Xiangke Wang

Adsorption and Desorption of U(VI) on Functionalized Graphene Oxides: A Combined Experimental and Theoretical Study

The adsorption and desorption of U(VI) on graphene oxides (GOs), carboxylated GOs (HOOC-GOs), and reduced GOs (rGOs) were investigated by batch experiments, EXAFS technique, and computational theoretical calculations. Isothermal adsorptions showed that the adsorption capacities of U(VI) were GOs > HOOC-GOs > rGOs, whereas the desorbed amounts of U(VI) were rGOs > GOs > HOOC-GOs by desorption kinetics. According to EXAFS analysis, inner-sphere surface complexation dominated the adsorption of U(VI) on GOs and HOOC-GOs at pH 4.0, whereas outer-sphere surface complexation of U(VI) on rGO was observed at pH 4.0, which was consistent with surface complexation modeling. Based on the theoretical calculations, the binding energy of [G(···)UO2](2+) (8.1 kcal/mol) was significantly lower than those of [HOOC-GOs(···)UO2](2+) (12.1 kcal/mol) and [GOs-O(···)UO2](2+) (10.2 kcal/mol), suggesting the physisorption of UO2(2+) on rGOs. Such high binding energy of [GOs-COO(···)UO2](+) (50.5 kcal/mol) revealed that the desorption of U(VI) from the -COOH groups was much more difficult. This paper highlights the effect of the hydroxyl, epoxy, and carboxyl groups on the adsorption and desorption of U(VI), which plays an important role in designing GOs for the preconcentration and removal of radionuclides in environmental pollution cleanup applications.

Adsorption of 4-n-Nonylphenol and Bisphenol-A on Magnetic Reduced Graphene Oxides: A Combined Experimental and Theoretical Studies

Adsorption of 4-n-nonylphenol (4-n-NP) and bisphenol A (BPA) on magnetic reduced graphene oxides (rGOs) as a function of contact time, pH, ionic strength and humic acid were investigated by batch techniques. Adsorption of 4-n-NP and BPA were independent of pH at 3.0- 8.0, whereas the slightly decreased adsorption was observed at pH 8.0-11.0. Adsorption kinetics and isotherms of 4-n-NP and BPA on magnetic rGOs can be satisfactorily fitted by pseudo-second-order kinetic and Freundlich model, respectively. The maximum adsorption capacities of magnetic rGOs at pH 6.5 and 293 K were 63.96 and 48.74 mg/g for 4-n-NP and BPA, respectively, which were significantly higher than that of activated carbon. Based on theoretical calculations, the higher adsorption energy of rGOs + 4-n-NP was mainly due to π-π stacking and flexible long alkyl chain of 4-n-NP, whereas adsorption of BPA on rGOs was energetically favored by a lying-down configuration due to π-π stacking and dispersion forces, which was further demonstrated by FTIR analysis. These findings indicate that magnetic rGOs is a promising adsorbent for the efficient elimination of 4-n-NP/BPA from aqueous solutions due to its excellent adsorption performance and simple magnetic separation, which are of great significance for the remediation of endocrine-disrupting chemicals in environmental cleanup.

Environmental Remediation and Application of Nanoscale Zero-Valent Iron and Its Composites for the Removal of Heavy Metal Ions: A Review.

The presence of heavy metals in the industrial effluents has recently been a challenging issue for human health. Efficient removal of heavy metal ions from environment is one of the most important issues from biological and environmental point of view, and many studies have been devoted to investigate the environmental behavior of nanoscale zerovalent iron (NZVI) for the removal of toxic heavy metal ions, present both in the surface and underground wastewater. The aim of this review is to show the excellent removal capacity and environmental remediation of NZVI-based materials for various heavy metal ions. A new look on NZVI-based materials (e.g., modified or matrix-supported NZVI materials) and possible interaction mechanism (e.g., adsorption, reduction and oxidation) and the latest environmental application. The effects of various environmental conditions (e.g., pH, temperature, coexisting oxy-anions and cations) and potential problems for the removal of heavy metal ions on NZVI-based materials with the DFT theoretical calculations and EXAFS technology are discussed. Research shows that NZVI-based materials have satisfactory removal capacities for heavy metal ions and play an important role in the environmental pollution cleanup. Possible improvement of NZVI-based materials and potential areas for future applications in environment remediation are also proposed.

Macroscopic and Microscopic Investigation of U(VI) and Eu(III) Adsorption on Carbonaceous Nanofibers.

