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The combination of metal − organic frameworks (MOFs) and magnetic nanoparticles has been demonstrated their prospective for sequestration of heavy metal or radionuclides. In this work, five kinds of magnetic metal − organic framework including ZIF-67@Fe3O4, ZIF-8@Fe3O4, CuBTC@Fe3O4, MIL101(Fe)@Fe3O4, and MOF74@Fe3O4, was prepared and used as novel highly-efficient scavengers for the capture of U(VI) from aqueous environment. These magnetic nanocomposites before and/or after U(VI) capture were characterized by FTIR, SEM–EDS, XRD, TEM, BET, and XPS analyses. A batch technique was applied for the capture of U(VI) using these magnetic nanocomposites at different operating parameters. The isotherm and kinetic data were more accurately fitted by the Langmuir and pseudo-second-order models, respectively. The ZIF-8@Fe3O4 and ZIF-67@Fe3O4 exhibit the potential to capture U(VI) from aqueous solutions with a maximum monolayer adsorption capacity of 441 and 1058 mg/g–1, respectively, by fitting the equilibrium data to the Langmuir model. The excellent adsorption capacity and the response to the magnetic field made these novel nanocomposites auspicious candidates in nuclear waste management. [ABSTRACT FROM AUTHOR]

The separation and recycling of rare earth elements (REEs) are very important owing to the high demand, limited resource, specific usages, and environmental issues. In this work, two-dimensional Ti3C2Tx MXene was introduced to remove REEs (Nd(III) and La(III)) from water, and its physicochemical properties were conducted by HRTEM, SEM–EDS, XRD, FTIR, and XPS. Various parameters, such as initial pH, REEs initial concentration, contact time, and temperature, were investigated by batch experiment, respectively. Furthermore, the adsorption kinetic and isotherm were examined to analyze the adsorption behavior and adsorption mechanism. Nd(III) and La(III) have a good affinity with Ti3C2Tx MXene surface functional groups (-F, -OH, and containing oxygen groups). The maximum adsorption capacities of Ti3C2Tx MXene for Nd(III) and La(III) were 229.85 mg/g and 175.83 mg/g at T = 333 K, respectively. The adsorption data of Nd(III) on Ti3C2Tx MXene fitted well with the Freundlich isotherms model and pseudo-second-order kinetic model. However, the best fitting for La(III) adsorption on Ti3C2Tx MXene was described by both pseudo-first-order and pseudo-second-order model. Thermodynamic study of Nd(III) and La(III) adsorption on Ti3C2Tx MXene showed that the reaction was a spontaneous and endothermic process. These results indicated Ti3C2Tx MXene had a great potential in extracting REEs from an aqueous solution. [ABSTRACT FROM AUTHOR]

The increasing usage of an important heavy metal chromium for industrial purposes, such as metallurgy, electroplating, leather tanning, and other fields, has contributed to an augmented level of hexavalent chromium (Cr(VI)) in watercourses negatively impacting the ecosystems and significantly making Cr(VI) pollution a serious environmental issue. In this regard, iron nanoparticles exhibited great reactivity in remediation of Cr(VI)-polluted waters and soils, but, the persistence and dispersion of the raw iron should be improved. Herein, this article utilized an environment-friendly celite as a modifying reagent and described the preparation of a novel composites namaly celite decorated iron nanoparticles (C-Fe0) and evaluation of C-Fe0 for the sequestration performance of Cr(VI) from aqueous solution. The results indicated that initial Cr(VI) concentration, adsorbent dosage, and especially solution pH are all critical factors to control C-Fe0 performance in Cr(VI) sequestration. We demonstrated that C-Fe0 could achieve a high Cr(VI) sequestration efficiency with an optimized adsorbent dosage. Fitness of the pseudo-second-order kinetics model with data indicated that adsorption was the rate-controlling step and chemical interaction controlled Cr(VI) sequestration on C-Fe0. The adsorption isotherm of Cr(VI) could be the best depicted by Langmuir model with a monolayer adsorption. The underlying sequestration path of Cr(VI) by C-Fe0 was then put forward, and the combined effect of adsorption and reduction implied the potentials of C-Fe0 in Cr(VI) removal. [ABSTRACT FROM AUTHOR]

