Divalent Lead in Aqueous Solution Changes the Surface Morphology of Dolomite and Inhibits Dissolution

: In groundwater systems, heavy metal ions as solutes (e.g., Pb) can adsorb onto the surface of calcite group rocks and inﬂuence their dissolution processes. The dolomite surface was examined using ﬁeld emission scanning electron microscopy (SEM) and various characterization tools and the changes in water chemistry indexes were reviewed throughout the dissolution process. Pb adsorption on the dolomite surface was evident after 15 days of exposure to 1 mg/L or 50 mg/L divalent Pb salt solutions; the Pb surface phase was mainly PbCO 3 with an octahedral ligand structure. SEM images show that dolomite in divalent Pb salt solutions can lead to the surface morphology exhibiting curved dissolution steps. In the closed system with Pb (1 mg/L), the total alkalinity and conductivity of the solution were lower than in the Pb-free system, and the pH difference was slight, indicating that the dibasic metal ion Pb inhibited the dolomite dissolution process. Combined with the composition of the ﬁnal solid phase, it is suggested that the dolomite surface preferentially adsorbs Pb 2+ on the active sites and that the newly grown solid phase is PbCO 3 possesses an octahedral ligand structure. Part of the surface-active site is occupied, resulting in a change in the dissolution proﬁle, and thus preventing further development of the dissolution proﬁle. Ultimately, the entire dolomite surface area is prevented from dissolution. The results of this study suggest that Pb 2+ may be an effective inhibitor of dolomite dissolution and may help to further reﬁne the geological carbon sink. solution composition groups. The slope of the total alkalinity curve for the solutions was large from 3 to 4 days at the beginning of the reaction, and slowly increased and stabilized after 4 days. In both gas-phase environments, the total alkalinity of the solution with deionized water as the initial solution was higher after 4 days of dissolution than that of the solution containing PbCl 2 , and the solution was re-dissolved by PbCl 2 treatment. The total alkalinity of the two reaction systems in the presence of Pb tended to be similar after 4 days of dissolution, and the trend in the total alkalinity of each solution corroborates the change in the conductivity of each solution in the previous section (Figures 8 and 9). pH, increased relative to the initial solution, changing slowly during the dissolution process. It was demonstrated that these processes are still present in the CO 2 system and the inhibition of dolomite dissolution by Pb in the CO 2 system is even more pronounced. On complex dolomite surfaces that possess incomplete deliquescent surfaces, fairly low concentrations (2.6 µ M) of Pb 2+ can inhibit dolomite dissolution in the near-equilibrium phase. The effect of Pb ions on the dissolution process of dolomite may be based on the fact that Pb is adsorbed to active sites on the surface of dolomite, preventing further development of the dissolution morphology and inhibiting the further dissolution of dolomite in the near-equilibrium phase. This work suggests that Pb 2+ can be an effective inhibitor of natural dolomite dissolution processes; therefore, experiments focusing on the inhibition of carbonate rock dissolution by surface sorbents have great potential in the study of natural karst process mechanisms in complex aqueous environments.


Introduction
Carbonate rocks are widely distributed in the lithosphere and are closely associated with human living space. They and their dissolution products also act as essential acid-base buffer components in the natural environment [1]. Carbonate dissolution processes are important environmental factors that profoundly affect both natural [2] and engineered systems [3,4]. Carbonate minerals are widespread in soils, sediments and bedrock, and calcite-dominated carbonate minerals are actively involved in Geochimica material cycles, such as the carbon-water-calcium cycle, which is further influenced by the dissolution of carbonate minerals in bedrock-saturated zones to form complex and extensive subsurface river network systems [5]. CO 2 (g) is often involved in the dissolution of carbonate minerals in the superficial karst zone, and this three-phase "water-rock-gas" interaction is the basis of the "atmosphere-soil-water-rock" carbon cycle [6]. The processes involved in the dissolution of carbonate rocks are considered to be the karst carbon sink component of the geological carbon sink but are generally considered to be carbon neutral [7]. The discovery of the "biological carbon pump" revealed that organisms can use bicarbonate in water as a source of carbon to synthesize organic carbon and one of the main sources of bicarbonate in water is carbonate dissolution involving CO 2 (g), so carbonate karst processes can achieve a net carbon sink through biological action [8]. Thus, the dissolution of minerals such as calcite and dolomite in karst environments directly or indirectly affects the global Geochimica material cycle, climate, and ecology. In recent decades, the dissolution of carbonate rocks has received much attention due to its universality and environmental significance.
The rate of dissolution of carbonate minerals and the factors controlling the dissolution process are the main elements of karst dynamics. Previous studies can be grouped into two areas. On the one hand, the differences caused by the pre-and post-differences in the elemental components contained in the liquid phase were used as a criterion to describe the mineral dissolution process under a particular set of conditions, so a set of surface complexation models consisting of surface adsorption and surface chemistry was developed. Oleg S. Pokrovsky et al. studied the dissolution process of divalent metal carbonate minerals (Ca, Mg, Sr, Ba, Mn, Fe, Co, Ni, Zn, Cd, and Pb), and the experimental results demonstrated that the formation of surface hydration complexes at 5 < pH < 8 and ∑CO 2 ≤ 10 −4 M determines the surface dissolution, and the dissolution rate of carbonate minerals is controlled by the metal by which the ionic surface hydration process is controlled. A theoretical model for the surface complexation of divalent metal carbonates was developed based on electrophoretic measurements and the correlation between aqueous and surface reaction stability constants [9]. The surface adsorption theory suggests that there is a diffusion layer at the interface between the liquid and solid phases and that the transfer of material between the two phases requires a diffusion layer, while ions in the liquid phase do not exist as separate ions but are hydrated to form various complexes. Oleg S. Pokrovsky et al. studied the dissolution kinetics of calcite, dolomite, and magnesite at different partial pressures of carbon dioxide (0-50 atm) and 25 • C. The experimental results showed that the dissolution rate was not proportional to H 2 CO 3 * (aq), but had a weak correspondence to the systemic partial pressure of carbon dioxide. The experiment also demonstrated the prediction of the surface complexation model proposed by Pokrovsky et al. for the dissolution process of dolomite and magnesite at a partial pressure of 50 atm CO 2 [10]. In subsequent experiments, Oleg S. Pokrovsky et al. broadened the carbonate dissolution environment to a range from acidic to neutral: 25 • C to 150 • C [11].
The dissolved surface complexation model emphasizes the hydration of the solid phase surface to produce a variety of surface complexes, and the distribution of these surface complexes on the solid phase surface better determines the face complexes on the solid phase surface. Carbonate minerals have crystal defects and dislocations, while surface features such as islands, cavities, kinks, and steps are formed during the dissolution of carbonate minerals. The mineral dissolution process can, therefore, be regarded as a surface morphological change process in which dissolution pits and steps formed by dislocations and defects develop, propagate, and annihilate each other. Lasaga A. C. and Luttge A. summarized this as a surface dissolution step-wave model. In addition to a dissolution mode with a net recession towards the normal surface, a step wave mode based on the surface morphology can also lead to layer exfoliation of the solid phase surface, and the two control surface dissolution [12]. This model, which relies on surface dissolution morphology, has been extended to Monte Carlo simulations for calcite-like mineral dissolution kinetics calculations [13]. As a result of this theoretical support, several experiments investigating the in situ dissolution processes of calcite-like minerals have also focused on the observation of changes in surface morphology development [14,15].
Metal ions such as Mn, Cd, Cu, and Pb are often studied as environmental toxins, aqueous solutions of these metal ions have a significant adsorption effect on carbonate surfaces [16,17], and the introduction of these 'exogenous cations' has an impact on carbonate dissolution. S.G. Terjsen et al. investigated the involvement of metal ions in the reaction of calcite with carbon dioxide to form bicarbonate and demonstrated that metal ions had an inhibitory effect, but the inhibition was not explained by the interference with the extension of the calcite surface steps. S.G. Terjsen et al. investigated the involvement of metal ions in the reaction of calcite with carbon dioxide to form bicarbonate and demonstrated that metal ions had an inhibitory effect, but the inhibition was not explained by the interference with the extension of calcite surface steps. It was shown that the adsorption inhibitor and the exogenous metal ions increased the concentration of carbonate ions on the surface and contributed to the reverse reaction. The authors concluded that these processes were responsible for the inhibition of dissolution, and, on this basis, developed a kinetic equation for dissolution that was in relatively good agreement with the actual measurements [18]. However, more experimental results from in situ atomic force microscopy (AFM) and vertical scanning (phase shift) interferometry (VSI) over the last decade have shown that water-soluble metal ions can influence the dissolution process of carbonate minerals, while exogenous metal ions have been observed to alter surface morphology during this process [19][20][21].
Pb is an environmental toxin but tends to be present in low concentrations under natural conditions due to reactions such as its complexation in water. This experiment aimed to build on previous studies to characterize the uptake of water-soluble Pb 2+ by dolomite powder (CaMg(CO 3 ) 2 being the main component), and its dissolution is affected in two ways under laboratory conditions. (1) In this experiment, natural dolomite powder after grinding was selected as the solid-phase sample, hoping to use the dolomite with an incomplete deconfined surface to study the adsorption process of Pb on its surface and the subsequent dissolution process. (2) The experiments were conducted by charging CO 2 until saturation as a CO 2 -sufficient environment and N 2 only as a CO 2 -insufficient environment, to simulate the absorption of lead ions by dolomite and the effect on dolomite dissolution in both environments. In this work, Fourier infrared spectroscopy (FTIR), laser Raman spectroscopy (Raman), X-ray photoelectron spectroscopy (XPS), and field emission scanning electron microscopy, coupled with energy spectroscopy (SEM + EDS), were used to investigate two issues related to the contact between the crushed surface of natural dolomite and metal ion solutions to determine the Pb adsorption process and its influence on the dissolution process of dolomite.

