Objective Calcium carbonate in calcareous soil can immobilize the heavy metal lead (Pb) by co-precipitation. However, soil acidification caused by fertilization and rhizosphere action inevitably affects the stability of calcium carbonate and its bound heavy metals.

Method Pb was selected for the study. The effects of pH and ionic strength on the co-precipitation of calcium carbonate with Pb were quantitatively investigated and the stability of calcium carbonate-Pb co-precipitates was studied using hydrochloric acid and organic acids (citric and malic acid).

Result The results showed that the increase of pH promoted the Pb immobilization by calcium carbonate, while the opposite was true for ionic strength. The stability of calcium carbonate-Pb coprecipitates was weaker with the increase of acidification pH. The extraction of Pb by organic acids was higher than that of hydrochloric acid, where citric acid was higher than malic acid. However, malic acid was more capable of extracting Pb under organic acid extraction at pH 5.5. XRD results indicated that the presence of Pb inhibited the transformation of vaterite to calcite. Ca L-edge XANES analysis revealed that vaterite was the primary phase responsible for Pb immobilization.

Conclusion Citric and malic acids have different effects on the stability of lead immobilized by calcium carbonate and are closely related to pH. The results can be useful for the study of the chemical transport and transformation behavior of heavy metal elements in the rhizosphere of calcareous soils.