Effects of Phosphate and Silicate Combined Application on Cadmium Form Changes in Heavy Metal Contaminated Soil

Pollution by heavy metal cadmium (Cd) in soil is still serious and control measures are constantly updated. In this paper, one indoor culture method was applied to investigate the effect of phosphate and thermo-activated nano silicate combined application on soil cadmium (Cd) speciation transformation. A total of 7 treatments were designed, which were: simulated cadmium-contaminated soil without phosphate and silicate recorded as the reference (CK) treatment; mixtures of 0.5%, 1.0%, and 2.0% soil heavy dose of potassium dihydrogen phosphate and 700 °C thermo-activated nano serpentine (potassium dihydrogen phosphate: thermo-activated nano serpentine ratio = 1:2) added to simulated cadmium-contaminated soil, denoted as nPS 700 -0.5, nPS 700 -1.0, and nPS 700 -2.0, respectively; and 0.5%, 1.0%, and 2.0% soil heavy dose of potassium dihydrogen phosphate and 700 °C thermo-activated nano zeolite mixture (potassium dihydrogen phosphate: thermo-activated nano zeolite ratio = 1:2) added to simulated cadmium-contaminated soil, denoted as nPF 700 -0.5, nPF 700 -1.0, and nPF 700 -2.0, respectively. The results showed that the combined application of potassium dihydrogen phosphate with thermo-activated nano serpentine or potassium dihydrogen phosphate with thermo-activated nano zeolite reduced the soil exchangeable Cd content to varying degrees and increased levels of carbonate-bound, Fe-Mn oxide-bound, organic-bound, and residual Cd forms to different degrees. In combined application of phosphate and thermo-activated nano silicate, the higher the dosage level, the greater the reduction of exchangeable Cd content and the better the effect on Cd-contaminated soil remediation: nPS 700 -2.0 > nPS 700 -1.0 > nPS 700 -0.5, nPF 700 -2.0 > nPF 700 -1.0 > nPF 700 -0.5 (N, P, S, and F represent nano, KH 2 PO 4 , serpentine, and zeolite, respectively, and 700 represents the activation temperature). At the same dosage level, the combined application of potassium dihydrogen phosphate and thermo-activated nano serpentine was more effective than that of potassium dihydrogen phosphate and thermo-activated nano zeolite in repairing Cd-contaminated soil (nPS 700 -2.0 > nPF 700 -2.0, nPS 700 -1.0 > nPF 700 -1.0, nPS 700 -0.5 > nPF 700 -0.5), which indicated that the combination of phosphate and thermo-activated nano silicate can passivate heavy metal cadmium (Cd) to a certain extent and promote the transformation of bioavailable Cd into an unusable state. The reason why potassium dihydrogen phosphate, zeolite, and serpentine can absorb heavy metal cadmium after entering the soil is because the silicate mineral itself can directly absorb cadmium. Second, after nano treatment and thermal activation, the specific surface areas and pores of the minerals increase, which enhances the adsorption performance. Third, because the pH value of the mineral itself is high, the pH value of the soil environment will rise, thereby transforming H 2 PO 4 − into PO 4 3− , which is conducive to the adsorption of Cd 2+ .