Incorporation of lead into pyromorphite: Effect of anion replacement on lead stabilization

Previous studies demonstrate that the leaching of heavy metals in unreliable waste forms causes serious environmental pollution and health concerns. Thus, research is focused on identifying an effective, safe strategy for disposing of metal-laden solid waste such as lead (Pb). This study evaluated the effect of anion replacement in the structure of pyromorphite (Pb-10(PO4)(6)Cl-2, a common mineral phase for Pb sequestering) on Pb stabilization. Phosphate (PO43-) at the tetrahedral pyromorphite site was simultaneously replaced by silicate (SiO44-) and sulphate (SO42-) in a controlled thermal treatment. The lattice expanded with the incorporation of additional SiO(4)(4-)and SO42-. Furthermore, the unit cell parameters of the solid solutions evolved linearly with an increase in the substitution degree (x in Pb-10(SiO4)(x)(SO4)(x)(PO4)((6-2x))Cl-2). This research also demonstrated that Pb distributed into amorphous in a PO43--deficient matrix, while asisite (Pb7SiO8Cl2) was formed when the matrix was dominated by SiO(4)(4-)and SO42-. The leaching results showed the isomorphous substitution in the target system rendered the products less durable towards acidic attack. Moreover, the fully isomorphous-substituted product (x = 3) showed more than two orders of magnitude lower leaching resistance than the PO43--rich phase (x = 0). The lattice expansion, resulting from the isomorphous substitution, suggested that a lower dissolution energy was required in a PO43--deficient matrix. The leaching kinetics pointed to a product with a lower apparent activation energy in the leaching process. The findings of this study provide unique insight into the design and optimization of waste forms for the immobilization of heavy metals.

Automated summary

This study evaluated the effect of anion replacement on the stability of lead in pyromorphite structure and found that isomorphous substitution decreased the durability of the products against acidic attack. The findings of this research provide important insights into the design and optimization of waste forms for heavy metal immobilization.