Dissolution Of Silicate Minerals Research Articles

Influence of long-term operation of supercritical carbon dioxide based enhanced geothermal system on mineralogical and microstructurally-induced mechanical alteration of surrounding rock mass

Chemico-microstructural and mineralogically-induced time-dependent mechanical alterations of the reservoir rock found in the peripheral zone of a CO2-based EGS is experimentally investigated. A series of uniaxial compressive and triaxial (under 30 MPa constant confining pressure with different temperatures; 25-300 °C) strength tests on granite specimens reacted in ScCO2+water medium for different time periods was performed by coupling acoustic emission (AE) technique. The results were compared with those for intact and water-reacted rock specimens in order to better understand this weakening phenomenon. Scanning electron microscopy (SEM) was also incorporated to evaluate the microstructural and mineralogical alterations.The results indicated that the mineralogical structure of Harcourt granite rock specimens was mainly altered by the dissolution of silicate minerals and plagioclase phase feldspar minerals. In addition, enhanced precipitation of secondary clay minerals including kaolinite, smectite, and illite was observed in the ScCO2+water-reacted rock specimens. The average compressive strength of ScCO2/water-reacted granite specimens found to be reduced with the increase of reaction period. Furthermore, 3.81%, 5.08%, 5.66% and 0.52% reductions in failure strength resulted under 30 MPa confining pressure in 6 months ScCO2/water-reacted granite specimens at 25 °C, 100 °C, 200 °C and 300 °C temperatures, respectively, compared to intact rock values at these temperatures. The post-failure analysis also revealed that the ScCO2/water reaction significantly alters the post-failure behaviour due to micro-mineralogically-induced weakening inside the rock pore structure. It was concluded that enhanced mineral precipitation and dissolution influence in time-dependent degradation of the surrounding rock mass which influence in long-term operation of the geothermal system.

Supercritical CO2-water-shale interactions and their effects on element mobilization and shale pore structure during stimulation

There are many advantages to using supercritical carbon dioxide (ScCO2) fracturing technology to exploit shale gas reservoirs in China, including minimal damage to the environment or formation, and displacing methane (CH4) in the adsorbed state. When ScCO2 enters fractures in the formation, ScCO2-water-shale interactions may affect the physicochemical properties of shale. In this study, a high-pressure reaction system was adopted to simulate ScCO2-water-shale interactions under ScCO2 stimulation conditions. The element mobilization and pore structure before and after the reaction were measured using ICP-MS, XRF. The results show that the major elements, including Ca, Mg, Na, K, and Al, exhibit varying degrees of mobilization after the interactions because of dissolution of carbonate and silicate minerals in shale samples. Compared with the major elements, trace elements have a lower mobility, quantified as <13.97%. The specific surface areas and pore volumes of two shale samples increase at different degrees after the reaction. The interactions have a more significant influence on the micropores. In addition, fractal features of the shale pore structure were analyzed. The fractal dimensions of the shale samples increase after the reaction, indicating that pore surface roughness increases, and pore structure morphology gradually transforms from regular to complex.

Using hydrochemical and isotopic data to determine sources of recharge and groundwater evolution in arid region from Eastern Desert, Egypt

In arid areas, identifying the recharge sources and hydrogeochemical evolution of groundwater is essential for effective water resource management. The hydrogeological, hydrogeochemical, and isotopic methods are used to understand the origin of water and associated groundwater recharge processes and conceptualize the groundwater flow within the major aquifer systems in the Idfu-Esna Area. In the study area, two main aquifers are studied: the unconfined Quaternary aquifer and the confined Nubian Sandstone aquifer. The groundwater from the two aquifers is characterized by distinct stable isotope signatures. This difference in isotopic compositions is interpreted in terms of difference in origin and recharge mechanisms. The recharge mechanisms are dominated by direct infiltration of precipitation, lateral flow from the surface water and leakage from deep aquifer system through the main faults. The hydrochemical results indicate that the dissolution of carbonate and silicate minerals, leaching of soils and/or ion exchange are the main processes controlling groundwater mineralization. Established conceptual model has clarified the groundwater hydrodynamics within the studied aquifer systems. Based on this model, groundwater mixing can occur between the different aquifers.

