Insolubility of Cr2O3 in Bioaccessibility Tests Points to Requirement for a New Human Oral Reference Dose for Trivalent Chromium

Bioaccessibility measurements have the potential to improve the accuracy of risk assessments and reduce the potential costs of remediation when they reveal that the solubility of chemicals in a matrix (e.g., soil) differs markedly from that in the critical toxicity study (i.e., the key study from which a toxicological or toxicity reference value is derived). We aimed to apply this approach to a brownfield site contaminated with chromium, and found that the speciation was Cr^III, using a combination of alkaline digestion/diphenylcarbazide complexation and X-ray absorption near edge structure analysis. The bioaccessibility of Cr₂O₃, the compound on which a reference dose for Cr^III is based, was substantially lower (<0.1%) than that of the Cr^III in the soils, which was a maximum of 9%, giving relative bioaccessibility values of 13,000% in soil. This shows that the reference dose is based on essentially an insoluble compound, and thus we suggest that other compounds be considered for toxicity testing and derivation of reference dose. Two possibilities are CrCl₃·6H₂O and KCr(SO₄)₂·12H₂O, which have been used for derivation of ecological toxicity reference values and are soluble at a range of dosing levels in our bioaccessibility tests.     

Microbe-aliphatic hydrocarbon interactions in soil: implications for biodegradation and bioremediation

Aliphatic hydrocarbons make up a substantial portion of organic contamination in the terrestrial environment. However, most studies have focussed on the fate and behaviour of aromatic contaminants in soil. Despite structural differences between aromatic and aliphatic hydrocarbons, both classes of contaminants are subject to physicochemical processes, which can affect the degree of loss, sequestration and interaction with soil microflora. Given the nature of hydrocarbon contamination of soils and the importance of bioremediation strategies, understanding the fate and behaviour of aliphatic hydrocarbons is imperative, particularly microbe-contaminant interactions. Biodegradation by microbes is the key removal process of hydrocarbons in soils, which is controlled by hydrocarbon physicochemistry, environmental conditions, bioavailability and the presence of catabolically active microbes. Therefore, the aims of this review are (i) to consider the physicochemical properties of aliphatic hydrocarbons and highlight mechanisms controlling their fate and behaviour in soil; (ii) to discuss the bioavailability and bioaccessibility of aliphatic hydrocarbons in soil, with particular attention being paid to biodegradation, and (iii) to briefly consider bioremediation techniques that may be applied to remove aliphatic hydrocarbons from soil.     

Do Current Standards of Practice in Canada Measure What is Relevant to Human Exposure at Contaminated Sites? II: Oral Bioaccessibility of Contaminants in Soil

Human health risk estimates for sites with contaminated soils are often based on the assumption that the outdoor soil sieved to < 250 μm is a reasonable surrogate for predicting exposures via incidental soil ingestion. In vitro bioaccessibility tests are also increasingly used to “improve” ingestion exposure predictions of contaminants at different sites. However, when considered in terms of factors that influence desorption of contaminants from particles and uptake into humans, available studies indicate that current “standards of practice” with respect to assays of oral bioaccessibility have a number of significant shortcomings, at least in Canada. These shortcomings are discussed and various factors that influence the assessment of bioaccessibility of contaminants in soils are examined. We finish with proposing some minimum data submission requirements to support the application and relevance of bioaccessibility assays at contaminated sites.     

Bioaccessibility of Cd,Cr, Cu,Mn, Ni,Pb, and Zn in Mediterranean mussel (Mytilus galloprovincialis Lamarck, 1819) along the southeastern Black Sea coast

The adverse effect of seafood consumption on human health is related to the bioaccessibility in contrast with the total heavy metal level in the tissues. In this study, bioaccessibility of Cd, Cr, Cu, Mn, Ni, Pb, and Zn in Mediterranean mussel (Mytilus galloprovincialis Lamarck, 1819) edible soft tissues collected along the southern Black Sea coast were investigated using in vitro digestion model. Total Pb concentration in the tissues was found to be higher than the maximum permissible limits set by European Commission. The bioaccessibility of metals in the tissues was found to be decreased in the order: Ni (83.11%) > Cu (80.47%) > Zn (76.86%) > Cd (73.32%) > Mn (69.11%) > Pb (61.07%) > Cr (58.44%). While significant positive linear regression was observed between total and bioaccessible concentrations for Mn, Pb, and Ni, significant negative linear regression was found for Cd (p < .05). The hazard quotients (HQ) calculated using bioaccessible amount were detected lower than the limit (HQ <1), which may not pose a potential hazard to humans reported by US Environmental Protection Agency. In sum, mussel consumption from sampling sites may cause a potential risk concerning human health especially for Cd and Pb in case of increased portion sizes.     

Assessing the Bioavailability and Risk from Metal-Contaminated Soils and Dusts

Exposure to contaminated soil and dust is an important pathway in human health risk assessment. Physical and chemical characteristics and biological factors determine the bioaccessibility/bioavailability of soil and dust contaminants. Within a single sample, contamination may arise from multiple sources of toxic elements that may exist as different species that impact bioavailability. In turn, the bioaccessibility/bioavailability of soil and dust contaminants directly impacts human health risk. Research efforts focusing on development and application of in vitro and in vivo methods to measure the bioaccessibility/bioavailability of metal-contaminated soils have advanced in recent years. The objective of this workshop was to focus on developments in assessing the bioaccessibility/bioavailability of arsenic-contaminated soils, metals’ contamination in urban Canadian residences and potential children's exposures to toxic elements in house dust, an urban community-based study (i.e., West Oakland Residential Lead Assessment), bioavailability studies of soil cadmium, chromium, nickel, and mercury and human exposures to contaminated Brownfield soils. These presentations covered issues related to human health and bioavailability along with the most recent studies on community participation in assessing metals’ contamination, studies of exposures to residential contamination, and in vitro and in vivo methods development for assessing the bioaccessibility/bioavailability of metals in soils and dusts.     

