Lead immobilization in tailings–based alkali–activated materials

Abstract

Our research explores the intricate mechanisms between lead (Pb) and alkali–activated materials (AAMs) derived from lead–zinc tailings. We delve into the lead immobilization process and its impact on material behavior. Our findings indicate that Pb2+ shortened the induction period and accelerated later hydration by offering additional nucleation sites. Notably, Pb2+ manifest a dual impact on the compressive strength of AAMs: a marginal increase at low concentrations (Pb2+ = 0.5%) and a decrease at higher ones (Pb2+ = 1.5%). Micro-analysis indicates that the hydrolysis of Pb2+ forms new materials that bridge larger pores within the AAMs matrix, resulting in reduced pore size. However, an excess of Pb2+ hampers gel polymerisation, impairing the gel’s structural integrity and the stability of immobilization. The interplay of these phenomena determines the overall strength of the material. Our leaching tests demonstrate that AAMs can effective immobilize Pb2+ at concentrations up to 1%, with a remarkable containment efficiency of 99.5% at 0.5% Pb2+. This is facilitated by several mechanisms, including the formation of insoluble precipitate, ion exchange with Na+/Ca2+, covalent bonding with Si, and physical encapsulation. These insights shed light on the promising applications of tailings–based AAMs for the effective immobilization of lead.