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, NiSO, NiS, 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 NiSO 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 NiSO were < 48% bioaccessible for a variety of gastric assays, which is much lower than all data for NiSO, 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.