A combined geochemical and μCT study on the CO2 reactivity of Surat Basin reservoir and cap-rock cores: Porosity changes, mineral dissolution and fines migration

Geological storage of CO2 generally involves injection of a CO2 stream into a high porosity and permeability reservoir, contained by one or more overlying low permeability formations. Sandstone reservoirs and associated cap-rocks of targeted CO2 storage sites therefore have distinct properties such as porosity and mineral contents. Their geochemical response or reactivity to injected supercritical CO2 and associated changes in porosity, and permeability affecting scaling, mineral trapping, injectivity, or migration can therefore be very different. Six drill core samples including quartz-rich sandstones, calcite cemented sandstones, and feldspar or clay-rich cap-rocks from a proposed demonstration site in the Surat Basin, Australia, were characterized before and after reaction with pure supercritical CO2 and low salinity formation water. The quartz-rich sandstones have low reactivity, and maintain high porosities with visible pore connectivity after reaction, they are unlikely to be affected by scaling. Kaolin and fine grain movement observed via μCT and SEM could have the potential to open or plug pores, potentially increasing or decreasing permeability and CO2 injectivity. Calcite cemented sandstones had the greatest measured change in porosity after reaction via calcite dissolution. Narrow angular channels were formed in the calcite cement around framework grains, extending through to the center of the sub-plug in the courser grained rock, and surface roughness increased. Solution pH was however quickly passivated. The highest concentrations of Ca, Mn, Sr, and Mg were released to solution from calcite dissolution. Clay (and feldspar) rich cap-rock core had mainly microporosity and the smallest initial pore throat diameters associated with clays. Small changes to μCT calculated porosities after reaction were related to a decrease in chlorite X-ray density, and dissolution of patchy carbonate minerals. Pores were disconnected in μCT images, except for some created horizontal connection along a sandy lamination in a cap-rock. Dissolved concentrations of Ca, Fe, Si, Sr, Mn, Li and Mg increased via dissolution of both carbonate and silicate minerals. Dissolved Ca, Fe, Mn and Mg from silicate minerals in the cap-rock were available for longer term mineral trapping of CO2. Potential increases in porosity and migration will be highest in the calcite cemented zones, while clay-rich cap-rocks could be expected to maintain integrity. There is a low likelihood of mineral trapping or scaling in the quartz rich lower Precipice Sandstone. Overlying rocks can provide Fe, Mg, Ca for mineral trapping of CO2 as ferroan carbonates such as siderite, ankerite and dolomite over longer time scales when pH is buffered.Changes to porosity, mineral content, and water chemistry after pure CO2 reaction observed here and in other published studies were dependent on mineral content and fluid accessibility. These results could be generalized to other sandstone reservoirs where it is expected to inject CO2. The results can also be used to validate geochemical models to build longer term predictions.

Tracing silicate weathering processes in the permafrost-dominated Lena River watershed using lithium isotopes

