Immobilization of Pb²⁺, Cd²⁺, Sr²⁺ and Ba²⁺ ions using calcite and aragonite

The uptake of Pb²⁺, Cd²⁺, Sr²⁺ and Ba²⁺ ions by calcite and aragonite were investigated in detail at room temperature using the batch method. The reactions of strontianite and witherite with the heavy metal ions were also examined in order to clarify their selectivity on different carbonate compounds. The Pb²⁺ ions were fixed as cerussite by both calcite and aragonite. Although Cd²⁺ ions reacted with aragonite to form otavite, Cd²⁺ did not react with calcite. The reactions of calcite and aragonite with Sr²⁺ and Ba²⁺ ions was very sluggish and yielded strontianite and witherite, respectively. The orders of the selectivity of the heavy metal ions for carbonate compounds were determined to be as follows: rmPb²⁺ > Sr²⁺ > Ba²⁺ >> Cd²⁺ for calcite; Pb²⁺ > Cd²⁺ > Sr²⁺ > Ba²⁺ for aragonite; Cd²⁺ >Pb²⁺ > Ba²⁺ for strontianite; Pb²⁺ >Cd²⁺ > Sr²⁺ for witherite. The immobilization of the heavy metal ions using carbonate compounds proceeded by dissolution and recrystallization mechanism rather than by a topotactic reaction.