Congcong Ding

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.

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.

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.

Mutual effect of U(VI) and Sr(II) on graphene oxides: evidence from EXAFS and theoretical calculations

The competitive interaction of U(VI) and Sr(II) on graphene oxides (GOs) was studied by batch techniques, EXAFS analysis and DFT calculations. The batch results indicated that decreased sorption of Sr(II) on GOs was observed at C[U(VI)] 0.2 mmol L−1, whereas the presence of Sr(II) did not affect U(VI) sorption on GOs. The increased sorption of Sr(II) at C[U(VI)] > 0.2 mmol L−1 resulted from the new available sites provided by the precipitated U(VI) or adsorbed hydrolyzed U(VI) species according to EXAFS analysis. The occurrence of a U–C shell in the absence/presence of Sr(II) indicated that U(VI) tended to form inner-sphere surface complexes with GOs. For the Sr(II) interaction, a Sr–C shell was observed at a low U(VI) concentration, but not formed at a high U(VI) concentration, indicating the shift of inner-sphere to outer-sphere surface complexes with increasing U(VI) concentration. According to DFT calculation, the binding energy of GO–U(VI) (e.g., −40.3 kcal mol−1 for inner-sphere coordination) was significantly lower than that of GO–Sr(II) (−16.4 kcal mol−1), demonstrating that U(VI) was preferentially bound to GOs relative to Sr(II). These findings can provide a reliable prediction of the transport and fates of U(VI) and Sr(II) at the water–GO interface and open doorways for the application of GOs.

The efficient enrichment of U(VI) by graphene oxide-supported chitosan

Graphene oxide-supported chitosan (GO-Ch) composites were synthesized using a covalent method for U(VI) adsorption and were characterized by scanning electron microscopy (SEM), Fourier transform infrared (FT-IR) spectroscopy, thermogravimetric analysis (TGA), differential thermal analysis (DTA) and extended X-ray absorption fine structure (EXAFS). The characteristic results indicated that Ch was successfully grafted onto GO. The adsorption of U(VI) on GO-Ch was investigated under different environmental conditions. The adsorption kinetics showed that the adsorption of U(VI) on GO-Ch followed the pseudo-second-order equation. The maximum adsorption capacity of U(VI) on GO-Ch at pH 4.0 and T = 303 K calculated from the Langmuir model was 225.78 mg g−1. Thermodynamic parameters calculated from temperature-dependent adsorption isotherms suggested that U(VI) adsorption on GO-Ch was an endothermic and spontaneous process. The batch desorption indicated U(VI) cannot be completely desorbed from GO-Ch without intervention, suggesting the irreversible adsorption of U(VI) on GO-Ch. The analysis of FT-IR spectra suggested that the interaction mechanism of U(VI) on GO-Ch was mainly chemical adsorption by –NH2 and –COOH groups. According to EXAFS analysis, the peaks at ∼2.9 A can be satisfactorily fitted by the U–C/N shell, revealing the formation of inner-sphere surface complexes. It is demonstrated that the GO-Ch nanocomposite can be a promising material for the preconcentration and solidification of U(VI) from large volumes of aqueous solution.

Simultaneous sorption and reduction of U(VI) on magnetite–reduced graphene oxide composites investigated by macroscopic, spectroscopic and modeling techniques

Magnetite–reduced graphene oxide (M–rGO) composites with different mass% (from 33% to 93%) magnetite contents were successfully synthesized via an in situ chemical precipitation method. The composites of M–rGO were characterized by SEM, XRD, FTIR, and XPS techniques. Macroscopic, spectroscopic and modeling techniques were used to study the mechanism for U(VI) removal by M–rGO. The results revealed that the high performance of M–rGO toward U(VI) removal resulted from the contribution of both sorption and reduction mechanisms. The reduction of U(VI) to U(IV) by M–rGO increased with increasing content of magnetite (Fe3O4), as evidenced by the XPS analysis. The kinetics model further suggested that the reduction reaction happened after the sorption of U(VI) on M–rGO. U(VI) was adsorbed on M–rGO via outer-sphere and inner-sphere surface complexation with oxygen-containing groups, whereas the inner-sphere surface complexation dominated with the increasing content of Fe3O4 on M–rGO due to the increased sorption sites (i.e., FeOH). The findings herein highlight the elucidation of the interaction mechanisms between M–rGO and U(VI), which is of significance in predicting the U(VI) removal properties of M–rGO and designing versatile adsorbents in environmental cleanup.