Mechanism and thermodynamics of scorodite formation by oxidative precipitation from arsenic-bearing solution.

Arsenic pollution is a challenging environmental issue caused by arsenic-bearing wastes from nonferrous metallurgy. Oxidative precipitation via introducing O2 into an ionic Fe(II)-As(V) solution is an advanced method for arsenic immobilization. However, the underlying mechanism is still not well understood. This study proposed a mechanism for scorodite formation by oxidative precipitation, and its thermodynamics were calculated using Gaussian software. Scorodite formation was divided into three stages: precursor formation (3-90 min), oxidative conversion (90-270 min) and crystallization (270-720 min) from the variation in precipitates and solution characterization and parameters such as initial pH, arsenic concentration, and ferrous dosage. In the scorodite formation mechanism, the precursors originate from the coordination polymerization of aqueous Fe(H2O)62+ and H2AsO4-, which contributes to the oxidative conversion of coordinated polymers ([Fe(H2O)4(H2O)]nn+) to basic Fe(H2O)2AsO4 until regular octahedral crystals are formed via nucleation and growth during crystallization. The ΔrGmθ for polymerization varied from -491.96 kJ mol-1 to -33.30 kJ mol-1, and the ΔrGmθ of oxidative conversion changed from -982.16 kJ mol-1 to -224.82 kJ mol-1, demonstrating the feasibility in scorodite formation. This research is significant for understanding scorodite formation in As(V) solutions. It can provide schemes for controlling and modifying the conditions of arsenic-bearing waste immobilization in the laboratories and industries.

Immobilizing arsenic-enriched wastewater from utilization of crude antimony oxides as scorodite using a novel multivalent iron source.

Arsenic-enriched wastewater (A-EW) is a hypertoxic sewage from the utilization of crude antimony oxides in lead anode slime metallurgy. In traditional methods, the H+ accumulation inhibits the arsenic immobilization during scorodite synthesis. In this study, a novel multivalent iron source comprised of Fe(OH)3 and FeSO4·7H2O was proposed to resolve the adverse effects of pH fluctuation during immobilizing A-EW as scorodite. Various approaches, such as scanning electron microscopy and X-ray photoelectron spectroscopy, were applied to characterize the synthesized scorodite. This work was divided into two parts. In thermodynamics, HnAsO4(3-n)- (n = 1, 2, 3) and Fe(OH)n(3-n)+ (n = 0, 1, 2, 3) can feasibly coprecipitate as scorodite according to their △rGm,Tθ ranged from -111.10 kJ mol-1 to -33.53 kJ mol-1. In experimental research, A-EW was immobilized as scorodite by optimizing conditions as initial pH = 2.0, molar ratio of Fe to As = 1.2, molar ratio of Fe(II) to Fe(III) = 4:6, arsenic concentration = 40 g/L, and temperature = 95 °C. The arsenic precipitation ratio is 99.60%, and the micromorphology of synthesized scorodite presents a regular octahedron having size of 5-10 µm. The low leachability of As (0.41 mg/L) in toxicity characteristic leaching procedure (TCLP) confirmed that the prepared scorodite is nonhazardous. The solution pH is stable at 2.0 as the H+ depletion (0.5660 mol) by Fe(OH)3 dissolution and Fe2+ oxidization balanced with that (0.5657 mol) generated from As(V)-Fe(III) coprecipitation. In general, the A-EW was effectively immobilized by proposed multivalent iron source, and can be potentially applied to safely dispose other industrial effluents, such as high arsenic leachates and arsenic-bearing waste acid from nonferrous metallurgy.

P-Doped Cotton Stalk Carbon for High-Performance Lithium-Ion Batteries and Lithium-Sulfur Batteries.

