Toxicity of Data-Poor Metals in Soil-Plant Systems

这项拟议的工作将研究生态上“数据贫乏”的稀土元素(REE)和铂族元素(PCE)。钯(Pd)和铂(Pt)等PGE从汽车催化转化器以及开采贱金属和贵金属的过程中共同排放到环境中，从而沉积到土壤中。稀土对数字电子设备和能源技术至关重要，它们是提高作物产量的微量营养素；稀土的采矿开采量从1990年的5万吨/年增加到2010年的15万吨/年。兴奋效应是对压力的一种双相反应，即对测量终点的刺激和抑制都取决于浓度。这种反应在物种、反应和毒物中非常普遍，但毒性阈值的推导通常不包括足够低的暴露浓度来引起兴奋。如果在得出毒性阈值的数据中没有发现兴奋效应，那么由此产生的土壤质量指南(SQG)就不能保护环境健康。土壤中金属的生物可及性取决于土壤的物理和化学特性，已有研究表明，当归一化到土壤的ECEC、pH、粘粒百分比和OM等土壤特性时，土壤总[金属]的毒性阈值更协调。其目的是：确定植物在土壤中单独和整个混合物中暴露于稀土、铈(Ce)、钕(ND)、镧(La)和Y(Y)的毒性阈值；确定植物在土壤中单独和整个混合物中暴露于钯(Pd)和铂(Pt)中的毒性阈值；确定土壤中pGES和REE的生物可获得性及其控制方法；以及量化稀土元素和pGES对响应终点的刺激作用。有人说，“在生态毒理学中接受兴奋作用和重要性需要重大的概念变化和大量的新数据”。拟议中的研究将通过确定对这些数据贫乏的金属的兴奋剂量反应来应对这一挑战，如果没有这些数据，关于如何以及是否应该将兴奋作用纳入ERA框架的辩论就无法推进。物种或基因类型之间毒性阈值的差异是种群或群落因环境污染物，特别是亚致死暴露而改变结构的原因。植物对污染物的兴奋反应可能因物种不同而不同，几乎可以肯定的是，植物对环境毒素的敏感性存在差异，如果不能充分描述植物对环境污染物的刺激剂量反应，可能会导致NOEC高于“真正的NOEC”，这将是刺激反应的峰值。鉴于加拿大新矿产资源的开发(例如火环)和加拿大海外矿业公司的发展，加拿大必须填补这些数据的空白，以保护本国的环境，并成为采矿可持续性方面的全球领先者，这一点至关重要。拟议的工作将填补这一空白。保护不足的代价是生态系统结构和功能的丧失；过度保护会造成不必要的减少和补救，这会产生生产力成本。这项工作的第二个影响将是为可能发生在受污染地点的所有混合物建立毒性阈值--单一金属毒性阈值是否足以保护环境免受金属混合物的影响的问题可能是“不”的，为二元和三种金属混合物的全因数建立毒性阈值是资源受限的，因此“全混合物”毒性阈值填补了环境保护的一个重要数据空白。

The proposed work will work on Rare Earth Elements (REEs) and Platinum Group Elements (PGEs) which are ecologically ‘data-poor’. PGEs such as Palladium (Pd) and platinum (Pt) are co-emitted into the environment from automotive catalytic converters, as well as from mining both base- and precious metals, thus are deposited to soils. REEs are critical to digital electronic devices and energy technologies, and they are micronutrients that enhance crop production; mining extraction of REEs increased from 50 kt/year in 1990 to 150 kt/year in 2010. Hormesis is a response to a stress which is bi-phasic, i.e. both stimulation and inhibition of the measured endpoint depending on concentration. This response is extremely widespread across species, responses and toxic agents, but the derivation of toxicity thresholds typically doesn’t include low enough exposure concentrations to cause hormesis. If hormesis is not identified in the data from which toxicity thresholds are derived, then resulting soil quality guidelines (SQGs) are not protective of environmental health. Bioaccessibility of metals in soils depends on soil physical and chemical characteristics, and it has been demonstrated that toxicity thresholds for total soil [metal] were more harmonized when normalized to soil characteristics such as eCEC, pH, %clay and OM. The objectives are to: determine toxicity thresholds for plants exposed to the REEs Cerium (Ce), Neodymium (Nd), Lanthanum (La) and Yttrium (Y) in soil, singly and in whole mixtures; to determine toxicity thresholds for plants exposed to the PGEs Palladium (Pd) and Platinum (Pt) in soil, singly and in whole mixture; to identify the bioaccessibility of PGEs and REEs in soils and how that is controlled; and to quantify hormesis for REEs and PGEs for response endpoints. It has been said that “The acceptance of hormesis and essentiality in ecotoxicology requires major conceptual changes and considerable new data”. The proposed research will respond to that challenge by identifying hormetic dose-responses to these data-poor metals, without which the debate on how and if hormesis should be accommodated in the ERA framework as questioned can’t advance. Variation in toxicity thresholds among species or genotypes is the reason why populations or communities shift structure in response to environmental contaminants, particularly for exposures that are sub-lethal. Hormetic responses by plants to contaminants, which may be differential among species, are almost certainly part of variation in sensitivity to environmental toxins, and failure to adequately characterize hormetic dose responses to environmental contaminants could result in a NOEC higher than the “true NOEC”, which would be the peak of the hormetic response. Given the development of new mineral resources in Canada (e.g. The Ring of Fire) and by Canadian mining companies abroad, it is crucial that Canada fill the data gaps for these data poor metals for its own environmental protection, as well as being a demonstrable global leader in sustainability of mining. The proposed work will fill that gap. The cost of under-protection is loss of ecosystem structure and function; over-protection imposes needless reduction and remediation which have productivity costs. A second impact of the work will be the establishment of toxicity thresholds for the whole mixtures likely to occur at contaminated sites – the question of whether single-metal toxicity thresholds adequately protect the environment from metal mixtures is likely “no”, establishing toxicity thresholds for a full factorial of binary and tertiary metal mixtures is resource –prohibitive, so ‘whole mixture’ toxicity thresholds fill an important data gap for environmental protection.