Investigation of the Solution Complexation Behavior of the Rare Earth Elements with Naturally Occurring Organic Ligands in Natural Terrestrial Waters

[0001086]约翰内森了解环境中稀土元素（REE）和其他重金属的化学行为对于预测它们对环境的影响以及它们在环境中的命运和运输至关重要。影响天然水体中这些和其他重金属的最重要因素之一是溶液络合。例如，溶液络合作用可以控制环境中重金属的迁移性、有效溶解度、反应性和毒性。不幸的是，金属与自然发生的有机配体的形成，以及它与这些过程和总金属浓度的直接联系，对大多数微量金属，包括稀土元素，知之甚少。虽然地球化学模型确实存在，可以预测天然水体中稀土与无机配体（即CO32-、p43 -、SO42-、OH-、CI-）的络合，但它们与天然水体中天然存在的有机配体的原位络合行为尚未得到专门研究。尽管与许多简单有机酸的稀土配合物的稳定常数与最强的稀土-无机（即碳酸盐）配合物的稳定常数相同或更大，但我们对稀土络合作用的理解仍然存在差异。因此，有机络合作用也可能控制着天然水体中稀土元素的形态，正如许多过渡金属阳离子所显示的那样。因此，考虑到对天然水体中稀土与有机配体的[原位]络合所知的很少，我们研究的主要目标是：(1)修改和开发一种特定的电化学技术，以(a)测量天然水体中溶解的稀土与天然存在的有机配体络合的比例，(b)测量这些天然存在的稀土-有机配体络合的强度（即条件稳定常数）；(2)应用改进的电化学方法测量了跨越天然水体pH范围、含有不同浓度无机络合配体和溶解有机碳浓度的水体中稀土的有机络合作用；(3)在早期平衡热力学模型的基础上，建立稀土溶液络合的定性模型，解决无机配体和天然有机配体之间的竞争问题。这些目标将采用实验室和实地相结合的方法来解决。我们将改进目前用于过渡金属的竞争配体平衡/吸附阴极溶出伏安法（CLE/ACSV）技术，以量化天然水中溶解的稀土元素的数量，这些稀土元素以天然有机配体的溶液配合物的形式存在。将收集天然水样进行浓度比较和对比，其中有机配体必须与无机碳酸盐配合物、中等浓度的中性pH水或无机络合配体和低DOC以及高DOC的酸性水竞争。我们关于稀土与天然有机配体络合的结果将与目前使用的平衡（无机配体物种形成）模型相结合。

0001086JohannessonUnderstanding the chemical behavior of the rare earth elements (REE) and other heavy metals in the environment is critical to predicting their impact on, as well as their fate and transport within, the environment. One of the most important factor affecting these and other heavy metals in natural waters is solution complexation. Solution complexation, for example, may exert controls on the mobility, effective solubility, reactivity, and toxicity of heavy metals in the environment. Unfortunately, metal speciation with naturally occurring organic ligands, and its direct connection to those processes and total metal concentrations, is only poorly known for most trace metals, including the REEs. Although geochemical models do exist that allow predictions to be made of REEs complexation with inorganic ligands (i.e., CO32-, P 4 3-, SO42-, OH-, CI- in natural waters, their in situ complexation behavior with naturally occurring organic ligands in natural waters has not been specifically studied. This disparity in our understanding of REE complexation exists despite the fact that stability constants for REE complexes with many simple organic acids are of the same magnitude, or greater, than stability constants for the strongest REE-inorganic (i.e., carbonate) complexes. Therefore, it is possible that organic complexation also controls the speciation of the REEs in natural waters, as has been shown to be the case for many transition metal cations. Consequently, considering what little is known about the [in situ] complexation of REEs with organic ligands in natural waters, the chief objectives of our study are: (1) modify and develop a specific electrochemical technique to (a) measure the fraction of dissolved REEs complexed with naturally occurring organic ligands in natural waters, and (b) measure the strength (i.e., conditional stability constants) of these naturally occurring REE-organic ligand complexes; (2) apply the modified electrochemical method to measure organic complexation of REEs in waters that span the pH range of natural waters and contain various concentrations of inorganic complexing ligands and dissolved organic carbon concentrations; and (3) develop a qualitative model of REE solution complexation that addresses competition between inorganic ligands and naturally occurring organic ligands and that builds upon an earlier equilibrium thermodynamic model. These objectives will be addressed using a combined laboratory and field approach. We will modify the competitive ligand equilibration/adsorptive cathodic stripping voltammetry (CLE/ACSV) technique currently used for transition metals to quantify the amount of dissolved REEs that occur in natural waters as solution complexes with naturally occurring organic ligands. Natural water samples will be collected to compare and contrast concentrations, where organic ligands must compete with inorganic carbonate complexes, neutral pH waters with moderate concentrations or inorganic complexing ligands and low DOC, and acidic waters with high DOC. Our results concerning REE complexation with natural organic ligands will be combined with an equilibrium (inorganic ligand speciation) model currently in use.