Dissolution Of Dolomite Research Articles

Carbonate mineral controls the transport of Cd from tailings to surrounding soils: An example from Cd-rich Niujiaotang Zn mine in Guizhou Province, Southwest China

Heavy metal pollution derived from the oxidation of sulfides in Pb-Zn mine tailings has been a common public concern. Although the oxidative dissolution of sphalerite will release cadmium into the soil environment, it is still unclear whether other minerals contain a large amount of cadmium and how much these minerals contribute to the surrounding environmental media. In this paper, XRD, particle size analyzer, SEM-EDS and improved BCR sequential extraction method were used to analyze cadmium rich zinc mine tailing and adjacent farmland soil in Duyun, Guizhou Province. The results show that the average contents of Pb, Zn, and Cd in tailing are 214 ± 71 mg/kg, 2668 ± 773 mg/kg, and 37.5 ± 9.97 mg/kg respectively. While in the surrounding soil, they were 519 ± 280 mg/kg, 3,779 ± 2614 mg/kg, and 30.8 ± 14.4 mg/kg respectively. The mineralogical composition of tailing is mainly dolomite and less pyrite. Cadmium in tailing is mainly in weak acid extractable state, indicating that cadmium mainly exists in dolomite of tailing. Pyrite in tailing generates acid through oxidation, which accelerates the dissolution of dolomite, thus releasing Cd from minerals and migrating to surrounding farmland, resulting in soil cadmium pollution. This study reveals an important process that has been neglected. Carbonate minerals play an important role in the migration of cadmium in the tailing of the Mississippi Valley-type (MVT) lead-zinc mine and the soils affected by the tailing.

Insight into the Process and Mechanism of Water-Rock Interaction in Underground Coal Mine Reservoirs Based on Indoor Static Simulation Experiments.

During the storage of underground water reservoirs in coal mines, water–rock interaction occurred between mine water and collapsed rocks, resulting in improved mine water quality, but the water–rock process and mechanism have not been clarified. In this study, six sets of simulated experiments were designed to investigate the water–rock interaction between two types of roof collapse rocks and different water samples in the Daliuta coal mine. The ion ratio method and multivariate statistics are used to reveal the process and mechanism of water–rock interaction during the experimental process from the perspective of input and output water chemical characteristics and rock properties, respectively. The results show that the ion concentration of the effluent water is controlled by water–rock interaction, and the water–rock process mainly involves the dissolution of halite, silicate, pyrite, calcite, dolomite, magnesia chlorite, and gypsum precipitation and is accompanied by ion exchange. Mineral dissolution and precipitation are the most important factors affecting the ion abundance in the effluent. There are differences in the reaction rate, degree of reaction, and dissolution process during the experimental process of fine sandstone and mudstone. Fine sandstone is more reactive than mudstone in terms of reaction rate and degree of reaction, and fine sandstone is dissolved from surface pores to inside and around, while mudstone is generated in new dissolved pores. We found that rock type, ion concentration in the input water, and rock reaction period influence the water–rock interaction during the experiment. The results provide a reference for clarifying the water–rock interaction during the storage of underground water reservoirs in coal mines and predicting the water quality of the effluent.

Seeded phosphorus retention in fixed-bed laboratory columns by the use of apatites

