Lukens Wayne W, Bucher Jerome I, Shuh David K, Edelstein Norman M
Actinide Chemistry Group, Chemical Sciences Division, Lawrence Berkeley National Laboratory, Berkeley, California 94720, USA.
Environ Sci Technol. 2005 Oct 15;39(20):8064-70. doi: 10.1021/es050155c.
Cementitious waste forms (CWFs) are an important component of the strategy to stabilize nuclear waste resulting from plutonium production by the U. S. Department of Energy. Technetium (99Tc) is an abundant fission product of particular concern in CWFs because of the high solubility and mobility of Tc(VII), pertechnetate (TcO4-), the stable form of technetium in aerobic environments. CWFs can more effectively stabilize 99Tc if they contain additives that chemically reduce mobile TcO4- to immobile Tc(IV) species. The 99Tc leach rate of reducing CWFs that contain Tc(IV) is much lower than that for CWFs that contain TcO4-. Previous X-ray absorption fine structure studies showed that Tc(IV) species were oxidized to TcO4- in reducing grout samples prepared on a laboratory scale. Whether the oxidizer was atmospheric O2 or NO3- in the waste simulant was not determined. In actual CWFs, rapid oxidation of Tc(IV) by NO3- would be of concern, whereas oxidation by atmospheric O2 would be of less concern due to the slow diffusion and reaction of O2 with the reducing CWF. To address this uncertainty, two series of reducing grouts were prepared using TcO4- containing waste simulants with and without NO3-. In the first series of samples, referred to as "permeable samples", the TcO4- was completely reduced using Na2S, and the samples were sealed in cuvettes made of polystyrene, which has a relatively large O2 diffusion coefficient. In these samples, all of the technetium was initially present as a Tc(IV) sulfide compound, TcSx, which was characterized by extended X-ray absorption fine structure (EXAFS) spectroscopy. The EXAFS data is consistent with a structure consisting of triangular clusters of Tc(IV) centers linked together through a combination of disulfide and sulfide bridges as in MoS3. From the EXAFS model, the stoichiometry of TcSx is TC3S10, which is presumably the compound generally referred to as "Tc2S7". The TcSX initially present in the permeable samples was steadily oxidized over 4 years. In the second series of samples, called "impermeable samples", the TcO4- was not initially completely reduced, and the groutsamples were sealed in cuvettes made of poly-(methyl methacrylate), which has a small O2 diffusion coefficient. In the impermeable samples, the remaining TcO4- continued to be reduced, presumably by blast furnace slag in the grout, as the samples aged. When the impermeable samples were opened and exposed to atmosphere, the lower-valent technetium species were rapidly oxidized to TcO4-.
水泥基废物固化体（CWFs）是美国能源部用于稳定钚生产过程中产生的核废料战略的重要组成部分。锝（99Tc）是CWFs中一种含量丰富且特别受关注的裂变产物，因为在有氧环境中，锝最稳定的形态高锝酸盐（TcO4-）中的Tc（VII）具有高溶解性和迁移性。如果CWFs含有能将可移动的TcO4-化学还原为不可移动的Tc（IV）物种的添加剂，那么它们就能更有效地稳定99Tc。含有Tc（IV）的还原性CWFs中99Tc的浸出率远低于含有TcO4-的CWFs。先前的X射线吸收精细结构研究表明，在实验室规模制备的还原性灌浆样品中，Tc（IV）物种被氧化成了TcO4-。但未确定氧化剂是大气中的O2还是废物模拟物中的NO3-。在实际的CWFs中，NO3-对Tc（IV）的快速氧化会引发关注，而大气中的O2由于其与还原性CWFs的扩散和反应缓慢，引发的关注较少。为了解决这一不确定性，使用含和不含NO3-的含TcO4-废物模拟物制备了两个系列的还原性灌浆。在第一系列样品（称为“可渗透样品”）中，使用Na2S将TcO4-完全还原，样品被密封在由具有相对较大O2扩散系数的聚苯乙烯制成的比色皿中。在这些样品中，所有的锝最初都以硫化物化合物TcSx的形式存在，通过扩展X射线吸收精细结构（EXAFS）光谱对其进行了表征。EXAFS数据与由Tc（IV）中心的三角形簇通过二硫键和硫桥组合连接在一起的结构一致，就像在MoS3中一样。根据EXAFS模型，TcSx的化学计量比为TC3S10，这可能就是通常所说的“Tc2S7”化合物。可渗透样品中最初存在的TcSX在4年多的时间里逐渐被氧化。在第二系列样品（称为“不可渗透样品”）中，TcO4-最初没有被完全还原，灌浆样品被密封在由具有小O2扩散系数的聚甲基丙烯酸甲酯制成的比色皿中。在不可渗透样品中，随着样品老化，剩余的TcO4-可能继续被灌浆中的高炉矿渣还原。当不可渗透样品打开并暴露在空气中时，低价态的锝物种迅速被氧化成TcO4-。