Increasing global temperatures are causing widespread changes in the Arctic, including permafrost thawing and altered freshwater inputs and trace metal and carbon fluxes into the ocean and atmosphere. Changes in the permafrost active layer thickness can affect subsurface water flow-paths and water-rock interaction times, and hence weathering processes. Riverine lithium isotope ratios (reported as δ7Li) are tracers of silicate weathering that are unaffected by biological uptake, redox, carbonate weathering and primary lithology. Here we use Li isotopes to examine silicate weathering processes in one of the largest Russian Arctic rivers: the Lena River in eastern Siberia. The Lena River watershed is a large multi-lithological catchment, underlain by continuous permafrost. An extensive dataset of dissolved Li isotopic compositions of waters from the Lena River main channel, two main tributaries (the Aldan and Viliui Rivers) and a range of smaller sub-tributaries are presented from the post-spring flood/early-summer period at the onset of active layer development and enhanced water-rock interactions. The Lena River main channel (average δ7Lidiss ∼ 19‰) has a slightly lower isotopic composition than the mean global average of 23‰ (Huh et al., 1998a). The greatest range of [Li] and δ7Lidiss are observed in catchments draining the south-facing slopes of the Verkhoyansk Mountain Range. South-facing slopes in high-latitude, permafrost-dominated regions are typically characterised by increased summer insolation and higher daytime temperatures relative to other slope aspects. The increased solar radiation on south-facing catchments promotes repeated freeze-thaw cycles, and contributes to more rapid melting of snow cover, warmer soils, and increased active layer thaw depths. The greater variability in δ7Li and [Li] in the south-facing rivers likely reflect the greater infiltration of melt water and enhanced water-rock interactions within the active layer.A similar magnitude of isotopic fractionation is observed between the low-lying regions of the Central Siberian Plateau (and catchments draining into the Viliui River), and catchments draining the Verkhoyansk Mountain Range into the Aldan River. This is in contrast to global rivers in non-permafrost terrains that drain high elevations or areas of rapid uplift, where high degrees of physical erosion promote dissolution of freshly exposed primary rock typically yielding low δ7Lidiss, and low-lying regions exhibit high riverine δ7Li values resulting from greater water-rock interaction and formation of secondary mineral that fractionates Li isotopes. Overall, the range of Li concentrations and δ7Lidiss observed within the Lena River catchment are comparable to global rivers located in temperate and tropical regions. This suggests that cryogenic weathering features specific to permafrost regions (such as the continual exposure of fresh primary minerals due to seasonal freeze-thaw cycles, frost shattering and salt weathering), and climate (temperature and runoff), are not a dominant control on δ7Li variations. Despite vastly different climatic and weathering regimes, the same range of riverine δ7Li values globally suggests that the same processes govern Li geochemistry – that is, the balance between primary silicate mineral dissolution and the formation (or exchange with) secondary minerals. This has implications for the use of δ7Li as a palaeo-weathering tracer for interpreting changes in past weathering regimes.

Ge and Si isotope signatures in rivers: A quantitative multi-proxy approach

Solutes derived from the dissolution of silicate minerals play a key role in Earth's climate via the carbon and other biogeochemical cycles. Silicon (Si) is a unique constituent of silicate minerals and a biologically important nutrient, so tracing its behavior in near-surface environments may provide important insights into weathering processes. However, Si released by weathering is variably incorporated into secondary mineral phases and biota, obscuring signals derived from primary weathering processes. Due to chemical similarities, Germanium (Ge) may help better understand the Si cycle and its relationship to chemical weathering. With this aim, we report new measurements of the concentration and isotopic composition of Ge for both the dissolved and particulate phases of a variety of global rivers. These measurements are combined with analyses of concentration and isotopic ratio of Si on the exact same sample set in order to make direct comparisons of the behavior of these two elements in natural river systems. With this dataset, we develop a new modeling framework describing the full elemental and isotopic systems of these solutes in rivers (i.e., Ge/Si, δ74Ge, and δ30Si). This multi-proxy approach allows us to ascertain the relative importance of biological versus mineral uptake in modulating the fluxes of these elements delivered to the modern ocean.Dissolved δ74Ge composition of rivers studied thus far range from 0.9 to 5.5‰ with a discharge-weighted global average of 2.6 ± 0.5‰. The Ge isotope composition of riverine suspended and bedload sediments is indistinguishable from silicate source rocks, which is consistent with mass balance expectations. The multi-proxy modeling suggests that, among the watersheds studied here, the isotopic fractionation of Si during secondary mineral phase precipitation (Δ30Sisec) ranges from −2.7 to −0.2‰, which removes between 19–79% of the initial dissolved Si, while between 12–54% is incorporated by biota. For Ge, modeling indicates that 79–98% of the dissolved load is incorporated into secondary mineral phases with a Δ74Gesec ranging from −4.9 to −0.3‰. The fractionation induced by biological uptake is calculated to range from −2.6 to −1.3‰ for Δ30Sibio and −0.7 ± 0.7‰ for Δ74Gebio. In addition to improving our understanding of the coupled Ge and Si cycles, our study provides a framework for using multiple isotopic tracers to elucidate the chemical behavior of solutes in natural waters.