This paper revealed the enhancement role of graphene oxide (GO) in reductive sequestration of UO22+, ReO4−, SeO42− and SeO32− on zero-valent iron (ZVI) using batch, column and X-ray spectroscopic experiments. First, we decorated nanoscale zero-valent iron (NZVI) on GO to obtain NZVI-GO, and the kinetics for adsorption of these radionuclides on NZVI-GO could be well described by the pseudo-second-order model, indicating a chemical interaction process. The isotherms for UO22+ adsorption on NZVI-GO could be fitted better by Freundlich model than that by Langmuir model. Besides, the presence of co-existing dissolved humic acid could significantly promoted the adsorption. In column experiments, the combination of GO to pack with ZVI and sand as reactive medium for removal of these radionuclides prolong the reaction longevity of the column packed with ZVI and sand as reactive medium. It might be the first report from XPS that the GO decoration facilitated NZVI reduced Re(VII), Se(VI) and Se(IV) into Re0 and Se(-II) with lower oxidation state, which could be not observed in bare NZVI system. So, due to the excellent electron mediation ability and adsorption capacity, GO demonstrated an important role in enhancing reductive sequestration of redox-sensitive radionuclides in a wide range of natural environments. [ABSTRACT FROM AUTHOR]

An ammonium/hydroxyl-enriched p-BN microrods were synthesized and the adsorption behavior of uranyl on p-BN was systematically studied. The adsorption kinetic and isotherm studies revealed the adsorption behavior is independent on electrostatic attraction or chemisorption. The results demonstrate uranyl hydrolyzed precipitated crystals are formed on the surface of p-BN at an optimized pH of 2.86 and the maximum adsorption capacity can reach 413 mg g−1 at T = 303 K. A hypothetical adsorption mechanism with a combination of surface adsorption and induced precipitation crystallization was proposed. [ABSTRACT FROM AUTHOR]

Herein, we used biochar pyrolyzed from rice straw to adsorb uranium (U) from aqueous solutions. The adsorption of U(VI) on biochar was strongly dependent on pH but independent on ionic strength. HA/FA enhanced the sorption at pH <6.8 while inhibited the sorption at pH >6.8. The sorption reached equilibrium within 3 h, which was not mediated by pH. The adsorption process was spontaneous and endothermic, and enhanced at higher temperature. However, the influence of temperature was negligible at low initial U(VI) concentrations. Therefore, biochar derived from rice straw may be a promising adsorbent for the removal of U(VI). [ABSTRACT FROM AUTHOR]

The X-ray absorption fine structure (XAFS) technology has exhibited a very unique application in the study of sorption mechanism, chemical species and microstructures of radionuclides at the natural solid-water interfaces. In this review, the interaction mechanism of radionuclides with clay minerals and nanomaterials under different environmental conditions are summarized from the XAFS spectroscopy analysis. The coordination number and the bond distances of radionuclides, the oxidation-reduction reactions, the influence of humic substances and microorganisms on the species and structures of radionuclides at molecule level are reviewed and compared. This review is helpful to understand the interactions of radionuclides with oxides, natural clay minerals and nanomaterials, which is also crucial to evaluate the physicochemical behaviors of radionuclides in the natural environment. [ABSTRACT FROM AUTHOR]

In this paper, the macroscopic interaction method and high resolution EXAFS technique with a bent crystal analyzer were combined to study Eu(III) interaction mechanism and microstructure with γ-MnOOH as a function of pH. The results indicated that Eu(III) interaction with γ-MnOOH was apparently dependent on pH but independent of ionic strength, suggesting the formation of inner-sphere surface complexation for Eu(III) onto γ-MnOOH. Results of EXAFS analysis indicated that Eu was surrounded by ∼9.0 O atoms in first coordination shell at R ≈ 2.40 Å, and second shell of Mn atoms at R ≈ 3.60 Å was observed for the three adsorption samples. These findings suggested formation of a bidentate surface complex with Eu(III) bonding by edge sharing to MnO-octahedron on γ-MnOOH surface. Both the macroscopic interaction data and the molecular level evidence of Eu(III) microstructure at the γ-MnOOH-water interface should be factored into better understanding the fate and mobility of Eu(III) and related radionuclides in the natural soil and water environment. [ABSTRACT FROM AUTHOR]

Titanate nanotubes (TNTs) have attracted great interest in multidisciplinary study since their discovery. The adsorption of thorium [Th(IV)] onto TNTs in the absence and presence of humic acid (HA)/fulvic acid (FA) was studied by batch technique. The influence of pH from 2.0 to 10.0, ionic strength from 0.001 to 0.1 mol L NaClO, and coexisting electrolyte cations (Li, Na, K) and antions (ClO, NO, Cl) on the adsorption of Th(IV) onto TNTs was tested. The adsorption isotherms of Th(IV) was determined at pH 3.0 and analyzed with Langmuir and Freundlich adsorption models, respectively. The results demonstrated that the adsorption of Th(IV) onto TNTs increases steeply with increasing pH from 2.0 to 4.0. Generally, HA/FA was showed to enhance Th(IV) adsorption onto TNTs at low pH values, but to reduce Th(IV) adsorption onto TNTs at high pH values. The adsorption of Th(IV) onto TNTs was also dependent on coexisting electrolyte ions in aqueous solution under our experimental conditions. The adsorption of Th(IV) onto TNTs is exothermic and spontaneous. The findings indicating that TNTs can be used as a promising candidate for the enrichment and solidification of Th(IV) or its analogue actinides from large volume solution in real work. [ABSTRACT FROM AUTHOR]

