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

由于农业径流，圣劳伦斯河中的稀土元素（ree）浓度正在增加，由此可以推断出圣劳伦斯流域土壤中稀土元素浓度的增加。由于加拿大东部土壤的pH值低于加拿大西部，土壤中稀土元素的潜在毒性正在增加，而稀土元素混合物对土壤生物群落的额外风险尚不清楚。铈（Ce）、镧（La）、钕（Nd）、钐（Sm）是轻稀土（lree），它们被植物作为二价离子吸收，但也可能作为三价离子转移。作为三价阳离子，镧系元素被认为取代了细胞外和细胞内结合位点上的二价钙，而镧被表征为超钙。稀土对植物的营养作用包括增加光合作用从而提高产量，提高养分的吸收和利用，增强呼吸和水分的吸收以及提高对胁迫的耐受性。提出的工作的第一个目标是扩大对陆生植物的稀土毒性阈值和终点的认识。这些阈值将在pH值、有效阳离子交换容量（eCEC）和主要土壤阳离子范围内的多种土壤中确定，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确定单一金属和混合物的这些阈值：当量（使用单一金属研究得出的影响浓度）和环境（基于地壳和采矿废物数据）。REE混合物的响应将使用一套统计模型来表征，这些模型可以确定响应是否遵循浓度添加、响应添加或一般线性模型。这些数据将为个体稀土元素创造潜能因子。这些数据还将提出可测试的假设，说明土壤中稀土元素的形成和运动可能如何改变植物对稀土混合物的名义暴露。第二个目标是通过使用批量提取、DGT设备和地球化学物种形成建模软件，确定土壤特征、土壤中生物可及的稀土元素和组织浓度之间的关系，探索土壤中稀土元素行为的这些可测试假设。第三个目标是将前两个目标的数据整合到植物毒性的blm型框架中，该框架可以单独或混合应用于土壤中的稀土元素，例如WHAM-FTOX。这将包括受土壤特征改变的稀土元素的溶液形态，稀土元素之间以及稀土元素与其他土壤阳离子之间对生物配体的竞争，以及通过土壤-植物连续体的扩散和散装运输如何改变暴露。该项目的数据将对加拿大监管机构提供稀土元素土壤质量指南具有实际用途。这些数据还将推进土壤金属对植物的毒性的基本知识，在溶液中是三价的元素。

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.