Influence Mechanism of Crystal Phase Composition on the Fixation of Zinc by Calcium Sulfate

Calcium sulfate coprecipitation is a widely adopted method for removing heavy metal ions in engineering. However, the commonly used dihydrate calcium sulfate (CaSO₄·2H₂O, DH) phase shows limited efficiency in removing Zn²⁺ ions. To enhance this process, we regulated the crystal phase of calcium sulfate and explored how its composition affects the mechanism of Zn²⁺ immobilization. Coprecipitation experiments indicate that as the molar fraction of hemihydrate calcium sulfate (CaSO₄·0.5H₂O, HH) increases in the mixture, the efficiency of Zn²⁺ immobilization by calcium sulfate improves. When the HH molar fraction surpasses 15.6%, the immobilization capacity levels off. Mechanistic studies show that Zn²⁺ is mainly immobilized through structural doping within the crystal's water channels rather than by surface adsorption. In both DH and HH, zinc ions occupy these water channels instead of replacing Ca²⁺ ions. The larger water channels in HH contribute to its greater capacity for zinc ion immobilization. However, high-concentration Na⁺ ions interfere with HH formation, preventing further increases in zinc ion immobilization capacity and causing it to reach equilibrium. This research offers theoretical insights for optimizing calcium sulfate coprecipitation in managing heavy metal pollution in water bodies.