Phosphorus retention in small- and medium-sized wastewater treatment plants is crucial to preventing the eutrophication of downstream catchments. One popular solution in combination with treatment wetlands is the use of reactive filters for phosphorus retention; however, identifying a suitable substrate is not an easy task in this process. Apatites have already proven to be an effective alternative for phosphorus retention, yet more in-depth research is needed. This article uses two natural apatite materials, NA1 and NA2, introduced in four fixed-bed laboratory columns to assess their phosphorus retention capacity. Various inflow conditions are set for the NA1 substrate to evaluate the impact of calcium and biomass development on performance. The substrates show high phosphorus retention (>16.8 g PO4-P/kg for NA1 and >17.5 g PO4-P/kg for NA2) as well as high kinetic rate coefficients (1.45 and 1.70 h−1 for NA1 and NA2, respectively), with performances above 80% for both substrates. The maximum phosphorus retention capacity is not attained at the end of the experiments, despite their long duration (230 days) and the short hydraulic residence times applied (∼2 h), thus suggesting a long-term removal capacity. The NA1 column fed with a calcium-deficient synthetic solution displays just slightly reduced kinetic rates, most likely due to calcite and dolomite dissolution from the media. The column fed with treated wastewater does not reveal any significant reduction in hydraulic conductivity due to biomass development. No loss of permeability due to chemical clogging was observed in the other columns. Scanning electron microscopy indicates that phosphorus retention occurs by the precipitation of amorphous calcium phosphate for both natural apatites, thereby clearly demonstrating the implementation of seeding mechanisms. Such a retention process is sustainable, which suggests it may proceed over even higher retention capacities.

Study on Mechanical Properties and Weakening Mechanism of Acid Corrosion Lamprophyre.

In order to study the weakening mechanism and mechanical behaviors of hard lamprophyre of Carboniferous Permian coal-bearing strata in China’s mining area, lamprophyre samples were subjected to static rock dissolution experiments with pH values of 0, 2, and 4. The acid corrosion mechanism of lamprophyre was revealed from the weight changes of samples, characteristics of solution ion concentration, and macro-mechanical properties. The experimental results show that reaction occurred between lamprophyre and acid solution. With the increasing concentration of H+, the reaction was more intense, the degree of acid etching was higher, and the weight loss was greater. The internal damage induced by acid etching results in the slow extension of the compaction stage of stress–strain curve of uniaxial compression, and the obvious deterioration of mechanical properties of the lamprophyre. The uniaxial compressive strength of the lamprophyre in the dry state is 132 MPa, which decreased to 39 MPa under the acid etching condition, showing significant mudding characteristics. Dolomite (CaMg(CO3)2 with 19.63%) and orthoclase (KAlSi3O8 with 31.4%) in lamprophyre are the major minerals constituents involved in acidification reaction. Photomicrograph recorded from SEM studies reveals that the dissolution effect was directly related to the concentration of H+ in the solution. The dissolution effect was from the surface to the inside. The small dissolution pores became larger and continuously expanded, then finally formed a skeleton structure dominated by quartz. The content of K+, Ca2+, and Mg2+ in the solution after acid etching reaction indicates that the acidified product of orthoclase is colloidal H2SiO3, which adhered to the surface of samples during acid etching and hinders the further acidification of minerals. The dissolution of dolomite and orthoclase under acidic conditions directly leads to the damage of their structure and further promotes the water–rock interaction, which is the fundamental reason for the weakening of the mechanical properties of lamprophyre.

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 PbCO3 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 Pb2+ on the active sites and that the newly grown solid phase is PbCO3 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 Pb2+ may be an effective inhibitor of dolomite dissolution and may help to further refine the geological carbon sink.

