Industrially Purified Nd Materials Identified by Distinct Mass-Dependent Isotopic Composition

N. Bothamy, A. Galy
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引用次数: 2

Abstract

Rare earth elements (REEs) are considered emerging anthropogenic pollutants. Anthropogenic lanthanum, cerium, samarium, and gadolinium alone, or excess of all the REEs have already been reported in the environment. In addition, it is only a matter of time for neodymium (Nd) of anthropogenic origin to be reported disseminated in the environment, given its growing demand for new technologies and its use in permanent magnets of wind turbine. So far, only in a few cases was the addition of anthropogenic Nd detected in soils and sediments by the measurements of REE concentrations. For this reason, we propose to use the Nd isotopic composition to help the distinction of pollution. The isotopic tracing of Nd using variations in the abundance of 143Nd from the radioactive decay of 147Sm (Nd-radiogenic composition) is one option. Here, we expand the Nd isotopic fingerprinting by the investigation of the stable Nd isotopic composition expressed as δxNd, the relative permil (%0) deviation from the isotopic composition of the pure Nd JNdi-1 reference standard. The measurement of δxNd used a MC-ICPMS (multi-collector inductively coupled plasma mass spectrometry) with sample-standard bracketing technique, allowing the determination of precise and accurate Nd isotopic variations. Our results show that Nd-magnets (Neo) and man-made purified Nd materials are not significantly different on average (respectively, δ148Nd of −0.105 ± 0.023 and −0.120 ± 0.141%0). More importantly, they are different from terrestrial rocks (δ148Nd of −0.051 ± 0.031%0). Moreover, the Nd-radiogenic composition of Neo can be highly variable, even when they come from a single supplier. In addition, the study of all Nd stable isotopic compositions demonstrates that irrespective of their natural origin (witnessed by their Nd-radiogenic composition), all Nd from rocks and man-made materials are related by mass-dependent isotopic fractionation laws. We also have defined a parameter, the Δ148−150Nd′, allowing the distinction of thermodynamic isotopic fractionation (the Δ148−150Nd′ is invariant) from kinetic isotopic fractionation (the Δ148−150Nd′ is negatively correlated with the δ148Nd). Such covariation is observed for anthropogenic materials that could be seen as small deficit in 150Nd (around 5 ppm/%0/amu), but too small to be consistent with nuclear field effect. On the other hand, the anthropogenic material defines a covariation in the Δ148−150Nd'–δ148Nd space in full agreement with the theoretical expectation from mass-dependent kinetic isotopic fractionation. The mass-dependent fractionation of Nd by chromatographic separation is also consistent with a kinetic isotopic fractionation. The purification of Nd from other light REEs by industrial processes involves chromatographic separation and, therefore, is likely to produce anthropogenic Nd with low values for δ148Nd associated with high values for Δ148−150Nd′. Both are resolvable with current MC-ICPMS technology and could be useful to trace incoming anthropogenic pollution in the environment. In soils, the combination of low values for δ148Nd with high values for Δ148−150Nd′ is likely to be an unambiguous pollution signal from the degradation in the environment of Neo or other industrial products, especially if this is associated with an Nd-radiogenic composition inconsistent with the surrounding rocks and soils. In contrast, the industrial residue of Nd purification could be characterized by high δ148Nd with low values for Δ148−150Nd′, and the leak or the discharge of such residue could also be unambiguously distinguished.
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用不同质量相关同位素组成鉴定工业纯化Nd材料
稀土元素被认为是新出现的人为污染物。环境中已经报道了单独的人为镧、铈、钐和钆,或所有稀土元素的过量。此外,鉴于钕对新技术的需求不断增长,以及其在风力涡轮机永磁体中的用途,据报道,人为来源的钕在环境中传播只是时间问题。到目前为止,只有在少数情况下,通过测量REE浓度,在土壤和沉积物中检测到人为Nd的添加。因此,我们建议使用Nd同位素组成来帮助区分污染。利用147Sm(Nd放射成因成分)放射性衰变中143Nd丰度的变化进行Nd同位素示踪是一种选择。在这里,我们通过研究以δxNd表示的稳定Nd同位素组成,即与纯Nd JNdi-1参考标准的同位素组成的相对允许偏差(%0),来扩展Nd同位素指纹图谱。δxNd的测量使用MC-ICPMS(多收集器电感耦合等离子体质谱法)和样品标准包围技术,从而能够准确测定Nd同位素的变化。我们的研究结果表明,钕磁体(Neo)和人造纯钕材料的平均差异不大(分别为δ148Nd−0.105±0.023和−0.120±0.141%0)。更重要的是,它们与陆地岩石(δ148钕−0.051±0.031%0)不同。此外,即使来自单一供应商,Neo的Nd放射成因成分也可能高度可变。此外,对所有Nd稳定同位素组成的研究表明,无论其自然来源如何(从Nd的放射成因组成来看),岩石和人造材料中的所有Nd都与质量依赖的同位素分馏定律有关。我们还定义了一个参数Δ148−150Nd′,允许区分热力学同位素分馏(Δ148−150 Nd′是不变的)和动力学同位素分馏(△148−150恩德′与δ148Nd负相关)。在150Nd(约5ppm/%0/amu)的人为物质中观察到这种协变,但其太小,与核场效应不一致。另一方面,人为物质在Δ148−150Nd’–δ148Nd空间中定义了协变,与依赖质量的动力学同位素分馏的理论预期完全一致。通过色谱分离的Nd的质量依赖性分馏也与动力学同位素分馏一致。通过工业过程从其他轻稀土中纯化Nd涉及色谱分离,因此,可能产生δ148Nd值较低、Δ148−150Nd′值较高的人为Nd。两者都可以通过当前的MC-ICPMS技术解决,并可用于追踪环境中的人为污染。在土壤中,δ148Nd的低值与Δ148−150Nd′的高值的组合可能是Neo或其他工业产品环境退化产生的明确污染信号,特别是如果这与Nd的放射性成分与周围岩石和土壤不一致有关。相反,Nd提纯的工业残留物可以以高δ148Nd为特征,Δ148−150Nd′的值较低,并且这种残留物的泄漏或排放也可以明确区分。
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