Materials and Methods
The experimental samples were made from ground natural rocks, which were extracted from a Pb-Zn sulfide mining area at the upper Shegongping right river in Shegongping Village, Dajing Yao Township, Lingchuan County, Guangxi Zhuang Autonomous Region, China. The perimeter rocks and sulfur-bearing Pb-Zn ore outside the mine area were collected separately and ground into a rock powder that could pass through 300 mesh. After X-ray diffraction, the main composition of the perimeter rock outside the mine area was dolomite with a small amount of barite, as shown in Figure 1.
the interference with the extension of calcite surface steps. It was shown that the adsorp tion inhibitor and the exogenous metal ions increased the concentration of carbonate ion on the surface and contributed to the reverse reaction. The authors concluded that thes processes were responsible for the inhibition of dissolution, and, on this basis, develope a kinetic equation for dissolution that was in relatively good agreement with the actua measurements [18]. However, more experimental results from in situ atomic force micros copy (AFM) and vertical scanning (phase shift) interferometry (VSI) over the last decad have shown that water-soluble metal ions can influence the dissolution process of car bonate minerals, while exogenous metal ions have been observed to alter surface mor phology during this process [19][20][21].
Pb is an environmental toxin but tends to be present in low concentrations unde natural conditions due to reactions such as its complexation in water. This experimen aimed to build on previous studies to characterize the uptake of water-soluble Pb 2+ b dolomite powder (CaMg(CO3)2 being the main component), and its dissolution is affecte in two ways under laboratory conditions. (1) In this experiment, natural dolomite powde after grinding was selected as the solid-phase sample, hoping to use the dolomite with a incomplete deconfined surface to study the adsorption process of Pb on its surface an the subsequent dissolution process. (2) The experiments were conducted by charging CO until saturation as a CO2-sufficient environment and N2 only as a CO2-insufficient env ronment, to simulate the absorption of lead ions by dolomite and the effect on dolomit dissolution in both environments. In this work, Fourier infrared spectroscopy (FTIR), lase Raman spectroscopy (Raman), X-ray photoelectron spectroscopy (XPS), and field emis sion scanning electron microscopy, coupled with energy spectroscopy (SEM+EDS), wer used to investigate two issues related to the contact between the crushed surface of natura dolomite and metal ion solutions to determine the Pb adsorption process and its influenc on the dissolution process of dolomite.

Materials and Methods
The experimental samples were made from ground natural rocks, which were ex tracted from a Pb-Zn sulfide mining area at the upper Shegongping right river in She gongping Village, Dajing Yao Township, Lingchuan County, Guangxi Zhuang Autono mous Region, China. The perimeter rocks and sulfur-bearing Pb-Zn ore outside the min area were collected separately and ground into a rock powder that could pass throug 300 mesh. After X-ray diffraction, the main composition of the perimeter rock outside th mine area was dolomite with a small amount of barite, as shown in Figure 1. In this experiment, a 1 L conical flask was chosen as the open system experimenta set-up and 1 g of dolomite powder and 1 g of Pb-Zn sulfide ore powder were immerse in 500 mL of deionized water and repeatedly sampled during the experiment. Howeve to maintain the experimental conditions of the closed system, which did not allow fo In this experiment, a 1 L conical flask was chosen as the open system experimental set-up and 1 g of dolomite powder and 1 g of Pb-Zn sulfide ore powder were immersed in 500 mL of deionized water and repeatedly sampled during the experiment. However, to maintain the experimental conditions of the closed system, which did not allow for repeated sampling, a 50 mL top-mouth bottle (sealed with a rubber gasket at the top) was used as the experimental set-up and 1 g of dolomite powder was immersed in 50 mL of reaction solution for batch experiments. The same batch was started at the same time, one bottle from the batch was taken out at 1 d intervals for sampling, and the experiment lasted for 15 d. The experimental set-ups of the closed system were placed in a water tank to isolate them from the atmosphere. The experimental group was set up to source the solution Pb ions. In the open system, Pb was sourced from both the solution and the ore, while in the closed system, Pb was only sourced from the solution, which was prepared using PbCl 2 dissolved in boiling water, and the concentration was set at 1 mg/L and 50 mg/L. At the same time, a redissolution experimental group was set up. The dolomite was immersed in a 1 mg/LPbCl 2 solution for 15 days and then removed and dried. The dolomites that were pre-treated with PbCl 2 solution were then immersed in deionized water for 15 days for the dissolution experiments. The karst process is often a three-phase water-rock-gas reaction where both O 2 and CO 2 are important controlling factors; therefore, in this experiment, the solution in the closed system is fed with N 2 or CO 2 before the experiment starts, to reduce the dissolved oxygen content of the solution and add the carbon source to the closed system.
Total alkalinity in the solution was determined using an alkalinity kit generated by MERCK, Germany, with an accuracy of 0.1 mmol/L. The conductivity and pH of the solution were determined using a portable multi-parameter water-quality analyser (PONSEL, France). X-ray diffraction patterns of the solid phase samples were obtained using a benchtop X-ray diffractometer (X'Pert3 Power). The instrument is a Cu target ceramic light tube, standard size design; scanning method is Theta/Theta goniometer, with vertical goniometer; rotation range during testing was from 10 • to 80 • . The surface morphology of the dolomite powder was obtained using a Field Emission Scanning Electron Microscope (JSM-7900M), coupled with an Oxford Instruments Energy Spectrometer (Ultim Max) to obtain a surface energy spectrum at 1.5 kV, 5 nA, and WD of approximately 10 mm. The infrared profiles of the powders were provided by an FTIR spectrometer (Nicolet) with specifications and technical specifications using the iS10, utilizing KBr as the press medium. The Raman profiles of the powders were provided by laser confocal Raman spectrometer (Thermo Fisher Scientific DXR, Waltham, MA, USA), using a 780 nm laser light source with a test range of 70-3600 cm −1 and a test temperature of 298 K. The photoelectron spectra of the samples were obtained using an American thermoelectric X-ray photoelectron spectrometer (ESCALAB 250Xi). The X-ray source was a monochromatized Al Ka (1486.6 eV) line source with full-spectrum pick-up energy of 100 eV in 1 eV steps and narrow-spectrum pick-up energy of 20 eV in 0.1 eV steps, tested at a vacuum of 10-19 mbar.

