The effect of particle size on oral bioavailability and bioaccessibility of soil Ni from different sources.

The goal of the work was to contribute to a unified approach to assessing the risk to human health of soil ingestion, for contaminated sites with elevated [Ni]. Robust relationships between in vitro bioaccessibility and in vivo bioavailability of Ni in various soils, with mechanistic understanding, would enable site-specific assessments of human exposure through soil ingestion. Four soils (three ultramafic Brunisols with geogenic Ni and one Organic soil with anthropogenic Ni) were sieved into PS < 10 µm, 10-41 µm, 41-70 µm, 70-105 µm, 105-150 µm, and 150-250 µm, the [Ni]T for which ranged from 560 to 103000 mg/kg. Mass fraction-adjusted [Ni]IVBA (SBRC gastric) for each soil fraction was similar whether calculated for all particles <250 µm or <150 µm %NiIVBA ranged from 3% to 16% of [Ni]T and %NiABA (accumulated Ni in urine, kidneys, and small intestine of Sprague Dawley rats gavaged with a soil) ranged from 0% to 0.49%. The correlation between these two measurements was weak (R2 = 0.06). Multiple linear dose response relationships attributing variation in %NiABA to %NiIVBA plus soil physicochemical parameters known to influence trace element availability in soils were developed. As many soil properties measured in this study were highly correlated, ridge regression enabled a predictive relationship where the effect of each parameter was its true contribution to variation in %NiABA. Using a ridge constant (k) of 0.012, %NiABA could be predicted from %NiIVBA adjusted for soil absorptive entities (OrgC, and Fe oxides (negative coefficients)) and soil pH (positive coefficient). %NiABA predicted from this relationship was very close to 1:1 with the observed %NiABA except at the lowest observed values which were lower than predicted. This study shows that as the conditions increasingly favour soil Ni solubility, more of the Ni was bioavailable; this generalization was true regardless of particle size or soil origin.

Derivation of a Ni bioaccessibility value for screening-level risk assessment of Ni substances in ingested materials including soils.

The objective of the present study was to derive a Ni bioaccessibility value for screening-level risk assessment of Ni substances in ingested materials including soils where multiple Ni substances are expected but not definitively identified. Broad ranges of Ni mass loading and dissolution time of a simple gastric assay were applied to pure Ni substances (removing the confounding factors of soil constituents on dissolution), thus broadening the applicability of the conclusions. The data were also used to support current knowledge of 'read across' for Ni substances. Release of Ni from pure manufactured Ni substances (Ni metal, NiO, NiSO4, Ni3S2, and NiS) was determined relative to Ni mass and substance surface area loading. Mass loadings ranged from 0.33 to 20.0 g Ni per L of 0.15 M HCl, and dissolution time ranged from 1 to 168 h. Proton exhaustion was indicated only at the highest loading (20 g/L) of NiO and Ni-M. Dissolution of substances other than NiSO4 was most likely limited by formation of intermediate products at the particle surface or particle agglomeration, impeding access to the principal Ni substance. The bioaccessibility of Ni for these substances was consistent with previously published data: substances other than NiSO4 were < 48% bioaccessible for a variety of gastric assays, which is much lower than all data for NiSO4, the usual reference substance. Thus, we suggest that Ni bioaccessibility data from gastric assays that are most relevant to human exposure can be relied upon to develop scientifically sound screening-level human health RA decisions for Ni contamination in soils and sediments in the absence of detailed Ni speciation.

Gastric bioaccessibility is a conservative measure of nickel bioavailability after oral exposure: Evidence from Ni-contaminated soil, pure Ni substances and Ni alloys.

