Pilot-scale remediation, resource utilization, and long-term stabilization of cadmium, lead and arsenic co-contaminated soil.

Soils contaminated with multiple heavy metals pose significant environmental and health risks due to their toxicity and persistence. Herein, the remediation efficiency of soil contaminated with complex mixtures of cadmium (Cd), lead (Pb), and arsenic (As) was investigated, along with the feasibility of resource utilization of the soil post-remediation through batch experiments, pilot-scale trials, and engineering applications. In the batch experiments, heavy metals (HMs) were effectively immobilized by the remediation agent, which consisted of a mixture of iron oxides and aluminum oxides. The remediation efficiency was positively linear correlated with the dose of the remediation agent. Considering the economy, a mass ratio of 10% of the remediation agent to soil was found to be the optimal solidification and stabilization of HMs. The leaching concentrations of Cd, Pb and As, were reduced from initial concentration of 517.9 µg/L, 1123.4 µg/L, and 73.7 µg/L to 85.7 µg/L, 427.5 µg/L, and 26.7 µg/L after solidification treatment. The stabilized soil was subsequently utilized for brick fabrication. At a 10% dose and a calcination temperature of 900 °C, the resulting bricks exhibited low heavy metal leachability, with Cd and Pb concentrations of only 2.5 µg/L and 3.6 µg/L, and As was not detected. X-ray diffractometer (XRD) analysis indicated that Pb was effectively incorporated into the feldspar structure, an common and environmental friendly mineral phase in the earth's crust. In the pilot-scale trials, results confirmed the effective of solidification of heavy metals, with leaching concentrations of Cd, Pb, and As all below 0.1 mg/L in the soil remediation process. Leachability assessments of the bricks confirmed that HMs were effectively immobilized within mineral crystal structures during high-temperature sintering, while compressive strengths remained consistently high, ranging between 30.6 MPa and 32.4 MPa, exceeding the standard requirement of 10 MPa. In the engineering application, long-term verification was conducted after treatment. Even 2.5 years after treatment, the concentrations of Cd, Pb and As, in the remediated soil remained significantly below the remediation target values. Therefore, brick fabricate not only achieved the solidification and stabilization of HMs in the soil but also realized the resource utilization of the remediated soil, having certain economic benefits and significant environmental benefits.