The use of carbon-based nanomaterials for the removal of radionuclides from aqueous solution.

Abstract

The safe decommissioning of nuclear reactors, worldwide, is a major challenge facing the nuclear industry. Operational activities associated with nuclear decommissioning are expected to generate large volumes of high-level aqueous nuclear waste. Therefore, there is a growing need to develop new treatment processes. Previous work has shown nanomaterials are effective sorption materials for radionuclide removal. However, minimal research has been carried out on hybrid sorption materials consisting of selective functional groups and carbon-based nanomaterials. In this research, functionalised multi-walled carbon nanotubes (MWCNTs) and graphene oxide (GO), were prepared for the removal of long-lived radionuclides, namely, uranium (U(VI)) from aqueous solution. A series of synthesis procedures were applied to unmodified forms of MWCNTs and GO to enhance selectivity towards U(VI). This was accomplished by attaching surface complexing functional groups, such as, COOH, OH and CONH, onto the surface of the nanomaterial. For this work, chemical functionalisation was confirmed by Raman spectroscopy and X-ray photoelectron spectroscopy (XPS). The former technique involved monitoring the ratio of the Raman D and G bands (ID/IG), before and after functionalisation, via statistical Raman mapping studies. Overall, an increase in the ID/IG ratio was observed for each functionalised material. This suggests that the chemical functionalisation procedure applied to unmodified MWCNTs and GO materials was successful in introducing an increased number of defect sites suitable for the attachment of functional groups. This finding was further supported by high resolution XPS studies, which confirmed the presence of select surface groups of interest onto each material. The sorption behaviour of functionalised MWCNTs and GO materials towards U(VI) was investigated in the form of batch sorption studies with solution analysis by inductively coupled plasma mass spectrometry (ICP-MS). The following parameters, which are known to influence sorption behaviour were investigated, namely, the effect of pH, contact time and competing ions (Mg, Co, Zn, Sr, Pb and Th). This was done to assess the suitability of each material for nuclear waste treatment. Initial studies revealed that functionalised MWCNTs had minimal selectivity and loading capacity (Qmax) for U(VI) in aqueous solution. The results showed for the best-performing MWCNTs material (COOH-f MWCNTs), the U sorption and the distribution co-efficient (Kd) decreased, in the presence of competing ions from 97.4 ± 1.6 to 12.6 ± 1.0 % and 3.1×104 to 1.4×102 mL g-1, respectively. Moreover, a Qmax of 34.01 mg g-1 was observed for COOH-f MWCNTs under optimal pH conditions. In terms of all the sorption materials tested, COOH-f GO, exhibited the highest selectivity (Kd of 3.7×103 mL g-1) for U(VI) in aqueous solution with a Qmax of 169.20 mg g−1. The sorption performance of COOH-f GO was tested using a high salinity aqueous nuclear waste sample (Sellafield, UK), which showed minimal selectivity for uranium (Kd of 1.0×102 mL g-1) even after pH-adjustment. Furthermore, a higher affinity for competing ions was observed with 85.0 ± 6.9 %, 82.5 ± 5.2 % and 87.5 ± 4.9 % of Mg, Sr and Pb removed by COOH-f GO. The effect of combining the high sorption capacity of COOH-f GO for uranium with magnetic separation (i.e. Fe3O4-f GO) was investigated by batch sorption studies using sequentially-extracted soil samples from the Laguna Sirven Deposit, Santa Cruz, Argentina. Overall, the results showed that Fe3O4-f GO can remove up to 86.7 ± 2.3 % of uranium from sample matrices relevant to uranium remediation. As such, the research described in this thesis could potentially find use in many future applications, including nuclear decommissioning and environmental remediation studies.