Lithium and its isotopes as tracers of groundwater salinization: A study in the southern coastal plain of Laizhou Bay, China

In the southern coastal plain of Laizhou Bay, due to intensive exploitation of groundwater since the early 1970s, the shallow aquifer has been severely influenced by saltwater intrusion, which causes the extraction to shift from shallow to deeper aquifer changing the hydrogeological condition greatly. This study was conducted to investigate the groundwater salinization using hydrochemistry and H, O and Li isotope data. Dissolved Li shows a linear correlation with Cl and Br in seawater, brine and saline groundwater indicating the marine Li source, whereas the enrichment of Li in surface water, brackish and fresh groundwater is impacted by dissolution of silicate minerals. The analyses of hydrochemistry and isotopes (H, O and Li) indicate that brine originated from seawater evaporation, followed by mixing processes and some water-rock interactions; shallow saline groundwater originated from brine diluted with seawater and fresh groundwater; deep saline groundwater originated from seawater intrusion. The negative correlation of δ7Li and Li/Na in surface water, brackish and fresh groundwater is contrary to the general conclusion, indicating the slow weathering of silicate minerals and hydraulic interaction between surface water and shallow groundwater in this area. The analyses of hydrochemistry and isotopes (Li, H and O) can well identify the salinity sources and isotope fractionation in groundwater flow and mixing, especially groundwater with high TDS. As both mixing with saltwater and isotope fractionation can explain the combination of high δ7Li and low TDS in brackish groundwater, isotope fractionation may limit their use in recognizing salinity sources of groundwater with low TDS.

Morphological instability of aqueous dissolution of silicate glasses and minerals

Understanding of aqueous dissolution of silicate glasses and minerals is of great importance to both Earth and materials sciences. Silicate dissolution exhibits complex temporal evolution and rich pattern formations. Recently, we showed how observed complexity could emerge from a simple self-organizational mechanism: dissolution of the silica framework in a material could be catalyzed by the cations released from the reaction itself. This mechanism enables us to systematically predict many key features of a silicate dissolution process including the occurrence of a sharp corrosion front (vs. a leached surface layer), oscillatory dissolution and multiple stages of the alteration process (e.g., an alteration rate resumption at a late stage of glass dissolution). Here, through a linear stability analysis, we show that the same mechanism can also lead to morphological instability of an alteration front, which, in combination with oscillatory dissolution, can potentially lead to a whole suite of patterning phenomena, as observed on archaeological glass samples as well as in laboratory experiments, including wavy dissolution fronts, growth rings, incoherent bandings of alteration products, and corrosion pitting. The result thus further demonstrates the importance of the proposed self-accelerating mechanism in silicate material degradation.

Evidence for the occurrence of hydrogeochemical processes in the groundwater of Khoy plain, northwestern Iran, using ionic ratios and geochemical modeling

