Kenneth J. Reimer

An inter-laboratory trial of the unified BARGE bioaccessibility method for arsenic, cadmium and lead in soil

The Bioaccessibility Research Group of Europe (BARGE) has carried out an inter-laboratory trial of a proposed harmonised in vitro physiologically based ingestion bioaccessibility procedure for soils, called the Unified BARGE Method (UBM). The UBM includes an initial saliva phase and simulated stomach and intestine compartments. The trial involved the participation of seven laboratories (five European and two North American) providing bioaccessibility data for As (11 samples), Cd (9 samples) and Pb (13 samples) using soils with in vivo relative bioavailability data measured using a swine model. The results of the study were compared with benchmark criteria for assessing the suitability of the UBM to provide data for human health risk assessments. Mine waste and slag soils containing high concentrations of As caused problems of poor repeatability and reproducibility which were alleviated when the samples were run at lower soil to solution ratios. The study showed that the UBM met the benchmark criteria for both the stomach and stomach & intestine phase for As. For Cd, three out of four criteria were met for the stomach phase but only one for the stomach & intestine phase. For Pb two, out of four criteria were met for the stomach phase and none for the stomach & intestine phase. However, the study recommends tighter control of pH in the stomach phase extraction to improve between-laboratory variability, more reproducible in vivo validation data and that a follow up inter-laboratory trial should be carried out.

Effects of Soil Composition and Mineralogy on the Bioaccessibility of Arsenic from Tailings and Soil in Gold Mine Districts of Nova Scotia

Bioaccessibility tests and mineralogical analyses were performed on arsenic-contaminated tailings and soils from gold mine districts of Nova Scotia, Canada, to examine the links between soil composition, mineralogy, and arsenic bioaccessibility. Arsenic bioaccessibility ranges from 0.1% to 49%. A weak correlation was observed between total and bioaccessible arsenic concentrations, and the arsenic bioaccessibility was not correlated with other elements. Bulk X-ray absorption near-edge structure analysis shows arsenic in these near-surface samples is mainly in the pentavalent form, indicating that most of the arsenopyrite (As(1-)) originally present in the tailings and soils has been oxidized during weathering reactions. Detailed mineralogical analyses of individual samples have identified up to seven arsenic species, the relative proportions of which appear to affect arsenic bioaccessibility. The highest arsenic bioaccessibility (up to 49%) is associated with the presence of calcium-iron arsenate. Samples containing arsenic predominantly as arsenopyrite or scorodite have the lowest bioaccessibility (<1%). Other arsenic species identified (predominantly amorphous iron arsenates and arsenic-bearing iron(oxy)hydroxides) are associated with intermediate bioaccessibility (1 to 10%). The presence of a more soluble arsenic phase, even at low concentrations, results in increased arsenic bioaccessibility from the mixed arsenic phases associated with tailings and mine-impacted soils.

Variability of bioaccessibility results using seventeen different methods on a standard reference material, NIST 2710

Bioaccessibility is a measurement of a substances solubility in the human gastro-intestinal system, and is often used in the risk assessment of soils. The present study was designed to determine the variability among laboratories using different methods to measure the bioaccessibility of 24 inorganic contaminants in one standardized soil sample, the standard reference material NIST 2710. Fourteen laboratories used a total of 17 bioaccessibility extraction methods. The variability between methods was assessed by calculating the reproducibility relative standard deviations (RSDs), where reproducibility is the sum of within-laboratory and between-laboratory variability. Whereas within-laboratory repeatability was usually better than (<) 15% for most elements, reproducibility RSDs were much higher, indicating more variability, although for many elements they were comparable to typical uncertainties (e.g., 30% in commercial laboratories). For five trace elements of interest, reproducibility RSDs were: arsenic (As), 22–44%; cadmium (Cd), 11–41%; Cu, 15–30%; lead (Pb), 45–83%; and Zn, 18–56%. Only one method variable, pH, was found to correlate significantly with bioaccessibility for aluminum (Al), Cd, copper (Cu), manganese (Mn), Pb and zinc (Zn) but other method variables could not be examined systematically because of the study design. When bioaccessibility results were directly compared with bioavailability results for As (swine and mouse) and Pb (swine), four methods returned results within uncertainty ranges for both elements: two that were defined as simpler (gastric phase only, limited chemicals) and two were more complex (gastric + intestinal phases, with a mixture of chemicals).

