Bioaccessibility of lead from lead-contaminated soil upon phosphate amendment using a physiologically-based extraction test

Lead is known to cause health problems in humans, especially children, and an effective in‐situ remediation option has been sought for years. Adding phosphoric acid (PA) to contaminated soil causes a reaction that binds the lead to phosphate to produce pyromorphite (Pb₅(PO₄)₃Cl), a form of lead believed to be non‐bioavailable; however, field trials have given varied results (Bosso et al 2008; Munksgaard and Lottermoser 2011; Tang et al. 2009). One explanation for these results might be the impact of the agent used to raise pH after phosphoric acid addition. In order to examine this explanation soil was collected from the Bonne Terre area in Missouri, which is known to have a high lead content due to past smelting activities. The soil was mixed with PA before calcium hydroxide and sodium hydroxide were added to the soil to neutralize the pH changes caused by the PA addition, and to determine whether the pH amendment impacted the rate of pyromorphite formation. The soil was then run through a physiologically‐based extraction test (PBET) that simulates a child’s stomach process to evaluate the success of the remediation attempt. The soil was monitored for a month after amendment addition, with all soil samples run through the PBET and a flame atomic absorption spectrometer to analyze the samples. Upon discovering that the change in concentration of extractable lead in soil was not statistically significant, an invitro test was conducted to discover what was occurring in the soil. Titration experiments were conducted based on the idea that pyromorphite was forming in the soil, but the low stomach pH was causing it to re‐dissolve. The titration experiments showed that below pH 3, pyromorphite dissolves, a hitherto overlooked phenomenon --Abstract, page iii