Adsorption Of Uranium (Ⅵ) From Aqueous Solution By Covalently Functionalized Graphene Oxide

Graphene oxide(GO) is a promising adsorbent due to the high specific surface area and abundant oxygen containing functional groups as well as easily covalent functionalization. Herein, the sulfonated graphene oxide(SGO), phosphorylated graphene oxides(PGO-1 and PGO-2) and oximated graphene oxide(AOGO) have been prepared by covalently modification with GO as the matrix structure. The physical and chemical properties of GO and functionalized GO were characterized by SEM, TEM and FT-IR, Zeta potential, Raman spectra, XPS and elemental analysis. The adsorptive properties of U(Ⅵ) on the graphenes were studied in detail as a function of solution pH, ionic strength, contact time, the initial U(Ⅵ) concentrations and temperature by a batch of static adsorption experiments. The adsorption kinetics, isotherms and thermodynamics have been studied. The selectivity, desorption and reuse properties of the graphenes for U(Ⅵ) have been also investigated.The sulfonated graphene oxide(SGO) was prepared by a simple method of vapor-solid transfer reaction. SGO exhibited thinner sheets. The amount of sulfonic acid groups on SGO was up to 1.35 mmol·g-1. The adsorptive properties of U(Ⅵ) on GO and SGO are strongly dependent on pH and nearly independent of ionic strength, suggesting that the both U(Ⅵ) adsorption processes are mainly dominated by inner-sphere surface complexation. The adsorption of U(Ⅵ) on GO and SGO agreed well with the pseudo-second-order kinetics and the Langmuir models, and the maximum adsorption capacity increased from 249.38 to 319.49 mg·g-1 after functionalization. Both adsorption reactions were spontaneous and endothermic processes. The selectivity for U(IV) is improved after modification with sulfonate groups. The U(Ⅵ) adsorbed on GO and SGO can be effectively eluted by 1.0 mol·L-1 HCl solution and the desorption rates were over 98%. The adsorptive property of U(Ⅵ) on SGO was stable and the adsorption capacity only decreased by 11% after five cycles of adsorption-desorption.Two kinds of phosphorylated graphene oxides(PGO-1 and PGO-2) were prepared by the in-situ phosphorylation method and the epoxide ring-opening reaction, respectively. PGO-1 and PGO-2 exhibited thinner sheets. The amount of phosphate groups on PGO-2 reach up to 2.42 mmol·g-1. The adsorptive properties of U(Ⅵ) on PGO-1 and PGO-2 are strongly dependent on pH and nearly independent of ionic strength, suggesting that the both U(Ⅵ) adsorption processes are mainly dominated by inner-sphere surface complexation. The adsorption of U(Ⅵ) on PGO-1 and PGO-2 agreed well with the pseudo-second-order kinetics and the Langmuir models, having the maximum adsorption capacities of 336.70 and 396.83 mg·g-1, respectively. Both adsorption reactions were spontaneous and endothermic processes. The selectivity for U(IV) was improved after modification with phosphate groups. The U(Ⅵ) adsorbed on PGO-1 and PGO-2 can be effectively eluted by 1.0 mol·L-1 HCl solution and the desorption rates were over 98%. The adsorptive properties of U(Ⅵ) on PGO-1 and PGO-2 were stable and the adsorption capacities only decreased by 10% and 13% after five cycles of adsorption-desorption, respectively.The oximated graphene oxide(AOGO) was prepared by the method of diazotization reaction. AOGO exhibited thinner sheets. The amount of oximido groups on AOGO was up to 1.96 mmol·g-1. The adsorptive property of U(Ⅵ) on AOGO is strongly dependent on pH and nearly independent of ionic strength, suggesting that the U(Ⅵ) adsorption process is mainly dominated by inner-sphere surface complexation. The adsorption of U(Ⅵ) on AOGO agreed well with the pseudo-second-order kinetics and the Langmuir models, having a maximum adsorption capacity of 495.05 mg·g-1. The adsorption reaction was an spontaneous and endothermic process. The selectivity for U(IV) is improved after modification with oximido groups. The U(Ⅵ) adsorbed on AOGO can be effectively eluted by 1.0 mol·L-1 HCl solution and the desorption rate was over 98%. The adsorptive property of U(Ⅵ) on AOGO was stable and the adsorption capacity only decreased by 12% after five cycles of adsorption-desorption.