Oral bioaccessibility (BAc) is a surrogate for the bioavailability (BAv) of a broad range of substances, reflecting the value that the approach offers for assessing oral exposure and risk. BAc is generally considered to have been validated as a proxy for oral BAv for the important soil contaminants Pb, Cd, and As. Here, using literature data for Ni BAc and BAv, we confirmed that Ni BAc (gastric only, with HCl mimicking stomach conditions) is a conservative measure of BAv for the oral exposure pathway. Measured oral BAv of Ni in soil was shown to be 50-100 times less than the simplest oral BAc estimates (%BAv = 0.012(%BAc) - 0.023 (r = 0.701, 95%CI [0.456, 0.847], n = 30)) in rats, demonstrating a significant conservatism for exposure assessment. The relationship between the oral BAv and BAc of nickel sulfate hexahydrate (NSHH) was comparable to that of soil, with measured oral BAv of NSHH (1.94%) being a small fraction of NSHH gastric BAc (91.1%). BAc and BAv reflect the underlying Ni speciation of the sample, with the bioaccessible leaching limits being represented by the highly soluble Ni salts and the poorly soluble Ni monoxide, and the environmental (e.g. soil properties) or gastric (e.g. food present) conditions. BAc has potential utility for chemical classification purposes because pure Ni substances can be grouped by %BAc values(using standardized methodologies for the relevant exposure routes), these groupings reflecting the underlying chemistry and speciation of the samples of substances tested here, with 0.008% %BAc for alloys (SS304, SS316, Inconel, Monel), <1% in green NiO and Ni metal massives, 0.9-23.6% for Ni powders, 9.8-22.7% for Ni sulfides, 26.3-29.6% for black oxidic Ni, and 82-91% for the soluble Ni salts. Oral BAc provides realistic yet conservative estimates of BAv for the hazard classification and risk assessment of Ni substances.

Modeling phytoremediation of aged soil Ni from anthropogenic deposition using Alyssum murale.

Several field-scale phytoextraction scenarios were created in a greenhouse study to investigate the feasibility of using Alyssum murale, to remediate three types of industrially Ni-contaminated soil (heavy clay, sand, organic muck) from Port Colborne, Ontario. The observed distribution of Ni mass between soil and aboveground vegetation was used in STELLA modeling software to predict timelines for the target soil Ni concentration, namely 1200 mg Ni/kg. Alyssum murale grown in sand would have a relatively constant pool of Ni available for plant uptake, which would not be the case for plants grown in organic muck and heavy clay. The maximum Ni extraction (%, plant Ni mass/soil Ni mass) was achieved in A. murale grown in unfertilized clay soil at the higher irrigation rate. Using these data, the STELLA model predicted that 246 years would be required to reduce soil Ni concentration in the most efficient combination of treatments to the remediation target. In addition, hypothetical A. murale Ni extraction in plant-soil systems optimized by manipulating soil chemistry and physical attributes, were modeled. The most optimized A. murale plant-soil systems for Ni extraction would require 9 years to achieve the same reduction, and it is not clear that this optimization can be achieved in the field. This study showed that phytoremediation using A. murale is not likely a time-sensitive approach for these soils.

Towards an exposure narrative for metals and arsenic in historically contaminated Ni refinery soils: Relationships between speciation, bioavailability, and bioaccessibility.

Archived soils contaminated with Ni, Cu, Co, and As from legacy operations of a nickel refinery at Port Colborne, Ontario, Canada were speciated using mineral liberation analysis. Four Ni minerals were identified as fingerprint compounds of the historical refinery emissions. Cu and Co were present in solid solution in these minerals due to their presence in the refinery's feed. The highest concentrations of Ni, Cu, Co, and As in these soils were 18,553, 1915, 196, and 79mg/kg, respectively, these elevated contaminant concentrations attesting to the importance of incidental soil ingestion to the oral exposure pathway in Port Colborne. The in vitro gastric bioaccessibility (BAc) was determined for these contaminants, as was in vivo oral bioavailability (BAv), using a mass balance approach in male Sprague-Dawley rats. In spite of the elevated soil concentrations of Cu, the BAv of this physiologically important metal could not be distinguished from that in commercial rat chow, suggesting low potential for exposure. Co and As also had low apparent BAv (<2%). For Ni, baseline oral BAv of naturally sourced dietary Ni was found to be approximately 2%, as was the oral BAv of Ni from nickel sulfate hexahydrate. The mass balances of NiSO4·6H2O were fully accounted-for in urine and feces after a single gavage dose, indicating little to no organ incorporation from this highly soluble salt. Therefore, the urinary estimates of Ni BAv for these soils were assumed to represent true BAv despite variable fecal recoveries. The high Ni concentrations enabled BAc-BAv relationships to be developed for these contaminated soils. For absolute bioavailability (ABA) and relative bioavailability (RBA) the relationships were: ABA=0.0116(BAc)-0.0479 and RBA=0.5542(BAc)-2.2817. These findings will advance the development of robust exposure narratives for soil metal contamination in Port Colborne and elsewhere.

