Tungstate removal from aqueous solution by nanocrystalline iowaite: An iron-bearing layered double hydroxide

Synthesis of easily renewable and recoverable magnetic PEI-modified Fe3O4 nanoparticles and its application for adsorption and enrichment of tungsten from aqueous solutions

Tungsten is a hazardous metal to human health and the environment, but it is also valuable. Previous studies have been limited to the adsorption and removal of tungsten, without considering its recovery and utilization. In this article, a renewable magnetic material, Fe₃O₄ nanoparticles coated by polyethyleneimine (Fe₃O₄@PEI NPs), is synthesized and used for the adsorption of tungsten in water. Tungsten adsorption experiments were conducted under different initial tungsten concentrations, contact times, solution pH values, and co-existing anions. The results show that Fe₃O₄@PEI NPs efficiently and rapidly adsorb tungsten from water, with a maximum adsorption capacity of 43.24 mg/g. Under acidic conditions (pH ∼2), the adsorption performance of the NPs maximized. This is because tungstate ions polymerize under such conditions to form polytungstic anions. These are attracted to the positively charged surface of Fe₃O₄@PEI NPs by electrostatic attraction, followed by complexation reactions with the surface hydroxyl and amino groups of NPs, as evidenced by multiple spectroscopic methods. The NPs can be recovered and renewed and provide a potential application for the enrichment and recycling of high-value tungsten (W(VI)).

Cr(VI) removal from contaminated waters using ultra-thin layered meixnerite

Cr(VI) is of great concern to public health and environmental safety due to its high toxicity. Here, we report a low-cost yet highly efficient method to prepare a novel LDH, ultra-thin layered meixnerite, which performed superiorly in treatment of aqueous Cr(VI) with little secondary pollution being induced. The produced ultra-thin layered meixnerite was composed of nanoparticles with a thickness of around 7 nm, less than 9 times the thickness of a single LDH layer. The XRD patterns of the ultra-thin layered meixnerite, in which the characteristic diffraction peaks of a typical LDH were weakened or even disappeared, confirmed the successful delamination. This special morphology of the ultra-thin layered meixnerite was not only helpful to its full dispersion in the Cr(VI)-bearing solutions but also facilitated the formation of more active sorption sites on its external surface. As a result, the maximum sorption capacity of UTLM for Cr(VI) removal was 480.9 mg g^-1, far higher than that of OM (196.9 mg g^-1). In addition to electrostatic attraction and anion exchange, the ultra-thin layered meixnerite could also become restacked during removal of aqueous Cr(VI) to generate inner-sphere complexation, finally inducing an enhanced Cr(VI) uptake. Furthermore, XPS analysis characterized the promotion of the break of Al-OH bond with the increase in temperature, and the Cr-O peak increased correspondingly from 29.69% at 25 °C to 48.77% at 85 °C, resulting that the ultra-thin layered meixnerite could remove Cr(VI) more effectively at higher reaction temperatures. Therefore, ultra-thin layered meixnerite is very suitable for future application in treatment of industrial wastewaters with elevated temperatures.

Efficient and Fast Removal of Aqueous Tungstate by an Iron-Based LDH Delaminated in L-Asparagine

High concentrations of tungstate in aqueous systems pose a severe threat to the environment and human health. This study explored the potential of iron-based LDHs to remove tungstate from water. To improve its tungstate uptake capacity, environment-friendly L-asparagine was used to delaminate iron-based LDH synthesized via a coprecipitation method. The successful delamination was proved by AFM, revealing that the thickness of the obtained nanoparticles was approximately 1–2 times that of a single LDH layer. XRD, TEM, and XPS analyses confirmed that the delaminated LDHs were amorphous and ultrathin and had surface defects within their nanosheets that acted as active sites, leading to a very fast tungstate sorption rate and superior tungstate uptake capacity. Notably, the original layered structure of the L-asparagine-treated LDH was recovered upon its reaction with tungstate-bearing solutions, and therefore, the high availability of aqueous tungstate to the interlayer regions during the structural restoration of the delaminated iron-based LDH contributed to its excellent capability of tungstate removal as well. In addition, the tungstate uptake by the delaminated iron-based LDH was not affected substantially by the presence of coexisting anions, implying that the strong inner-sphere complexation between the tungstate and LDH layers with defects (i.e., Fe-O bonds) was the primary mechanism responsible for the tungstate removal. The delamination process described in this paper was validated to be an effective way to enhance the immobilization of tungstate by iron-based LDHs without inducing secondary pollutions, and delaminated iron-based LDHs are promising to be used extensively in the practice of treating tungstate-rich waters.