The adsorption mechanism of U(VI) and Eu(III) on carbonaceous nanofibers (CNFs) was investigated using batch, IR, XPS, XANES, and EXAFS techniques. The pH-dependent adsorption indicated that the adsorption of U(VI) on the CNFs was significantly higher than the adsorption of Eu(III) at pH < 7.0. The maximum adsorption capacity of the CNFs calculated from the Langmuir model at pH 4.5 and 298 K for U(VI) and Eu(III) were 125 and 91 mg/g, respectively. The CNFs displayed good recyclability and recoverability by regeneration experiments. Based on XPS and XANES analyses, the enrichment of U(VI) and Eu(III) was attributed to the abundant adsorption sites (e.g., -OH and -COOH groups) of the CNFs. IR analysis further demonstrated that -COOH groups were more responsible for U(VI) adsorption. In addition, the remarkable reducing agents of the R-CH2OH groups were responsible for the highly efficient adsorption of U(VI) on the CNFs. The adsorption mechanism of U(VI) on the CNFs at pH 4.5 was shifted from inner- to outer-sphere surface complexation with increasing initial concentration, whereas the surface (co)precipitate (i.e., schoepite) was observed at pH 7.0 by EXAFS spectra. The findings presented herein play an important role in the removal of radionuclides on inexpensive and available carbon-based nanoparticles in environmental cleanup applications.

Simultaneous adsorption and reduction of U(VI) on reduced graphene oxide-supported nanoscale zerovalent iron

The reduced graphene oxide-supported nanoscale zero-valent iron (nZVI/rGO) composites were synthesized by chemical deposition method and were characterized by SEM, high resolution TEM, Raman and potentiometric acid-base titrations. The characteristic results showed that the nZVI nanoparticles can be uniformly dispersed on the surface of rGO. The removal of U(VI) on nZVI/rGO composites as a function of contact time, pH and U(VI) initial concentration was investigated by batch technique. The removal kinetics of U(VI) on nZVI and nZVI/rGO were well simulated by a pseudo-first-order kinetic model and pseudo-second-order kinetic model, respectively. The presence of rGO on nZVI nanoparticles increased the reaction rate and removal capacity of U(VI) significantly, which was attributed to the chemisorbed OH(-) groups of rGO and the massive enrichment of Fe(2+) on rGO surface by XPS analysis. The XRD analysis revealed that the presence of rGO retarded the transformation of iron corrosion products from magnetite/maghemite to lepidocrocite. According to the fitting of EXAFS spectra, the UC (at ∼2.9Å) and UFe (at ∼3.2Å) shells were observed, indicating the formation of inner-sphere surface complexes on nZVI/rGO composites. Therefore, the nZVI/rGO composites can be suitable as efficient materials for the in-situ remediation of uranium-contaminated groundwater in the environmental pollution management.

Novel fungus-Fe3O4 bio-nanocomposites as high performance adsorbents for the removal of radionuclides

The bio-nanocomposites of fungus-Fe3O4 were successfully synthesized using a low-cost self-assembly technique. SEM images showed uniform decoration of nano-Fe3O4 particles on fungus surface. The FTIR analysis indicated that nano-Fe3O4 was combined to the fungus surface by chemical bonds. The sorption ability of fungus-Fe3O4 toward Sr(II), Th(IV) and U(VI) was evaluated by batch techniques. Radionuclide sorption on fungus-Fe3O4 was independent of ionic strength, indicating that inner-sphere surface complexion dominated their sorption. XPS analysis indicated that the inner-sphere radionuclide complexes were formed by mainly bonding with oxygen-containing functional groups (i.e., alcohol, acetal and carboxyl) of fungus-Fe3O4. The maximum sorption capacities of fungus-Fe3O4 calculated from Langmuir isotherm model were 100.9, 223.9 and 280.8 mg/g for Sr(II) and U(VI) at pH 5.0, and Th(IV) at pH 3.0, respectively, at 303 K. Fungus-Fe3O4 also exhibited excellent regeneration performance for the preconcentration of radionuclides. The calculated thermodynamic parameters showed that the sorption of radionuclides on fungus-Fe3O4 was a spontaneous and endothermic process. The findings herein highlight the novel synthesis method of fungus-Fe3O4 and its high sorption ability for radionuclides.

Rationally designed 1D Ag@AgVO3 nanowire/graphene/protonated g-C3N4 nanosheet heterojunctions for enhanced photocatalysis via electrostatic self-assembly and photochemical reduction methods

1D Ag@AgVO3 nanowire/graphene/protonated g-C3N4 nanosheet (Ag@AgVO3/rGO/PCN) heterojunctions are fabricated via a simple electrostatic self-assembly process followed by a photochemical reduction method. In this hybrid structure, 1D Ag@AgVO3 nanowires penetrate through 2D nanosheets (graphene and PCN), forming a 3D hybrid photocatalyst, which is applied as an efficient visible light driven photocatalyst for organic pollutant degradation. Its enhanced photocatalytic activity is ascribed to the well-known electronic conductivity of 2D graphene, the intense visible light absorption of 1D Ag@AgVO3 nanowires, large surface areas and rapid photogenerated charge interface transfer and separation. Our results provide a facile way to fabricate hierarchical g-C3N4-based photocatalysts in a controlled manner and highlight promising prospects by adopting an integrative 1D and 2D nanomaterial strategy to design more efficient semiconductor-based composite photocatalysts with high photocatalytic activities and a wide spectral response toward environmental and energy applications.