Rare earth element patterns in a rapidly changing karst environment

Samples of surface water, groundwater and rocks were collected along the Louros River karst system in Greece to study the evolution of the chemical composition of groundwater. It was found that the hydrochemistry is closely related to the lithology of the host rocks. Emphasis was placed on the study of REEs due to the presence of phosphate-rich rocks in the area. A distinctive feature that was also taken into account was the high velocities of surface and groundwater in this rapidly changing karst environment.The carbonate formations of the Ionian Zone determine the hydrogeology of the catchment, particularly the Senonian, Vigla and Pantokrator limestones, through which groundwater mainly moves at relatively high velocities, play a key role. From the northernmost areas at the upper part of the catchment to the southernmost areas at the river's mouth, a multitude of springs, along the river, mark the main base level, above which a common aquifer is formed on either riverbed, hydraulically and locally connecting those geological formations.The chemical composition of the water is mainly controlled by the dissolution of calcite, dolomite and gypsum, with Ca–HCO3 being the dominant water type. Elevated pH values (>7), increased concentrations of dissolved oxygen and low concentrations of Fe and Mn indicate a karst system with oxidising alkaline waters. Groundwater type changes when it encounters gypsum in the area, or when the process of dedolomitisation takes place and, together with the phosphate minerals contributes to the release of REEs into the water. ΣREEs range from 0.22 in anhydrite to 33.8 ppm in the sandstones. The REE concentrations in the 0.45 μm-filtered surface and groundwater samples range between 50 and 260 ng/L. REEs concentration in groundwater is controlled by pH.The European shale normalised REE profiles of the rock samples are enriched in LREEs compared to HREEs and show a slightly negative Ce anomaly, which is explained by an oxidising sedimentation environment. The positive Eu anomaly of the same samples was attributed to the presence of phosphate minerals. Positive anomalies in both Ce and Eu were observed in the corresponding profiles of surface and groundwater samples. The effect of carbonate species, especially HCO3−, on surface and groundwater resulted in their enrichment in HREEs. In general, an enrichment of HREEs in surface and groundwater was observed in proportion to the distance covered.

Study on the Microstructure Evolution and Strength Damage Mechanism of Dolomite under Dissolution Condition

Dolomite is a common type of natural soluble rock. The strength of rock decreases under the action of corrosion, which has a significant impact on the self-stability and long-term safety of the tunnel surrounding the rock. To reveal the microscopic structure evolution and strength-damage law of carbonate rock caused by chemical corrosion, a series of tests such as rock chemical corrosion test, rock uniaxial compression test and electron microscope scanning test are conducted at different pH values on the dolomite of the Doushantuo Formation. The rock dissolution at different pH values exhibits four typical stages: the initial dissolution stage, secondary dissolution acceleration stage, stable dissolution rate stage and dissolution attenuation stage. During the dissolution process, the initial dissolution rate is 25.91 times that of the stable stage, and the maximum strength attenuation is 76.2% after 21 days of dissolution. For macroscopic failure, the rock is developed from 1 to 2 external fractures to multiple internal and external fractures and penetrated, and the specimen transforms from brittle to flexible. For microstructure, the sample exhibits corrosion characteristics along the joint surface, intensified corrosion at the edge, etc. The porosity increase rate is 0.6%/d; however, the length–width ratio of the pores is maintained at 1.7–1.85, indicating that the development rate of pores in different directions is similar. The results of this study have enriched the study of the dolomite dissolution mechanism and, in addition, have important reference value for the stability evaluation of tunnel surrounding rock in karst environments.

Identification of origin and runoff of karst groundwater in the glacial lake area of the Jinsha River fault zone, China

Karst groundwater plays important roles as a water supply and in sustaining the biodiversity and ecosystems of the eastern Qinghai–Xizang Plateau. Owing to the stratigraphic structure, high tectonic activity, and changeable climate of the region, the recharge source, runoff path, and dynamic characteristics of karst groundwater are highly complex, which poses challenges with regard to the protection of water resources and ecology. This study identified the origin and flow processes of karst groundwater in the glacial lake area of the Jinsha River fault zone using satellite remote sensing, hydrochemical and isotope analyses, and flow measurements. Results showed that active faults control the distribution of glacial lakes and the recharge, runoff, and discharge of karst groundwater. Glacial lake water is an important source of karst groundwater in the Jinsha River fault zone area. Specifically, glacial lake water continuously recharges the karst system via faults, fractures, and karst conduits, thereby maintaining the relative stability of karst spring flows. Through hierarchical cluster analysis, two main runoff conduits of karst water were distinguished: one along the Dingqu Fault and the other along the Eastern Zhairulong Fault, which together account for 59% of the total regional karst groundwater flow. The elevation difference between the recharge and discharge areas of the main karst springs is > 1000 m. Groundwater runoff is fast and residence time in the aquifer is short. The dissolution of calcite and dolomite mainly occurs during transit through the groundwater system, and cation exchange is weak. Therefore, the regional karst springs are predominantly HCO3−Ca·Mg type. To protect regional karst water resources and ecology, the monitoring and protection of glacial lakes should be strengthened.