Tungsten distribution and vertical migration in soils near a typical abandoned tungsten smelter

As an emerging contaminant, tungsten's distribution and speciation in soils are far from understood. In this study, two soil profiles near a typical abandoned tungsten smelter in Hunan Province, China were collected and investigated, to ascertain the binding and association of tungsten with different soil components and subsequently to understand its mobility. The data showed that past tungsten smelting activities resulted in elevated concentrations of both tungsten and arsenic in the soil profiles, both of which ranged from dozens of to a few hundred mg/kg. Nano-scale secondary ion mass spectrometry (NanoSIMS) was employed to quantify the distribution and association of tungsten with various other elements. Combined with sequential extraction and mineralogical analysis, the data from NanoSIMS showed that aluminosilicates including kaolinite and illite were the most important mineral hosts for tungsten, whereas arsenic was predominantly bound to iron (oxyhydr)oxides. Additional data from ¹³C nuclear magnetic resonance and X-ray photoelectron spectroscopy revealed that soil organic matter retained tungsten in deep soils (>70 cm) by binding tungsten through carboxyls on aromatic rings. Compared to arsenic, tungsten migrated deeper in the soil profiles, suggesting its higher mobility and potential risk to groundwater quality.

Natural dissolved organic matter (DOM) affects W(VI) adsorption onto Al (hydr)oxide: Mechanisms and influencing factors

Tungsten (W) is a contaminant with health implications whose environmental behaviors are not understood well. Sorption to mineral surfaces is one of the primary processes controlling the mobility and fate of W in soils, sediments, and aquifers. However, few papers published hitherto have not yet figured out the influences of dissolved organic matter (DOM) on this process. Here, we examine W(VI) adsorption behaviors onto Al (hydr)oxide (AAH) in the presence or absence of DOM derived from plant rhizosphere, using batch experiments coupled with X-ray photoelectron spectroscopy (XPS) and Fourier transform infrared spectroscopy (FTIR). The morphology and functional group analyses results show that DOM can facilitate the aggregation of AAH and block surface Al-OH groups. Coexisting DOM inhibits W(VI) adsorption onto AAH at acidic to neutral pH (4-7), and the presence of either Na ⁺ or PO₄^3- can exert a completely different impact on W(VI) adsorption. XPS and FTIR characterizations further demonstrate surface W complexes with the Al-OH groups of AAH and carboxyl groups of DOM. There is no reduction of W(VI) during the adsorption processes, and poly-tungstate species are formed on the surface of both AAH and AAH-DOM coprecipitates. This study provides the first evidence of the roles of natural DOM on W sequestration at the mineral-water surface, which has an important implication for the prediction of the migration and bioavailability of W in natural environments.

