Interaction of Th(IV) with graphene oxides: Batch experiments, XPS investigation, and modeling

Highlights

• The performance of GOs in Th(IV) immobilization was comprehensively investigated.

• Ionic strength affected the microscopic distribution of the adsorbed Th(IV) species.

• The strong acidic groups played a dominant role in Th(IV) sorption on GOs.

• HA enhanced the Th(IV) sorption on GOs mainly by the increase of the GO stability.

• The sorption capacity of GOs for Th(IV) was higher than those of common sorbents.

Abstract

The high surface area, the pH-dependent highly negative charge, and the abundant surface functional groups make graphene oxides (GOs) efficient for cation immobilization. In this study, the performance of GOs in Th(IV) immobilization as a function of contact time, pH, ionic strength, GO concentration, humic acid (HA) and temperature was investigated by batch experiments. The sorption data of pH effect at different ionic strengths and HA concentrations were fitted by diffusion layer model (DLM). The results showed that the whole sorption process was dependent on pH and independent of ionic strength, suggesting that surface complexation was the dominant sorption mechanism over ionic exchange. Ionic strength did not affect the macroscopic sorption of Th(IV) on GOs but did influence the microscopic distribution of the adsorbed Th(IV) species on GOs. The strong acidic groups played a dominant role in Th(IV) sorption on GOs. The adsorbed Th(IV) mainly presented as ≡ XOHTh^4 + and ≡ XOTh(OH) ^2 + on GO surfaces. XPS analysis further provided evidence that the oxygen-containing functional groups on GO surfaces participated in the sorption of Th(IV). HA enhanced the sorption of Th(IV) on GOs mainly by the increase of the stability of GOs rather than the increase in the negative surface charge of GOs or the formation of Th(IV)–HA–GO surface complexes. Based on the Langmuir isotherm model, the maximum sorption capacity of GOs was 252.5 μmol/g, which is higher than those of the common sorbents. This work highlights the possible application of GOs in Th(IV) contamination management.

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.