Thermodynamic control on chemical weathering of river bedrock at the Aba and Qamdo in Kaschin-beck disease region, eastern Tibet Plateau

Groundwater is the main source of water for living and agriculture in an area. To investigate the relationship between water environment and Kaschin-beck disease (KBD), we collected published data on water samples (data from the KBD areas of Aba Prefecture, Qamdo area, the Yellow River Basin region and the non-KBD area of the Yangtze River Basin region) for thermodynamic simulations to further analyze the hydrogeochemical properties. The analysis results show that: (1) The groundwater in Aba Prefecture and Qamdo area is weakly alkaline. (2) The groundwater type in Aba Prefecture and Qamdo area is Ca-HCO3− type. (3) The average concentration of cations Mg2+and Ca2+ can be expressed as Yellow River Basin > Yangtze River Basin > Aba Prefecture > Qamdo area. The average concentration of cation Na+and anion Cl− can be expressed as Yellow River Basin > Qamdo area > Yangtze River Basin>Aba Prefecture. The value of TDS can be expressed as Yellow River Basin > Qamdo area > Yangtze River Basin > Aba Prefecture. This may indicate that there is no direct relationship between KBD and water environment. (4) The dissolution of calcite and dolomite in quartzite controls the change of element content of water. (5) Analysis of the Gibbs distribution shows that most of the elements in the groundwater of the KBD areas (Aba Prefecture, Qamdo area and the Yelllow River Basin region) and non-KBD area (the Yangtze River Basin region) majorly come from rock weathering. (6) The overall quality of groundwater is good, the salinity is low, and it is suitable for agricultural irrigation.

Hydrogeochemical characteristics and processes of thermokarst lake and groundwater during the melting of the active layer in a permafrost region of the Qinghai–Tibet Plateau, China

Permafrost degradation and the development of thermokarst lakes are important factors driving the variability of regional hydrologic processes. Hydrogeochemical and isotopic analyses are important methods for investigating the hydrologic processes of thermokarst lakes. This study focused on comparing the chemical and hydrogeochemical characteristics between lake water and groundwater during the melting of the active layer in a typical thermokarst lake region on the Qinghai–Tibet Plateau (QTP). Ninety-five samples were collected during different periods of active layer melting and analyzed using statistical, isotope, hydrogeochemical, and modeling methods. Statistical results showed that the average concentrations of almost all ions were lower in lake water than in groundwater, with wider spatial variability in groundwater. The lake water is of the ClNa and HCO3-Ca type with low TDS (total dissolved solids), whereas groundwater is of the HCO3-Ca and mixed type (or transition type) with high TDS. The chemical types of the lake water and groundwater are mainly driven by rock weathering. In terms of the saturation index (SI), halite and gypsum are unsaturated dissolved, whereas dolomite and calcite are generally saturated. Evaporation significantly affects the chemical composition of groundwater, while the hydrochemical compositions of lake water are relatively stable under the joint control of evaporation, precipitation, surface runoff, and groundwater. The isotopic analysis results showed that the contribution of permafrost meltwater and precipitation to groundwater and lake water varied during different stages of active layer melting. According to hydrogeochemical modeling, the main chemical reactions in groundwater are the precipitation of calcite and the dissolution of halite, dolomite, and gypsum. The intensity of groundwater flow determines the degree of chemical reactions along the flow path at different stages of active layer melting. The findings can provide deeper insight into hydrogeochemical processes in thermokarst lake regions under the background of permafrost degradation.