Factors Controlling the Bioaccessibility of Arsenic(V) and Lead(II) in Soil

The relative oral bioaccessibility of labile Pb(II) and As(V) added to soils was investigated in a well-characterized soil using a physiologically based extraction test (PBET) to simulate metal solubility in a child's digestive system. The effect of soil and PBET (i.e., simulated stomach and small intestine) pH, soil metal concentration, soil to solution ratio, and soil-metal aging time were investigated. Arsenic bioaccessibility was relatively unaffected by a variation in simulated stomach and small intestine pH over the range 2 to 7 and soil pH over the range 4.5 to 9.4. In contrast, Pb(II) bioaccessibility was strongly dependent on both the simulated stomach, small intestine, and soil pH, showing enhanced sequestration and decreased bioaccessibility at higher pH values in all cases. Although the bioaccessibility of Pb(II) was constant over the concentration range of approximately 10 to 10,000?mg/kg, the As(V) bioaccessibility significantly increased over this concentration range. The bioaccessibility of both arsenic and lead increased as the soil-to-solution ratio decreased from 1:40 to 1:100. Additional lead sequestration was not observed during 6 months of soil aging, but As(V) bioaccessibility decreased significantly during this period.     

An In Vitro Procedure for Estimation of Lead Relative Bioavailability: With Validation

The relative bioaccessibility leaching procedure (RBALP) is a simple, reproducible, and rapid in vitro procedure for estimating the in vivo (juvenile swine) relative bioavailability (RBA) of lead in solid media. Control of pH, temperature, and agitation are the most critical parameters of this in vitro procedure. The performance of the method was evaluated by triplicate analyses of each of 19 different test substances by the author and three independent laboratories, and comparison of the results to relative bioavailability (RBA) values measured in vivo. The results indicate that the RBALP measurements are strongly correlated with the in vivo RBA values (r² = 0.924, p < 0.0001), with an average absolute error of 10% and an average predictive error of 20%. Comparison of results within and between laboratories indicates that the procedure is highly reproducible, with inter-and intra-laboratory coefficients of variation of 4% and 6%, respectively, and within-sample precision of approximately 7%. Based on the results reported here, the RBALP can be effective in providing reliable estimates of lead RBA as predicted by the immature swine model. The method may also be valuable in evaluating site-specific differences in bioaccessibility, assessing remedial technologies intended to reduce lead RBA, providing a screening mechanism for futurein vivo studies, and providing insight into the chemical and physical factors that control lead bioavailability.     

A Hypothetical Cyclic Pathway for the Metabolism of Odd-carbon n-Alkanes or Propionyl-CoA via Seven-carbon Tricarboxylic Acids in Yeasts

An extensive study has been undertaken to elucidate the physiological significance of threo-D_s-2-methylisocitric acid produced mainly from odd-carbon n-alkanes by a mutant strain of Candida lipolytica. The mutant strain showed slower growth responses to odd-carbon n-alkanes, especially of shorter chain-length, and failed to utilize this acid as sole carbon source, whereas the parent strain and many other yeasts tested were able to utilize this acid. About one half of yeasts tested accumulated this acid extracellularly. Under a thiamine-deficient condition, amounts of pyruvate produced by the parent strain from odd-carbon n-alkanes were ten times as large as those from even-carbon n-alkanes. A scheme for the partial oxidation of propionyl-CoA to pyruvate via C₇-tricarboxylic acid by yeasts was supposed. This scheme may offer suggestion on the metabolism of propionyl-CoA by other living organisms. A hypothetical pathway of citrate accumulation from odd-carbon n-alkane was also presented.     

Content of Inorganic and Organic Pollutants and Their Mobility in Bottom Sediment from the Orlík Water Reservoir (Vltava River,Czech Republic)

The content of both inorganic and organic pollutants, in sediment from the Orlík reservoir (Vltava river, Czech Republic), and the main tributaries, was determined to assess the total loads of the pollutants in the upper layer of sediment (0–30 cm), and the potential bioaccessibility and bioavailability of these pollutants. Organic pollutants showed non-hazardous levels of polycyclic aromatic hydrocarbons and polychlorinated biphenyls. Elevated phosphorus content indicated the possible eutrophization potential; however, significant correlation of phosphorus with iron and aluminum suggested a high immobilization rate of this element with iron and aluminum cations. Risk elements in the sediment were separated according to their potential environmental risk: (1) cobalt, nickel, copper, lead, vanadium, and chromium showed both low levels and relatively low mobility, i.e. low environmental risk; (2) the arsenic levels were significantly elevated (exceeding the maximum allowable limits for land application of sediment) but its mobility was low; (3) cadmium and zinc exceeded the limits and showed high mobility and bioaccessibility. These two elements also showed high bioavailability for the bivalves Unio tumidus and Anodonta anatina. The results showed differences in pollutant loads in sediment along the lake, and different input of pollutants via the individual tributaries.