Heterojunction-Promoted Sodium Ion Storage of Bimetallic Selenides Encapsulated in a Carbon Sheath with Boosted Ion Diffusion and Stable Structure.

Although metallic chalcogenides are deemed as attractive sodium anode materials recently, the electrochemical performance is severely confined by the liability of structural collapse and sluggish ion diffusion kinetics. Herein a composite of carbon-encapsulated bimetallic selenides MoSe2-Sb2Se3 was prepared by a hydrothermal method on the basis of abundant reaction sites, high activity, an extra built-in electric field generated from heterointerfaces, and synergistic effects between the different components. Equally important, the carbon coating is effective to support the structural stability by restraining the vast volumetric variation to achieve the purpose of improving the cycling performance. The density functional theory calculation results indicate that the band gap is narrowed and that the work function is decreased on the interface of the MoSe2-Sb2Se3 heterojunction, leading to an additional driving force stemming from the introduction of the built-in electric field and the formation of the Sb-Se (Se from MoSe2) bond. Therefore, the resultant composite presents increased reaction kinetics and good electrochemical properties by acquiring a capacity of 376.0 mA h g-1 over 580 cycles at 2.0 A g-1 for the half-cell and 276 mA h g-1 over 750 cycles at 2 A g-1 for the full-cell. This work highlights bimetallic selenides with facilitated ion transferability with high performance.

Co-transport of U(VI) and gibbsite colloid in saturated granite particle column: Role of pH, U(VI) concentration and humic acid.

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.

Co-transport of U(VI), humic acid and colloidal gibbsite in water-saturated porous media.

The release of uranyl from uranium tailing sites is a widely concerned environmental issue, with limited investigations on the effect of coexistence of various colloids. Gibbsite colloids extensively exist, together with ubiquitous humic substances, in uranium polluted waters at tailing sites, due to high concentration of dissolved Al in acid mine drainage. In this context, we investigated the co-transport of U(VI), gibbsite colloids and humic acid (HA) as a function of pH and ionic strength at a U(VI) concentration (5.0 × 10-5 M) relevant within mine tailings and related waste. It was found that, owing to electrostatic attraction, gibbsite colloids and HA associated with each other and transported simultaneously regardless of U(VI) presence. Besides the impact of pH and ionic strength, whether gibbsite colloids facilitated U(VI) transport depended on HA concentration. Gibbsite colloids impeded U(VI) transport at relatively low HA concentration (≤5 mg L-1), because associated colloids loaded with U(VI) were positively charged which favored colloid retention on negatively charged quartz sand in the column. U(VI) together with gibbsite colloids and low concentration HA was completely blocked at natural pH and/or high ionic strength. At relatively high HA concentration (20 mg L-1), however, the associated colloids showed negative zeta potential which facilitated U(VI) transport because of repulsion between negatively charged colloids and quartz sand. Meanwhile, high concentration of HA dramatically accelerated the transport of gibbsite colloids. These results implied that gibbsite colloids might imped U(VI) migration at uranium tailing sites unless the aquifers are enriched with abundant humic substances.

Co-transport of U(VI) and akaganéite colloids in water-saturated porous media: Role of U(VI) concentration, pH and ionic strength.

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.