The effect of calcium on the removal of Cd2+ in the formation of biogenic secondary iron minerals

Cadmium is a toxic heavy metal found in acid mine drainage. It hinders plant and animal growth and accumulates in human organs. In this study, through shake flask experiments, an iron-rich, sulphate-rich environment was simulated, and Acidithiobacillus ferrooxidans was used to mediate the formation of secondary high-iron minerals to explore the effect of calcium ions on the removal of Cd²⁺ from that environment. Four treatment systems were used: "Blank", "Ca²⁺-30 mg/L", "Fe/K = 3,Ca²⁺-30 mg/L", and "Fe/K = 3". The results showed that Cd²⁺ with an initial concentration of 20 mg/L was effectively removed in each treatment system. The removal efficiencies of Cd²⁺ in each treatment were 23.46%, 18.42%, 52.88%, and 45.76% respectively. The quantity and type of minerals determined the removal efficiency of Cd²⁺. The Fe/K = 3 treatment system can significantly increase the amount of mineral formation and improve the removal efficiency of Cd²⁺. In the Ca²⁺-30 mg/L, Fe/K = 3 treatment system, the biological oxidation ability was the strongest, and the removal effect of Cd²⁺ was the best under the combined action of K⁺ and Ca²⁺. Co-precipitation was the main way to remove Cd²⁺ during the formation of biogenic secondary iron minerals, and the removal amount was 5.64 to 14.83 times that of adsorption. Biogenetic secondary iron minerals showed high values in repairing heavy metal pollution. This study provides a theoretical basis for treating heavy metals in acid mine drainage.