobic & anaerobic atmospheres [ABSTRACT FROM AUTHOR]

Environmental colloids play crucial roles in the transport of environmental pollutants in porous media by acting as pollutant carriers. In this work, the dispersion stability and correlated transport of kaolinite colloid were investigated as a function of solution pH, solution ionic strength, and concentration of humic acid (HA), the roles of kaolinite colloid in driving Eu(III) transport were discussed. The results showed that the dispersion of kaolinite colloid was favorable at alkaline and extremely acidic pH values, the trend of aggregation with varying pH was critically reversed at pH ∼3.2 due to the transformation of surface electrical properties. Cations with higher valence and mineral affinity showed a more significant contribution in inducing colloid aggregation, which was generally in accordance with the Schulze-Hardy rule and Hofmeister series. HA greatly increased the colloid stability by altering the surface electrostatic potential and steric effect. The Derjguin-Landau-Verwey-Overbeek (DLVO) model suggested that the electrostatic force between colloidal particles controlled the aggregation and destabilizing trend of colloid, and the theoretically calculated critical coagulation concentration was consistent with that determined from kinetic aggregation experiments. The roles of kaolinite colloid in driving Eu(III) transport varied under different conditions, and the transport behavior was highly correlated with the dispersion stability trend of colloid. These results can provide an enhanced understanding of the environmental fate of kaolinite colloid as well as commensal pollutants. Image 1 • Kaolinite colloid is dispersion favorable under high pH and extreme low pH. • Ions with high valence and mineral affinity are significant in inducing aggregation. • Humic acid greatly increases the dispersion stability of kaolinite colloid. • Transport of colloid and Eu(III) is related with the dispersion stability of colloid. [ABSTRACT FROM AUTHOR]

The photocatalytic reduction of U(VI) is recognized as an economical and effective way for U(VI) removal/recovery from solutions. To improve the photocatalytic activity of TiO 2 under visible light, TiO 2 was hydrogenated by NaBH 4 to generate Ti3+ self-doped black TiO 2 (BT n). The self-doped Ti3+ alongside oxygen vacancies (Ov) could act as interband level to increase visible light capture and reduce the recombination of photogenerated carriers. The obtained BT n samples showed high performance for U(VI) elimination under near neutral conditions, and held an outstanding anti-interference for U(VI) over competing metal cations and anions. Methanol and ethanol could act as sacrificial donors, being favorable for the photocatalytic reduction of U(VI), while the presence of EDTA inhibited the photoreduction of U(VI). The BT n photocatalysts showed relatively high stability and reusability during the photocatalysis and elution processes. The XPS, TEM and XRD results revealed that U(VI) was photo-reduced to form UO 2 on the surface of BT n. This work may serve as an important reference for improving the photocatalytic reactivity of TiO 2 as well as for the efficient removal/recovery of U(VI) from aqueous solutions. Image 1 • Ti3+ self-modification black TiO 2 (BT) was prepared by simple reduction method. • BT showed high efficiency for U(VI) adsorption and photocatalytic reduction under visible light. • Ti3+ and oxygen vacancies in BT enhanced the reactivity of U(VI) reduction. • The photocatalytic reduction of U(VI) was highly selective to coexisting ions. • The photoconversion process of U(VI) was addressed using XRD, XPS, and TEM. Ti3+ self-doped black TiO 2 were synthesized, which exhibited excellent photocatalytic performance and anti-interference for the reduction of U(VI) under visible light. [ABSTRACT FROM AUTHOR]

The determination of the speciation and binding types of metals is critical to assessing mobility and bioavailability in soils and sediments. The current study determined the speciation of Zn and Cd in contaminated sediments in the Baiyin district using X-ray absorption fine structure (XAFS) and micro-X-ray fluorescence (μ-XRF). The results revealed that the speciation of Zn in sediments was quite complicated and related to the mineralogical composition and physiochemistry in different locations, with speciation including Zn2+, Zn–Al layered double hydroxide (LDH), Zn primary minerals (sphalerite and franklinite), Zn-Calcite, and Zn-Ferrihydrite. In comparison to Zn, the speciation of Cd in sediments was found to be simpler and less influenced by the geochemical properties of the samples. The μ-XRF analysis indicated a strong correlation between the distribution of Cd and hot spots of Ca and Zn. The XAFS analysis confirmed that the dominant form of Cd was adsorbed on calcite and illite, suggesting a high level of bioavailability, mobility, and potential environmental risk. The results also showed that Zn and Cd coexisted and had a synergistic effect in the calcite and clay minerals of contaminated sediments. This study is the first report on the in-situ speciation of heavy metals in alkaline and calcareous sediment of north China, and the use of spectroscopy techniques allowed for the reliable identification of the speciation of Zn and Cd. [Display omitted] • The concentrations and properties of Zn and Cd in sediment were investigated. • Zn species were complex and related to the properties of different sediment. • Cd represented simple speciation with CdS, and Cd adsorbed on calcite and illite. • Zn and Cd coexisted and represented a synergistic effect in calcareous sediment. [ABSTRACT FROM AUTHOR]

