Dissolution and precipitation of calcite in different water environments

Dissolution and precipitation of calcite are the basis of various karst geological processes, but the dissolution process and solubility of calcite under different water environmental conditions need to be further studied. In this paper, the dissolution of natural calcite (CaCO₃) was studied in different types of water at room temperature. The analysis of chemical composition and the characterization of X-Ray Diffraction (XRD) and Scanning Electron Microscope (SEM) showed that when calcite respectively dissolved in pure water, air-saturated water, CO₂-saturated water, the HCl solution of initial pH=3 and the NaOH solution of initial pH=9, both of its chemical composition and crystal structure did not change significantly; the aqueous Ca concentrations increased with time and attained an equilibrium or steady state, respectively reaching the values of 0.4444-0.4696 mmol·L⁻¹, 0.4020-0.4154 mmol·L⁻¹, 0.5739-0.6597 mmol·L⁻¹, 1.0981 mmol·L⁻¹ and 0.4489 mmol·L⁻¹, 4,080 hours after dissolution. For the dissolution of calcite in pure water, air-saturated water, CO₂-saturated water and the HCl solution of initial pH=3, the pH values increased with time and reached 8.06-8.40 at the end of test. For the dissolution in the NaOH solution of initial pH=9, the pH values increased rapidly to 9.71 with time, and then decreased slowly to an equilibrium or stable state around 8.31. The calculation results with the PHREEQC software indicated that after the dissolution in pure water, air-saturated water, CO₂-saturated water, the HCl solution of initial pH=3 and the NaOH solution of initial pH=9 for 2,640−4,080 hours, the aqueous solutions were undersaturated with all the possible minerals and did not precipitate to form aragonite (Saturation Index, SI=−0.09-−0.30), dolomite (SI=−2.03-−3.79), magnesite (SI=−3.00-−4.77) and other carbonate minerals. The solubility product (K_sp) and the Gibbs free energy of formation (ΔG_f˚) for calcite (CaCO₃) were estimated from the dissolution in pure water to be 10^{−8.48±0.08}−10^-8.48±0.13 and 1,129.82±0.51-−1,129.87±0.76 kJ·mol⁻¹, respectively. The solubility product (K_sp) of calcite (10^−8.48) is higher than that of rhodochrosite (10^−10.58), otavite (10⁻¹²), spherocobaltite (10^−9.98) and smithsonite (10⁻¹⁰), indicating that the carbonate precipitation method can be applied to fix heavy metals such as Mn, Cd, Co and Zn in soils and waters, which can effectively reduce their mobility and bioavailability in the environment. After dissolution in the seawater collected from Guangxi Beihai and the groundwater from Guilin Yanshan for 4,080 hours, the surface scanning analysis of Energy Dispersive Spectrometer (EDS) showed that calcite samples contained the components of (Ca_0.96Mg_0.06) CO₃ and (Ca_0.99Mg_0.01) CO₃, respectively. The slight increase in Mg content was related to the higher Mg concentration in the seawater (1,056 mg·L⁻¹) and the groundwater (6.70 mg·L⁻¹). In general, after 4,080 hours of dissolution, the pH variation in different types of natural water was listed in the order of river water (10.31) > seawater (8.80) > groundwater (8.03-8.28); the variation of Ca concentrations in different types of natural water was listed in the order of seawater > groundwater > river water; the variation of the total concentrations of HCO₃+CO₃ was in the order of groundwater > seawater > river water. For the dissolution in natural water, Ca and Mg concentrations in the river water collected from Guilin Lijiang River initially increased to a steady state and then decreased; the SI values of calcite, aragonite and dolomite increased slowly with time, and attained a saturated or oversaturated state after 1,680-hour, 1,920-hour and 2,160-hour, respectively. Thedecrease in Ca and Mg concentrations of the river water indicated that the precipitation of calcite, aragonite and dolomite might occur. For the dissolution in the seawater collected from Guangxi Beihai, the seawater was always saturated or oversaturated with calcite, aragonite, dolomite and magnesite, and the SI values decreased slowly at the beginning and then increased slowly after 720 hours, and finally repectively reached 0.84-1.06, 0.65-0.88, 2.40-2.95 and 0.60-0.76 after 2,640 hours. For the dissolution in the groundwater collected from Guilin Yanshan, the groundwater was always oversaturated or saturated with calcite and aragonite, while it was always saturated or undersaturated with dolomite and magnesite. The SI values decreased slowly at the beginning, then increased slowly after 720 hours, and finally reached 0.14-0.22, −0.06-0.02, −0.25-−0.03 and −1.36-−1.26, respectively after 2,640 hours. For the dissolution in the groundwater collected from Guilin Yaji, the groundwater was always oversaturated or saturated with calcite and aragonite, while it was always saturated or undersaturated with dolomite and magnesite. The SI values decreased slowly at the beginning and then increased slowly after 12 to 48 hours, but decreased slightly once again to a steady state after 480 hours and respectively reached 0.20-0.35, 0.02-0.15, −0.17-−0.04 and −1.50-−1.32 after 2,640 hours. The results further illustrate the significant influence of the water environment on calcite dissolution and can provide a reference for the geochemical simulation of karst geological processes.