Graphene oxide/polypyrrole composites for highly selective enrichment of U(VI) from aqueous solutions

Graphene oxide/polypyrrole (GO/PPy) composites were synthesized via a dielectric barrier discharge (DBD) plasma technique in nitrogen conditions, and characterized by scanning electron microscopy (SEM), Raman spectroscopy, thermal gravimetric analysis (TGA), Fourier transformed infrared spectroscopy (FT-IR) and X-ray photoelectron spectroscopy (XPS). The sorption of U(VI) ions on GO/PPy composites from aqueous solutions was investigated as a function of contact time, pH, ionic strength and U(VI) initial concentrations. The sorption capacity of U(VI) on GO/PPy composites was much higher than those of U(VI) on GO, PPy and many other materials of today. The sorption of U(VI) on GO/PPy composites obeyed the Langmuir model, and was mainly attributed to surface complexation via the coordination of U(VI) ions with oxygen- and nitrogen-containing functional groups. The selectivity sorption of U(VI) ions on GO/PPy composites in the presence of other metal ions (i.e., Co(II), Ni(II), Cd(II), Sr(II), Zn(II)) indicated an overall preference for U(VI) ions. Moreover, the GO/PPy composites could be regenerated through the desorption of adsorbed U(VI) ions by using 1.0 M HCl solution, and cycling reused without an obvious decrease of sorption capacity. All these performances indicate that GO/PPy composites are suitable materials for the highly selective removal and preconcentration of U(VI) ions from aqueous solutions in environmental pollution management.

Competitive sorption of Pb(II), Cu(II) and Ni(II) on carbonaceous nanofibers: A spectroscopic and modeling approach.

The competitive sorption of Pb(II), Cu(II) and Ni(II) on the uniform carbonaceous nanofibers (CNFs) was investigated in binary/ternary-metal systems. The pH-dependent sorption of Pb(II), Cu(II) and Ni(II) on CNFs was independent of ionic strength, indicating that inner-sphere surface complexation dominated sorption Pb(II), Cu(II) and Ni(II) on CNFs. The maximum sorption capacities of Pb(II), Cu(II) and Ni(II) on CNFs in single-metal systems at a pH 5.5±0.2 and 25±1°C were 3.84 (795.65mg/g), 3.21 (204.00mg/g) and 2.67 (156.70mg/g)mmol/g, respectively. In equimolar binary/ternary-metal systems, Pb(II) exhibited greater inhibition of the sorption of Cu(II) and Ni(II), demonstrating the stronger affinity of CNFs for Pb(II). The competitive sorption of heavy metals in ternary-metal systems was predicted quite well by surface complexation modeling derived from single-metal data. According to FTIR, XPS and EXAFS analyses, Pb(II), Cu(II) and Ni(II) were specifically adsorbed on CNFs via covalent bonding. These observations should provide an essential start in simultaneous removal of multiple heavy metals from aquatic environments by CNFs, and open the doorways for the application of CNFs.

Formation of Fe3O4@MnO2 ball-in-ball hollow spheres as a high performance catalyst with enhanced catalytic performances

While the synthesis of heterogeneous catalysts is well established, it is extremely challenging to fabricate complex hollow structures with mixed transition metal oxides. Herein, we report a facile in situ growth process of SiO2@Fe3O4@MnO2, followed by an etching method to synthesize a hierarchical hollow structure, namely Fe3O4@MnO2 ball-in-ball hollow spheres (Fe3O4@MnO2 BBHs). The as-prepared Fe3O4@MnO2 BBHs were applied to degrade methylene blue (MB) by catalytic generation of active radicals from peroxymonosulfate (PMS), exhibiting the merits of excellent catalytic performance, easy separation, good stability and recyclability. In this architecture, the degradation process can be divided into three layers. The outer hierarchical MnO2 nanosheets could accumulate and transport the pollutants by electrostatic interactions and catalyze the generation of active radicals for degradation. Both the inner MnO2 nanosheets and the outer Fe3O4 hollows could produce active radicals to accelerate the pollutant degradation. The active catalytic sites also existed in the inner Fe3O4 hollows, which could further degrade the highly concentrated pollutants in the hollows. This work provides new strategies for the controllable synthesis of complex hollow structures and their application in environmental remediation.