In vitro formation of pyromorphite via reaction of Pb sources with soft-drink phosphoric acid

Pyromorphite (Pb5(PO4)3Cl) is a Pb mineral whose rapid kinetic formation and thermodynamic stability (Geochim. Cosmochim. Acta 37 (1973) 367, Geochim. Cosmochim. Acta 38 (1974) 887) has gained much attention in recent years as a mechanism to diminish Pb toxicity. Our research examined the in vitro formation of pyromorphite (Ksp=10−25) to significantly reduce Pb bioavailability via the reaction of PbCl2 and Pb paint with cola soft drinks (regular, caffeine-free, and diet) that contain phosphoric acid. X-ray diffraction spectroscopy was employed to identify and quantify the crystal phases. The experiments consisted of a two-phase experimental design to simulate stomach and gastrointestinal (GI) conditions comprised of an initial reaction system (stomach) at pH 2.0 for 1 h at 37 °C and second-phase conditions (GI tract) at pH 7.0 for 5 h at 37 °C. The results showed that available Pb in solution was quickly transformed to pyromorphite by a precipitation mechanism. The PbCl2 experiments observed over 90% Pb removal from solution within 30 s and nearly all (>93–100%) of the Pb was converted to pyromorphite within 6 h. Similar results were observed in the Pb-paint system in which all of the available Pb initially available in solution was stabilized as pyromorphite within a short reaction period. While approximately 69% of the total Pb in the paint was converted to pyromorphite, there were minimal amounts of Pb in solution. It appeared that the dissolution mechanism allowing Pb in the paint to be released into the cola solution was repressed and was attributed to precipitation of pyromorphite on the surfaces of Pb-paint particles that inhibited further Pb release. The results of this research suggest that the in vivo formation of pyromorphite in children that ingest Pb-contaminated material (soil, dust, paint) with a phosphate source, such as a cola soft drink, may significantly reduce Pb bioavailability and allow passage of the pyromorphite material through the GI tract with minimal Pb absorption into the body.