Recovery of U(VI) from aqueous media with layered double hydroxides of Zn and Al, intercalated with complexones

The ability of layered double hydroxides (LDHs) with DTPA and EDTA complexones and carbonate ions in the interlayer space to sorb U(VI) was compared. The U(VI) sorption values correlate with the dissociation stability of the U(VI) complexes with these ligands in solution. The synthesized hydrotalcite-like sorbents intercalated with complexones showed high performance in sorption treatment of water to remove U(VI), irrespective of its speciation in natural waters (including di- and tricarbonate complexes). The carbonate form of LDH recovers only cationic uranyl species.     

Sorption of U(VI) from aqueous solutions with carbon sorbents

The sorption characteristics of ordinary and oxidized sorts of synthetic (SKN) and kernel (KAU) carbons and also carbon fabric oxidized with HNO₃ (AUTo) with respect to U(VI) were studied. The influence of solution pH on the sorption capacity of carbon materials with respect to uranium was elucidated. The influence of chlorine and sulfate anions on the sorption rate and sorption capacity was studied. Based on kinetic curves and sorption isotherms of uranyl ions and their derivatives, possible mechanisms of uranium adsorption with carbon sorbents were considered. It was shown that carbon sorbents can be used for treatment of aqueous media, among them drinking water, to remove U(VI) compounds.     

Synthesis and characterization of layered double hydroxides (LDHs) with intercalated chromate ions

Chromate intercalated layered double hydroxides (LDHs) having the formula M^II₆M′^III₂(OH)₁₆CrO₄·4H₂O (M^II = Ca, Mg, Co, Ni, Zn with M′^III = Al and M^II = Mg, Co, Ni with M′^III = Fe) have been prepared by coprecipitation. The products obtained are replete with stacking disorders. DIFFaX simulations show that the stacking disorders are of three kinds: (i) turbostratic disorder of an originally single layered hexagonal (1H) crystal, (ii) random intergrowth of polytypes with hexagonal (2H) and rhombohedral (3R) symmetries and (iii) translation of randomly chosen layers by (2/3, 1/3, z) and (1/3, 2/3, z) leading to stacking faults having a local structure of rhombohedral symmetry. IR spectra show that the CrO₄²⁻ ion is incorporated either in the T_d or in the C_3v symmetry. The interlayer spacing in the latter case is 7.3 Å characteristic of a single atom thick interlayer showing that the CrO₄²⁻ ion is grafted to the metal hydroxide slab. On thermal treatment, the CrO₄²⁻ ion transforms into Cr(III) and is incorporated into the spinel oxide or phase separates as Cr₂O₃. In the LDH of Mg with Al, Cr(III) remains in the MgO lattice as a defect and promotes the reconstruction of the LDH on soaking in water. In different LDHs, 18-50% of the CrO₄²⁻ ion is replaceable with carbonate anions showing only partial mineralization of the water-soluble chromate. The extent of replaceable chromates depends upon the solubility of the corresponding LDH, which in turn is determined by the solubility of the MCrO₄. These studies have profound implications for the possible use of LDHs for chromate amelioration in green chemistry.     

Layered double hydroxides with atomic-scale defects for superior electrocatalysis

Atomic composition tuning and defect engineering are effective strategies toenhance the catalytic performance of multicomponent catalysts by improvingthe synergetic effect; however, it remains challenging to dramatically tune the active sites on multicomponent materials through simultaneous defect engineeringat the atomic scale because of the similarities of the local environment. Herein,using the oxygen evolution reaction (OER) as a probe reaction, we deliberatelyintroduced base-soluble Zn(II) or Al(III) sites into NiFe layered double hydroxides(LDHs), which are one of the best OER catalysts. Then, the Zn(II) or Al(III) siteswere selectively etched to create atomic M(II)/M(III) defects, which dramaticallyenhanced the OER activity. At a current density of 20 mA·cm^?2, only 200 mV overpotential was required to generate M(II) defect-rich NiFe LDHs, which is the best NiFe-based OER catalyst reported to date. Density functional theory(DFT) calculations revealed that the creation of dangling Ni–Fe sites (i.e., unsaturated coordinated Ni–Fe sites) by defect engineering of a Ni–O–Fe site at the atomic scale efficiently lowers the Gibbs free energy of the oxygen evolutionprocess. This defect engineering strategy provides new insights into catalysts atthe atomic scale and should be beneficial for the design of a variety of catalysts.