Rapid population growth, industrialization, and agricultural expansion in the Khoy area (northwestern Iran) have led to its dependence on groundwater and degradation of groundwater quality. This study attempts to decipher the major processes and factors that degrade the groundwater quality of the Khoy plain. For this purpose, 54 groundwater samples from unconfined and confined aquifers of the plain were collected in July 2017 and analyzed for major cations and anions (Na, K, Ca, Mg, HCO3, SO4, and Cl), minor ions (NO3 and F), and Al. Magnesium and bicarbonate were identified as the dominant cation and anion, respectively. Several ionic ratios and geochemical modeling using PHREEQC indicated that the most important hydrogeochemical processes to affect groundwater quality in the plain were weathering and dissolution of evaporitic and silicate minerals, mixing, and ion exchange. There were smaller effects from evaporation and anthropogenic factors (e.g., industries). Results showed that the high salinity of the groundwater in the northeast area of the plain was due to the high solubility of the evaporitic minerals, e.g., halite and gypsum. Reverse ion exchange and the contribution of mineral dissolution were more significant than ion exchange in the northeastern part of the plain. Elevated salinity of the groundwater in the southeast was attributed mostly to reverse ion exchange and somewhat to evaporation.

Dynamics of self-reorganization explains passivation of silicate glasses

Understanding the dissolution of silicate glasses and minerals from atomic to macroscopic levels is a challenge with major implications in geoscience and industry. One of the main uncertainties limiting the development of predictive models lies in the formation of an amorphous surface layer––called gel––that can in some circumstances control the reactivity of the buried interface. Here, we report experimental and simulation results deciphering the mechanisms by which the gel becomes passivating. The study conducted on a six-oxide borosilicate glass shows that gel reorganization involving high exchange rate of oxygen and low exchange rate of silicon is the key mechanism accounting for extremely low apparent water diffusivity (∼10−21 m2 s−1), which could be rate-limiting for the overall reaction. These findings could be used to improve kinetic models, and inspire the development of new molecular sieve materials with tailored properties as well as highly durable glass for application in extreme environments.

Experimental partitioning of Ca isotopes and Sr into anhydrite: Consequences for the cycling of Ca and Sr in subseafloor mid-ocean ridge hydrothermal systems

The elemental and isotopic mass balance of Ca and Sr between seawater and the oceanic crust at mid-ocean ridge (MOR) hydrothermal systems integrates various physiochemical processes in the subseafloor, such as dissolution of primary silicate minerals, formation of secondary minerals, and phase separation in the subseafloor. In particular, the precipitation and recrystallization of anhydrite are recognized as important processes controlling the Ca and Sr elemental and isotope composition of high temperature vent fluids and coexisting ocean crust, and yet, little experimental data exist to constrain the mechanism and magnitude of these critical geochemical effects. Thus, this study experimentally examines Sr/Ca partitioning, Ca isotope fractionation, and the rate of exchange between anhydrite and dissolved constituents. These experimental constraints are then compared with Sr/Ca and Ca isotope compositions of anhydrite and vent fluids sampled from the TAG hydrothermal system. Accordingly, anhydrite precipitation and recrystallization experiments were performed at 175, 250, and 350 °C and 500 bar at chemical conditions characteristic of active MOR hydrothermal systems. Experimental data suggest that upon entrainment and recharge of seawater into MOR hydrothermal systems anhydrite will rapidly precipitate with a Ca isotopic composition that is depleted in the heavy isotope compared to the hydrothermal fluid. The magnitude of the Ca isotope fractionation, Δ44/40Ca(Anh-Fluid), is temperature dependent, −0.45, −0.22, and −0.02‰, for 175, 250, and 350 °C, respectively, but likely indicative of kinetic effects. Utilization of a 43Ca spike in solution was implemented to quantify the time-dependent extent of isotope exchange during anhydrite recrystallization at chemical equilibrium. These data indicate that the rate of exchange is a function of temperature, where 12, 46, and 45% exchange occurred within 1322, 867, 366 h at 175, 250, and 350 °C, respectively. The partitioning of Sr/Ca between anhydrite and constituent dissolved species during precipitation depends greatly on the saturation state of the hydrothermal fluid with respect to anhydrite at each experimental temperature, KD(Anh-Fluid) = 1.24–0.55 at 175–350 °C, broadly similar to results of earlier experimental observations by Shikazono and Holland (1983). Equilibrium KD(Anh-Fluid) values were estimated by taking explicit account of time dependent magnitude of exchange, yielding values of 0.43, 0.36, 0.29 at 175, 250, and 350 °C, respectively. Coupling these experimental constraints with the temperature gradient inferred for high temperature MOR hydrothermal systems suggests that the Ca isotope and Sr elemental composition of anhydrite formed near the seafloor will retain the composition derived upon initial formation conditions, which is indicative of disequilibrium. In contrast, at greater depths and at higher temperatures, anhydrite will reflect close to equilibrium Sr/Ca partitioning and Ca isotope fractionation conditions. The experimental and natural data presented in this study can be used to further understand the effect of anhydrite precipitation during hydrothermal circulation in the oceanic crust and on the chemical and isotopic composition of seawater on geologic timescales.