Biomass as a carbon material source is the characteristic of green chemistry. Herein, a series of hierarchical P-doped cotton stalk carbon materials (HPCSCMs) were prepared from cheap and abundant biowaste cotton stalk. These materials possess a surface area of 3463.14 m2 g-1 and hierarchical pores. As lithium-ion battery (LIB) anodes, the samples exhibit 1100 mAh g-1 at 0.1 A g-1 after 100 cycles and hold 419 mAh g-1 at 1 A g-1 after 1000 cycles, with nearly 100% capacity retention. After HPCSCMs are loaded with sulfur (S/HPCSCMs), the samples (S/HPCSCMs-2) deliver a discharge capacity of 413 mAh g-1 at 0.1 A g-1 after 100 cycles as lithium-sulfur (Li-S) battery cathodes. This excellent electrochemical performance can be attributed to P in carbon networks, which not only provides more active sites, but also improves electrical conductivity.

Fe2O3/MoO3@NG Heterostructure Enables High Pseudocapacitance and Fast Electrochemical Reaction Kinetics for Lithium-Ion Batteries.

Transition metal oxides (TMOs) hold great potential for lithium-ion batteries (LIBs) on account of the high theoretical capacity. Unfortunately, the unfavorable volume expansion and low intrinsic electronic conductivity of TMOs lead to irreversible structural degradation, disordered particle agglomeration, and sluggish electrochemical reaction kinetics, which result in perishing rate capability and long-term stability. This work reports an Fe2O3/MoO3@NG heterostructure composite for LIBs through the uniform growth of Fe2O3/MoO3 heterostructure quantum dots (HQDs) on the N-doped rGO (NG). Due to the synergistic effects of the "couple tree"-type heterostructures constructed by Fe2O3 and MoO3 with NG, Fe2O3/MoO3@NG delivers a prominent rate performance (322 mA h g-1 at 20 A g-1, 5.0 times higher than that of Fe2O3@NG) and long-term cycle stability (433.5 mA h g-1 after 1700 cycles at 10 A g-1). Theoretical calculations elucidate that the strong covalent Fe-O-Mo, Mo-N, and Fe-N bonds weaken the diffusion energy barrier and promote the Li+-ion reaction to Fe2O3/MoO3@NG, thereby facilitating the structural stability, pseudocapacitance contribution, and electrochemical reaction kinetics. This work may provide a feasible strategy to promote the practical application of TMO-based LIBs.

A Knitting Copolymerization Strategy to Build Porous Polytriazolium Salts for Removal of Anionic Dyes and MnO4.

Although considerable efforts have been devoted to novel ionic porous networks (IPNs), the development of them in a scalable manner to tackle the issues in pollutant treatment by adsorption remains an imminent challenge. Herein, inspired by natural spider webs, a knitting copolymerization strategy is proposed to construct analogue triazolium salt-based porous networks (IPN-CSUs). It is not only convenient to incorporate the cationic motifs into the network, but easy to control over the contents of ionic pairs. The as-prepared IPN-CSUs displays a high surface area of 924 m2 g-1 , a large pore volume of 1.27 cm3 g-1 and abundant ionic sites, thereby exhibiting fast adsorption rate and high adsorption capacity towards organic and inorganic pollutants. The kinetics and thermodynamics study reveal that the adsorption followed a pseudo-second-order kinetic model and Langmuir isotherm model correspondingly. Specifically, the maximum adsorption capacity of the IPN-CSUs is as high as 1.82 mg mg- 1 for permanganate ions and up to 0.54 mg mg-1 for methyl orange, which stands out among the previously reported porous adsorbents so far. It is expected that the strategy reported herein can be extended to the development of other potential efficient adsorbents in water purifications.

One-pot construction of nitrogen-rich polymeric ionic porous networks for effective CO2 capture and fixation.

Facile preparation of ionic porous networks (IPNs) with large and permanent porosity is highly desirable for CO2 capture and transformation but remains a challenge. Here we report a one-pot base-mediated construction of nitrogen-rich IPNs through a combination of nucleophilic substitution and quaternisation chemistry from H-imidazole. This strategy, as proven by the model reactions of 1H-imidazole or 1-methyl-1H-imidazole with cyanuric chloride, allows for fine regulation of porosity and physicochemical properties, leading to nitrogen-rich IPNs featuring abundant ionic units and radicals. The as-prepared networks, termed IPN-CSUs, efficiently capture CO2 (80.1 cc g-1 at 273 K/1 bar) with an ideal CO2/N2 selectivity of 139.7. They can also effectively catalyse the cycloaddition reaction between CO2 and epoxides with high yields of up to 99% under mild conditions (0.1 MPa, 298 K), suggesting their possible applications in the fields of both selective molecular separation and conversion. Unlike the previously known strategies generally involving single coupling chemistry, our strategy combining two coupling routes in one pot appears to be unique and potentially applicable to other building blocks.