Molybdenum Release Triggered by Dolomite Dissolution: Experimental Evidence and Conceptual Model.

The injection of oxygenated water into anoxic aquifers during managed aquifer recharge (MAR) can cause the mobilization of metal(loid)s. Here, we study the processes controlling MAR-induced molybdenum (Mo) release in dolomitic aquifers. Sequential chemical extractions and energy dispersive X-ray spectroscopy combined with scanning electron microscopy point to an association of Mo with easily soluble sulfurized organic matter present in intercrystalline spaces of dolomites or directly incorporated within dolomite crystals. The easily soluble character was confirmed by a batch experiment that demonstrated the rapid mobilization of Mo, dissolved organic carbon, and sulfur. The type and time of batch solution contact with the sulfurized organic matter impacted the release of Mo, as demonstrated by a 36% increase in Mo concentrations when shaking was intensified. Based on the experimental results, a conceptual model for the release of Mo was formulated, where (i) the injection of oxygenated water causes the oxidation of pyrite in the aquifer matrix, and (ii) the associated release of protons (H+) induces the dissolution of dolomite as a buffering reaction, which (iii) enhances the accessibility of the injectant to intercrystalline and incorporated sulfurized organic matter within dolomite, causing the release of Mo.

Authigenic Gypsum Precipitation in the ARAON Mounds, East Siberian Sea

Authigenic gypsum has been observed in marine methane hydrate-bearing sediments throughout the last decade. However, changes in mineral composition and gypsum precipitation in methane emission environments have not yet been reported in the Arctic. Expeditions aboard R/V ARAON revealed several mound structures described as active seeps, which were given the name ARAON Mounds (AMs). Core sediments from the AMs provide an excellent opportunity to research authigenic mineral production in the Arctic methane environment. We identified sedimentary units and investigated the mineral composition of gravity cores from the AMs and a background site. The background core ARA09C-St13, obtained between the mound structures, contains five sedimentary units that extend from the Chukchi Rise to Chukchi Basin, and core sediments from the AMs contain three sedimentary units in the same order. The fundamental difference between AMs and the background site is the lack of dolomite and abundance of gypsum in AMs. This gypsum precipitated authigenically in situ based on its morphological features. Precipitation was more closely associated with the absence of dolomite than the location of the sulfate–methane transition according to the vertical distribution of gypsum in the sediment. Chemical weathering and gypsum overgrowth were confirmed on dolomite surfaces recovered from the AMs, suggesting that dolomite dissolution is the primary source of Ca for gypsum precipitation. Dissolution of biological carbonates and ion exclusion may provide Ca for gypsum precipitation, but this mechanism appears to be secondary, as gypsum is present only in sedimentary units containing dolomite. The main sources of sulfate were inferred to be oxidation of H2S and disproportionation of sulfide, as no sulfide other than gypsum was observed. Our findings reveal that gypsum precipitation linked to methane emission in the Arctic Ocean occurs mainly in dolomite-rich sediments, suggesting that gypsum is a suitable proxy for identifying methane hydrate zones in the Arctic Ocean.

OPTIMIZATION AND KINETICS STUDIES OF THE DISSOLUTION OF DOLOMITE IN SULPHURIC ACID (H2SO4) VIA BOX-BEHNKEN EXPERIMENTAL DESIGN

