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（REE）的浓度正在增加，由此可以推断圣劳伦斯河流域土壤中的稀土元素浓度增加。 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，稀土混合物对土壤生物群的额外风险未知。铈（Ce）、镧（La）、钕（Nd）、钐（Sm）是轻稀土元素（LREE），它们以二价离子的形式被植物吸收，但也可以三价离子的形式迁移。 作为三价阳离子，镧系元素被认为从细胞外和细胞内结合位点置换二价Ca，其中La被表征为超钙。 稀土元素对植物的营养效应包括增加光合作用，从而提高产量，增强营养物质的吸收和利用，增强呼吸作用和水分吸收，以及增强对胁迫的耐受性。 拟议的工作的第一个目标是扩大知识的稀土毒性阈值陆生植物和端点。 这些阈值将在多种土壤中确定，其范围包括pH值、有效阳离子交换容量（eCEC）和主要土壤阳离子，这些因素预计会改变稀土元素的毒性。 将使用两种不同类型的浓度比确定单一金属和混合物的阈值：等毒性（使用单一金属研究得出的效应浓度）和环境（基于地壳和采矿废物数据）。将使用一套统计模型来表征REE混合物的响应，该模型确定响应是否遵循浓度相加、响应相加或一般线性模型。 这些数据将创建单个稀土元素的效价因子。这些数据还将创建可验证的假设，如何稀土元素的形态和运动在土壤中可能会修改名义上的暴露于稀土混合物的植物。第二个目标是探索这些可测试的假设，稀土元素在土壤中的行为，通过确定土壤特性，生物可获得的稀土元素在土壤中，和组织浓度之间的关系，使用批量提取，DGT设备和地球化学形态建模软件。 第三个目标是将前两个目标的数据整合到一个BLM类型的植物毒性框架中，该框架可以单独或混合应用于土壤中的稀土元素，如WHAM-FTOX。这将包括受土壤特征影响的稀土元素溶液形态、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大量运输如何改变暴露。 该项目的数据将为加拿大监管机构提供实际用途，为稀土元素的土壤质量指南提供信息。 这些数据也将推进土壤金属毒性的基础知识，植物的元素是三价溶液。

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.