Enhancing lithium ion diffusion kinetic in hierarchical lithium titanate@erbium oxide from coating to doping via facile one-step co-precipitation.

Hierarchical lithium titanate@erbium oxide (Li4Ti5O12@Er2O3) microspheres from coating to doping were successfully synthesised by a simple and scalable one-step co-precipitation method. Microscopic observations revealed that the Li4Ti5O12@Er2O3 microspheres present a well-defined hierarchical structure and that Li4Ti5O12 is coated by the Er2O3 layer. The X-ray photoelectron spectroscopy (XPS) results demonstrate that partial Ti4+ is reduced to Ti3+ and induces oxygen vacancy because partial Er3+ dope into octahedral 16d Li+/Ti4+ sites of Li4Ti5O12. Owing to the hierarchical microsphere structure, Er2O3 coating, and Er3+ doping, the material exhibits excess rate capacity (183.7 mAh g-1 at 30C). The hierarchical microsphere structure shortens the diffusion pathways for Li+ ions. The Er2O3 coating on the surface reduces the adverse interface reaction. Importantly, oxygen vacancy induced by Er3+ doping enhances Li+ ion diffusion kinetics and offers extra space to store Li+ ions, which endows this sample with excess rate capacity.

Micro-nano NiO-MnCo2O4 heterostructure with optimal interfacial electronic environment for high performance and enhanced lithium storage kinetics.

This manuscript provides an in situ synthesis method for the self-assembly of a heterostructured NiO-MnCo2O4 micro-nano composite with a poriferous shell. The special shell structure effectively alleviated the volume variation and subsequently enhanced the diffusivity of ions in the cycling process for cyclic stability. The inner spaces among the stacked nanoparticles are conducive to electrolyte infiltration and the transfer of ion/electrons with low concentration polarization. Consequently, the optimized NiO-MnCo2O4 exhibited excellent cycle stability (718.8 mA h g-1 after 1000 cycles at 2 A g-1) and highly recoverable rate performance. On gaining insight into the heterointerface structure, it was indicated that the optimal interfacial electronic environment in the presence of the nickel content plays a key role in creating lattice defects and active sites to increase the ion diffusion rate, electron conductivity and unlock extra pseudocapacitance for ion storage. The excellent capabilities from the optimal heterointerface environment will promote the development of high-energy applications of LIBs.

The effect of precursor speciation on the growth of scorodite in an atmospheric scorodite synthesis.

In this study, we propose a growth pathway of scorodite in an atmospheric scorodite synthesis. Scorodite is a non-direct product, which is derived from the transformation of its precursor. Different precursor speciation leads to different crystallinity and morphology of synthesized scorodite. At 10 and 20 g l-1 initial arsenic concentration, the precursor of scorodite is identified as ferrihydrite. At 10 g l-1 initial arsenic concentration, low arsenic concentration is unfavourable to the complex between arsenate and ferrihydrite, inhibiting the transformation of ferrihydrite into scorodite. The synthesized scorodite is 1-3 µm in size. At 20 g l-1 initial arsenic concentration, higher arsenic concentration favours the complex between arsenate and ferrihydrite. The transformation process is accessible. Large scorodite in the particle size of 5-20 µm with excellent crystallinity is obtained. However, the increasing initial arsenic concentration is not always a positive force for the growth of scorodite. When initial arsenic concentration increases to 30 g l-1, Fe(O,OH)6 octahedron preferentially connects to As(O,OH)4 tetrahedron to form Fe H 2 As O 4 2 + or FeHAs O 4 + ion. Fe-As complex ions accumulate in solution. Once the supersaturation exceeds the critical value, the Fe-As complex ions deprotonate and form poorly crystalline ferric arsenate. Even poorly crystalline ferric arsenate can also transform to crystalline scorodite, its transformation process is much slower than ferrihydrite. Therefore, incomplete developed scorodite with poor crystallinity is obtained.