This work determined the influence of humic acid (HA) and fulvic acid (FA) on the interaction mechanism and microstructure of Ni(II) onto diatomite by using batch experiments, X-ray photoelectron spectroscopy (XPS), and extended X-ray absorption fine structure (EXAFS) methods. Macroscopic and spectroscopic experiments have been combined to see the evolution of the interaction mechanism and microstructure of Ni(II) in the presence of HA/FA as compared with that in the absence of HA/FA. The results indicated that the interaction of Ni(II) with diatomite presents the expected solution pH edge at 7.0, which is modified by addition of HA/FA. In the presence of HA/FA, the interaction of Ni(II) with diatomite increased below solution pH 7.0, while Ni(II) interaction decreased above solution pH 7.0. XPS analysis suggested that the enrichment of Ni(II) onto diatomite may be due to the formation of (≡SO)Ni. EXAFS results showed that binary surface complexes and ternary surface complexes of Ni(II) can be simultaneously formed in the presence of HA/FA, whereas only binary surface complexes of Ni(II) are formed in the absence of HA/FA, which contribute to the enhanced Ni(II) uptake at low pH values. The results observed in this work are important for the evaluation of Ni(II) and related radionuclide physicochemical behavior in the natural soil and water environment. [ABSTRACT FROM AUTHOR]

This work examines the sequestration of Cu(II) by sorption process onto plasma-induced polyaniline (PANI)-grafted multiwalled carbon nanotubes (denoted as MWCNTs/PANI) prepared by an plasma-induced grafting technique. The role of a variety of environmental conditions such as pH, ionic strength, natural organic matter (NOM) in the sorption of Cu(II) onto MWCNTs/PANI is studied. The results indicate that the sorption is strongly dependent on pH but independent of ionic strength. A positive effect of NOM on Cu(II) sorption is found at pH <7.5, whereas a negative effect is observed at pH >7.5. The sorption isotherms in the absence and presence of NOM can be better described by Freundlich model than Langmuir model. Sorption isotherms of Cu(II) at higher initial NOM concentrations are higher than those at lower NOM concentrations. The thermodynamic data calculated from temperature-dependent sorption suggest that the sorption is spontaneous and enhanced at higher temperature. Results of this work suggest that MWCNTs/PANI may be a promising candidate for cost-effective treatments of Cu(II)-contaminated wastewaters. [ABSTRACT FROM AUTHOR]

The sorption behavior and microscopic sequestration mechanisms of radionuclide Ni(II) on mordenite as a function of aging time, ionic strength, initial Ni(II) concentrations, solid content and coexistent electrolyte ions were investigated by the combination of batch and EXAFS techniques. Macroscopic experiment results show that the sorption of Ni(II) is dependent on ionic strength at pH<7, and independent of ionic strength at pH>7. The sorption percentage of Ni(II) on mordenite increases with increasing solid content, while the sorption capacity decreases as solid content increases. The presence of different electrolyte ions can enhance or inhibit the sorption of Ni(II) on mordenite in various degrees. EXAFS analysis results of the samples under three different ionic strengths suggest that the retained Ni(II) in these samples exists in an octahedral environment with six water ligands. In the initial period of rapid uptake, the sorption of Ni(II) is dominated by the formation of inner-sphere surface complexes. As aging time increases, Ni(II) sequestration behavior tends to be mainly controlled by the formation of Ni phyllosilicate co-precipitates and/or Ni(OH)(s) precipitates. Results for the second shell fit of the sample prepared at an initial Ni(II) concentration of 100 mg/L indicate the possible formation of Ni polynuclear surface complexes. Both the macroscopic sorption data and the molecular level evidence of Ni(II) surface speciation at the mordenite/water interfaces should be factored into better predictions of the mobility and bioavailability of Ni(II) in environment mediums. [ABSTRACT FROM AUTHOR]

