Direct Two-Dimensional Time Series Observation of pH Distribution around Dissolving Calcium Carbonate Crystals in Aqueous Solution

Abstract

Monitoring in situ local pH changes to dissolving/growing crystal surfaces is critical to decipher the related mechanisms. To date, however, reliable techniques to acquire such measurements remain under-developed. Here we report the first successful attempt to observe time-resolved, two-dimensional distribution of pH around crystals dissolving in aqueous solutions. The local pH change around dissolving calcium carbonate polymorphs (calcite and aragonite) was visualized by using the fluorescent probe 8-hydroxypyrene-1,3,6-trisul-fonic acid HPTS under an optical microscope. The observations unambiguously showed that pH increased and saturated rapidly immediately adjacent to the reactive surfaces on the cleavage rhombs of calcite. Approximately 1 mm away, however, the increase slowed down significantly, generating a steep pH gradient at the crystal–water interface. Aragonite, which has rough fractured surfaces in contrast to the well cleaved calcite, dissolved much faster and showed a stronger tendency for the pH profile to trace the outer shape of crystal grains. These results indicate that this technique is sensitive enough to detect small differences in dissolution between polymorphs and surface conditions and may be used as an effective tool to visualize and understand various reactions in aqueous solutions including environmental weathering and material erosion.

Formation and Phase Selection of CaCO3 in the Intervention of Lignin Monomer Model Compounds

“…Besides acid concentration, ambient temperature and pH may also affect the formation of CaCO 3 . [ 32–35 ] As shown in Figure a–c, the crystal form of the generated ACCs was vaterite when temperature was 25 °C. This was always kept till the temperature increased to 80 °C in the PCA, FA, and SA solutions.…”

Replacement of Calcium Carbonate Polymorphs by Cerussite

“…In the acidic Pb 2+ -containing solution, calcium carbonate minerals undergo a proton-mediated dissolution process, releasing Ca 2+ and bicarbonate (HCO 3 – ) (eq ). In an acidic bulk solution (with initial pH = 2.7 and final pH ≤ 4.5), bicarbonate can transform to the thermodynamically more stable carbonic acid (H 2 CO 3(aq) ) (eq ), yet it is expected to be the dominant species near the carbonate mineral surface where pH is higher . Within the pore space, therefore, it is likely that dissolved Pb 2+ reacted with bicarbonate, leading to the formation of cerussite (eq ).…”

“…45 In an acidic bulk solution (with initial pH = 2.7 and final pH ≤ 4.5), bicarbonate can transform to the thermodynamically more stable carbonic acid (H 2 CO 3(aq) ) (eq 2), yet it is expected to be the dominant species near the carbonate mineral surface where pH is higher. 46 Within the pore space, therefore, it is likely that dissolved Pb 2+ reacted with bicarbonate, leading to the formation of cerussite (eq 3). This interface-mediated dissolution/precipitation mechanism 26,27 explains the observation that the overall morphologies of calcium carbonate polymorphs were preserved during their replacement by cerussite (Figure 2).…”