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

了解稀土元素(REE)和其他重金属在环境中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去向和迁移至关重要。影响天然水体中这些重金属和其他重金属的最重要因素之一是溶液络合作用。例如，溶液络合作用可以控制环境中重金属的流动性、有效溶解度、反应性和毒性。不幸的是，金属与天然有机配体的形态及其与这些过程和总金属浓度的直接联系，对于包括稀土在内的大多数痕量金属来说，知之甚少。虽然已有的地球化学模型可以预测稀土与天然水中无机配体(即CO32-、P43-、SO42-、OH-、CI-)的络合作用，但它们与天然有机配体的原位络合行为尚未得到具体研究。尽管许多简单有机酸的稀土络合物的稳定常数与最强的稀土-无机(即碳酸盐)络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存在这种差异。因此，有机络合作用也可能控制天然水中稀土的形态，正如许多过渡金属阳离子所显示的那样。因此，考虑到天然水中稀土与有机配体的[原位]络合知之甚少，本研究的主要目标是：(1)改进和发展一种特定的电化学技术，以(A)测量天然水中溶解的稀土与天然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物的强度(即条件稳定常数)；(2)应用改进的电化学方法测量跨越自然水域pH范围且含有不同浓度的无机络合配体和溶解有机碳的水域中稀土的有机络合作用；(3)发展了稀土溶液络合的定性模型，该模型考虑了无机配体和天然有机配体之间的竞争，并建立在早期的平衡热力学模型的基础上。这些目标将采用实验室和实地相结合的方法来实现。我们将修改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)，以定量天然水中以天然有机配体络合物形式存在的溶解稀土的量。将收集天然水样以比较和对比浓度，其中有机配体必须与无机碳酸盐络合物竞争，中等浓度的中性pH水或无机络合配体和低DOC的水，以及高DOC的酸性水。我们关于稀土与天然有机配体络合的结果将与目前正在使用的平衡(无机配体形态)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.