Comparative study between CaAl-Cl LDH and conventional immobilizing agents for reducing ecological and health risks of soil Cd

Soil Cadmium (Cd) has posed a great threat to ecological security and human health in recent years. The exposure under the direct oral ingestion of contaminated soils has been the main contributor of Cd to the human body. The immobilization is an advisable technology to reduce the activity and bioavailability of Cd for the remediation of the Cd-contaminated soil. However, several challenges still remain for conventional immobilizing agents, such as the limited and weak immobilization mechanism, low efficiency and stability. Fortunately, layered double hydroxides (LDHs) can be a new promising type of functional material with great adsorption capability on heavy metals. But, only a few studies were focused on the application of soil remediation, regardless of the potential application to the health risk of soil Cd. In this comparative study, two conventional immobilizing agents (Ca(OH)2 and Ca(H2PO4)2) and one promising LDH (hydrocalumite, CaAl-Cl LDH) were used to remediate the soils with Cd contamination. Four typical soils (red soil, brown soil, cinnamon soil, and black soil) were collected from the cities of Fuzhou, Beijing, Nanjing, and Gongzhuling in China, and then artificially contaminated to the Cd concentration of 180 mg/kg. Subsequently, the three immobilizing agents were applied on the land under the pre-determined optimal patterns. Specifically, the mass ratios of Cd to the immobilizing agent were 1:100, 1:500, and 1:1000, respectively. A comprehensive analysis was also made to determine the effects on the specific soil physicochemical property (soil pH), Cd fractionation (ecological risk), and oral bioaccessibility of soil Cd (health risk) after three-month immobilization. The relevant mechanisms were further explored. Note that there was no evaluation of the health risk of soil Cd using the total concentration of Cd in the soil, due to the overestimation. Furthermore, the Cd bioaccessibility was utilized from the advanced in vitro test, where a PBET model was modified referring to the IVG model. An accurate simulation was then achieved in the digestion processes in the human gastrointestinal tract. Results showed that the soil pH increased greatly after the CaAl-Cl LDH remediation, further facilitating the Cd immobilization. In terms of Cd fractionation in soil, the Ca(OH)2, Ca(H2PO4)2, and CaAl-Cl LDH all significantly reduced the mobility factor of soil Cd (i.e., the proportion of exchangeable and carbonates-bound Cd fractions to the total Cd), with an average decrease of 16.1%, 56.9%, and 29.2%, respectively. As such, better capabilities were obtained to reduce the crop uptake of soil Cd and the ecological risk to the ambient environment. More importantly, the CaAl-Cl LDH more effectively reduced the oral bioaccessibility and the health risk of soil Cd, compared with the rest. The average decrease was 19.2% in the gastric phase, which was 3.11 and 1.99 times those of Ca(OH)2 and Ca(H2PO4)2, respectively. Besides, the decrease was 33.0% in the small intestinal phase, which was 5.99 and 2.72 times those of the conventional. A series of mechanisms were also proposed for the Cd immobilization, including surface complexation, interlayer anion exchange-adsorption, dissolution-precipitation, and isomorphous substitution. Therefore, the CaAl-Cl LDH demonstrated great potential to immobilize the soil Cd in a super-stable manner. It is also highly expected for wide application in real cases. Anyway, the finding can provide a scientific basis and valuable reference for the development, improvement, and selection of the immobilization agents/technologies, in order to better remediate the Cd-contaminated soils.