Hydrochemical Characteristics and Multivariate Statistical Analysis of Natural Water System: A Case Study in Kangding County, Southwestern China

The utilization for water resource has been of great concern to human life. To assess the natural water system in Kangding County, the integrated methods of hydrochemical analysis, multivariate statistics and geochemical modelling were conducted on surface water, groundwater, and thermal water samples. Surface water and groundwater were dominated by Ca-HCO3 type, while thermal water belonged to Ca-HCO3 and Na-Cl-SO4 types. The analyzing results concluded the driving factors that affect hydrochemical components. Following the results of the combined assessments, hydrochemical process was controlled by the dissolution of carbonate and silicate minerals with slight influence from anthropogenic activity. The mixing model of groundwater and thermal water was calculated using silica-enthalpy method, yielding cold-water fraction of 0.56–0.79 and an estimated reservoir temperature of 130–199 °C, respectively. δD and δ18O isotopes suggested that surface water, groundwater and thermal springs were of meteoric origin. Thermal water should have deep circulation through the Xianshuihe fault zone, while groundwater flows through secondary fractures where it recharges with thermal water. Those analytical results were used to construct a hydrological conceptual model, providing a better understanding of the natural water system in Kangding County.

Dual slag filters for enhanced phosphorus removal from domestic waste water: performance and mechanisms

The phosphorus (P) removal of five combinations of dual filters consisting of blast furnace slag (BFS), argon oxygen decarburisation slag (AOD) and electric arc furnace slag (EAF) was evaluated in column experiments with domestic waste water. The columns were fed with waste water for 24 days. The column with only EAF had the best P removal performance (above 93% throughout the experiment). The speciation of the bound P was evaluated by P K-edge X-ray absorption near-edge structure (XANES) spectroscopy. In all five columns, the main P species of the slag packed in the outlet chamber was amorphous calcium phosphate (ACP). In samples from the inlet chambers, the contributions from crystalline Ca phosphates, P adsorbed on gibbsite and P adsorbed on ferrihydrite were usually much greater, suggesting a shift of P removal mechanism as the waste water travelled from the inlet to the outlet. The results provide strong evidence that P was predominantly removed by the slags through the formation of ACP. However, as the pH decreased with time due to the progressively lower dissolution of alkaline silicate minerals from the slag, the ACP was rendered unstable and hence redissolved, changing the P speciation. It is suggested that this process strongly affected the lifespan of the slag filters. Of the slags examined, EAF slag had the best P removal characteristics and BFS the worst, which probably reflected different dissolution rates of alkaline silicates in the slags.