Results
After exposing the dolomite surface to the solution, the solid phase was collected and dried to obtain non-in situ SEM images. According to the experimental setup, compared to the open system, the solid phase samples in the closed system had no Pb-Zn sulfide ore fractions, are single in composition, and are minimally affected by the external environment, which facilitates the detection of solid-phase surface features; therefore, only the hydrochemical processes in the open system experimental group were examined.  Figure 2 shows the characteristic XPS peaks in dolomite in the closed system after 15 days of exposure to 1 mg/L and 50 mg/LPbCl 2 solutions. The two photoelectron peaks between 135 eV and 145 eV are close to the stronger characteristic peaks corresponding to elemental Pb in previous studies, namely the Pb4f 5/2 and Pb4f 7/2 peaks at 143 eV and 138 eV [22]. Increasing the concentration of PbCl 2 in the solution resulted in a subsequent increase in the intensity of the two characteristic peaks.
According to previous XPS studies on Pb compounds, Pb will show the photoelectron peak's chemical shifts when forming compounds [23][24][25]; therefore, in this paper, the Pb4f 7/2 peak in the XPS peak spectrum of dolomite exposed to PbCl 2 solution was split and fitted. The results are shown in Figure 3, and the reference data of Pb chemical shifts are shown in Table 1. eV [22]. Increasing the concentration of PbCl2 in the solution resulted in a subsequent increase in the intensity of the two characteristic peaks. According to previous XPS studies on Pb compounds, Pb will show the photoelectron peak's chemical shifts when forming compounds [23][24][25]; therefore, in this paper, the Pb4f7/2 peak in the XPS peak spectrum of dolomite exposed to PbCl2 solution was split and fitted. The results are shown in Figure 3, and the reference data of Pb chemical shifts are shown in Table 1.   According to previous XPS studies on Pb compounds, Pb will show the photoelectron peak's chemical shifts when forming compounds [23][24][25]; therefore, in this paper, the Pb4f7/2 peak in the XPS peak spectrum of dolomite exposed to PbCl2 solution was split and fitted. The results are shown in Figure 3, and the reference data of Pb chemical shifts are shown in Table 1.    Figure 4 shows the X-ray diffraction patterns of the dolomite powder under different experimental conditions. The four sets of XRD data show that the composition of the strong peaks in the solid phase is still dominated by strong peaks in CaMg(CO 3 ) 2 , with peaks in BaSO 4 or (Ba, Pb)SO 4 . The overall peak distribution characteristics did not significantly change. In the XRD pattern of the dolomite exposed to PbCl 2 , peaks consistent with the standard PbCO 3 peak positions only occurred at 20.1960 • , 49.3789 • , 54.8250 • , and 56.2238 • . However, the peak intensities are relatively low and there were no strong peaks in the peak profile corresponding to the several strong peaks in the standard PbCO 3 pattern. The four characteristic peaks correspond to the standard diffraction peak card as PDF00-047-1734:PbCO 3 , while, at 27.9850 • , the strong peak only occurs in the system containing Pb, where it corresponds to the standard diffraction peak card as PDF 00-005-0568:CaCO 3 .  Figure 4 shows the X-ray diffraction patterns of the dolomite powder under different experimental conditions. The four sets of XRD data show that the composition of the strong peaks in the solid phase is still dominated by strong peaks in CaMg(CO3)2, with peaks in BaSO4 or (Ba, Pb)SO4. The overall peak distribution characteristics did not significantly change. In the XRD pattern of the dolomite exposed to PbCl2, peaks consistent with the standard PbCO3 peak positions only occurred at 20.1960°, 49.3789°, 54.8250°, and 56.2238°. However, the peak intensities are relatively low and there were no strong peaks in the peak profile corresponding to the several strong peaks in the standard PbCO3 pattern. The four characteristic peaks correspond to the standard diffraction peak card as PDF00-047-1734:PbCO3, while, at 27.9850°, the strong peak only occurs in the system containing Pb, where it corresponds to the standard diffraction peak card as PDF 00-005-0568:CaCO3. (a) charged with N2; (b) charged with N2 and exposed to Pb; (c) charged with CO2; (d) charged with CO2 and exposed to Pb; the solid vertical line is the standard dolomite (PbCO3) peak position; the dashed vertical line is the standard calcite (CaCO3) peak position.

FTIR, Raman Results of Dolomite before and after the Dissolution
The mid-infrared spectra of dolomite powder exposed to PbCl2 solution were compared with those of dolomite powder that were pre-treated and then redissolved; see (a) charged with N 2 ; (b) charged with N 2 and exposed to Pb; (c) charged with CO 2 ; (d) charged with CO 2 and exposed to Pb; the solid vertical line is the standard dolomite (PbCO 3 ) peak position; the dashed vertical line is the standard calcite (CaCO 3 ) peak position.

FTIR, Raman Results of Dolomite before and after the Dissolution
The mid-infrared spectra of dolomite powder exposed to PbCl 2 solution were compared with those of dolomite powder that were pre-treated and then redissolved; see Figure 5. The two sets of mid-infrared spectra share the same peak distribution characteristics: two sharp peaks at 729 cm −1 and 881 cm −1 , a broader and stronger absorption band at 1440 cm −1 , and three smaller, weaker peaks in the high-frequency region, located at 1819 cm −1 , 2524 cm −1 , and 2896 cm −1 .
The Raman shifts in the dolomite powder exposed to PbCl 2 solution are similar to those of dolomite; however, at 168 cm −1 , 292 cm −1 , and 718 cm −1 , the Raman shifts are shifted in the short-wave direction compared to the initial dolomite powder. The pretreated and redissolved dolomite powder has the same Raman shifts as the latter; see Figure 6.

SEM Images of Dolomite Surfaces and EDS Results
Previous observations of calcite surface dissolution morphology using SEM or atomic force microscopy (AFM) were generally made on more complete and flat surfaces, cut out by extended cleavage surfaces, such as calcite {1014} surfaces [26][27][28]. In contrast, the mechanically crushed dolomite powder surface morphology mainly consisted of rhombohedralshaped pits with linear macroscopic steps, which were formed by the mechanical breaking of the multilayered cleavage surfaces. The morphology resulting from mechanical crushing is densely packed on the surface, mostly combining to form irregular pits and folded macro-steps connected by rhombic edges and straight lines (Figure 7a,b). Increasing the concentration of PbCl 2 in the solution to 50 mg/L, the dolomite surface showed only curved steps distributed on a flatter cleavage surface, in addition to the obvious macroscopic steps formed by mechanical fragmentation (Figure 7b). At the same time, CO 2 was introduced to the Pb-containing solution, the dolomite was immersed, and "curved steps" consisting of folded lines were observed on the surface of the dolomite. Additionally, adjacent rhombohedral-shaped pits were combined into one irregular pit (Figure 7e). During the dissolution of dolomite in Pb 2+ solution, none of the SEMs observed the appearance of Pb 2+ -containing precipitation morphology on the dolomite surface, which is inconsistent with the appearance of ellipsoidal mound precipitation morphology on the surface in an earlier study [29]. The dolomite powder energy spectra before and after the reaction determined its main component to be CaMg(CO 3 ) 2 , and the surface energy spectra of dolomite exposed to Pb 2+ solutions showed the presence of Pb components in specific areas ( Table 2). The locations of the specific regions are shown in Figure 7f. On the dolomite surface exposed to Pb 2+ solution, the energy spectra in Table 3 show that the apparent Pb apparent concentration is greater in the stepped areas of the dolomite surface than in the flatter areas.  The Raman shifts in the dolomite powder exposed to PbCl2 solution are similar to those of dolomite; however, at 168 cm −1 , 292 cm −1 , and 718 cm −1 , the Raman shifts are shifted in the short-wave direction compared to the initial dolomite powder. The pretreated and redissolved dolomite powder has the same Raman shifts as the latter; see   The Raman shifts in the dolomite powder exposed to PbCl2 solution are similar to those of dolomite; however, at 168 cm −1 , 292 cm −1 , and 718 cm −1 , the Raman shifts are shifted in the short-wave direction compared to the initial dolomite powder. The pretreated and redissolved dolomite powder has the same Raman shifts as the latter; see  after the reaction determined its main component to be CaMg(CO3)2, and the s ergy spectra of dolomite exposed to Pb 2+ solutions showed the presence of Pb co in specific areas ( Table 2). The locations of the specific regions are shown in Fig the dolomite surface exposed to Pb 2+ solution, the energy spectra in Table 3 sho apparent Pb apparent concentration is greater in the stepped areas of the dolom than in the flatter areas.
Water 2022, 14, x FOR PEER REVIEW

