Separation and Purification of Trace Neptunium from Hundred-gram Granite

The study of the separation method of neptunium in environmental sample is of great significance in the context of radioactive survey and nuclear emergency monitoring. In China, the radioactive waste disposal site and the nuclear fuel cycle facility are located in northwest, and the main rock type is granite. To study the distribution of radionuclides in granite, it is necessary to study the dissolution of the granite and the separation of neptunium to facilitate the detection of trace neptunium. Commonly, HF and inorganic acids such as HNO₃ and HClO₄ were used as digestion. However, the addition of HF could form insoluble Mg-Ca-Al fluoride in the solution, so that trace neptunium will be mixed into the precipitation, resulting in the loss of neptunium. Ammonium bifluoride could effectively digest insoluble accessory minerals in rocks, avoiding the precipitation of insoluble fluoride and realizing the complete digestion of granite. Since the amount of ammonium bifluoride is 4-6 times that of rocks, the solution contains a certain amount of fluoride ions. However, Np⁴⁺ has strong complexation with F^-, so it is difficult to separate and purify neptunium by ion exchange resin, TTA extraction. In this paper, the acid digestion method was used to realize the complete dissolution of granite of the order of 100 grams. A new kind of absorbent was synthesized by impregnating DMDODGA into polymethyl methacrylate resin (CMG20). Under the conditions of different concentrations of nitric acid, hydrochloric acid, hydrofluoric acid and mixed acid, the adsorption performance of DMDODGA/CMG20 for Np(Ⅳ) was studied. The results show that the adsorbent is able to complex Np(Ⅳ) in the range of 0.5-4 mol/L nitric or hydrochloric acid, with the distribution coefficient up to 10⁵ mL/g. According to the characteristics of adsorption neptunium in hydrofluoric acid, the DMDODGA separation process was proposed, which realized the enrichment of neptunium from large volume rock solution. By using Dowex 1×4 anion exchange separation, the purpose of removing complexants in the stripping of DMDODGA separation process and concentration of neptunium is achieved. The method was verified by separating neptunium from 100 grams of granite. The results show that the granite solution is clear and free of residues. The yield of neptunium in the process is 87.6%, and decontamination factors of rock matrix elements and uranium are greater than 1.0×10⁵. Neptunium is concentrated 200 times in the process. This method realizes the complete dissolution of siliceous rocks on the order of 100 grams. It can enrich and purify the femtogram quantities of neptunium in a large amount of rock dissolution liquid without masking F^- and addition of carriers. This method provides a new technical support for environmental radioactive survey and monitoring.