Open image in new window

     

Sorption of U(VI) from aqueous solutions on layered double hydroxides of Mg,Al, and Nd

Sorption of U(VI) from aqueous solutions of various compositions on layered double hydroxides (LDHs) of Mg, Al, and Nd, and also on layered double oxides (LDOs) of Mg and Al was studied. Experiments on sorption of UO₂²⁺ carbonate complexes from aqueous solution on LDO-Mg-Al showed that [UO₂(CO₃)₃]⁴⁻ ions are weakly captured by LDH-Mg-Al formed by contact of LDO-Mg-Al with water. With an increase in the uranium concentration in solution, K _d of U(VI) decreases. For 3.3 × 10⁻³ M [UO₂(CO₃)₃]⁴⁻ solutions, after 24-h contact of the solid and liquid phases, the distribution coefficients do not exceed 1.0 ml g⁻¹ at V/m = 50 ml g⁻¹. From aqueous nitrate solutions, U(VI) is sorbed on LDH-Mg-Al-NO₃ poorly: K _d of U(VI) does not exceed 1.0 ml g⁻¹ at 24-h contact of the solid and liquid phases and V/m = 50 ml g⁻¹. At the same time, U(VI) is efficiently sorbed from aqueous 10⁻³ M UO₂(NO₃)₂ solutions on LDH-Mg-Al and LDH-Mg-Nd with CO₃²⁻ and OH⁻ ions in the interlayer space. After 15-min contact of the solid and liquid phases, K _d of U(VI) exceeds 5 × 10³ ml g⁻¹. With an increase in the UO₂²⁺ concentration to 10⁻¹ M, K _d of U(VI) decreases considerably, becoming lower than 25 ml g⁻¹, but increases to ∼150 ml g⁻¹ with an increase in the contact time of the solid and liquid phases to 24 h. The effect of the Na⁺, Ca²⁺, Cl⁻, and SO₄²⁻ ions and of pH of the initial solution on the U(VI) sorption from aqueous UO₂(NO₃)₂ solutions on LDH-Mg-Al-CO₃ was examined.     

Desorption of U(VI) from montmorillonite with aluminum and iron hydroxides deposited on its surface

Decontamination of uranium-containing samples based on montmorillonite from the Cherkassy deposit, with Al(III) and Fe(III) hydroxides (weight fraction with respect to metal 0 and 5%: M-0, MAl-5, and MFe-5) deposited on its surface, by desorption with complexing agents (EDTA, citric acid, oxalic acid, sodium carbonate) was studied. With respect to efficiency of U(VI) desorption from the sample surface, the ligands can be ranked in the following order: citric acid > oxalic acid > ETDA ≥ sodium carbonate. In neutral solutions, citric acid is the most effective and environmentally suitable. From M-0 and MAl-5 samples, U(VI) is desorbed completely, whereas in the case of MFe-5 about 15% of U is irreversibly bound to the surface.     

Layered double hydroxide of Zn and Al,intercalated with hexacyanoferrate(II) ions,as a sorbent for removing cesium radionuclides from aqueous solutions

Sorption of cesium radionuclides from aqueous solutions with layered double hydroxide of Zn and Al, intercalated with hexacyanoferrate(II) ions, was studied. The effect of Na⁺, K⁺, and Ca²⁺ on the degree of cesium removal from aqueous solutions was determined, and high selectivity of the sorbent was demonstrated.     

Coconut coir as biosorbent for Cr(VI) removal from laboratory wastewater

A high cost-effective treatment of sulphochromic waste is proposed employing a raw coconut coir as biosorbent for Cr(VI) removal. The ideal pH and sorption kinetic, sorption capacities, and sorption sites were the studied biosorbent parameters. After testing five different isotherm models with standard solutions, Redlich-Peterson and Toth best fitted the experimental data, obtaining a theoretical Cr(VI) sorption capacity (SC) of 6.3 mg g(-1). Acid-base potentiometric titration indicated around of 73% of sorption sites were from phenolic compounds, probably lignin. Differences between sorption sites in the coconut coir before and after Cr adsorption identified from Fourier transform infrared spectra suggested a modification of sorption sites after sulphochromic waste treatment, indicating that the sorption mechanism involves organic matter oxidation and chromium uptake. For sulphocromic waste treatment, the SC was improved to 26.8+/-0.2 mg g(-1), and no adsorbed Cr(VI) was reduced, remaining only Cr(III) in the final solution. The adsorbed material was calcinated to obtain Cr(2)O(3,) with a reduction of more than 60% of the original mass.     

Montmorillonite modified with polyethylenimines as a sorbent for recovering U(VI) from wastewaters

Clay minerals modified with polyethylenimines (Mn-t-PEI) show promise for sorption treatment of wastewaters to remove U(VI) and for U(VI) preconcentration in determination of its low concentrations. Conditions are found for preparing modified sorbents: The optimal pH range for PEI sorption to prepare the modified sorbents is ≥9.0, which suggests electrostatic interaction of PEI with montmorillonite. The maximal sorption of the linear and branched polymers on montmorillonite, calculated from the sorption isotherms, is 0.32–0.38 g PEI/g mineral. The U(VI) sorption value on natural montmorillonite is considerably lower than on Mn-t-PEI, where the isotherm at low U(VI) concentrations is practically linear and the degree of U(VI) sorption exceeds 97% (depending on the uranium content).     

