A Biotic Ligand Model for Phytotoxicity of Rare Earth Elements in Mixture

由于农业径流，圣劳伦斯河中稀土元素的浓度正在增加，由此可以推断圣劳伦斯盆地土壤中的土壤浓度增加。由于加拿大东部土壤的土壤pH值低于加拿大西部，土壤中稀土的毒性潜力正在增加，稀土混合物对土壤生物群的额外风险尚不清楚。Ce(Ce)、La(La)、Nd(Nd)、Sm(Sm)是轻稀土元素，它们以二价离子的形式被植物吸收，但也可能以三价离子的形式转移。作为三价阳离子，La被认为取代了细胞内和细胞外结合位置上的二价Ca，La的特征是超钙。稀土对植物的营养效应包括提高光合作用和产量，提高养分的吸收和利用，增强呼吸和水分吸收，以及增强对胁迫的耐受性。拟议工作的第一个目标是扩大对陆地植物和终点的稀土毒性阈值的了解。这些阈值将在pH、有效阳离子交换量(ECEC)和主要土壤阳离子的范围内的多个土壤中确定，这些因素预计会改变稀土的毒性。将使用两种不同类型的浓度比率来确定单一金属和混合物的这些阈值：等毒浓度(使用单一金属研究得出的有效浓度)和环境浓度(根据地壳和采矿废物数据)。稀土混合物的响应将使用一套统计模型来表征，这些模型识别响应是否遵循浓度添加、响应添加或一般线性模型。这些数据将产生单个稀土元素的效力系数。这些数据还将建立可检验的假设，即土壤中稀土的形态和运动可能如何改变植物对稀土混合物的名义暴露。第二个目标是利用批量提取、DGT设备和地球化学形态模拟软件，通过确定土壤特性、土壤中可生物获得的稀土和组织浓度之间的关系，来探索这些可检验的稀土在土壤中的行为假说。第三个目标是将前两个目标的数据整合到BLM型植物毒性框架中，该框架可应用于土壤中单一或混合的稀土元素，如WHAM-FTOX。这将包括由土壤特性改变的稀土的溶液形态，稀土之间以及稀土与其他土壤阳离子之间的竞争，对生物配体的竞争，以及通过土壤-植物连续体的扩散和大量迁移可能如何改变暴露。该项目的数据将对加拿大监管机构提供稀土的土壤质量指导方针具有实际用途。这些数据还将促进关于土壤金属对植物毒性的基础知识，这些元素在溶液中是三价的。

Concentrations of rare earth elements (REEs) are increasing in the St. Lawrence River, resulting from agricultural runoff, from which increased soil concentrations in the St. Lawrence basin soils can be inferred. As soil pH is lower in eastern Canadian soils than in western Canada, the potential for toxicity of REEs in soil is growing, and the additional risk of REE mixtures to soil-dwelling biota, unknown. Cerium (Ce), Lanthanum (La), Neodymium (Nd), Samarium (Sm) are light REEs (LREEs) which are taken up by plants as divalent ions but may be translocated as trivalent ions. As trivalent cations, the lanthanides are thought to displace the divalent Ca from extra- and intra-cellular binding sites, with La being characterized as super-calcium. The nutrient effects of REEs on plants include increased photosynthesis and thus yield, enhanced uptake and utilization of nutrients, enhanced respiration and uptake of water and increased tolerance to stress. The first objective of the proposed work is to expand the knowledge of REE toxicity thresholds for terrestrial plants and endpoints. These thresholds will be determined in multiple soils that range in pH, effective cation exchange capacity (eCEC) and major soil cations, factors that are expected to vary the toxicity of REEs. These thresholds will be determined for both single metal and mixtures using two different types of concentration ratios: equitoxic (using the effect concentrations derived from single metal studies) and environmental (based on crustal and mining waste data). The REE mixtures responses will be characterized using a suite of statistical models that identify whether the responses follow concentration addition, response addition or a general linear model. These data will create the potency factors for the individual REEs. The data will also create testable hypotheses of how REE speciation and movement in the soil might be modifying the nominal exposure of plants to REE mixtures. The second objective is to explore these testable hypotheses of REE behavior in the soil by identifying the relationships among soil characteristics, bioaccessible REEs in soils, and tissue concentrations using batch extraction, DGT devices and geochemical speciation modelling software. The third objective is to integrate the data from the first two objectives into a BLM-type framework for plant toxicity that can be applied to REEs in soil, singly or in mixture, such as WHAM-FTOX. This will include the solution speciation of the REEs as modified by soil characteristics, competition among REEs and between REEs and other soil cations, for the biotic ligand, and how diffusive and bulk transport through the soil-plant continuum might modify exposure. The data from this project will be of practical use to Canadian regulators to inform soil quality guidelines for REEs. The data will also advance fundamental knowledge of soil metal toxicity to plants for elements that are trivalent in solution.