The synthesis of calcium arsenate@iron arsenate coating materials and their application for arsenic-containing wastewater treatment.

The current method of treating arsenic-containing wastewater is mainly to use a calcium method to synthesize stable calcium arsenate. It is easy to cause reverse dissolution by rain or other erosion, releasing arsenic into the natural world and polluting soil and groundwater. So, calcium arsenate is not an ideal material for removing and immobilizing arsenic. Iron arsenate (FeAsO4) is much better than calcium arsenate because of its stability and acid resistance. In this study, calcium arsenate@iron arsenate coating materials were synthesized. From the results of the XRD and SEM analyses, it was shown that calcium arsenate was coated by an iron arsenate shell which consisted of nanoparticles. The stability of the coating materials was determined using the Toxicity Characteristic Leaching Procedure (TCLP). The results showed that the concentrations of As for CaHAsO4 and Ca3(AsO4)2 were 744 mg L-1 and 302.2 mg L-1, respectively. Arsenic was not detected through the TCLP tests for CaHAsO4@FeAsO4 and Ca3(AsO4)2@FeAsO4 coating materials, and the best coating condition was confirmed to be an Fe/As molar ratio of 4 : 1, pH of 4, and temperature of 50 °C. The stability of the materials showed a significant improvement. The results indicated that calcium arsenate materials could be converted to coating materials by using ferric salts. The coating materials had excellent stability in an aqueous solution. Thus, the coating was suitable for the removal and immobilization of arsenic in industrial applications. This work provided a new way to treat arsenic-containing wastewater, which was simple and economical. This method has potential for use in the field of wastewater treatment containing arsenic.

Correction: The synthesis of calcium arsenate@iron arsenate coating materials and their application for arsenic-containing wastewater treatment.

[This corrects the article DOI: 10.1039/C9RA05278J.].

The Synthesis of the Pomegranate-Shaped α-Fe2O3 Using an In Situ Corrosion Method of Scorodite and Its Gas-Sensitive Property.

The release of hazardous gas increases with the development of industry. The research of gas-sensitive materials has attracted attention. Nanoscale iron oxide (α-Fe2O3) is one of the research hotspots of gas-sensitive materials because it is a cheap, non-toxic semiconductor material. In this study, pomegranate-shaped α-Fe2O3 was synthesized using an in situ corrosion method of scorodite. Spherical-shaped α-Fe2O3 nanoparticles were included in the octahedral shells. The forming process of the structure was analyzed by a variety of measurements. The shell was formed first through the deposition of Fe(OH)3, which was produced by hydrolyzing scorodite. Then, the corrosion was continued and Fe(OH)3 precipitation was produced below the shell. The particles aggregated and formed spheres. The pomegranate-shaped α-Fe2O3 was formed when the scorodite was hydrolyzed completely. The gas-sensing properties of α-Fe2O3 were investigated. The results showed that pomegranate-shaped α-Fe2O3 was responsive to a variety of gases, especially xylene. The value of Ra/Rg was 67.29 at 340 °C when the concentration of xylene was 1000 ppm. This indicated the pomegranate-shaped α-Fe2O3 has potential application as a xylene gas sensor.

Optimized Synthesis of Nitrogen and Phosphorus Dual-Doped Coal-Based Carbon Fiber Supported Pd Catalyst with Enhanced Activities for Formic Acid Electrooxidation.

