Prevention of CaCO 3 scale formation by trace amounts of copper (II) in comparison to zinc (II)

Abstract

The current paper describes the influence of trace amounts of Cu 2+ ions on the precipitation of calcite in an aqueous and supersaturated CaCO 3/NaCl system and in natural water of moderate hardness. Furthermore the influence of Zn 2+ ions on CaCO 3 precipitation in the CaCO 3/NaCl system was examined and the results were compared. The resulting reduction of the Ca 2+ concentration was recorded as a function of time by measuring the electric conductivity and the pH value. Because of the metal species present in the solution, two competing processes take place. Neutral CuCO 3 0(aq) and ZnCO 3 0(aq) complexes are adsorbed onto the active growth sites of the CaCO 3 surface and ,thus, further crystal growth is inhibited. As a result of nucleation catalysis, Cu(OH) 2 particles cause the accelerated formation of CaCO 3 nuclear crystals, which counteracts the inhibition process. At Cu 2+ concentrations > 1.40 mg/L, Cu(OH) 2 is transformed into stable and crystalline malachite, at which no catalysed nucleation can take place. This results in a significant increase of the induction period and inhibition, while the efficiency attained is about 90% for Cu 2+ contents > 2.00 mg/L. For an addition of Zn 2+ ions, ZnCO 3 is precipitated and acts as strong accelerator of heterogeneous nucleation of CaCO 3, resulting in an enhanced growth rate which dominantly counteracts the inhibition effect. The results may be explained, that copper ions are more effective for scale prevention than zinc ions.