Interaction of Th(IV) with graphene oxides: Batch experiments, XPS investigation, and modeling
Introduction
With the rapid development of the nuclear industry, understanding the physicochemical behaviors of radionuclides, notably long-lived radionuclides, has been of great environmental concern in peaceful utilization of nuclear energy [1], [2], [3]. Radionuclides with high mobility in contaminated water will transfer into soil and end up in plant materials. The radionuclides which enter the food chain will potentially produce serious damage to human beings [4]. Among these nuclear waste and fission products, Th(IV) has been considered as a potential nuclear fuel by converting Th(IV) into 233U. Additionally, Th(IV) is also regarded as an analog for other + 4 valence actinides because it has a stable tetravalent state in nature [5], [6]. Th(IV) can cause progressive and irreversible damage to cells, lymph nodes, lungs, livers, pancreases, and bones because of its high toxicity and radioactivity even at low concentration [7]. Therefore, the separation and enrichment of Th(IV) from aqueous solutions are crucial in protecting environment and reutilizing thorium as a resource.
The sorption, diffusion and migration of radionuclides on various clay minerals, oxides and manmade nanomaterials such as alumina [8], carbon nanotubes [9], sepiolite [10], zeolite [11], rectorite [12], attapulgite [13], activated carbon fiber [14], kaolinite [15], goethite [16], regosols [17], chitosan [18], hematite [19] and bentonite [20] have been studied extensively using different experimental techniques and computational theoretical calculations [21], [22], [23]. Many researchers still settled down to find new adsorbent materials with the best performance in immobilization of radionuclides [10], [11], [12], [13], [14], [15], [16], [17], [18], [19], [20], [21], [22].
Besides the high surface area (theoretical value of 2620 m2/g), graphene oxides (GOs) have a wide range of functional groups such as epoxy, hydroxyl, and carboxyl groups and are highly negatively charged over a wide range of pH values, thus GOs were regarded as potential materials for the sequestration of radionuclides and other heavy metal ions [24], [25], [26]. Humic acid (HA), which is widely present in the natural environment, has strong complexation ability with various radionuclides because of its abundant oxygen-containing functional groups. HA can also be adsorbed on GOs via π–π interaction and hydrophobilic interaction, which may change the physicochemical properties of GOs and consequently influences the performance of GOs for radionuclide removal [27]. Multiple studies have focused on the effect of HA on the sorption of heavy metal ions (Pb(II) and Ni(II)) or radionuclides (U(VI), Sr(II), and Eu(III)) on rectorite and anatase. These results showed that HA enhanced the sorption at low pH and inhibited sorption at high pH [28], [29], [30]. However, the application of GOs as sorbents in the removal of Th(IV) in the presence of HA has not been roundly explored. Research on Th(IV) sorption onto GOs is important and useful in estimating the potential application of GOs in radionuclide immobilization.
Herein, GOs prepared from graphite through a modified Hummers method were used as sorbents to remove Th(IV) from aqueous solutions. Th(IV) was selected as a chemical analog for tetravalent actinides [31]. The effects of contact time, solution pH, ionic strength, GO concentration, HA and temperature on Th(IV) sorption were comprehensively studied. The experimental data of Th(IV) sorption on GOs as a function of pH at different ionic strengths or HA concentrations were modeled by diffusion layer model (DLM) with the aid of FITEQL 4.0 software to provide insight into the sorption mechanism of Th(IV) on GOs. In addition, the interaction mechanism of Th(IV) with GOs was determined through X-ray photoelectron spectroscopy (XPS) analysis.
Section snippets
Materials and sorbent preparation
HA was extracted from a piece of soil from the Gansu Province in China, and the main constituents of the HA were listed as follows: C 60.44%, H 3.53%, N 4.22%, O 31.31%, and S 0.50% [32].
GOs were prepared using a modified Hummers method from flake graphite (average particle diameter of 20 μm, 99.95% purity, Qingdao Tianhe Graphite Co. Ltd., China) with concentrated H2SO4, KMnO4, and 30% H2O2 as the oxidants [33]. GOs were dispersed in Milli-Q water (18.2 MΩ/cm) to prepare a stock suspension (100
Characterization of GOs
Fig. 1a shows the SEM image of the synthesized GOs. The nature of graphene can be confirmed by the typical ripples presented on the GO surfaces. The TEM image (Fig. 1b) reveals that the film of GOs is transparent and form scrolls and that the wrinkles are primarily located on the edge of the GOs.
The variation of the ZP values of GOs as a function of pH was measured and shown in Fig. 1c. The value of ZP is dependent on the surface coverage of charging species at a given pH and was determined by
Conclusion
In this study, the batch experiments combined with XPS investigation and DLM modeling were employed to study the performance of GOs in Th(IV) immobilization. The results of batch experiments showed that the sorption kinetics of Th(IV) on GOs followed a pseudo-second-order model. Th(IV) sorption on GOs strongly depended on pH, the sorbent concentration, and temperature, while independent of ionic strength, suggesting the formation of surface complexes was the dominant mechanism for Th(IV)
Acknowledgments
The financial supports that came from NSFC (grant nos. 21307135, 41273134, 21377132, 21477133, 91326202, and 21225730) are acknowledged.
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