Development of a Pd-based catalyst with highly active and durable properties for formic acid oxidation reaction at the anode remains an important matter of interest in the research community. Herein, we have designed novel coal-based carbon fibers (Coal-CFs) with dicyandiamide (DCD) as nitrogen (N) source, triphenylphosphine (TPP) as phosphorus (P) source dual-doped to support Pd catalysts (Pd/NP-Coal-CFs(DCD/TPP)), which exhibit superior catalytic performance toward formic acid oxidation reaction. Mass activity of formic acid oxidation of Pd/NP-Coal-CFs(DCD/TPP) catalyst is 536.6 mA·mg-1Pd, which is 2.5 times higher than that of Pd/Coal-CFs catalyst. The higher specific surface areas, exclusive electron transport path, and the high synergistic interaction of N and P are the favorable phenomena for catalytic performance. The addition of coal not only increases the abundant defects sites but also makes the utilization of coal with high added value. This N and P dual-doped catalyst inspires an idea for promoting applications in practical fuel cells.

Self-Assembled Sandwich-like Vanadium Oxide/Graphene Mesoporous Composite as High-Capacity Anode Material for Lithium Ion Batteries.

Sandwich-like V2O5/graphene mesoporous composite has been synthesized by a facile solvothermal approach. The crystalline structure, morphology, and electrochemical performance of the as-prepared materials have been investigated in detail. The results demonstrate that the 30-50 nm V2O5 particles are homogeneously anchored on conducting graphene sheets, which allow the V2O5 nanoparticles to be wired up to a current collector through the underlying conducting graphene layers. As an anode material for lithium ion batteries, the composite exhibits a high reversible capacity of 1006 mAh g(-1) at a current density of 0.5 A g(-1) after 300 cycles. It also exhibits excellent rate performance with a discharge capacity of 500 mAh g(-1) at the current density of 3.0 A g(-1), which is superior to the performance of the vanadium-based materials reported previously. The electrochemical properties demonstrate that the sandwich-like V2O5/graphene mesoporous composite could be a promising candidate material for high-capacity anode in lithium ion batteries.

Synthesis, characterization and fluorescence properties of Eu(III) and Tb(III) complexes with novel mono-substituted beta-diketone ligands and 1,10-phenanthroline.

Two novel pyridine-2,6-dicarboxylic acid derivatives of mono-beta-diketone, methyl 6-benzoylacetyl-2-pyridinecarboxylate (MBAP) and 6-benzoylacetyl-2-pyridinecarboxylic acid (BAPA) and their Eu(III) and Tb(III) complexes were synthesized and characterized by elemental analysis, FT-IR, (1)H NMR and TG-DTG. Moreover, their Eu(III) and Tb(III) complexes using 1,10-phenanthroline as a secondary ligand were prepared and characterized. The luminescence properties of these complexes in solid state were investigated in detail. The results suggested that Tb(III) complexes exhibit more efficient luminescence than Eu(III) complexes, the fluorescence intensity of Ln(III) complexes with BAPA is about twice as strong as that of Ln(III) complexes with MBAP, the fluorescence of mono-beta-diketone complexes using 1,10-phenanthroline as a secondary ligand was prominently higher than that of complexes without adding 1,10-phenanthroline, and the ligand BAPA is an excellent sensitizer to Eu(III) and Tb(III) ion.

Diffusion and kinetics of solvent-free reaction between molecular crystals by time-resolved powder UV-Vis reflection spectrum.

Traditionally, chemical reaction between solids has been considered to typically occur on a geological time scale without the benefit of high temperature, due to diffusion block in the solids. However, recent advancements have revealed that many solvent-free reactions between molecular crystals can quickly occur at room or near-room temperature. These reactions have raised a novel scientific question as to how the reactive species can overcome the diffusion-controlled kinetic limitations under such moderate conditions. From time-resolved powder UV-vis reflection spectra and optical micrographs with the reaction between dimethylglyoxime and Ni(Ac) 2.4H 2O and the reaction between hexamethylenetetramine and CoCl 2.6H 2O as models, we found that the solvent-free reaction really occurs at an intermediate state between the solid state and the liquid state. Formation of the liquid phase provides a convenient approach to diffusion of reactive species, whereas formation of a solid product layer hampered the transfer of reactive species. Both factors led to a broad reactive rate band in the long reaction region. The results have explained the diffusion mechanism of the fast reaction between the molecular crystals under moderate conditions.