The effects of pH, contact time and natural organic ligands on radionuclide Eu(III) adsorption and mechanism on titanate nanotubes (TNTs) are studied by a combination of batch and extended X-ray absorption fine structure (EXAFS) techniques. Macroscopic measurements show that the adsorption is ionic strength dependent at pH < 6.0, but ionic strength independent at pH > 6.0. The presence of humic acid (HA) / fulvic acid (FA) increases Eu(III) adsorption on TNTs at low pH, but reduces Eu(III) adsorption at high pH. The results of EXAFS analysis indicate that Eu(III) adsorption on TNTs is dominated by outer-sphere surface complexation at pH < 6.0, whereas by inner-sphere surface complexation at pH > 6.0. At pH < 6.0, Eu(III) consists of ∼ 9 O atoms at R ≈ 2.40 Å in the first coordination sphere, and a decrease in N with increasing pH indicates the introduction of more asymmetry in the first sphere of adsorbed Eu(III). At long contact time or high pH values, the Eu(III) consists of ∼2 Eu at R ≈ 3.60 Å and ∼ 1 Ti at R ≈ 4.40 Å, indicating the formation of inner-sphere surface complexation, surface precipitation or surface polymers. Surface adsorbed HA/FA on TNTs modifies the species of adsorbed Eu(III) as well as the local atomic structures of adsorbed Eu(III) on HA/FA-TNT hybrids. Adsorbed Eu(III) on HA/FA-TNT hybrids forms both ligand-bridging ternary surface complexes (Eu-HA/FA-TNTs) as well as surface complexes in which Eu(III) remains directly bound to TNT surface hydroxyl groups (i.e., binary Eu-TNTs or Eu-bridging ternary surface complexes (HA/FA-Eu-TNTs)). The findings in this work are important to describe Eu(III) interaction with nanomaterials at molecular level and will help to improve the understanding of Eu(III) physicochemical behavior in the natural environment. [ABSTRACT FROM AUTHOR]

In this work, a series of batch experiments were carried out to investigate the effect of various environmental factors such as contact time, pH, ionic strength, coexisting electrolyte ions, humic substances and temperature on the sorption behavior of illite towards Cu(II). The results indicated that Cu(II) sorption on illite achieved equilibrium quickly. The pH- and ionic strength-dependent sorption suggested that Cu(II) sorption on illite was dominated by ion exchange or outer-sphere surface complexation at pH < 7, whereas the pH-dependent and ionic strength-independent sorption indicated that the sorption process was mainly attributed to inner-sphere surface complexation at pH > 7. A positive effect of humic substances on Cu(II) sorption was found at pH < 6.5, whereas a negative effect was observed at pH > 6.5. The Langmuir and Freundlich models were used to simulate the sorption isotherms of Cu(II) at three different temperatures of 293, 313, and 333 K. The thermodynamic parameters (Δ H, Δ S, and Δ G) of Cu(II) sorption on illite were calculated from the temperature dependent sorption isotherms, and the results indicated that the sorption of Cu(II) on illite was endothermic and spontaneous. From the experimental results, it is possible to conclude that illite has good potentialities for cost-effective treatments of Cu(II)-contaminated wastewaters. [ABSTRACT FROM AUTHOR]

The effects of pH, ionic strength, competing ions and initial metal concentrations on the uptake behavior and mechanism of radioactive Ni(II) onto MnO was investigated using a combination of classical macroscopic methods and the extended X-ray absorption fine structure (EXAFS) spectroscopy technique. The results indicated that the uptake of Ni(II) on MnO is obviously dependent on pH but independent of ionic strength, which suggested that the uptake of Ni(II) onto MnO is attributed to an inner-sphere surface complex rather than an outer-sphere surface complex. EXAFS analysis shows that the hydrated Ni(II) is adsorbed through six-fold coordination with an average Ni-O interatomic distance of 2.04 ± 0.01 Å. It can be inferred from the EXAFS analysis that the inner-sphere surface complex of Ni(II) onto MnO is involved in both edge-sharing and corner-sharing linkages. Both the macroscopic uptake data and the molecular level evidence of Ni(II) surface speciation at the MnO-water interfaces should be factored into better prediction of the bioavailability and mobility of Ni(II) in soil and water environment. [ABSTRACT FROM AUTHOR]

Bentonite has been extensively studied because of its strong sorption ability and low permeability. In this work, the Na-bentonite from Gaomiaozi County (China) has been characterized by XRD, FTIR and acid-base titration. The sorption of Eu(III) on Na-bentonite in the absence/presence of humic acid (HA) was studied at T = 25 ± 2 °C and in 0.01 mol/L NaClO4 solution. The effects of pH, HA, contact time and initial Eu(III) concentrations were also investigated. The results indicate that the sorption of Eu(III) on Na-bentonite was dependent on pH values. The presence of HA had little effect on Eu(III) sorption at low pH values, but decreased Eu(III) sorption at high pH values. X-ray absorption fine structure spectroscopy (XAFS) was applied to characterize the local structural environment of the adsorbed Eu(III) on bare Na-bentonite and HA-bentonite hybrids. The results indicate that Eu(III) was bound to O atoms at a distance of about 2.39 Å at pH 4.15. The results are crucial for the evaluation of the sorption and migration of other trivalent lanthanides and actinides in bentonite as backfill materials. [ABSTRACT FROM AUTHOR]