Sorption of U(VI) onto layered double hydroxides and oxides of Mg and Al,prepared using microwave radiation

Layered double hydroxides of Mg and Al, containing CО₃^2– ions in the interlayer space (LDH-Mg-Al-CО₃), and layered double oxides of Mg and Al (LDO-Mg-Al) were prepared using microwave radiation (MWR). The use of MWR allows not only acceleration of the synthesis of both LDH and LDO, but also preparation of compounds with high kinetic characteristics of the U(VI) sorption. The degree of U(VI) sorption (α) from 10^–2 M aqueous U(VI) solutions at a sorption time of 4 h and V/m = 50 mL g^–1 exceeds 99.0%. In sorption from more concentrated (10^–1 M) aqueous U(VI) solutions under similar conditions, α on all the samples does not exceed 37.5%.     

Mechanochemically assisted synthesis of pristine Ca(II)Sn(IV)-layered double hydroxides and their amino acid intercalated nanocomposites

Syntheses of Ca(II)Sn(IV)-layered double hydroxides (LDHs) are attempted by the traditional co-precipitation as well as mechanochemical methods. Both the co-precipitation method and the one-step milling operation proved to be unsuccessful; these methods only produced physical mixtures of hydroxides and carbonates of the two metal ions. However, a two-step milling operation (dry milling followed by milling in the presence of minute amount of water) led to successful synthesis, verified by a range of characterisation methods. Surprisingly, it was found that ball-milling was not even necessary; the reaction proceeded on manual grinding of the components in an agate mortar with a pestle. The preparation of nanocomposites through intercalation of the anions of cystine or valine into Ca(II)Sn(IV)-LDH could also be achieved by the two-step milling method verified again by a range of instrumental methods.     

Magnetic Sorbents for Removing U(VI) from Aqueous Media

Sorption of U(VI) onto magnetic sorbents from aqueous media was studied. The limiting value of the U(VI) adsorption onto magnetite and magnetic potassium zinc hexacyanoferrate(II) at pH 5.0, calculated from the Langmuir equation, is 125 and 102 μmol g^–1, respectively. Natural components of waters, namely, Na⁺ and Ca²⁺ ions and fulvic acids at concentrations of up to 200 mg dm^–3, do not noticeably influence the U(VI) sorption onto magnetic sorbents. Owing to high selectivity (degree of recovery 87–91%) and possibility of separating the solid phase by magnetic separation, these sorbents show promise for removing U(VI) from mineralized mine water.     

Removal of chloride ion from aqueous solution by ZnAl-NO(3) layered double hydroxides as anion-exchanger

The layered double hydroxides (LDHs) containing nitrate as the interlayer anion has a high anion-exchange capacity in the presence of appropriate anions. In the light of this, ZnAl-NO(3) LDHs have been employed to remove chloride ion from aqueous solution in a batch mode. The influences of conditions for chloride ion uptake, including dosage of LDHs, pH of aqueous solution, and temperature on anion-exchange have been investigated, respectively. The thermodynamic parameters including Gibbs free energy (DeltaG(0)), standard enthalpy change (DeltaH(0)), and standard entropy change (DeltaS(0)) for the process were calculated using the Langmuir constants. It was found from kinetics test that the pseudo-second order kinetics model could be used to well describe the uptake process. An E(a) value of 10.27 kJ/mol provides evidence the anion-exchange process. The explanation of anion-exchange phenomenon has also been supported by X-ray diffraction and FT-IR spectra.     

Surface-modified Phanerochaete chrysosporium as a biosorbent for Cr(VI)-contaminated wastewater

To improve the removal efficiency of heavy metals from wastewater, the surface of a fungal biomass was modified to obtain a high-capacity biosorbent for Cr(VI) in wastewater. The effects of pH, initial concentration, and sorption time on Cr(VI) removal by polyethylenimine (PEI)-modified Phanerochaete chrysosporium were investigated. The biomass adsorption capacity was significantly dependent on the pH of the solution, and the optimum pH was approximately 3.0. The maximum removal for Cr(VI) was 344.8 mg/g as determined with the Langmuir adsorption isotherm. Pseudo-first-order Lagergren model is better than pseudo-second-order Lagergren model when simulating the kinetic experiment results. Furthermore, an amount of Cr(VI) was reduced to Cr(III), indicating that some reactions occurred on the surface of the biomass leading to the reduction of Cr(VI). The point of zero potential for the modified biomass increased from an initial pH of 3.0 to a much higher value of 10.8, indicating that the PEI-modified biomass is better than the pristine biomass for adsorption of anionic adsorbates. Results showed that the PEI-modified biosorbent presented high efficiency in treating Cr(VI)-contaminated wastewater.     

