[Reconstructed Soil Vertical Profile Heavy Metal Cd Occurrence and Its Influencing Factors]

Aiming at the problem of heavy metal Cd pollution in mining wasteland under the high background value and superposition area of the historical mining source, a total of 30 samples were collected from the reclaimed soil of a historical sulfur mine in Southwest China. The total contents of Cadmium (Cd), Chromium (Cr), Nickel (Ni), Arsenic (As), and Mercury (Hg) in soil samples were analyzed. Scanning electron microscopy-electron diffraction spectroscopy (SEM-EDS) and BCR method for continuous extraction of heavy metals were used to analyze the occurrence of Cd under different soil thicknesses. The effectiveness of heavy metal Cd was evaluated by the risk assessment coding method (RAC), and the ratio of secondary phase and primary phase (RSP) was obtained. In addition, the changes in Cd morphology under different physicochemical properties were discussed. The results show that the concentration factor of Cd in the five heavy metals is the highest over 4. Through the morphological characteristics, it can be found that there are mainly subangular and angular particles in the samples. The resulting energy spectrum characteristics at different depths were similar, and all sampled contained Cd, Fe, S, and As. Isomorphs of Cd element were observed. According to the potential risk assessment of Cd, the RAC results show that the risk is medium or below, while the RSP results may lead to serious pollution except for the bottom layer. The total amount of heavy metals is the most important factor affecting the distribution of Cd forms, followed by pH, organic matter, and CEC. The results of this study can provide scientific basis for further understanding the activity, migration rule, biological toxicity, and occurrence form of Cd in profile soil.