Abstract: U(VI) sorption on goethite was investigated as functions of pH, solid-to-liquid ratio (m/V), ionic strength and U(VI) concentration by a batch experimental method. Effects of phosphate, carbonate and fulvic acid (FA) on U(VI) sorption were examined. It was found that the sorption of U(VI) increases from 0% to 100% over the pH range of 2.5–4.5 and keeps constant in the high pH range. The sorption of U(VI) on goethite is insensitive to ionic strength. Different surface complexes in the framework of double-layer model were examined for fitting the sorption of U(VI) on goethite. A model with two mononuclear inner-sphere surface complexes, ≡SOUO2 + and ≡SOUO2OH, was found capable of reproducing the pH sorption edges, the sorption isotherms and the sorption data with variable m/V in this study. The proposed model can also interpret the pH sorption edge collected at without considering any ternary surface complexes of carbonate. Moreover, it was found that the presence of phosphate at relatively high concentration (6×10−4 mol/L) promotes U(VI) sorption. The presence of FA of 20mg/L has little effect on the sorption of U(VI) on goethite. [Copyright &y& Elsevier]

• Illite colloid and phosphate display synergistic promotion on uranium transport. • Phosphate precipitates with U(VI) to form intrinsic colloids for transport. • Blocking coagulation of colloids was responsible for ternary co-transport. • Retardation strategy of U(VI) by phosphate mineralization should be more discreet. The phosphorus-containing reagents have been proposed to remediate the uranium contaminated sites due to the formation of insoluble uranyl phosphate mineralization products. However, the colloids, including both pseudo and intrinsic uranium colloids, could disturb the environmental fate of uranium due to its nonnegligible mobility. In this work, the transport pattern and micro-mechanism of uranium coupled to phosphate and illite colloid (IC) were investigated by combining column experiments and micro-spectroscopic evidences. Results showed that uranium transport was facilitated in granular media by forming the intrinsic uranyl phosphate colloid (such as Na-autunite) when the pH > 3.5 and C Na+ < 10 mM. Meanwhile, the mobility of uranium depended greatly on the typical water chemistry parameters governing the aggregation and deposit of intrinsic uranium colloids. However, the attachment of phosphate on illite granule increased the repulsive force and enhanced the dispersion stability of IC in the IC-U(VI)-phosphate ternary system. The non-preequilibrium transport and retention profiles, HRTEM-mapping, as well as TRLFS spectra revealed that the IC enhanced uranium mobility by forming the ternary IC-uranyl phosphate hybrid, and acted as the coagulation preventing agent for uranyl phosphate particles. This observed facilitation of uranium transport resulted from the formation of intrinsic uranyl phosphate colloids and IC-uranyl phosphate hybrids should be taken into consideration when evaluating the potential risk of uranium migration and optimizing the in-situ mineralization remediation strategy for uranium contaminated environmental water. [Display omitted] [ABSTRACT FROM AUTHOR]

Understanding the in-situ transport behavior of U(VI) in granitic formations is of considerable interest for geological disposal of high-level radioactive wastes (HLW). In this context, the co-transport of U(VI) and representative naturally-occurring colloids, i.e., humic acid (HA) and gibbsite colloid (GC), was studied in granite column as a function of pH, U(VI) concentration and HA amount. It was found that, in addition to pH, co-transport of U(VI) and GC was also controlled by U(VI) concentration, the effect of which can be transport-facilitating and transport-impeding for U(VI) at relatively low concentration (2.0 × 10−6 mol/L) and for U(VI) at high concentration (5.0 × 10−5 mol/L), respectively. HA can present opposite effects on GC transport depending on HA amount. The transport-impeding effect by small amount of HA (5 mg/L) is due to strong aggregation between GC and HA from electrostatic attraction and complexation, whereas the transport-facilitating effect by big amount of HA (20 mg/L) is because of the complete HA coating which stabilizes associated colloids and alters surface charge from positive to negative. In ternary co-transport systems, a similar HA-dependent effect was also observed for both U(VI) and GC regardless of presence of high concentration U(VI). Besides the application of the Derjaguin–Landau–Verwey–Overbeek (DLVO) theory, the mechanisms behind binary and ternary co-transport of U(VI), GC and HA were also analyzed by assessing the evolutions of zeta potential and particle size in the column effluents. Finally, a two-site non-equilibrium model and a two-site kinetic attachment/detachment model were applied to describe the breakthrough curves of U(VI) and individual/combined colloids, respectively. The findings of this study indicated that combined effects of GC and HA on radionuclides transport is dominated by the amount of HA, and a facilitating transport of radionuclide can be expected in the underground environment rich in humic acid. Unlabelled Image • GC exhibits opposite effects on U(VI) transport depending on U(VI) concentration. • Co-transport of GC and HA is controlled by the amount of HA. • The combined effect of GC and HA on U(VI) transport is dominated by HA amount. [ABSTRACT FROM AUTHOR]

