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[MIS14-05] Influence of needle-like ZnO single crystals upon precipitation and polymorphic transition of calcium carbonate crystals
Keywords:calcium carbonate crystals, scale, ZnO single crystals upon precipitation
Among our interests in a broad discipline of crystallographic studies, understanding calcium carbonate crystals, comprising about 4% of the Earth’s crust and playing a significant role in the long-term global CO2 cycle, is essential. However, calcium carbonate’s crystal growth and polymorphic transition under various chemical and environmental conditions are poorly understood. From an engineering perspective, the calcium carbonate scale generated in water pipes and heat exchangers causes serious problems such as pipe blockage and corrosion (Kioka and Nakagawa, 2021). This study investigates the effect of needle-like ZnO single crystal materials on the precipitation and polymorphic transition of calcium carbonate crystals by laboratory experiments.
We prepared CaCO3 crystal precipitations by mixing NaHCO3 solution and CaCl2×2H2O solution (300 mM). Stainless steel substrate (SUS304) specimens were coated with needle-like ZnO single crystals/silicone composite and immersed for 30, 60, 120, and 180 minutes. After immersion, we washed and dried the specimens, calculated the weight changes of specimens, and analyzed the precipitated CaCO3 crystals using SEM. We evaluated the amount of precipitated crystals and polymorphic transition of CaCO3 under different immersion conditions, including fluid velocities, temperature, and CaCO3 concentration.
Our results showed that the weight increase of the ZnO-coated specimens was smaller than that of the reference specimens under all studied conditions. The average weight-based inhibition effectiveness of CaCO3 precipitation was 60.5% at 20°C, 48.7% at 60°C, 55.1% at 20°C in a stirred condition, and 68.2% in a 50% concentration of CaCO3. We suggest that the undulated surface microstructure of the ZnO coating surface lowered the surface energy of the specimens, making it difficult for crystals to adhere to the surface and resulting in the inhibition effect. The air “pocket” facilitated within the submerged coating structure prevents crystal from coming into contact with the liquid phase in the early stages of crystal formation. Also, the reference specimen showed the highest ratio of calcite in all conditions, due to the enhanced effect of calcite precipitation by the substrate of stainless steel we studied. On the other hand, in the ZnO-coated specimens, precipitations of calcite and aragonite were predominant at 20℃ and 60℃, respectively. This suggests that the ZnO coating counteracted the effect of leached ions the substrate material, and the crystalline polymorphs deposited according to the solution temperature.
We prepared CaCO3 crystal precipitations by mixing NaHCO3 solution and CaCl2×2H2O solution (300 mM). Stainless steel substrate (SUS304) specimens were coated with needle-like ZnO single crystals/silicone composite and immersed for 30, 60, 120, and 180 minutes. After immersion, we washed and dried the specimens, calculated the weight changes of specimens, and analyzed the precipitated CaCO3 crystals using SEM. We evaluated the amount of precipitated crystals and polymorphic transition of CaCO3 under different immersion conditions, including fluid velocities, temperature, and CaCO3 concentration.
Our results showed that the weight increase of the ZnO-coated specimens was smaller than that of the reference specimens under all studied conditions. The average weight-based inhibition effectiveness of CaCO3 precipitation was 60.5% at 20°C, 48.7% at 60°C, 55.1% at 20°C in a stirred condition, and 68.2% in a 50% concentration of CaCO3. We suggest that the undulated surface microstructure of the ZnO coating surface lowered the surface energy of the specimens, making it difficult for crystals to adhere to the surface and resulting in the inhibition effect. The air “pocket” facilitated within the submerged coating structure prevents crystal from coming into contact with the liquid phase in the early stages of crystal formation. Also, the reference specimen showed the highest ratio of calcite in all conditions, due to the enhanced effect of calcite precipitation by the substrate of stainless steel we studied. On the other hand, in the ZnO-coated specimens, precipitations of calcite and aragonite were predominant at 20℃ and 60℃, respectively. This suggests that the ZnO coating counteracted the effect of leached ions the substrate material, and the crystalline polymorphs deposited according to the solution temperature.