Environ. Sci. Technol. 2007, 41, 4813-4820
Assessment of the Solubility and Bioaccessibility of Barium and Aluminum in Soils Affected by Mine Dust Deposition S . S . S H O C K , * ,† B . A . B E S S I N G E R , † Y. W. LOWNEY,§ AND J. L. CLARK| Exponent, 15375 SE 30th Place, Suite 250, Bellevue, Washington, 98007, Exponent, Boulder, Colorado, and Teck Cominco Alaska Incorporated, Anchorage, Alaska
Barium is a heavy metal to which human and animal receptors may be exposed in various settingssfor example, in mineral extraction industries where the mining and milling of ores occurs. Aluminum is also an element abundant in soil and dust to which human and animal receptors may be exposed in association with such industries. This study investigated the solubility and bioaccessibility of barium and aluminum in simulated gastric fluids using an in vitro test method previously validated for lead. Soil samples were collected from the vicinity of a mine and transport road that generated fugitive dust containing barium as barite (BaSO4). It was found that barium bioaccessibility in different tundra soil and fugitive dust source materials varied greatly, between 0.07 and 66.0%, depending on sample location, grain size, solid-to-fluid ratio used in the in vitro experiments, and the analytical method selected for determining total barium concentrations in the sample substrates. For X-ray fluorescence spectrometry (XRF) analytical methods and a solid-to-fluid ratio of 1:100, barium bioaccessibility from the barite-rich mine waste rock and gyro crusher ore dust source materials was very low (0.07-0.36%). By contrast, the bioaccessibility of barium in tundra soil samples affected by fugitive dust deposition ranged from 3.8 to 19.5%. The relative solubility of barium measured in the simulated gastric fluids of this study is consistent with time-dependent dissolution of barite in mine waste rock and ore dust, and the presence of more soluble chemical forms in tundra soil. Laboratory XRF analysis was the only analytical method used in this study that accurately characterized total barium concentrations for all sample substrates. Aluminum bioaccessibility was distinguished from barium bioaccessibility by its generally lower values and smaller dependence on grain size and solid-to-fluid ratios. The range of aluminum bioaccessibility values (0.31-4.0%) is consistent with the predominance of aluminum in relatively insoluble aluminosilicate minerals.
* Corresponding author e-mail:
[email protected]; phone: (425) 519-8722; fax: (425) 519-8799. † Exponent, Bellevue, WA. § Exponent, Boulder, CO. | Teck Cominco Alaska Incorporated. 10.1021/es0703574 CCC: $37.00 Published on Web 06/07/2007
2007 American Chemical Society
Introduction It has been established that the chemical form of a metal present in soil and dust is an important factor in its availability for uptake by human and ecological receptors, because solubility in the gastrointestinal (GI) tract is directly dependent on its speciation (1, 2). The default reference dose (RfD) for estimating human toxicity and the toxicity reference values (TRVs) for assessing toxicity to ecological receptors for barium are currently based on studies where subjects were orally exposed to highly soluble forms of barium, i.e., barium chloride or barium hydroxide (3-8). However, the predominant chemical form of barium in soil (barite, BaSO4) is considerably less soluble than the forms used in these studies (9). Thus, failure to consider the low bioavailability of the forms of barium predominantly found in soil (bioavailability being the fraction of the ingested dose that reaches the bloodstream) can result in calculations indicating that barium presents a potential risk, when in fact exposures do not pose a significant risk. To improve the accuracy of risk assessment, the bioavailability of the forms of barium in site-specific media, relative to the bioavailability of the forms used in the toxicity studies (i.e., the relative bioavailability) needs to be taken into account. Although the traditional approach for estimating bioavailability has been with expensive in vivo studies with laboratory animals, in vitro extraction tests have been developed as a more cost-effective means of estimating bioavailability for improving exposure estimates for use in risk assessment (2). In vitro extraction tests measure bioaccessibility, which is the fraction of a substance that is soluble in a simulated gastric environment, and thus provide an estimate of the amount available for absorption into the bloodstream. The objective of the present study was to assess the bioaccessibility of barium in various soil media affected by fugitive dust from mining operations where barite was generated as a waste byproduct. The approach was to simulate the interaction of soil and dust with gastric juices using an in vitro method previously developed and validated for assessing the relative oral bioavailability of lead using paired in vitro and in vivo studies (1, 10-12). Because grain size and solid-to-fluid ratios have been shown to affect bioaccessibility values (1, 2, 11, 13), these parameters were varied in the study. Bioaccessibility testing for aluminum was also conducted. Although the results of this study are focused on soils and dust near a mining site where barite is present in waste rock, they may also be applicable to other exposure scenarios (e.g., oil and gas well drilling and/or the manufacture of various barium compounds). Site Background. The principal focus of this investigation was the bioaccessibility of barium in soil and dust near the DeLong Mountain Regional Transportation System (DMTS) road and Red Dog Mine. The DMTS is a transport road that connects the Red Dog Mining District in western Alaska to its port facility 50 miles to the west. Previous investigations have shown that fugitive dust has been generated along the DMTS as a result of the escape of ore concentrates from haul trucks and tracking of mine soil and dust onto the road (14, 15). Barium concentrations in ore concentrates were found to range from 2400 to 2700 mg/kg, which are higher than concentrations in background soil by a factor of 10 (background mean was 315 mg/kg; 15). In addition, barium concentrations are even higher in the ore (on the order of 3-7%), waste rock (on the order of 10-14%), and tailings (on the order of 3-7%) generated during the mining and milling process at the mine. VOL. 41, NO. 13, 2007 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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TABLE 1. Summary of Barium Concentrationsa XRF concentration in soil
soil ID
grain size
3050B/ICP-OESb conc. in soil (mg/kg)
tundra-road tundra-road tundra-road tundra-mine tundra-mine tundra-mine main waste road main waste road main waste road gyro crusher dust gyro crusher dust gyro crusher dust