Dissolution Processes in Open Systems
The open-system experimental groups in this experiment refer to dissolution groups that can exchange substances with the laboratory environment. These groups are set with a large liquid-to-solid ratio, with 2 g of solid corresponding to 500 mL of water, and are designed to enable repeated water samples to be taken for a variety of water chemistry parameters. Additionally, the rock samples for this experiment were taken downstream of the Pb-Zn ore mine, where the atmospheric environment provides dissolved oxygen to promote oxidation reactions in the sulfur-containing components of Pb-Zn ore, lowering the pH of the solution and enhancing the chemical weathering of the rock. In the open experimental group, Pb was supplied to the reaction system by both the solid phase and the liquid phase, with the solid phase source deriving from the dissolution of the sulfur-bearing fraction of Pb-Zn ore and the liquid phase source coming from the artificial addition of different concentrations of PbCl 2 solution.
When the dolomite powder was exposed to PbCl 2 solution for 15 days, the pH value continued to decrease, the conductivity (Conduc.) increased as dissolution proceeded and the total inorganic carbon of the solution similarly increased; when comparing different initial concentrations of PbCl 2 solution, the total inorganic carbon (TIC) of the solution similarly varied between the two groups, with the pH curve in the 50 mg/L PbCl 2 experimental groups being higher than that of the 1 mg/L PbCl 2 experimental group, while the solution conductivity curve was low.

Dissolution Processes in Closed Systems
The closed system experimental group was set up to ensure that the experimental system was as isolated as possible from the external environment during the dissolution process, so 50 mL glass bottles with lids were chosen as the experimental vessel for the batches. Therefore, the solid-liquid ratio of the closed system group was much higher than that of the open system group. The reactions were carried out simultaneously in the same batch over 15 days. One bottle from each batch was taken out every 24 h for sampling and testing. The solid phase of the experimental group was only dolomite powder, with N 2 or CO 2 passed in to approximate the gas phase component, and the liquid phase was a 1 mg/L PbCl 2 solution. Two groups were set up with or without Pb in solution (XXdolomite system; XX-Pb + dolomite system). Dolomite powder was exposed to PbCl 2 for 15 days, dried, and then redissolved in deionized water. The above process was used as a pretreatment re-dissolution experiment for the group (XX-dolomite(Pb)). CO 2 passed into water will produce carbonic acid, which will further ionize to provide H + to the solution. Comparing the different gas-phase conditions, the pH of the solution decreased with CO 2 ; this promoted the dolomite dissolution, resulting in a higher conductivity and total alkalinity than the experimental group with N 2 . In both gas-phase atmospheres, the conductivity of the solution with an initial solution of deionized water was higher during the 15-day dissolution process than both the solution containing PbCl 2 and the solution that was re-dissolved after the PbCl 2 treatment. The conductivity of the solutions in the two reaction systems with Pb was similar during the 15-day dissolution process, and the conductivity of the solution with PbCl 2 as the initial solution in the N 2 atmosphere was slightly higher than that of the solution that was re-dissolved after pretreatment with PbCl 2 . The change in pH during dissolution is mainly related to the composition of the incoming gas, and the pH during dissolution is similar for different solution composition groups. The slope of the total alkalinity curve for the solutions was large from 3 to 4 days at the beginning of the reaction, and slowly increased and stabilized after 4 days. In both gas-phase environments, the total alkalinity of the solution with deionized water as the initial solution was higher after 4 days of dissolution than that of the solution containing PbCl 2 , and the solution was re-dissolved by PbCl 2 treatment. The total alkalinity of the two reaction systems in the presence of Pb tended to be similar after 4 days of dissolution, and the trend in the total alkalinity of each solution corroborates the change in the conductivity of each solution in the previous section (Figures 8 and 9). 22, 14, x FOR PEER REVIEW 15 days, dried, and then redissolved in deionized water. The above pretreatment re-dissolution experiment for the group (XX-dolomite CO2 passed into water will produce carbonic acid, which will vide H + to the solution. Comparing the different gas-phase conditio tion decreased with CO2; this promoted the dolomite dissolution, conductivity and total alkalinity than the experimental group with atmospheres, the conductivity of the solution with an initial solutio was higher during the 15-day dissolution process than both the solu and the solution that was re-dissolved after the PbCl2 treatment. Th solutions in the two reaction systems with Pb was similar during t process, and the conductivity of the solution with PbCl2 as the ini atmosphere was slightly higher than that of the solution that was r treatment with PbCl2. The change in pH during dissolution is main position of the incoming gas, and the pH during dissolution is sim tion composition groups. The slope of the total alkalinity curve for t from 3 to 4 days at the beginning of the reaction, and slowly increase 4 days. In both gas-phase environments, the total alkalinity of the so water as the initial solution was higher after 4 days of dissolution th containing PbCl2, and the solution was re-dissolved by PbCl2 treatm ity of the two reaction systems in the presence of Pb tended to be s dissolution, and the trend in the total alkalinity of each solution co in the conductivity of each solution in the previous section (Figures

Dolomite Exposure to Pb 2+
The initial solution configured for this experiment contained a lead of 1 mg/L, which can be converted to 4.86 × 10 −6 mol/L. According to the hydroxide, when the solution reaches equilibrium (i.e., the precipitatio concentration is 2.94 × 10 −15 = 5.42 × 10 −8 mol/L, at which point the H + con solution can be calculated from kw to be 1.85 × 10 −7 , which means that t 6.73. In other words, when the solution pH is greater than 6.73, the solu rated concerning lead hydroxide and vice versa. The initial lead soluti ment was configured by dissolving lead chloride in hot water, and its in termined to be 6.33, which is less than 6.73, so the initial solution was lead hydroxide. The initial solution pH is weakly acidic and the solid p

Dolomite Exposure to Pb 2+
The initial solution configured for this experiment contained a lead ion concentration of 1 mg/L, which can be converted to 4.86 × 10 −6 mol/L. According to the K sp value of lead hydroxide, when the solution reaches equilibrium (i.e., the precipitation limit) the OH − concentration is 2.94 × 10 −15 = 5.42 × 10 −8 mol/L, at which point the H + concentration in the solution can be calculated from kw to be 1.85 × 10 −7 , which means that the solution pH is 6.73. In other words, when the solution pH is greater than 6.73, the solution is supersaturated concerning lead hydroxide and vice versa. The initial lead solution in this experiment was configured by dissolving lead chloride in hot water, and its initial pH was determined to be 6.33, which is less than 6.73, so the initial solution was not saturated for lead hydroxide. The initial solution pH is weakly acidic and the solid phase dolomite in this experiment should more readily react with the acidic solution. The reaction equation is as follows: CaMg(CO 3 ) 2 + 2H + = Ca 2+ + Mg 2+ + 2HCO 3

