In Situ Formation of Pyromorphite Is Not Required for the Reduction of in Vivo Pb Relative Bioavailability in Contaminated Soils
Citation:
Juhasz, A., D. Gancarz, C. Herde, S. McClure, K. Scheckel, AND E. Smith. In Situ Formation of Pyromorphite Is Not Required for the Reduction of in Vivo Pb Relative Bioavailability in Contaminated Soils. J. Schnoor (ed.), ENVIRONMENTAL SCIENCE AND TECHNOLOGY. ACS Publications, Washington, DC, 48(12):7002-7009, (2014).
Impact/Purpose:
Lead (Pb) is a ubiquitous contaminant found in the environment as a result of a variety of anthropogenic activities. Due to the well documented relationship between Pb exposure and impaired intellectual development in young children, a number of Pb-containing products (e.g., fuel, paint) were phased out in order to reduce Pb emissions to the environment and to reduce potential human exposure to these Pb sources. Although changes in Pb bioaccessibility may be assessed using in vitro assays as a surrogate measure for Pb RBA, it has been suggested that the reduction in Pb bioaccessibility may occur due to parameters associated with the extraction test (i.e., gastric phase pH) which induce pyromorphite formation and not the treatment itself.12 However, other research suggests that in vitro assays have no experimental artefact influence on determining Pb bioaccessibility with pure reagent chemicals due to excessive organic ligands in solution. As a result, direct evidence of the effect of phosphate amendments on Pb RBA should be assessed using in vivo studies supplemented with indirect lines such as spectroscopic analysis. Accordingly, the aim of this study was to determine the effect of phosphate treatments on Pb RBA (using an in vivo mouse model) and Pb speciation (using XAS) in three distinct Pb-contaminated soils. It was hypothesised that although chloropyromorphite formation may not occur in situ due to unfavourable soil pH conditions, a reduction in Pb RBA may occur due to in vivo chloropyromorphite formation.
Description:
The effect of phosphate treatment on lead relative bioavailability (Pb RBA) was assessed in three distinct Pb-contaminated soils. Phosphoric acid (PA) or rock phosphate were added to smelter (PP2), nonferrous slag (SH15), and shooting range (SR01) impacted soils at a P:Pb molar ratio of 5:1. In all of the phosphate amended soils, Pb RBA decreased compared to that in untreated soils when assessed using an in vivo mouse model. Treatment effect ratios (i.e., ratio of Pb RBA in treated soil divided by Pb RBA in untreated soil) ranged from 0.39 to 0.67, 0.48 to 0.90 and 0.03 to 0.19 for PP2, SH15, and SR01, respectively. The decrease in Pb RBA following phosphate amendment was attributed to the formation of poorly soluble Pb phosphates (i.e., chloropyromorphite, hydroxypyromorphite, and Pb phosphate) that were identified by X-ray absorption spectroscopy (XAS). However, a similar decrease in Pb RBA was also observed in untreated soils following the sequential gavage of phosphate amendments. This suggests that in vivo processes may also facilitate the formation of poorly soluble Pb phosphates, which decreases Pb absorption. Furthermore, XAS analysis of PA-treated PP2 indicated further in vivo changes in Pb speciation as it moved through the gastrointestinal tract, which resulted in the transformation of hydroxypyromorphite to chloropyromorphite.
