Toxicity of Data-Poor Metals in Soil-Plant Systems

点击翻译按钮获取中文摘要

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.