−
(1) Disregarding how the carbonic acid equilibrium of the solution causes the dissolved CO 3 2− to change, 1 mol of H + in the solution is always able to dissolve 1 mol of CO 3 2− from the dolomite into the solution. At this point, the concentration of carbonate in the solution at equilibrium can be calculated from the K sp of lead carbonate and the initial lead concentration in the solution to be 3.02 × 10 −8 mol/L. There is no carbonate present in the initial solution, so the carbonate in the solution comes from the reaction between H + and dolomite, the volume of the solution remains unchanged during the reaction, and the volume of the solution is assumed to be 1L for the convenience of calculation. Then, according to Equation (1), the consumption of 3.02 × 10 −8 mol of H + in solution reacts with dolomite. Only then can the CO 3 2− reach 3.02 × 10 −8 mol. The initial solution pH is 6.33 and, after consuming 3.02 × 10 −8 mol H + , the H + concentration becomes 4.38 × 10 −7 and the pH becomes 6.35. 6.35 < 6.73, at which point the solution is saturated with lead carbonate but not with lead hydroxide. As the dolomite dissolves, the solution is reduced in H + and increased in CO 3 2− , which means that the solution is oversaturated with lead carbonate and still not saturated with lead hydroxide during the reaction from pH = 6.35 to pH = 6.73. Therefore, this paper argues that the initial lead solution first reaches lead carbonate supersaturation as the dolomite dissolves, a state that facilitates the generation of lead carbonate on the dolomite surface. The solution is then supersaturated with lead hydroxide only as the dolomite dissolves. If a lead carbonate precipitate is generated first, reducing the concentration of lead ions in the solution, the solution will likely remain unsaturated for lead hydroxide. Therefore, the lead on the surface of the dolomite should primarily become lead carbonate rather than lead hydroxide.
Regarding the possible presence of sulfate in the solution, we believe that the only source of sulfate present in the solution is the dissolution of the barite component of the solid phase impurities. Although the K sp of barite is much greater than that of dolomite but under weakly acidic conditions, H + promotes the dissolution reaction of dolomite, which rapidly and releases large quantities of carbonate into the solution and consumes H + . The K sp of PbSO 4 is 1.82 × 10 −8 , while that of lead carbonate is 1.46 × 10 −13 , which shows that the K sp of lead carbonate is much smaller than that of barium sulfate; therefore, in this work we assume that the solution is first supersaturated with lead carbonate rather than lead sulfate under the premise that the source of carbonate in the solution is sufficient. However, we cannot exclude the possible generation of PbSO 4 , which is reflected in the results of the XPS experiments, but we still believe that the dolomite surface production is mainly PbCO 3 .
The solution in the reaction system was measured using inductively coupled plasma emission spectrometry (ICP-OES) after 1 day. The Pb concentration in the solution was found to be below the confidence range of the instrument. This experimental result indicates that, after 1 day of dolomite reaction in either 1 mg/L PbCl 2 solution or 50 mg/L PbCl 2 solutions, the Pb in the aqueous solution was converted to the solid phase and the remaining Pb in the solution was less than 50 µg/L. XPS results of dolomite powder in 1mg/L PbCl 2 solution and 50 mg/L PbCl 2 solution were compared and revealed characteristic peaks at 143 eV and 138 eV for both samples, which were close to the Pb4f 5/2 and Pb4f 7/2 standard peak positions, respectively, identifying the appearance of a Pb-containing solid phase on the dolomite surface after exposure to the Pb-containing solution for 15 days. Using the XPS chemical shift peak positions of the previous standard Pb-containing compounds as a reference, we carried out a splitting of Pb4f 7/2 . The splitting results indicate that the Pbcontaining solid phase on the dolomite surface mainly consists of PbCO 3 with a little PbSO 4 ; see Figure 3. A small amount of PbSO 4 is presumed to be produced from the reaction of the impurity BaSO 4 with Pb in the dolomite. Comparing the two sets of data, we can see that the intensity of the Pb4f peak in the dolomite surface is also higher for the more concentrated Pb solution, and the lower PbCO 3 equilibrium constant (log KPbCO 3 = −13.13, T = 298 K) means that the solution is relatively supersaturated for the solid phase; therefore, we believe that the enhanced Pb4f peak signal is the result of more PbCO 3 precipitation. In experiments on the adsorption of Pb on calcite {10 − 14} cleavage surfaces, calcite cleavage surfaces immersed in high concentrations of Pb solutions generated more PbCO 3 precipitates, thereby increasing the intensity of the corresponding Pb peaks in XPS [30]. When SEM observations were carried out in conjunction with EDS surface composition surface analysis and compared with the regional electron energy spectroscopy data of dolomite exposed to deionized water, the dolomite surface showed precipitation of Pb components after exposure to PbCl 2 solution for 15 d, which corroborated the results obtained by XPS. XRD showed the same variation in surface composition to some extent. The total diffraction peaks were still characterized by a solid phase consisting of dolomite with associated barite and, in the Pb-bearing group, the characteristic peaks in the PbCO 3 fraction and the characteristic peaks in the BaSO 4 fraction appeared; see Figure 4. The Pb-bearing dolomite powder diffraction data showed characteristic peaks in PbCO 3 that were not present in the non-Pb group, but the positions of the peaks in the X-ray diffraction spectra and the standard PbCO 3 peaks rarely corresponded to each other, which is different from the experimental results, where PbCO 3 replaced CaCO 3 in a highly concentrated Pb 2+ solution [29]. Notably, a characteristic CaCO 3 diffraction peak was found at 27.9850 • from Pb-bearing dolomite, and CaCO 3 reprecipitation may reflect a coupled dissolutionprecipitation process due to the uneven dissolution of the dolomite [31].
The SEM observations of the dolomite surface in the previous section show that the surface is a powder fractured dolomite surface rather than a conventional pure crystal cut flat surface, but the same changes in surface morphology are observed: in the non-Pb solution the dolomite surface morphology is generally composed of rhombohedralshaped pits and macroscopic steps, whereas after 15 days in the Pb containing solution, the dolomite surface is characterized by a large reduction in the number of steps and pits and curved steps on a flatter surface were observed (Figure 7c,e). As seen in Figure 7a,b, the morphology formed by mechanical breakage is characterized by relatively small curvature and similarly shaped steps on each deconstruction surface within a region, as if similarly shaped steps were stacked on top of each other. We attribute this similar morphology with the above characteristics to mechanical damage. In contrast with experiments that were conducted and observed on the artificially cut deconstruction surface of calcite, the surface of the dolomite raw rock powder has more pits or steps formed by mechanical fragmentation, and the surface consists of multiple incomplete deconstruction surfaces. Such a complex surface makes it impossible to accurately distinguish between mechanically broken and dissolved forms. Regardless of the presence or absence of Pb solution in the system, as the dissolution process proceeds, the surface morphologies formed by mechanical forces will develop, lengthen, and annihilate each other, eventually resulting in increasing flat areas. Independent and relatively large curvature steps with relatively long lengths occur in flat areas, as well as pits with some curvature around the perimeter and connecting multiple pits (Figure 7e). This surface morphology is similar to that observed on artificially cut surfaces. Athanasios Godelitsas observed calcite solution surfaces exposed to 1000 mg/L aqueous lead solution for one month. In situ experimental results showed the transformation of straight steps into curved steps on the deconstruction surface, the transformation of small adjacent diamond-shaped solution pits into large irregular solution pits, and the appearance of mound-like precipitation at the steps [32]. On the one hand, the specific morphologies on the surface of dolomite exposed to Pb solutions are similar to those observed by Athanasios Godelitsas; on the other hand, the surface of dolomite (calcite type) with intact solution surfaces after dissolution in pure water consists of regular rhombohedral-shaped pits with straight and folded steps, which are different from the morphological features observed on the surface of dolomite exposed to Pb solutions. Comparing images of the initial protolith surface with those of dolomite exposed to Pb solution, this paper concludes that the appearance of curved steps and irregular solution pits with a certain curvature on the surface of dolomite exposed to Pb solution is not entirely attributable to dissolution following mechanical fragmentation. The surface morphology of any origin is influenced by the dissolution process and these morphologies are altered by the presence of Pb in solution. Thus, similar to artificially cut surfaces, the presence of Pb on incomplete dissociation surfaces after mechanical fragmentation during the dissolution process can also lead to the appearance of curved dissolution steps and irregular dissolution pits as specific surface morphologies.

The "Sweet Spot" of Adsorption on Dolomite Surfaces
CHAI Rukuan used the characteristics of suspended bonds and surface energy theory to establish a molecular dynamics model and quantitatively calculate the suspended bond density and surface free energy for different forms of calcite surface in the model. The results show that the suspended bond density and surface free energy of surface vacancies and surface projections are greater than those of ideal planes. The surface vacancies and protrusions have more active sites, which correspond to stronger reactivity; therefore, water molecules are preferentially adsorbed at the surface vacancies and protrusions [33]. Pb 2+ ionic radius is larger than Ca 2+ , so when Pb 2+ is adsorbed onto CaMg(CO 3 ) 2 crystals by ion exchange, it is necessary to consider not only the dehydration of the metal ion but also the rejection of Pb 2+ by the crystal lattice [12]. According to Figure 10, the number of first nearest neighbours is three at the kink (Figure 10 (3) and two at the step ( Figure 10 (2)). The high number of vacant sites with high first nearest neighbours has more unbonded surfaces, meaning that the high levels of free energy promote the dehydration of metal ions in solution and the formation of ionic bonds at this position; however, Pb 2+ is repelled by the lattice because its ionic radius is large compared to Ca 2+ . Combining these two reasons, it can be qualitatively assumed that the kink and step may be the 'sweet spot' for Pb adsorption on the dolomite surface. Based on the results of the EDS assay of the dolomite surface area exposed to 50 mg/L Pb 2+ solution, the apparent concentration of Pb in the selected dolomite surface step areas was higher than that in the flatter areas. This result is consistent with the above assumptions.
not entirely attributable to dissolution following mechanical fragmentation. The surface morphology of any origin is influenced by the dissolution process and these morphologies are altered by the presence of Pb in solution. Thus, similar to artificially cut surfaces, the presence of Pb on incomplete dissociation surfaces after mechanical fragmentation during the dissolution process can also lead to the appearance of curved dissolution steps and irregular dissolution pits as specific surface morphologies.

