The role of chloride in calcium precipitation from drinking groundwater

BACKGROUND

Chloride is present in many water sources. In addition, the use of hypochlorite as a disinfectant for drinking water increases its chloride concentration. However, the effect of this prevalent ion on Ca precipitation in drinking water is poorly described, and its effect, as a function of the Ca/Mg ratio, which drives Ca precipitation, has not been studied. Accordingly, Ca precipitation was studied using batch assays at different concentrations of chloride and hypochlorite for Ca/Mg ratios of 4.0 and 1.1. The assays were performed at 20 and 25 °C. Chemical speciation analysis was performed using Visual MINTEQ software, and precipitates derived from the process were analyzed by X-ray diffraction.

RESULTS

Ca precipitation was influenced by the Cl⁻ concentration, causing an oversaturation of CaCO₃ with a trend to a maximum. The highest Ca precipitation was achieved at a Ca/Mg ratio of 4.0, obtained at an initial Cl⁻ concentration of 50 mg L⁻¹, both at 20 and 25 °C. For a Ca/Mg ratio of 1.1, the highest precipitation occurred at initial Cl⁻ concentrations of 10–15 mg L⁻¹ at 20 °C and 20 mg L⁻¹ at 25 °C. In ClO⁻ experiments, Ca precipitation increases proportionally with hypochlorite dosage. Greater Ca precipitation is observed at Ca/Mg = 4.0 compared to Ca/Mg = 1.1. The proportion of aragonite or calcite in the precipitates obtained depends on the presence of Cl⁻ and ClO⁻ as well as on the Ca/Mg ratio.

CONCLUSION

Chloride affects Ca precipitation by oversaturation, and the precipitation of CaCO₃ species depends on the concentration of Mg. © 2024 Society of Chemical Industry (SCI).