Background: The recovery of any valuable component from dolomite as a double carbonate mineral depends on its dissolution efficiency. Aim: This study aimed to optimize and provide a simplified novel approach to the kinetics of dolomite dissolution in sulphuric acid solution using the Box-Behnken experimental design. Methods: The dolomite sample was dissolved in a sulphuric acid solution at seventeen different experimental conditions. The residue containing impurities was removed via filtration, while precipitation was carried out at the optimum conditions. Results and Discussion: The relationship between the independent and dependent variables best fits into the two-factor interaction model with a coefficient of determination of 0.9492, adjusted R² of 0.9187, and predicted R² of 0.7514. The total residual sum of 3x10-13 and adequate precision of 18.769 show that the predicted dissolution efficiency is much closer to the experimental values. The analysis of variance revealed that the individual effect of acid concentration, temperature, and dissolution time all positively contribute to the dissolution. The interactive effect of acid concentration with temperature and the interactive effect of temperature with dissolution time also positively influences the dissolution efficiency. Following the dissolution of dolomite in sulphuric acid, a white precipitate was formed at room temperature, which dissolved back at a temperature of 70 oC, agitation speed of 900 revolutions per minute, and within 10 minutes. A predictive approach using a two-factor interactive model was applied to generate the kinetic data. Conclusions: The established model equation is suitable for predicting dolomite dissolution in sulphuric acid. The application of the shrinking core model to the generated data shows that the reaction between dolomite and sulphuric acid is film diffusion control with a first-order reaction (0.6587) and activation energy of 27.5 KJmol-1k-1.

Groundwater Quality Affected by the Pyrite Ash Waste and Fertilizers in Valea Calugareasca, Romania

The aim of the study was to assess the groundwater quality in a rural area affected by the abandoned pyrite ash waste dumps. The abundance of major ions in groundwater depends largely on the nature of the rocks, climatic conditions, and mobility. To evaluate geochemical processes, 30 groundwater samples collected from Valea Calugareasca, Prahova County, Romania, were analyzed for the major anions (NO3−, SO42−, Cl−, HCO3−, and F−) and cations (Ca2+, Mg2+, Na+, and K+), which are naturally highly variable due to climatic and geographical location conditions. Ca2+, Na+, Mg2+, and K+ varied between 118 and 275 mg/L, 32 and 160 mg/L, 12.2 and 78.4 mg/L, and 0.21 and 4.48 mg/L, respectively. NO3− levels exceeding the World Health Organization (WHO) limit of 50 mg/L were identified in 17% of the groundwater samples, mainly as result of fertilizers applied to agricultural activities. The hydrogeochemical study identified dolomite dissolution and halite precipitation as natural sources of ions as well as the presence of pyrite as a source of SO42− ions in 60% of the samples. The sulfate content varied between 125 and 262 mg/L. Bicarbonate and chloride concentrations varied between 202 and 530 mg/L and 21 and 212 mg/L. The saturation index indicates the contribution of Ca2+ ions in the groundwater samples came from some processes of dissolving rocks such as aragonites (values between 1.27 and 2.69) and calcites (values between 1.43 and 2.82). Negative halite values indicated that salt accumulation results from precipitation processes. Only 10% of the analyzed groundwater samples were suitable for human consumption, the samples being situated on the hill, far away from the pyrite ash waste dumps and agricultural land.

Hydrogeochemical processes in aquifers of volcano-sedimentary origin using inverse modeling

The study area is located in the Trans-Mexican Neovolcanic Belt. A complex hydrogeological system was approached, composed of metamorphic rocks, covered by sedimentary rocks of marine origin and folded, covered by tertiary clastic rocks and a volcanic sequence of Quaternary age. The main objective of this research was to carry out a hydrogeochemical characterization to identify hydrogeochemical processes. The interaction with different geological units and anthropogenic activities produces different water types (facie Ca–Mg–Na–HCO3–SO4 in the urban area, Ca–Na–Mg–HCO3 agricultural area). It was determined that the local flow contributes 55%, the intermediate 26%, and the regional 19% with ternary mixtures model. With speciation diagrams, the conditions of chemical equilibrium in the aquifer are determined. Given this scenario, inverse modeling was applied in a cross-section to identify water interaction with the different types of geological materials through which it circulates and anthropogenic activities. The inverse modeling in cross-section 71-61, from the Pico de Orizaba recharge zone, indicates that the main processes present are altering silicates and clays (albite, illite, Ca-montmorillonite, gibbsite; dolomite, halite and gypsum dissolution; calcite and kaolinite formation). In cross-section 61-51, the model shows albite, gibbsite, Ca-montmorillonite alteration; dolomite and halite dissolution; also, calcite, gypsum, kaolinite, and illite formation. In cross-section 51-53, the modeling indicates albite, gibbsite, illite, Ca-montmorillonite alteration; halite and dolomite dissolution; also, gypsum, calcite, kaolinite, formation; NH3 (g), NH4, N (2) (g), ion exchange of NaX. The organic matter plays an essential role in REDOX conditions due to anthropogenic activities of the study area.