The "Sweet Spot" of Adsorption on Dolomite Surfaces
CHAI Rukuan used the characteristics of suspended bonds and surface energy theory to establish a molecular dynamics model and quantitatively calculate the suspended bond density and surface free energy for different forms of calcite surface in the model. The results show that the suspended bond density and surface free energy of surface vacancies and surface projections are greater than those of ideal planes. The surface vacancies and protrusions have more active sites, which correspond to stronger reactivity; therefore, water molecules are preferentially adsorbed at the surface vacancies and protrusions [33]. Pb 2+ ionic radius is larger than Ca 2+ , so when Pb 2+ is adsorbed onto CaMg(CO3)2 crystals by ion exchange, it is necessary to consider not only the dehydration of the metal ion but also the rejection of Pb 2+ by the crystal lattice [12]. According to Figure 10, the number of first nearest neighbours is three at the kink (Figure 10 (3) and two at the step (Figure 10 (2). The high number of vacant sites with high first nearest neighbours has more unbonded surfaces, meaning that the high levels of free energy promote the dehydration of metal ions in solution and the formation of ionic bonds at this position; however, Pb 2+ is repelled by the lattice because its ionic radius is large compared to Ca 2+ . Combining these two reasons, it can be qualitatively assumed that the kink and step may be the 'sweet spot' for Pb adsorption on the dolomite surface. Based on the results of the EDS assay of the dolomite surface area exposed to 50 mg/L Pb 2+ solution, the apparent concentration of Pb in the selected dolomite surface step areas was higher than that in the flatter areas. This result is consistent with the above assumptions. The surface position elements represent the macroscopic morphological features on the surface, as shown in Figure 11, while the dissociation energy of these sites varies and the dissolved surface exfoliation process can be decomposed into surface morphological processes, such as dislocation into cavities, kink progression into steps and the mutual annihilation of step waves, in addition to the overall surface exfoliation process along the The surface position elements represent the macroscopic morphological features on the surface, as shown in Figure 11, while the dissociation energy of these sites varies and the dissolved surface exfoliation process can be decomposed into surface morphological processes, such as dislocation into cavities, kink progression into steps and the mutual annihilation of step waves, in addition to the overall surface exfoliation process along the normal direction. Therefore, the active sites in the surface morphology (point errors, nuclei of dissolution pits, positive and negative kinks, etc.) control the development of the surface morphology and can play a role in controlling the surface dissolution exfoliation process [34].
SEM observations did not reveal the appearance of significant precipitation at the steps. In contrast, in experiments using Atomic Force Microscopy (AFM) correlation, an ellipsoidal hill-shaped precipitation morphology appeared on the calcite surface at the dissolution steps in a 100 µM Pb solution [30]. AFM observed the growth in heterogeneous nucleated PbCO 3 at the contact interface and a 'pyramidal' structure on the surface when calcite was dissolved in a 5 mM Pb solution [35]. The SEM images of this experiment do not show the morphology of PbCO 3 precipitation on the surface. However, this work still suggests that a new solid phase of PbCO 3 or Pb-bearing carbonate solid solution is present on the dolomite surface. One reason for this is that experimental XPS and XRD data on surface composition corroborate the hypothesis that a new Pb-bearing solid phase was generated. The AFM images from the TENG H also did not show significant morphological features of a new solid-phase precipitation formation at the dissolution step [36]. This inconsistency may be due to inconsistencies in the magnification and resolution of the instruments used for observation. normal direction. Therefore, the active sites in the surface morphology (point errors, nuclei of dissolution pits, positive and negative kinks, etc.) control the development of the surface morphology and can play a role in controlling the surface dissolution exfoliation process [34]. SEM observations did not reveal the appearance of significant precipitation at the steps. In contrast, in experiments using Atomic Force Microscopy (AFM) correlation, an ellipsoidal hill-shaped precipitation morphology appeared on the calcite surface at the dissolution steps in a 100 μM Pb solution [30]. AFM observed the growth in heterogeneous nucleated PbCO3 at the contact interface and a 'pyramidal' structure on the surface when calcite was dissolved in a 5 mM Pb solution [35]. The SEM images of this experiment do not show the morphology of PbCO3 precipitation on the surface. However, this work still suggests that a new solid phase of PbCO3 or Pb-bearing carbonate solid solution is present on the dolomite surface. One reason for this is that experimental XPS and XRD data on surface composition corroborate the hypothesis that a new Pb-bearing solid phase was generated. The AFM images from the TENG H also did not show significant morphological features of a new solid-phase precipitation formation at the dissolution step [36]. This inconsistency may be due to inconsistencies in the magnification and resolution of the instruments used for observation.

Form of Pb Presence on Dolomite Surfaces
The common crystal structure of PbCO3 is not the same octahedral ligand structure as calcium carbonate or calcium magnesium carbonate, implying that PbCO3 with a nonoctahedral ligand structure has different crystal properties to dolomite. Neil C. Sturchio showed that the substitution of Pb for Ca can form the new solid-phase PbCO3, implying that Pb enters the Ca site in the calcite octahedral coordination structure, and that Pb migrates from the outermost layer to the interior of the solid solution, which is the nearsurface layer of dolomite [37]. Infrared spectroscopy and Raman spectroscopy are different in principle, but both can reflect the elemental composition and crystal structure of a solid phase through the molecular structure and vibrational patterns of molecular groups within the solid phase. Figure 5 shows the IR spectra of dolomite exposed to PbCl2 solution and dolomite pretreated with PbCl2 solution before being dissolved again in deionized water, with similar peak positions. Referring to the results of previous studies, the distribution of the peak positions of these two peaks is consistent with that of typical dolomite IR spectra [38], and no obvious spectral peaks appear at the PbCO3 IR spectral peaks, indicating that the solid phase is still internally dominated by dolomite and the crystal structure is still the octahedral ligand structure of dolomite. Figure 6 similarly