Extraction of U(VI) from Perchlorate Solutions with Diantipyrylmethane Derivatives

Extraction of U(VI) from perchlorate solutions with diantipyrylmethane derivatives in chloroform and dichloroethane is studied. The composition of extractable complexes is [UO₂L₂](ClO₄)₂. The extraction constants are estimated. The maximal extractive power is shown by diantipyrylmethane in dichloroethane. Introduction of substituents at the methylene carbon of the extractant molecule makes the extraction worse.     

Effect of aluminum and iron hydroxides deposited on the montmorillonite surface on the U(VI) sorption

Sorption of U(VI) on montmorillonite (M) of the Cherkassy deposit, with Al(III) and Fe(III) deposited on its surface in various amounts (1 and 5 wt %), was studied. The presence of such hydroxides increases the U sorption in the range pH 4.0–8.0. The U(VI) sorption increases in the order M < [M + 1% Al(III)] < [M + 5% Al(III)] < [M + 1% Fe(III)] < [M + 5% Fe(III)]. The U sorption isotherms on all the sorbents were obtained. The effect exerted on the sorption by artificial (ethylenediaminetetraacetic acid) and natural (fulvic acids) complexing agents was examined. Original Russian Text ? G.N. Pshinko, A.A. Bogolepov, S.A. Kobets, A.A. Kosorukov, 2009, published in Radiokhimiya, 2009, Vol. 51, No. 2, pp. 187–190.     

Sorption of U(VI) from sulfate solutions with spherically granulated chitosans

Sorption of U(VI) from sulfate solutions with spherically granulated chitosans was examined. It was shown that the best sorption and kinetic characteristics are exhibited by freshly formed and cross-linked chitosan granules. The total static exchange capacity for the freshly formed granules was 0.7 mmol of U(VI) per gram of dry sorbent. The diffusion coefficients and similarity criteria describing the diffusion process parameters were calculated. It was found that, irrespective of the method by which the granules were prepared (freshly formed, cross-linked, air-dried), the dominating process is U(VI) sorption on the surface, mainly controlled by external diffusion. Physicomechanical tests of the films of the irradiated chitosan and cellophane (absorbed dose 100–250 kGy) revealed higher strength characteristics of the irradiated chitosan.     

Effects of EDTA and NTA on sorption of U(VI) on the clay fraction of soil

Sorption behavior of U(VI) is studied in model systems containing montmorillonite with various concentrations of humic acids on its surface and complexants (disodium salts of ethylenediaminetetraacetic and nitrilotriacetic acids). Humic acids deposited onto the surface of montmorillonite enhance the U(VI) sorption at pH <7.0. The effect of complexants on the U(VI) sorption is rather intricate because of the possibility of formation of some surface uranium complexes, whose nature depends on the nature and concentration of the complexant in solution, pH, and humic acid concentration on the surface of montmorillonite.     

The removal and recovery of Cr(VI) by Li/Al layered double hydroxide (LDH)

Hexavalent Cr has been identified as one of the toxic metals commonly present in industrial effluents. Among the treatment techniques developed for removing Cr(VI) from waste waters, sorption is most commonly applied, due to its simplicity and efficiency. However, few adsorbents can be recycled and reused cost-effectively. In this study, the removal and recovery of Cr(VI) from water using Li/Al LDH was investigated. The removal of Cr(VI) by Li/Al LDH was evaluated in a batch mode. The results demonstrated that Cr(VI) adsorption onto Li/Al LDH occurs by replacing the Cl(-) that originally exists in the interlayer of the adsorbent. The degree of Cr(VI) adsorption observed for Li/Al LDH was relatively high and the process occurred rapidly; however, a portion of adsorbed Cr(VI) was gradually desorbed, due to the Li de-intercalation of Li/Al LDH. Lithium de-intercalation from Li/Al LDH with interlayer Cl(-) and interlayer Cr(VI) follows the first order kinetics and has the activation energies of 76.6 and 41.5 kJ mol(-1), respectively. The properties of thermal unstability and the high adsorption capacity of Li/Al LDH may lead to the development of an innovative technique for the removal of Cr(VI) from Cr(VI)-containing wastewater. That is, Li/Al LDH may be used as an effective adsorbent for the adsorption of Cr(VI) in an ambient environment. Following the adsorptive process, the adsorbed Cr(VI) may be released, using heated water to treat the Cr(VI)-containing Li/Al LDH particles. Through this hydrothermal treatment of the used adsorbent, Cr(VI) can be recovered and the solid product (gibbsite) can be recycled for further use.