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 (BAy) 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 BAy for the important soil contaminants Pb, Cd, and As. Here, using literature data for Ni BAc and BAy, we confirmed that Ni BAc (gastric only, with HCI mimicking stomach conditions) is a conservative measure of BAy for the oral exposure pathway. Measured oral BAy 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 BAy and BAc of nickel sulfate hexahydrate (NSHH) was comparable to that of soil, with measured oral BAy of NSHH (1.94%) being a small fraction of NSHH gastric BAc (91.1%). BAc and BAy 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, 263-29.6% for black oxidic Ni, and 82-91% for the soluble Ni salts. Oral BAc provides realistic yet conservative estimates of BAy for the hazard classification and risk assessment of Ni substances. (C) 2020 The Authors. Published by Elsevier Ltd.