Form of Pb Presence on Dolomite Surfaces
The common crystal structure of PbCO 3 is not the same octahedral ligand structure as calcium carbonate or calcium magnesium carbonate, implying that PbCO 3 with a nonoctahedral ligand structure has different crystal properties to dolomite. Neil C. Sturchio showed that the substitution of Pb for Ca can form the new solid-phase PbCO 3 , implying that Pb enters the Ca site in the calcite octahedral coordination structure, and that Pb migrates from the outermost layer to the interior of the solid solution, which is the nearsurface layer of dolomite [37]. Infrared spectroscopy and Raman spectroscopy are different in principle, but both can reflect the elemental composition and crystal structure of a solid phase through the molecular structure and vibrational patterns of molecular groups within the solid phase. Figure 5 shows the IR spectra of dolomite exposed to PbCl 2 solution and dolomite pretreated with PbCl 2 solution before being dissolved again in deionized water, with similar peak positions. Referring to the results of previous studies, the distribution of the peak positions of these two peaks is consistent with that of typical dolomite IR spectra [38], and no obvious spectral peaks appear at the PbCO 3 IR spectral peaks, indicating that the solid phase is still internally dominated by dolomite and the crystal structure is still the octahedral ligand structure of dolomite. Figure 6 similarly shows the Raman spectra of dolomite exposed to PbCl 2 solution and dolomite pretreated with PbCl 2 solution before being dissolved again in deionized water, adding the Raman spectrum of the initial dolomite as a control. All three show spectral peaks at 291 cm −1 and 331 cm −1 due to the alternating distribution of Ca 2+ and Mg 2+ in dolomite, causing two vibrational modes of bending outside the carbon and oxygen planes [39]. The three peaks are also similar in other peak positions, indicating that the solid phase is still dominated by dolomite within the solid phase and the crystal structure is still the octahedral ligand structure of dolomite [40]. However, for the dolomite exposed to PbCl 2 solution, the Raman spectra of both peaks at 168 cm −1 , 292 cm −1 , and 718 cm −1 are shifted in the short-wave direction compared to the initial dolomite. The peak at 168 cm −1 corresponds to the crystal internal lattice vibration, that at 292 cm −1 corresponds to the CO 3 2− group out-of-plane bending vibration mode and that at 718 cm −1 corresponds to the group in-plane bending vibration mode. The Raman shifts produced by the above three vibrational modes are proportional to the bond energy of the C-O bond. For carbonatite minerals, the larger the ionic radius of the metal ion, the smaller the bond energy of the C-O bond, and the corresponding Raman shift is shifted in the short-wave direction [41]. Comparing the positions of the three Raman shift spectral peaks, it can be concluded that the surface crystal structure of the dolomite that was in contact with the solution is still the octahedral ligand structure of dolomite, but the Pb ions with larger ionic radii than Ca and Mg occupy the position of Ca or Mg in the original octahedral ligand structure; see Figure 12.
internal lattice vibration, that at 292 cm corresponds to the CO3 group out-of-plane bending vibration mode and that at 718 cm −1 corresponds to the group in-plane bending vibration mode. The Raman shifts produced by the above three vibrational modes are proportional to the bond energy of the C-O bond. For carbonatite minerals, the larger the ionic radius of the metal ion, the smaller the bond energy of the C-O bond, and the corresponding Raman shift is shifted in the short-wave direction [41]. Comparing the positions of the three Raman shift spectral peaks, it can be concluded that the surface crystal structure of the dolomite that was in contact with the solution is still the octahedral ligand structure of dolomite, but the Pb ions with larger ionic radii than Ca and Mg occupy the position of Ca or Mg in the original octahedral ligand structure; see Figure 12. No peaks of sufficient intensity appeared at the peak positions of the Raman and IR spectra corresponding to the non-octahedral ligand PbCO3. This suggests that the new solid phase generated on the surface of dolomite surface to the PbCl2 solution was mainly the octahedral ligand PbCO3. For polycrystalline powder X-ray diffraction, different crystal structures correspond to different diffraction peak spectra, and the XRD test results show that the positions of the diffraction peaks of the dolomite exposed to PbCl2 solution are mostly different from the dolomite (PbCO3) displayed on the PDF card, with only a few characteristic peaks in the same position. PbCO3 with an octahedral ligand structure is similar to PbCO3 with a non-octahedral ligand structure; however, due to the difference in their crystal structures, they exhibit different diffraction peaks. Therefore, as in the previous paper, we can deduce that the main product on the dolomite surface after exposure to PbCl2 solution is PbCO3 with an octahedral ligand structure, and the proportion of nonoctahedral ligand structure of dolomite is so low that its characteristics are difficult to detect by various solid-phase characterization methods. No peaks of sufficient intensity appeared at the peak positions of the Raman and IR spectra corresponding to the non-octahedral ligand PbCO 3 . This suggests that the new solid phase generated on the surface of dolomite surface to the PbCl 2 solution was mainly the octahedral ligand PbCO 3 . For polycrystalline powder X-ray diffraction, different crystal structures correspond to different diffraction peak spectra, and the XRD test results show that the positions of the diffraction peaks of the dolomite exposed to PbCl 2 solution are mostly different from the dolomite (PbCO 3 ) displayed on the PDF card, with only a few characteristic peaks in the same position. PbCO 3 with an octahedral ligand structure is similar to PbCO 3 with a non-octahedral ligand structure; however, due to the difference in their crystal structures, they exhibit different diffraction peaks. Therefore, as in the previous paper, we can deduce that the main product on the dolomite surface after exposure to PbCl 2 solution is PbCO 3 with an octahedral ligand structure, and the proportion of nonoctahedral ligand structure of dolomite is so low that its characteristics are difficult to detect by various solid-phase characterization methods.