Highly efficient sorption of molybdenum from tungstate solution with modified D301 resin

The separation of molybdenum (Mo) from tungstate solution is a bottleneck problem in tungsten (W) metallurgy, and it hinders the development of high-purity tungsten materials. In this research, a modified D301 resin was used to adsorb and separate molybdenum from tungstate solution. The maximum sorption capacity (Q e) of modified D301 for MoS4 2- was found to be 428 mg g-1 and the separation coefficient (β) was 108.9 when the contact time was 4 h and the reaction temperature was 25 °C and the pH value of the tungstate solution was 7.2. The sorption process conforms to Langmuir isotherm models and the quasi-second-order kinetic model. The sorption mechanism was also discussed, which was a single layered spontaneous sorption process. Theoretical calculations infer bonding behavior between the N atom on the resin and the S atom on the MoS4 2- molecule. The sorption energy is -7.67 eV, which indicated that the sorption process is stable chemical sorption. The desorption experiment showed that more than 90% molybdenum could be desorbed from the loaded resin when the concentration of sodium hydroxide solution was 5 w%. Finally, after three-stage sorption-desorption, almost all molybdenum in the solution was adsorbed, achieving better separation of tungsten and molybdenum.

Simultaneous removal of harmful anions from geothermal waters using OH- intercalated Mg-Fe-LDH: batch and field column studies

Unlike collectively treatable industrial wastewaters where only one or a few pollutants have concentrations much higher than the relevant standards, geothermal waters, in which multiple harmful constituents coexist, are usually discharged dispersedly, provoking a big challenge for their effective treatment. Here, a Mg/Fe layered double hydroxide with OH^- intercalated (Mg-Fe-OH-LDH) was synthesized in a mechanochemical way and then applied in the treatment of various types of high-temperature geothermal waters in western Yunnan (China) containing a variety of harmful anions (As, Sb, W, and F) and inducing local environmental pollution. Due to the endothermic nature of removal of aqueous As, Sb, W, and F by Mg-Fe-OH-LDH, the original high temperatures of the geothermal waters could promote their sorption effectively. Batch sorption experiments demonstrated that over 94% and 80% of the As and W removal amounts could be reached within the first 10 and 20 min, respectively. On-site column experiments confirmed that the sorbent could remove the targeted harmful constituents from the investigated geothermal waters efficiently. In fact, the performance of the sorbent in the column studies was even better than that in the batch experiments, which can be ascribed to the continuous impetus for sorption caused by the concentration gradient in the flowing sorption system. Specifically, Mg-Fe-OH-LDH displayed the best sorption performance for As(V) among various harmful constituents, and the sorption of As along with W and F was little affected by the coexisting common anions in the geothermal waters, including Cl^-, SO₄^2-, and HCO₃^-/CO₃^2-. In contrast, the removal of Sb(V) from geothermal waters may be impeded to a certain extent by SO₄^2- and CO₃^2-, which possessed stronger electronegativity or smaller ionic radii. This study is the first attempt to apply Mg-Fe-LDH in treatment of geothermal waters with multiple harmful constituents and sheds a light on providing a practical approach for field treatment of geothermal water-derived pollution.

Biosorption of tungstate onto garlic peel loaded with Fe(III), Ce(III), and Ti(IV)

In present study, garlic peel (GP) was modified by loading with Fe(III), Ti(IV), and Ce(III) through a cation exchange process, i.e., nGP-COOH + Mn+ = (nGP-COO)-Mn+ + nH+ (M = Fe, Ce, Ti), which could adsorb tungstate effectively under the weakly acidic conditions. The optimal initial pH for maximum adsorption of W(VI) was determined at 1~3 for Ti-GP, 1~4 for Fe-GP, and 3 for Ce-GP, respectively; and at pH 2.5, the corresponding maximum adsorption capacity for Fe-GP, Ti-GP, and Ce-GP was evaluated as 91.5 mg/g, 83 mg/g, and 84 mg/g tungsten respectively. Coexisting anions like chloride, sulfate, and carbonate showed little effect on tungsten adsorption, while fluoride and phosphate inhibited the adsorption drastically. The column adsorption showed that the breakthrough point for Ce-GP, Ti-GP, and Fe-GP was 180 min, 200 min, and 270 min respectively. And 0.1 mol/L NaOH effectively eluted the adsorbed tungsten, and concentration of the eluted solution had almost 6, 19.9, and 22 factors of the initial tungstate concentration correspondingly.