Hydrochemical Characteristics and Groundwater Quality Assessment Using an Integrated Approach of the PCA, SOM, and Fuzzy c-Means Clustering: A Case Study in the Northern Sichuan Basin

Groundwater is a precious resource that is vital for human life. Widespread human activities occur in the Sichuan basin due to befitting climate and geological conditions, inducing a series of groundwater contamination. In this article, hydrochemical analysis, principal component analysis (PCA), self-organizing map (SOM), and fuzzy c-means clustering (FCM) were integrated to reveal the hydrochemical process and assess groundwater quality in the northern part of the Sichuan Basin based on a collection of 203 groundwater samples. The groundwater hydrochemical types were dominated by the HCO3-Ca type. The PCA results show both natural and anthropogenic factors contributed to the hydrochemical compositions. The combination of the SOM and FCM classifies neurons into two categories: the first category where NO2− and NH4+ are most similar, perhaps as anthropogenic sources of pollution, which pose serious threats to human health; and the second category, where the total dissolved solids, Ca2+, Na+, Cl−, SO42, Mg2+, and K+ are most similar, explained as the influence of natural factors. The ion source was determined by water–rock interactions: Na+ mainly comes from the dissolution of silicate rocks, while Ca2+, Mg2+, and HCO3− from the dissolution of calcite and dolomite. Cation exchange was recognized in the water–rock interactions. The achievements would provide a significant reference for groundwater protection in the Sichuan Basin.

CO2-Water-Rock Interactions in Carbonate Formations at the Tazhong Uplift, Tarim Basin, China

The effects of CO2-water-rock interactions on the injectivity and safety of CO2 geological storage have drawn wide attention. The geochemical reaction mechanisms in carbonate formations after CO2 injection are still controversial. To better understand the transformation of injected CO2 in carbonates and the involved geochemical reactions, we first conducted autoclave experiments reproducing the in-situ conditions of the Lianglitage Formation, Yingshan Formation, and Qiulitage Formation at the Tazhong Uplift in the Tarim Basin. We then established a batch model using TOUGHREACT-ECO2H, validated with the experimental results, to simulate the long-term CO2-water-rock interactions. It was found that the initial mineral compositions and water chemistry have important effects on the CO2-water-rock interactions in carbonate formations. The experiment results show that the dissolution of calcite and dolomite dominates in the early reaction period. However, we still observed some secondary minerals, such as ankerite, montmorillonite, calcite, and dolomite. The CO2-water-rock reactions can be more dramatic when the contents of calcite and dolomite in carbonates are closer. Moreover, the long-term simulation results show that calcite, magnesite, and hematite are the main formed secondary minerals, whereas dolomite is the major dissolved mineral. This study is helpful for a better understanding of the CO2 mineral trapping mechanism in carbonate formations.