Bioaccessibility estimates by gastric SBRC method to determine relationships to bioavailability of nickel in ultramafic soils.

Frameworks for human health risk assessment often include the opportunity to correct the estimate of exposure for bioavailability, which could be predicted from bioaccessibility. Lead and As are the only metallic elements for which bioavailability and bioaccessibility have been correlated across a spectrum of mineralogy and particle types. The objective of the present study is to correlate in vivo bioavailability with ex vivo bioaccessibility for elevated Ni in soils of ultramafic origin and explore attribution of any variation in this correlation to mineralogical characterization of the Ni. Ultramafic soils were field collected in British Columbia, CA. Rietveld quantitative X-ray diffraction was used for the characterization and quantification of crystalline materials containing Ni. Bioaccessible Ni was determined using the in vitro method developed by the Solubility/Bioaccessibility Research Consortium. Bioavailable Ni was determined by gavage dose of the soils to Sprague-Dawley rats. Urine and feces were collected every 24 h. At the end of 72 h, the animals were humanely sacrificed using carbon dioxide as per the approved animal care protocol. All organs were harvested, washed and preserved. Fecal elimination of gavaged Ni ranged from 35 to 95% including positive control. Relative bioavailability (RBA) ranged from 5 to 18%. In vitro bioaccessibility (IVBA) of soil Ni ranged from 0 to 17%; IVBA explained 86% of the variation in RBA. Normalizing both axes to soil olivine accounted for an additional 10% of the variation in RBA. For risk assessment of Ni contaminated soils, IVBA would be a useful and cost effective tool in estimating exposure of mammals through ingestion of soil particles, with some additional benefit of considering Ni mineralogy.

Comparison of nickel speciation in workplace aerosol samples using sequential extraction analysis and X-ray absorption near-edge structure spectroscopy.

There is a pressing need to further develop speciation knowledge of Ni workplace aerosols as the Zatka sequential extraction method used until now to speciate workplace Ni exposures has limitations. Here we compare the Zatka and XANES methods and evaluate XANES spectroscopy as a more appropriate and accurate technique for identifying nickel species in workplace aerosols. XANES spectroscopy is capable of identifying unique Ni species in the unaltered samples. Our findings indicate some significant departures in speciation assignment between the Zatka and XANES methods. In particular, the Zatka method can overestimate the soluble Ni fraction and it may underestimate the sulphidic and metallic fractions in some samples. Of particular importance, XANES is able to identify component sulphidic species. This information can lead to more accurate exposure matrices and more refined epidemiological analysis of respiratory cancer causation in sulphidic Ni processing.

In vitro estimates of bioaccessible nickel in field-contaminated soils, and comparison with in vivo measurement of bioavailability and identification of mineralogy.

This study determined nickel (Ni) bioaccessibility in weathered smelter-contaminated soils, separately for particle-sized fractions using two in vitro methods: simulated gastrointestinal digestion (PBET) and PBET followed by absorption by Caco-2 cells. Relative bioavailability of Ni in soils was determined in vivo using rats, validating in vitro estimates; a mineralogical basis of variation in bioavailability/bioaccessibility among soils was explored. In vitro assays identified the same difference in bioaccessibility for Ni among particle size fractions. PBET estimates were more precise, thus likely to be more useful in discerning differences among soils. In vivo bioavailability for Ni was below limit of detection for the small soil particles, and 31% and 56% for the larger particles. The relative bioavailability calculated from this work suggests that risk from ingesting Ni-contaminated soils could be overestimated by between 2- and 50-fold if the estimates of exposure are not adjusted for the lower bioavailability of weathered Ni originating from smelter emissions. The overestimation that would occur by using total Ni is greatest for the particle size that is most likely to adhere to the hands of children, demonstrating the importance of particle-size separation of soils for bioavailability determination and risk assessment.