The synergistic action mechanisms of ternary industrial waste stabilized lead ion contaminated soil

In this study, ternary industrial waste comprising red mud, carbide slag and phosphogypsum was combined with cement to act as a curing agent for the remediation of Pb2+-contaminated soil. The mixing proportions of the four materials were determined based on the resulting mechanical and permeability properties as well as the pH and leached Pb2+ concentration of the solidified soil. Furthermore, variations in the compressive strength, pH of leachate, permeability coefficient and leached Pb2+ concentration with the curing age were studied. Additionally, scanning electron microscopy, X-ray diffraction, sequential extraction and mercury intrusion porosimetry analyses were performed to characterize the mineral constituents and microstructures of the solidified soil. The experimental results reveal that when cement, red mud, carbide slag, and phosphogypsum were added at mass percentages of 10 %, 2.5 %, 1.25 % and 1.25 %, respectively, the strength of the solidified soil increased and the permeability coefficient reached 10−7 cm/s. In this case, the pH value was 11.91, and the leached Pb2+ concentration was 0.403 mg/L, which comply with the environmental safety standards. Moreover, with increasing curing age, the strength increased, the permeability coefficient decreased, pH slightly increased and leached Pb2+ concentration decreased. The results revealed that the stabilization mechanism of the Pb2+-contaminated soil depends on the adsorption and physical encapsulation of the hydration products as well as the formation of a small amount of lead compounds, thus converting Pb2+ from a highly soluble state to a less reactive state.