The dynamic uptake of lead and its radionuclides by natural and synthetic aluminium-phosphate-sulfates

The ability of aluminium-phosphate-sulfate (APS) phases to preferentially sorb lead and its radionuclides, particularly ²¹⁰Pb, from metallurgical processing streams has been recently recognised empirically. This suggests that APS minerals may be suitable for the removal of lead from environmental and anthropogenic processes. We investigated the Pb sorption capabilities of APS with different Ca:Sr and SO₄:PO₄ ratios over a range of aqueous Pb concentrations (10–1000 ppm) and pH (1.5–5.5) typical of metallurgical processes and acid drainage conditions. Through a combination of characterization techniques including electron probe microanalysis, (laser ablation-) inductively coupled plasma mass spectrometry and X-ray absorption spectroscopy, we confirm the rapid (< 8 h) incorporation of Pb into the crystal lattice of APS phases. We also provide a mechanistic pathway for the sorption mechanism, with Pb sorption favoured at pH 3.5–5.5 via the direct replacement of lattice-bound Ca by Pb within the APS crystal structure. The observed Pb-incorporation dynamics of APS minerals, along with their insolubility and high thermodynamic stabilities, support the use of APS minerals as a novel agent for the uptake of Pb, radiogenic and nonradiogenic, from process-, surface-, and groundwaters. Since Pb quickly enters the crystal structure of environmentally stable APS minerals, these phases have much potential for long-term storage of Pb waste, and in particular for sequestration of the highly radioactive ²¹⁰Pb isotope enriched in U-bearing geological materials.