Effect of particle size on arsenic bioaccessibility in gold mine tailings of Nova Scotia.

Tailings samples from the Goldenville and Montague abandoned gold mines in Nova Scotia, Canada were subjected to bioaccessibility tests to examine the effects of the choice of particle size fraction on the bioaccessibility of arsenic. The proportion of finer grains (<150μm) in this sample set varied from 6.0 to 66wt.%. Samples were sieved to <250, <150, and <45μm particle size fractions. The arsenic bioaccessibility ranged from less than 1.0 to 48%, but no systematic variation was observed (p>0.13) precluding the association of greater percent arsenic bioaccessibility with a specific particle size fraction, method or site. On the other hand, the highest bioaccessible arsenic concentrations (up to 5200mgkg(-1)) were consistently observed in samples sieved to the <45μm particle size, for both the physiologically based extraction test and a glycine-buffered bioaccessibility method (in 89 and 87% of samples tested, respectively). This was due to higher total arsenic concentrations in the same particle size fraction. Grain maps obtained by X-ray absorption spectroscopy indicate that samples with the highest percent arsenic bioaccessibility contain amorphous pentavalent arsenic distributed throughout the sample as well as grains coated with pentavalent arsenic. Arsenic bioaccessibilities lower than 10% were found in samples with encapsulated arsenopyrite and some grains composed primarily of pentavalent arsenic. The <45μm particle size fraction appears to yield conservative (protective) estimates of the bioaccessible dose of arsenic, but wide variations exist in particle size distribution and arsenic bioaccessibility between samples. As well, sieving to <45μm may exclude potentially relevant particles by restricting the study to an average particle size that is smaller than the average size of particles found on human hands, and may unduly influence the resulting bioaccessibility measurements.

Bioaccessibility and speciation of arsenic in country foods from contaminated sites in Canada

Arsenic in foods obtained through foraging or hunting (country foods) in contaminated areas has not been reported; moreover the chemical form (arsenic speciation) is not known. Bioaccessibility extractions can be used to extract the arsenic from samples, giving information about the arsenic that is available for absorption into humans. Bioaccessibility of arsenic was measured in country foods (berries, other plants, mushrooms and hares) collected from contaminated sites in Canada. Arsenic speciation in the bioaccessibility extracts was also determined. Arsenic concentrations in berries ranged from 0.06 to 21 mg/kg, and Labrador tea contained 1.9 mg/kg of arsenic (all wet weight). Arsenic concentrations (wet weight) ranged up to 46 mg/kg in mushrooms, but they were much lower in hare muscle tissue (0.007 to 0.6 mg/kg). Percent bioaccessibility was lowest in berries and plants (means of 12-45%), where the arsenic species were mostly toxic inorganic arsenic. Bioaccessibility was higher in mushrooms and hare meat (means of 22-76%), where along with toxic inorganic arsenic, substantial proportions of less toxic organoarsenic species were measured, including non-toxic arsenobetaine. The speciation patterns were highly variable in both mushrooms and hare meat. Toxic forms of arsenic are present in country foods collected from contaminated areas, but the amounts vary according to and within each sample type. Therefore testing should ideally be carried out for new sample types and locations to estimate exposures to humans.

Addressing arsenic bioaccessibility in ecological risk assessment: A novel approach to avoid overestimating risk

The risk of arsenic exposure to deer mice (Peromyscus maniculatus) living in areas of naturally and anthropogenically elevated arsenic levels was determined using three separate calculations of arsenic daily intake: Estimated daily intake (EDI), bioaccessible EDI (BEDI), and actual daily intake (ADI). The present work is of particular interest, because the risk assessments were determined for animals naturally exposed to arsenic. Gastric fluid extraction was used to obtain bioaccessibility data for soil and plant samples collected from three study sites (background, mine forest, and tailings) in Yellowknife (NT, Canada). Calculations using the EDI indicated that deer mice living in tailings habitat (average soil arsenic concentration, 1,740 +/- 2,240 microg/g) should have been experiencing serious health effects as a result of their exposure to arsenic. Using BEDI and ADI in the risk assessment calculation, however, resulted in an order-of-magnitude decrease in calculated risk. In addition, results calculated using the BEDI and ADI were not significantly different, suggesting that using bioaccessibility provides a more realistic estimate of potential risk. The present results provide evidence that the use of EDI in traditional risk assessments may seriously overestimate the actual risk, which in some instances may result in expensive and unnecessary clean-up measures.

