Competitive adsorption of Sr(II) and U(VI) on graphene oxide investigated by batch and modeling techniques

Surface modified and functionalized graphene oxide membranes for separation of strontium from aqueous solutions

Graphene oxide-strontium (GO-Sr) composites prepared under different ambient conditions were characterized using morphological and spectroscopic techniques to optimize the uptake of Sr from aqueous solutions. These studies indicated that interactions among GO and Sr²⁺ ions are highly sensitive to size and aging of GO sheets, as well as pH of the ambience. Further, the extent of Sr uptake on GO sheets was found to be largely influenced by relative fractions of the associated -COOH, -OH, C-O-C functional groups and sp²-C domains. Membranes prepared using various forms of GO were evaluated for their Sr separation ability and, a window of parameters for optimum separation of Sr has been proposed. Among the variety of membranes studied, those made up of fresh and large GO sheets were found to exhibit superior Sr adsorption capacity (~296 mg/g) at limited GO mass. Further, adsorption efficiency of these GO membranes was observed to deteriorate with aging of GO sheets and rise of GO mass on membrane. The membrane based filtration procedure introduced in present work facilitates to provide a lamellar structure of GO sheets with abundant surface area, diverse and accessible sites for Sr²⁺ ion uptake and offer high Sr adsorption efficiencies.

Manganese dioxide-loaded mesoporous SBA-15 silica composites for effective removal of strontium from aqueous solution

Manganese dioxide-loaded mesoporous SBA-15 silica (MnO₂/SBA-15) composites with short pore length were aprepared for the first time by simply immersing SBA-15 into a KMnO₄ and MnCl₂ mixed solution. Adsorption of Sr²⁺ from aqueous solution by using the MnO₂/SBA-15 was investigated by varying the pH, contact time, initial Sr²⁺ concentration, MnO₂ content and temperature. The adsorption process was rapid during the first 40 min and reached equilibrium in 120 min. The Sr²⁺ adsorption capacity increased with increasing pH, MnO₂ content and temperature, and the adsorption capacity of SBA-15 was significantly improved by the loading of MnO₂. Moreover, the experimental adsorption data were analyzed using different equilibrium isotherm, kinetic and thermodynamic models. The results showed that the isotherm data were well-described by the Langmuir model. The maximum Sr²⁺ adsorption capacity was determined to be 75.1 mg g^-1 at 283 K based on the Langmuir model. The analyzed kinetic data indicated that the Sr²⁺ adsorption process was well fitted by the pseudo-second order model. Furthermore, the thermodynamic parameters of adsorption were also determined from the equilibrium constant values obtained at different temperatures. The results suggested that the adsorption process was spontaneous and endothermic, and the overall mechanism of Sr²⁺ adsorption was a combination of physical and chemical processes.

Removal of radionuclides from acidic solution by activated carbon impregnated with methyl- and carboxy-benzotriazoles

The removal efficiencies of metals commonly used to model the fate and transport of aqueous uranium and radioactive its daughter products, were observed on activated carbons impregnated with different benzotriazole derivatives. Acidic solutions containing U(VI), Sr(II), Eu(III), and Ce(III) were used to determine the immobilization potential of carboxybenzotriazole (CBT) and methylbenzotriazole (MeBT), where these derivatives were sorbed to different types of granular activated carbon (GAC). This sorption behavior can be predicted by Redlich–Peterson model. Flow-through column tests showed that the immobilization of uranium and some of its daughter products, significantly improves in response to oxidized GACs saturated with carboxybenzotrzole (CBT), which reached a maximum elimination for U(VI) at 260 BV, Eu(III) at 114 BV, Ce(III) at 126 BV, and Sr(II) at 100. MeBT significantly desorbed from GAC under acidic conditions. Trace amounts of CBT were observed in some column effluents, but this did not appear to alter the effectiveness of metal removal, regardless of the model radionuclide studied. These results suggest that enhanced immobilization of selected metals on GAC, can be achieved by impregnating oxidized activated carbon with carboxylated benzotriazoles, and that metal removal efficiency on this media, is related to their valence and ionic radius in acidic environments.

Graphene oxide-facilitated uranium transport and release in saturated medium: Effect of ionic strength and medium structure

Natural subsurface environment is a complex heterogeneous system. To investigate the effect of ionic strength (IS) and heterogeneity on the transport and remobilization of graphene oxide (GO)-facilitated uranium (U(VI)) in saturated porous media, column experiments were performed by the injection of U(VI) alone and U(VI)+GO mixtures into homogeneous and heterogeneous porous media under low and high ionic strength (1 and 50 mM) conditions, and then the columns were successively flushed with background solution and DI water. Results showed that when U(VI) only was introduced into the columns, IS had little effect on the migration of U(VI) alone in both media and the presence of preferential flow in heterogeneous media slightly enhanced the mobility of U(VI). As U(VI)+GO mixtures were injected into the columns, GO showed strong mobility at low IS and high released peak at high IS. The appearance of GO significantly enhanced U(VI) transport in both media. Under low IS condition, the mobility of U(VI) was significantly enhanced at the injection phase, and the medium heterogeneity further promoted the amount of GO-sorbed U(VI) transport. At high IS, less GO-sorbed U(VI) was observed during injection phase, and a large amount of retained GO-sorbed U(VI) were released with GO remobilization during water flushing phase, and the release showed the longer-tailing phenomenon and the release amount was more pronounced in heterogeneous media. The findings in this study showed that the coupled effect of solution chemistry and media heterogeneity played important roles on GO-facilitated U(VI) transport and release in soil and groundwater system.

Preparation of alginate fibers coagulated by calcium chloride or sulfuric acid: Application to the adsorption of Sr2

The adsorption behavior of Sr²⁺ over alginate (Alg) fibers prepared by wet spinning was investigated. Different grades of sodium alginate (Alg-Na) were chosen. The Alg fibers were obtained by coagulation of 1% H₂SO₄ (Alg-acid) or 5% CaCl₂ (Alg-Ca) solutions. In addition, the Sr²⁺ adsorption percentages of the spherical Alg-Ca beads with a 0.672-mm-diameter was 70.64% which was significantly lower than that Alg-Ca fibers (79.49%). These results suggested that the fibrous shape is more suitable than the spheres as an adsorbent from sea water. For Sr²⁺ adsorption capacities using different Alg fibers, the Alg-acid fibers obtained from 12% IL-2 and 8% I-2 grade solutions reached adsorption equilibrium at 99.88 and 99.27%, respectively, within 3 min. However, the Alg-Ca fiber obtained from 8% I-2 grade solution reached equilibrium at 80.01% within 30 min. Moreover, the Alg-acid fiber obtained from 8% I-2 grade solution adsorbed up to 34 mg/g of Sr²⁺ at an initial concentration of 1700 mg/L solutions. However, when Sr²⁺ co-existing cations (Ca²⁺, Na⁺, and mixtures of them) the adsorption capacity of the Alg-Ca fiber obtained from 8% I-2 grade solution slightly decreased since the egg-box structure of Alg-Ca fiber favored the selective Sr²⁺ adsorption and subsequent ion exchange with Ca²⁺.

Analysis of competitive binding of several metal cations by graphene oxide reveals the quantity and spatial distribution of carboxyl groups on its surface

GO contains two different types of binding sites, bonding to which results either in high or low NMR relaxivity of resulted Gd³⁺–GO and Mn²⁺–GO solutions.