Sustainable stabilization/solidification of Cd and Pb in industrially heavy metal-contaminated site soils using a novel binder incorporating bone meal and fly ash

This study recycles two abundant and low-cost industrial by-products, bone meal and fly ash, to develop a novel and sustainable binder (MPS) for the stabilization/solidification (S/S) of cadmium (Cd) and lead (Pb) in contaminated soils collected from a smelter site. The investigation encompasses analyses of soil pH, unconfined compressive strength (UCS), Cd/Pb leachability, and microstructure properties of MPS-stabilized soils and compares the S/S performances of MPS with conventional and widely used cement (graded PO 42.5). The findings revealed that MPS-stabilized soils maintained notably lower pH values than their cement-stabilized counterparts. MPS exhibited excellent strength performance approaching cement, and only 6% MPS is needed to meet the strength standard (> 350 kPa). Moreover, the Cd stabilization performance of MPS in contaminated site soils was also comparable to that of cement. 12% MPS-stabilized soils after 7-day curing equaled 28-day-cured 10% cement-stabilized soils in Cd leaching, meeting the limit. Notably, MPS could achieve the required Cd leachability at lower pH levels than cement, making it a low-alkalinity and sustainable binder. Additionally, MPS-stabilized soils showed enhanced acid-buffering capacities. Microscopic tests, including X-ray Diffraction (XRD), scanning electron microscopy (SEM), and energy dispersive spectroscopy (EDS) analyses demonstrated the formation of magnesium phosphate-based products, hydrotalcite, magnesium silicate hydrate gel, and Cd/Pb phosphate-based products as the predominant chemical mechanisms contributing to strength development and Cd/Pb stabilization, respectively. The study demonstrates the efficacy of MPS in stabilizing real contaminated soils from industrial sites and shows its potential as a promising binder for on-site S/S practices.