Method variables affecting the bioaccessibility of arsenic in soil

Arsenic bioaccessibility tests are now being commonly used in risk assessment. However, concerns remain about the reliability of such tests because the bioaccessibility of arsenic from soil may be susceptible to soil composition (including iron concentration), as well as method considerations such as varying liquid-to-solid ratios and the chosen buffer system. In this study, arsenic-contaminated tailings and soils were tested to compare two bioaccessibility methods: one that uses glycine as a buffer, and a second that is more physiologically based. With the glycine-buffered method, arsenic and iron bioaccessibility increased in the presence of a higher buffer concentration at higher liquid-to-solid ratios, whereas the results of physiologically-based tests were unaffected by variations in these parameters. In the glycine-buffered system, interactions between iron and glycine may influence the concentration of arsenic in solution, which may not be consistent with human gastrointestinal conditions. The choice of a physiologically-based method may be more appropriate to achieve representative arsenic bioaccessibility values toward estimating risks to human health.

Bioaccessibility of mercury in selected Ayurvedic medicines

Five Ayurvedic medicines with mercury concentrations of 85mg/kg and higher were characterized with respect to their speciation and their bioaccessibility. X-ray absorption spectroscopy revealed that the mercury in the Ayurvedic medicines was inorganic and best matched to cinnabar, even in samples that had been hypothesized to contain mercury through plant sources only. The bioaccessibility (bioaccessible concentrations and percent bioaccessibility) was measured using two methods: a two-phase physiologically based extraction test (PBET gastric, G and gastric+intestinal phase, GI); and the fed organic estimation human simulation test (FOREhST). The percent bioaccessibility of mercury in all Ayurvedic samples was very low (<5%), corresponding to the low solubility of cinnabar, but it increased with increasing dissolved organic carbon content of the bioaccessibility solutions (PBET-G<PBET-GI<FOREhST). Filtration of FOREhST solutions reduced the bioaccessible mercury concentrations to undetectable values for most of the Ayurvedic samples. Incorporation of percent relative bioaccessibility of mercury into risk calculations decreased daily intake estimates by 29-900 times, and reduced them to acceptable levels for three of the five medicines.

Bioaccessibility Extractions for Contaminant Risk Assessment

In the process of performing risk assessments for humans or animals, it has been shown that the assumption that all of a contaminant in an ingested matrix poses risk is not always accurate. Specifically, not all of the contaminant is bioavailable (absorbed by the body). Bioaccessibility, which is the fraction of a contaminant that dissolves in gastrointestinal conditions, is used to estimate bioavailability. This is because in vitro laboratory extractions can be used, since they have the potential to be cost effective, commercially viable, and comparable to bioavailability measurements. Bioaccessibility extractions were developed from the fields of pharmaceutical dissolution testing, nutrition, and safety, based on simulated gastric and intestinal fluids developed for those applications. In the development of a number of different methods, some groups have focused on simple methods with few chemicals that can be easily conducted in a laboratory, and others have targeted maximum physiological representativeness with its accompanying complexity (e.g., dynamic systems and incorporation of microorganisms). Bioaccessibility of inorganic contaminants is most susceptible to changes in pH, and organic contaminant bioaccessibility is most affected by organic content of the extraction fluid. Validation of several methods, accomplished by finding significant relationships between bioaccessibility and bioavailability results, has been demonstrated for arsenic, lead, and cadmium, and comparisons for organic contaminants indicate that several methods are promising. Regulatory agencies should be able to develop frameworks that allow bioaccessibility extractions to be relevant for use in risk assessment.

Arsenic Speciation, Distribution, and Bioaccessibility in Shrews and Their Food

Shrews (Sorex cinereus) collected at a historic mine in Nova Scotia, Canada, had approximately twice the arsenic body burden and 100 times greater daily intake of arsenic compared with shrews from a nearby uncontaminated background site. Shrews store arsenic as inorganic and simple methylated arsenicals. Much of the arsenic associated with their primary food source, i.e., small invertebrates, may be soil adsorbed to their exoskeletons. A physiologically based extraction test estimated that 47xa0±xa02% of invertebrate arsenic is bioaccessible in the shrew gastrointestinal tract. Overall, shrews appear to be efficient at processing and excreting inorganic arsenic.