Australian Nuclear Science and Technology Organisation, Lucas Heights, NSW 2234, Australia; School of Biological, Earth and Environmental Sciences, UNSW Sydney, NSW, 2052, Australia.
Australian Nuclear Science and Technology Organisation, Lucas Heights, NSW 2234, Australia.
Sci Total Environ. 2022 Jul 15;830:154706. doi: 10.1016/j.scitotenv.2022.154706. Epub 2022 Mar 21.
Increasing concentrations of Rare Earth Elements (REE) plus yttrium (REY) are entering the environment due to human activities. The similar chemical behaviour across the whole REY, i.e. the lanthanide series (lanthanum to lutetium) and yttrium, allows their use as tracers, fingerprinting rock-forming processes and fluid-rock interactions in earth science systems. However, their use in fingerprinting waste and particularly low-level radioactive waste has not received much attention, despite the direct use of REE in the nuclear industry and the traditional use of REE as proxies to understand the environmental mobility of the actinide series (actinium to lawrencium). The highly instrumented low-level radioactive waste site at Little Forest (Australia) allows a detailed REY study, investigating interactions with local strata, neighbouring waste forms and shallow groundwater flows. Groundwater samples and solids from cored materials were recovered from 2007 to 2012 from the study site and regional baseline sites in the same geological materials. The REY in water samples were analysed by automated chelation pre-concentration (SeaFast, ESI) followed by ICP-MS determination, while solid samples were analysed using Neutron Activation Analysis (NAA) and X-ray fluorescence scanning (ITRAX). Solid rocks showed no REY departed from typical Upper Crust compositions in either Little Forest or regional background sites. Shallow groundwater from ~4-5 m, at or slightly below waste trench levels, showed water-waste interaction as a marked enrichment, relative to shale-normalised patterns, in samarium, europium and gadolinium, with depleted yttrium. Leachate samples from the neighbouring urban landfill show different REY normalised patterns. REY distribution changes with depth through increased interaction with shales and sandstones. Variations in pH and redox conditions lead to widespread precipitation of Fe-hydroxides, which scavenge REY with differential uptake by precipitating solids, resulting in increases in Y and higher Y/Ho ratio in the groundwater along the flow path. Our study revealed that the Little Forest low-level radioactive waste has a REY fingerprint different to that of groundwater in surrounding land uses. REY can be used to fingerprint diverse waste sources, assess the mobility of lanthanides inferring the mobility of selected actinides, and to trace the fate of REY during groundwater recharge. The approach presented can refine source allocation and trace pollutant mobility in current and legacy urban, mixed and radioactive waste sites around the world.
由于人类活动，环境中稀土元素（REE）加钇（REY）的浓度正在增加。整个 REY（即镧系元素（镧至镥）和钇）具有相似的化学行为，这使得它们可被用作示踪剂，用于地球科学系统中对造岩过程和流体-岩石相互作用进行指纹识别。然而，尽管 REE 在核工业中被直接使用，并且 REE 传统上被用作了解锕系元素（锕至镧系元素）环境迁移性的示踪剂，但它们在示踪废物方面，尤其是在低放废物方面的应用并没有受到太多关注。在澳大利亚的小森林（Little Forest）高度仪器化的低放废物场，可以进行详细的 REY 研究，调查与当地地层、相邻废物形式和浅层地下水流动的相互作用。2007 年至 2012 年，从小森林研究点和同一地质材料的区域基线点采集地下水样本和岩芯材料中的固体。水样中的 REY 采用自动螯合预浓缩（SeaFast，ESI）后进行电感耦合等离子体质谱法（ICP-MS）测定，而固体样品则采用中子活化分析（NAA）和 X 射线荧光扫描（ITRAX）进行分析。固体岩石显示，无论是在小森林还是区域背景点，REE 都没有偏离上地壳的典型组成。在废物沟附近约 4-5 米深、略低于废物沟水平的浅层地下水中，相对于页岩归一化模式，铥、铕和钆出现明显的富集，钇则出现亏损，表明水-废物相互作用。来自附近城市垃圾填埋场的浸出液样品显示出不同的 REY 归一化模式。随着与页岩和砂岩相互作用的增加，REY 的分布随深度而变化。pH 值和氧化还原条件的变化导致广泛的 Fe-氢氧化物沉淀，这些沉淀通过沉淀固体的不同吸收来吸附 REE，从而导致地下水沿流动路径中 Y 增加和 Y/Ho 比值升高。我们的研究表明，小森林低放废物的 REE 特征与周围土地利用的地下水不同。REY 可用于对不同废物源进行指纹识别，评估镧系元素的迁移性，从而推断选定锕系元素的迁移性，并追踪地下水补给过程中 REE 的命运。本研究提出的方法可以改进全球当前和遗留的城市、混合和放射性废物场的源分配和污染物迁移性追踪。