Effect of Pb 2+ on the Dissolution Process of Dolomite
Atmospheric CO 2 is actively involved in the dissolution of carbonate rocks, and the partial pressure of CO 2 controls the dissolution process and solubility to a certain extent [11]. CO 2 is involved in the stoichiometric equilibrium of the water-rock-gas reaction, which provides a carbon source to the system and is closely related to pH. In this experiment, pH, total alkalinity, and conductivity were used as indicators of the degree of carbonate dissolution. CO 2 is constantly replenished by the external environment in the open experimental system, whereas, in the closed experimental system, CO 2 is consumed as dissolution proceeds [42]. In the open experimental system, the pH decreases with time and the total inorganic carbon values of the high concentration PbCl 2 solution are similar to those of the low-concentration PbCl 2 solution during the whole experimental cycle of 15 days. The Pb source for the open experimental system was not only derived from the added PbCl 2 solution but also from the Pb-zinc ore containing Pb. The Pb-Zn ore dissolved with the dolomite, probably because the dissolution of the sulfur fraction changed the pH of the solution, causing the pH to decrease as dissolution proceeded. Under acidic conditions with a high H+ content in the solution, dolomite dissolution is controlled by pH. In the closed experimental system, the pH increased as dissolution proceeded. In the group charged with CO 2 to reach CO 2 saturation, the HCO 3 − and CO 3 2− in solution were supplied by both CO 2 and solid-phase dolomite as the reaction proceeded to near equilibrium. The total alkalinity and conductivity of the solution were much higher than in the non-CO 2 charged group, while the pH was lower than in the non-CO 2 charged group, indicating that CO 2 is involved in and contributes to the dissolution of dolomite. A comparison of the water chemistry data for the same gas groups with and without Pb elements shows that the total alkalinity of the dolomite dissolution process in the Pb-containing solution is smaller than the total alkalinity of the dolomite dissolution process in the Pb-free solution in the non-CO 2 charged system. The water chemistry trends in the CO 2 -sufficient system are still similar to that of the uncharged system, and the difference in total alkalinity and conductivity is more pronounced in the group with or without Pb. John W. Morse systematically summarized the study of carbonate dissolution kinetics, summarizing previous studies linking solutions to carbonate surface dissolution morphology and suggesting that dissolution inhibitors have a profound effect on dissolution kinetics in the near-equilibrium phase of carbonates [43]. As the whole system was in a closed state, not many factors affected the dolomite dissolution. Following the change in the pH of the whole dissolution process, 1mg/L of PbCl 2 solution did not affect the pH of the system, so the Pb in the solution did not affect the system dissolution process by changing the pH of the solution. The conductivity of the solution was positively correlated with dolomite dissolution, so the difference in solution conductivity also demonstrated an inhibitory effect of Pb on the near-equilibrium phase of dolomite dissolution.
Based on the results of this experiment's solid and liquid phases, the previous hypothesis is confirmed. Pb adsorbs or precipitates at the active sites on the dolomite surface. The ionic bonding of the newly formed PbCO 3 or Ca, Mg, and Pb carbonate solid solutions possesses a sufficiently strong bonding energy to hinder the dissolution process at these sites (e.g., kink extension and deepening of the dissolution step extension), ultimately leading to the inhibition of dolomite dissolution in the near-equilibrium phase. The morphological features observed in this experiment are similar to the structure followed by Hongmei Tang [44]. These generated PbCO 3 occupied active sites, causing the steps to deepen and expand at an uneven rate, allowing for linear steps to transform themselves into curved steps or adjacent linear steps to combine to create curved steps. Similarly, irregular dissolution pits formed on the surface; see Figure 7e.
More importantly, the initial dolomite surface in this experiment was mechanically fragmented and the surface morphology existed on multiple incomplete deconfined surfaces, as opposed to the more complete and flat surfaces cut out of the extended deconfined surfaces chosen in previous experiments. It is not possible to quantitatively compare the mass, effective surface area, and active sites on the surface of the solid-phase material used in each of the previous experiments. However, a powder with the same amount of material must possess many more active sites than the whole crystal, and this difference may have several orders of magnitude. Therefore, we can qualitatively assume that there are more active sites present on the surface of the powder than on artificially cut planes. By comparing Figure 7b with previous AFM or SEM images, it can be seen that the mechanically crushed dolomite surface selected for this experiment is far more complex than the artificially cut out calcite crystal surface, implying that the surface-active site density in this experiment is much higher than that of the artificially cut surface. Jin Ma investigated the variation in effective surface area (ESA) during the dissolution of a dolomite-CO 2 -water system. When only the active sites on the dolomite surface were involved in the dissolution reaction, results from image-based stochastic Monte Carlo simulations showed that ESA nearly linearly increased [45]. Unlike previous experiments, this experiment used dolomite powder as the sample, which can be approximated as a complete infiltration of the solid phase surface after immersion in the solution. At this point, only the active sites on the dolomite surface controlled the effective surface area of dolomite. According to Jin Ma's Monte Carlo simulations of dolomite, the ESA will increase linearly as the reaction proceeds, meaning that powdered dolomite with more initial active sites will have a higher ESA than artificially cut dolomite surfaces during the entire dissolution process. The present experiments, characterized by a low Pb 2+ concentration (2.6 µM) with a high density of active sites on the surface, still exhibited the inhibition of dolomite dissolution by Pb 2+ . We suggest that the reason for this is that Pb 2+ binds preferentially to the active sites on the surface. These active sites control the dissolution of adjacent areas and, by occupying these active sites, Pb 2+ controls the dissolution of adjacent areas, so that a small amount of Pb 2+ controls the dissolution of a 'large' area. The solid phase characterizations result also indicates that the main product on the dolomite surface is the octahedral ligand PbCO 3 . Pb 2+ in solution preferentially adsorbs to the active sites on the wider surface rather than aggregating at a few sites and then producing non-octahedral ligands of PbCO 3 in heterogeneous nucleation. Therefore, the inference where Pb preferentially binds to the surface-active site may explain why no precipitation morphology was observed in SEM. In summary, similar to experiments with artificially cut crystal faces at relatively high molar concentrations of Pb solutions, the 2.6 µM (1 mg/L) Pb solution still acts as a solubility inhibitor for complex dolomite surfaces that possess incomplete cleavage surfaces. The preferential binding of Pb2+ causes this phenomenon at the active site and leads to the formation of octahedral ligands of PbCO 3 by occupying Ca 2+ or Mg 2+ positions. These new solids rely on their strong ionic bonds to hinder the development of regional dissolution morphology and ultimately inhibit the dissolution of dolomite in the near-equilibrium phase.
Calcite carbonates also dissolve slowly in pure water, with H + in solution crossing the liquid-phase diffusion layer and combining with carbonate on the surface of the solid phase, with the resulting HCO 3 − breaking away from the lattice, and Ca 2+ also eventually breaking away from the lattice. Therefore, the involvement of atmospheric CO 2 in the chemical weathering of carbonates is essentially a process whereby the dissolution of atmospheric CO 2 leads to an increase in the concentration of H + in water, which in turn promotes the dissolution of the solid phase. The process noted by the red arrow in Figure 13 is widely regarded as a net carbon sink for karst, where atmospheric CO 2 can be very slowly dissolved in water in the absence of carbonate dissolution. Atmospheric CO 2 dissolution and calcite carbonate dissolution are interrelated by H + and, therefore, contribute to each other's positive reactions. Therefore, previous studies on the disturbance term accounting for atmospheric CO 2 carbon sinks have focused on H + that is "not generated by dissolved atmospheric CO 2 ". Such H + is generally referred to as "exogenous acid"; however, exogenous acid may consist of two components. One component is the H + substituted into the system by "exogenous water", which means that exogenous water with a low pH sinks into the system; the other is the reaction of non-carbonate minerals such as S and N in the solid phase with water or the presence of oxygen addition, which can produce H 2 SO 4 or HNO 3 , thus increasing the H + concentration in the solution. However, according to the inferences drawn in this paper, when exogenous water carries large amounts of heavy metal ions (Pb, etc.) into the system, these metal ions preferentially bind to the active sites on the surface of calcite group carbonates, forming MCO 3 with higher bonding energy. H + must still bond to CO 3 2− on the lattice before it can leave the lattice and enter the solution, and higher bond energies mean harder bond breakage; therefore, these MCO 3 act as inhibitors of calcite group carbonates. As mentioned earlier, atmospheric CO 2 solutions are facilitated in association with the chemical weathering of the calcite group, so when the calcite carbonate dissolution process is inhibited, it can be assumed that the amount of atmospheric CO 2 dissolved into the water is correspondingly reduced. This means that the heavy metal ions M 2+ in the exogenous water reduces the total karst carbon sink by inhibiting the calcite group dissolution process. In other words, the process by which heavy metal ions from outside the groundwater system enter the system and adsorb on the calcite group carbonate surface will diminish the karst carbon sink potential of the whole system. sumed that the amount of atmospheric CO2 dissolved into the water is correspondingly reduced. This means that the heavy metal ions M 2+ in the exogenous water reduces the total karst carbon sink by inhibiting the calcite group dissolution process. In other words, the process by which heavy metal ions from outside the groundwater system enter the system and adsorb on the calcite group carbonate surface will diminish the karst carbon sink potential of the whole system.

Conclusions
This experiment investigated the interaction between solutions at concentrations of 1mg/L and 50mg/L PbCl2 and natural dolomite powder. XPS, XRD results show that Pb is significantly absorbed on the crushed surface of dolomite in deionized water and that high concentrations of Pb solutions lead to the formation of more solid-phase PbCO3 on the surface. The FT-IR, Raman results show that Pb is mainly present on the solid surface as PbCO3 or (Pb, CaMg)(CO3)2 solid solution in an octahedral ligand structure. SEM + EDS results show that Pb influences the dissolution morphology of the dolomite surface, influenced by Pb from straight steps and rhombic dissolution pits, shifting to curved steps and irregular dissolution pits, and PbCO3 tends to precipitate in the dissolution step area. The results of the hydro-chemical data show that total inorganic carbon in the open system is controlled by H + in solution, whereas the presence of solution Pb in the closed system in either the CO2-sufficient or -insufficient system reduces the total solution alkalinity and conductivity during dissolution, while the pH increases relative to the initial solution and Figure 13. Kinetics of calcite group carbonate dissolution in a solid−liquid−gas three−phase system.

Conclusions
This experiment investigated the interaction between solutions at concentrations of 1 mg/L and 50 mg/L PbCl 2 and natural dolomite powder. XPS, XRD results show that Pb is significantly absorbed on the crushed surface of dolomite in deionized water and that high concentrations of Pb solutions lead to the formation of more solid-phase PbCO 3 on the surface. The FT-IR, Raman results show that Pb is mainly present on the solid surface as PbCO 3 or (Pb, CaMg)(CO 3 ) 2 solid solution in an octahedral ligand structure. SEM + EDS results show that Pb influences the dissolution morphology of the dolomite surface, influenced by Pb from straight steps and rhombic dissolution pits, shifting to curved steps and irregular dissolution pits, and PbCO 3 tends to precipitate in the dissolution step area. The results of the hydro-chemical data show that total inorganic carbon in the open system is controlled by H + in solution, whereas the presence of solution Pb in the closed system in either the CO 2 -sufficient or -insufficient system reduces the total solution alkalinity and conductivity during dissolution, while the pH increases relative to the initial solution and changes slowly during the dissolution process. The results of the hydro-chemical data indicate that the total H + controls inorganic carbon in the open system in the solution. In contrast, the presence of Pb solution in the closed system, in either the CO 2 -sufficient or -insufficient system reduced the total solution's alkalinity and conductivity, and the pH, increased relative to the initial solution, changing slowly during the dissolution process. It was demonstrated that these processes are still present in the CO 2 system and the inhibition of dolomite dissolution by Pb in the CO 2 system is even more pronounced. On complex dolomite surfaces that possess incomplete deliquescent surfaces, fairly low concentrations (2.6 µM) of Pb 2+ can inhibit dolomite dissolution in the near-equilibrium phase. The effect of Pb ions on the dissolution process of dolomite may be based on the fact that Pb is adsorbed to active sites on the surface of dolomite, preventing further development of the dissolution morphology and inhibiting the further dissolution of dolomite in the near-equilibrium phase. This work suggests that Pb 2+ can be an effective inhibitor of natural dolomite dissolution processes; therefore, experiments focusing on the inhibition of carbonate rock dissolution by surface sorbents have great potential in the study of natural karst process mechanisms in complex aqueous environments.

Conflicts of Interest:
The authors declare no conflict of interest.