Chemical Evolution Process and Quality Assessment of Seepage Groundwater in Tunnel Crossing Gas-Bearing Coal Seams

The chemical characteristics of groundwater in the gas coal seam section of the tunnel have rich geological significance. To study the chemical evolution process and controlling factors of groundwater in the gas-bearing coal seam section of the tunnel, and the influence of tunnel coal removal on the groundwater quality, field investigations and laboratory experiments were carried out on the groundwater in the coal measure strata and the high-gaseous section and surrounding aquifers. Through hydrochemical analysis, correlation analysis, hydrochemical simulation, and other methods, the chemical origin of groundwater in coal measure strata and other aquifers was revealed. The water quality of groundwater was analyzed by water quality index (WQI), sodium adsorption rate (SAR), percentage of soluble sodium (Na+%), and participation of sodium carbonate (RSC). The results show that the water samples of each aquifer are weakly alkaline. Influenced by formation lithology and mineral redox reactions, the genesis of coal formation water is more complex than that of natural karst groundwater. The water chemistry characteristics show obvious differences among aquifer groups, with low correlation of HCO3-, Ca2+, and Mg2+. The central drainage ditch is dominated by dolomite dissolution, and the sloping shaft side ditch is dominated by calcite dissolution, and ion exchange effects are prevalent in different water sources. In terms of irrigation water quality, the permeability index (PI), magnesium hazard (MH), and Kelly index (KI) calculations show that 45.8% of the water samples are in the “unsuitable” condition. This study helps to fully understand the quality of coal seam water in tunnels and can provide a reference for groundwater utilization.

Dynamics of weight loss of dolomite dropouts at different stages of dissolution in <em>Albic Retisol</em>

In a laboratory experiment in 2020, the dynamics of the decrease in mass of large particles of dolomite dropouts in acidic Albic Retisol was established. Dolomite dropouts were collected from the landfill of a road construction factory. The results of the laboratory experiment showed that after 114 days of composting, the loss of dolomite mass ranged from 8.7 to 34.2 % of its initial content. The loss of mass of particles during composting from 114 to 224 days slowed down and fluctuated, depending on the variant, from 2.2 to 5.1 % of the initial mass of dolomite. The mechanism of weathering was considered and the factors enhancing the rate of dissolution of dolomite dropouts in the soil were revealed. Linear empirical dependencies of the rate of dissolution of dolomite in soil at different stages of the experiment were developed. Clustering of the developed models was carried out according to the absolute values ​​of the rate of decrease in the mass of particles in the vessels. When selecting the dose of application of large particles of dolomite for reclamation of acidic soils and duration of their action, it is recommended to take into account the duration of time the dolomite spent in the landfill.

Geochemical modeling of CO2 injection and gypsum precipitation at the Ketzin CO2 storage site

Gypsum crystals are found at the well perforation of observation well Ktzi 202 of the test site for CO2 storage at Ketzin, Germany. XRD analysis confirms pure gypsum. Fluid samples before and after CO2 injection are analyzed. Geochemical modeling is conducted to identify the mechanisms that lead to gypsum formation. The modeling is carried out with PHREEQC and Pitzer database due to the high salinity of up to 5 mol per kg water. Due to their significantly higher reactivity compared to other minerals like silicates, calcite, dolomite, magnesite, gypsum, anhydrite, and halite are considered as primary mineral phases for matching the observed brine compositions in our simulations. Calcite, dolomite, and gypsum are close to saturation before and after CO2 injection. Dolomite shows the highest reactivity and mainly contributes to buffering the brine pH that initially decreased due to CO2 injection. The contribution of calcite to the pH-buffering is only minor. Gypsum and anhydrite are no geochemically active minerals before injection. After CO2 injection, gypsum precipitation may occur by two mechanisms: (i) dissociation of CO2 decreases activity of water and, therefore, increases the saturation of all minerals and (ii) dolomite dissolution due to pH-buffering releases Ca2+ ions into solution and shifts the mass action to gypsum. Gypsum precipitation decreases with increasing temperature but increases with increasing partial CO2 pressure. Our calculations show that calcium sulfate precipitation increases by a factor of 5 to a depth of 2000 m when Ketzin pressure and temperature are extrapolated. In general, gypsum precipitation constitutes a potential clogging hazard during CO2 storage and could negatively impact safe site operation. In the presented Ketzin example, this threat is only minor since the total amount of gypsum precipitation is relatively small.