方解石在不同水环境中的溶解与沉淀作用

Dissolution and precipitation of calcite in different water environments

方解石的溶解与沉淀是各种岩溶地质作用的基础,但对其在不同水环境条件下的溶解过程和溶解度有待深入研究。文章通过实验研究了天然方解石(CaCO_(3))在不同水环境中的溶解作用。结果表明,方解石在纯净水、空气饱和水、CO_(2)饱和水、初始pH=3的溶液和初始pH=9的溶液中溶解时,Ca浓度随着溶解时间的增加呈现缓慢升高并趋于稳定,在溶解4080 h后达到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)和0.4489 mmol·L^(−1);方解石(CaCO_(3))在纯净水中溶解时溶度积(Ksp)为10−8.48±0.08~10−8.48±0.13,吉布斯生成自由能ΔGf˚[CaCO_(3)]为−1129.82±0.51~−1129.87±0.76 kJ·mol^(−1)。在天然水中溶解时,桂林漓江水中Ca和Mg的浓度在达到饱和状态后呈现下降趋势;广西北海海水相对于碳酸盐矿物处于饱和或过饱和状态;桂林雁山和丫吉地下水相对于方解石、文石和白云石处于饱和或过饱和状态。研究结果进一步证明了水环境对方解石溶解的显著影响,研究结果可为岩溶地质作用的地球化学模拟提供参考。

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(CaCO_(3))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,CO_(2)-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,CO_(2)-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,CO_(2)-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(Ksp)and the Gibbs free energy of formation(ΔGf˚)for calcite(CaCO_(3))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(Ksp)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(Ca0.96Mg0.06)CO_(3) and(Ca0.99Mg0.01)CO_(3),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 HCO_(3)+CO_(3) 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.