Pyromorphite formation from montmorillonite adsorbed lead

The reaction of Pb-adsorbed montmorillonite with aqueous solutions of PO₄ and Cl ions results in the decrease in phosphate concentration associated with the formation of a new phase - pyromorphite Pb₅(PO₄)₃Cl. Pyromorphite crystals range in size from hundreds of nm to several tens of μm, depending on the PO₄, K, and Ca concentrations in the reacting system. A strong ion-exchange effect of K⁺ and Ca²⁺ cations on desorption of Pb²⁺ from Pb-adsorbed montmorillonite was observed. Also, a high concentration of cations leads to a rapid desorption of Pb and the formation of fine pyromorphite crystals. In contrast, low PO₄, K and Ca concentrations result in the formation of relatively large euhedral crystals. Final Pb concentrations are much lower in experimental sets than in control experiments with no phosphate present.