Bioaccessibility of Lanthanide Metals. M. F. Shibata, K. M. Johnson, and W. Gorham, Tetra Tech, Inc., ENSR, Inc.
Lanthanide metals (also known as rare earths) are a group of 15 elements that are used in electronic, automotive, environmental, and medical technologies. U.S. mining operations supply raw material for one-third of the world’s need for lanthanides. A risk assessment was performed recently to evaluate the potential for adverse human health and ecological effects that may occur as a result of expanding lanthanide-mining operations. A key factor in estimating human and biotic exposures was the bioaccessibility of lanthanide metals. Bioaccessibility is a measure of the amount of an administered dose that is available for uptake across cell membranes (i.e., uptake into an organism) and is typically expressed as a proportion or percentage of the administered dose. Site-specific exposures to lanthanide metals were estimated using the proportion of the total concentration that is soluble under simulated gastrointestinal tract conditions (including stomach pH, mixing, and emptying rates). This in vitro method has been validated, primarily for lead (Ruby et al., 1993, 1996) in tests conducted with miniature swine. Soil, indoor carpet dust, wastewater solids, and pipeline scale samples were collected at the mining facility. Under conditions that simulate the mammalian stomach, the bioaccessibility of the most prevalent lanthanide metals ranged from 1 to 16 percent for soils and indoor carpet dust (i.e., environmental media that have not undergone acid leaching to extract lanthanide metals); and, from 36 to 91 percent for wastewater solids and pipeline scale (i.e., environmental media that have undergone acid leaching to extract lanthanide metals). The following bioaccessibility factors were used in the human health and ecological risk assessment (in conjunction with newly derived toxicity data for lanthanide metals): (1) six percent for pre-leachate media and (2) 68 percent for post-leachate media.
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