Preparation of lightweight aggregates using waste soil contaminated with heavy metals: Physical properties, microstructure and verification of heavy metal solidification through different leaching tests

Abstract

Considerable volumes of polluted soil are being discharged around the world resulting in serious pollution. As both Si and Al are important components in the framework of a lightweight aggregate (LWA), polluted soil can be used as a raw material for preparing the LWA. In this study, LWAs were prepared from polluted soil containing Cu, Cr, Ni, and Pb. Tests were conducted to assess their macroscopic, microscopic, and heavy-metal leaching properties. The mechanism underlying heavy metal dissolution and a method to accelerate this dissolution were systematically studied under the condition of monolithic leaching of the LWAs. The solidification state and leaching process of heavy metals under different conditions of the LWA were investigated. The experimental results indicated that the LWA prepared from polluted soil met the LWA standard and new crystalline phases were formed during the sintering process. At the same time, long-term leaching tests and accelerated leaching tests showed that the leaching concentrations of the four heavy metals were all lower than the safety limits specified in Chinese standards. The experiments revealed that the pH, temperature, and ultrasonication could accelerate the dissolution of heavy metals. In particular, ultrasonication caused the heavy metals to exhibit the effects of long-term leaching in a short time. Nevertheless, leaching amount did not exceed the limits specified by the standard, thus improving the our understanding of heavy metal solidification in LWAs. These results could significantly guide the construction industry in establishing an evaluation method for heavy metal dissolution that conforms to the service environment of LWAs, enabling the safe management of solid wastes and the sustainable development of light aggregates.