Effects of metal stabilizers on soil hydraulic characteristics and mobility of cadmium

This study investigated the effect of typical stabilizers on hydraulic properties, immobilization, and leachate characteristics based on the diffusive gradient thin-films technique (DGT) and a leaching experiment. Three types of stabilizers were classified based on various characteristics of soil field capacity (θ_f), and their immobilization effects were as follows: (i) θ_f increased and the immobilization of Cd was achieved with nanohydroxyapatite, increasing θ_f by 19.36% and decreasing the bioavailable Cd by 78.84%; (ii) the increasing θ_f conversely inhibited cadmium stabilization. Straw biochar averagely promoted θ_f by 17.39%, while the stabilization was suppressed; (iii) other stabilizers (zeolite, montmorillonite, and sepiolite) had no significant effect on θ_f and immobilization. It is suggested that stabilization depends on chemical mechanisms and is probably also affected by hydraulic mechanisms. The first types of stabilizers formed precipitates with poor solubility, and the strong affinity of heavy metals to soil particles can account for that the increasing θ_f had a negligible influence on the dissolution equilibrium of the heavy metals. Attapulgite also belongs to this type. The second and third types of stabilizers primarily adsorbed cadmium through ion exchange, resulting in the relatively easy heavy metal release. Increasing θ_f facilitated the desorption of heavy metals in the case of the second stabilizer type. However, the inconspicuous change in θ_f caused by the third stabilizer type had no impact on stabilization. Moreover, Cd leaching was positively correlated with bioavailable Cd and soil permeability. Heavy metal migration induced by colloids less than 90 nm in coarse biochar treatments deserves further research.