[Hydrocalumite Passivation Effect and Mechanism on Heavy Metals in Different Cd-Contaminated Farmland Soils]

Hydrocalumite (Ca-Al-LDHs) is a new type of layered composite metal hydroxide that has a large specific surface area, high anion exchange performance, and high stability. This study focuses on the application of hydrocalumite to remediate different Cd-contaminated farmland soils. These were collected from the Lanping County in the Yunnan Province (highly polluted), Kunshan City in the Jiangsu Province (medium polluted), and Nanjing City in the Jiangsu Province (lowly polluted). Changes in the available Cd, Pb, Zn, and the morphological transformations of these heavy metals in the three soils were investigated through a passivation experiment; moreover, the immobilization mechanism of hydrocalumite was explored by X-ray diffraction (XRD) and Fourier transform infrared spectroscopy (FTIR). The results showed that hydrocalumite could increase the soil pH and reduce the content of available Cd, Pb, and Zn:the maximum reduction in the available Cd reached 97.7%, 96.3%, and 91.8% in each of the three polluted soils, respectively. The easily exchangeable heavy metals were converted into carbonates, as well as into Fe-Mn oxide organic and residual forms following the addition of hydrocalumite:the passivation effect was more evident in the highly Cd-polluted soil than in the low and medium Cd-polluted soils. Since hydrocalumite possess several adsorption sites, the presence of carbonate impurities and reactive groups (e.g., hydroxyl and carboxyl groups) easily coordinated with Cd, Pb, and Zn can lead to a considerable reduction of heavy metal availability in the soils. Therefore, we conclude that hydrocalumite can be effectively applied to the remediation of farmland soils characterized by different Cd pollution levels.