Abstract In this study, the incorporation of U(VI) into CaCO 3 under different aging times and U(VI) concentrations was studied by combining batch experiments, X-ray diffraction (XRD), attenuated total reflection Fourier transformed infrared spectroscopy (ATR-FTIR), and extended X-ray absorption fine structure (EXAFS) approaches. Batch sorption experiments showed that the sorption of U(VI) on calcite was strong pH-dependence, and high pH was beneficial for U(VI) sorption possibly due to the electrostatic attraction between positively charged calcite and negatively charged uranyl tri-carbonate species. XRD patterns showed that the [104] facet of calcite shifted toward low angle at pH ∼10.0, which indicated that the uranyl tri-carbonate species of U(VI) possibly diffused into calcite lattice by replacing Ca atoms, and then induced the expansion of calcite crystal cell. The incorporation of U(VI) into CaCO 3 showed that the uptake of U(VI) gradually decreased within the first 200 h, and then significantly increased with the increasing aging time. U(VI) incorporation into CaCO 3 might experience vaterite, transition from vaterite to calcite, and calcite stages, which were confirmed by XRD, ATR-FTIR, and X-ray absorption near-edge structure (XANES) spectroscopy. As the U(VI) concentration increased, the transition time from vaterite to calcite correspondingly increased, indicating that U(VI) incorporation into CaCO 3 can stabilize vaterite phase. EXAFS analyses suggested that the local structure of uranyl moiety was changing during the incorporation process, and the species of U(VI) incorporation into vaterite was similar to uranyl carbonates, however indeed different from the species of uranyl tri-carbonate presented in calcite. Graphical abstract Image 1 Highlights • U(VI) sorption on calcite was studied using batch and spectroscopies. • The incorporation of U(VI) into CaCO 3 were explored under different aging times and U(VI) concentrations. • XANES, EXAFS and ATR-FTIR were used to explore the incorporation mechanism of U(VI) into CaCO 3. • The incorporation of U(VI) into CaCO 3 experienced three stages. • The species of U(VI) in vaterite is UO 2 CO 3 differing from the uranyl tri-carbonate in calcite. [ABSTRACT FROM AUTHOR]

Abstract Sorption of U(VI) onto TiO 2 as functions of pH, ionic strength, contact time, soil humic acid (SHA), solid-to-liquid ratio and temperature was studied under ambient conditions using batch and spectroscopic approaches. The sorption of U(VI) on TiO 2 was significantly dependent on pH and ionic strength. The presence of SHA slightly enhanced the sorption of U(VI) on TiO 2 below pH 4.0, while it inhibited U(VI) sorption in the higher pH range. U(VI) sorption on TiO 2 was favored at high temperatures, and the sorption process was estimated to be endothermic and spontaneous. Reduction of U(VI) to lower valent species was confirmed by X-ray photo-electron spectroscopy analysis. It is very interesting to find that U(VI) sorption on TiO 2 was promoted in solutions with higher back-ground electrolyte concentrations. In the presence of U(VI), higher back-ground electrolyte made more TiO 2 particles aggregate through (001) facets, leading more (101) facets to be exposed. Therefore, the reduction of U(VI) was enhanced by the exposed (101) facets and more U(VI) removal was observed. Graphical abstract Unlabelled Image [ABSTRACT FROM AUTHOR]

Abstract Remediating uranium contamination becomes a worldwide interest because of increasing uranium release from mining activities. Due to ubiquitous presence of pyrite and the application of iron-based technology, colloidal iron oxy-hydroxides such as akaganéite colloid (AKC) extensively exist in uranium polluted water at uranium tailing sites. In this context, we studied individual and co-transport of U(VI) and AKC in water-saturated sand columns at 50 mg/L AKC and environmentally relevant U(VI) concentrations (5.0 × 10−7 ∼ 5.0 × 10−5 M). It was found that, in addition to the impact of pH and ionic strength, whether AKC facilitated U(VI) transport depended on U(VI) concentration as well. The presence of AKC facilitated U(VI) transport at relatively low U(VI) concentration (5.0 × 10−7 ∼ 5.0 × 10−6 M), which was due to the strong adsorption of U(VI) on AKC and faster transport of AKC than that U(VI) as observed in their individual transport experiments. At relatively high U(VI) concentrations (5.0 × 10−5 M), however, AKC impeded U(VI) transport because U(VI) of high concentration decreased AKC colloidal stability and increased AKC aggregation and attachment. Thus, U(VI) and AKC co-transport was even blocked completely at relatively high pH and ionic strength. The mechanisms behind the co-transport of U(VI) and AKC were also confirmed by assessing the evolutions of aqueous pH and AKC zeta potential and particle size distribution in the column effluents. A two-site non-equilibrium model and a two-site kinetic attachment/detachment model well-described the breakthrough curves of U(VI) and AKC, respectively. Knowledge generated from this study provides a thorough understanding of uranium transport in the absence/presence of AKC, and brings new insights into the influence of contaminant concentration on co-transport in the presence of colloids. Graphical abstract Image 1 Highlights • Akaganéite colloids inhibit U(VI) transport at relatively high U(VI) concentration. • U(VI) concentration is a vital factor in regulating its co-transport with AKC. • U(VI) adsorption increases AKC size and impedes AKC transport. [ABSTRACT FROM AUTHOR]