Demarcation of fluoride vulnerability zones in granitic aquifer, semi-arid region, Telengana, India

This study has been carried out in the granitic aquifer of Maheshwaram watershed, Telengana, India. In this study, groundwater sample data of 8 years were analyzed for the fluoride content with other chemical quality parameters. The correlation and factor analysis were employed to understand the mechanisms for fluoride (F) enrichment as well as the hydrochemistry of the area. These analyses addressed that the observed groundwater quality was due to water-rock interaction in the aquifer and fluoride is coming from the dissolution of fluorite and other silicate minerals like biotite and hornblende by the groundwater. Land use/land cover (LULC) study from 2002 to 2008 revealed there were significant positive changes in build-up land and negative changes in vegetation cover after 2003. The main agriculture (paddy) has been reduced to 0.97 km2 in 2008 from 2.39 km2 in 2003. The studied watershed has been characterized on the basis of F concentration into safe, transition, and unsafe groups following the WHO and BIS guidelines. The temporal variation of the three groups showed that 57.6% area of the watershed was in unsafe zone in 2000–2003, but 69.2% of the area became safe in 2006–2009. It has been found that F concentration reduced in 12.59% of the area (became safe from unsafe) accompanied by the reduction of paddy field area. After validation with present (2016) fluoride concentrations, it was found that 16.28% are vulnerable in near future. The results of this study showed that (a) the safe and unsafe zones of fluoride concentrations vary with time with the changes in other parameters associated with it like crop pattern and (b) vulnerable zone can be identified based on the susceptibility to change of safe and unsafe zones. Such studies are useful for planning and management purposes.

Impure CO2 reaction of feldspar, clay, and organic matter rich cap-rocks: Decreases in the fraction of accessible mesopores measured by SANS

During CO2 geological storage, low porosity and permeability cap-rock can act as a structural trap, preventing CO2 vertical migration to overlying fresh water aquifers or the surface. Clay and organic matter rich shales, fine-grained sandstones and mudstones often act as cap-rocks and may contain substantial sub-micron porosity. CO2-brine-rock interactions can open or close pore throats through dissolution, precipitation or migration of clay fines or grains. This could affect CO2 migration if the porosity is accessible, with unchanging or decreasing accessible porosity favourable for trapping and integrity. Two cap-rock core samples, a clay and organic-rich mudstone and a more organic-lean feldspar-rich fine grained sandstone, from a well drilled for a CO2 storage feasibility study in Australia were experimentally reacted with impure CO2 (+SO2, O2) and low salinity brine at reservoir conditions. Mercury injection capillary pressure indicated that the majority of pores in both cores had pore throat radii ~ 5–150nm with porosities of 5.5–8.4%. After reaction with impure CO2-brine the measured pore throats decreased in the clay-rich mudstone core. Dissolution and precipitation of carbonate and silicate minerals were observed during impure CO2 reaction of both cores via changes in water chemistry. Scanning electron microscopy identified macroporosity in clays, mica and amorphous silica cements. After impure CO2-brine reaction, precipitation of barite, Fe-oxides, clays and gypsum was observed. Ion leaching from Fe-rich chlorite was also apparent, with clay structural collapse, and fines migration. Small-angle neutron scattering measured the fraction of total and non-accessible pores (~10–150nm radii pores) before and after reaction. The fraction of pores that was accessible in both virgin cap-rocks had a decreasing trend to smaller pore size. The clay-rich cap-rock had a higher fraction of accessible pores (~0.9) at the smallest SANS measured pore size, than the feldspar rich fine-grained sandstone (~0.75). Both core samples showed a decrease in SANS accessible pores after impure CO2-water reaction at CO2 storage conditions. The clay-rich cap-rock showed a more pronounced decrease. After impure CO2-brine reaction the fraction of accessible pores at the smallest pore size was ~0.85 in the clay-rich cap-rock and ~0.75 in the feldspar-rich fine-grained sandstone. Reactions during impure CO2-brine-rock reaction have the potential to close cap-rock pores, which is favourable for CO2 storage integrity.

CO2 mineral trapping in fractured basalt

Fractures in basalt can provide substantial surface area for reactions, and limited mass transfer in fractures can allow accumulation of cations to form carbonate minerals in geologic carbon sequestration. In this study, flood basalt and serpentinized basalt with engineered fractures were reacted in water equilibrated with 10MPa CO2 at 100°C or 150°C for up to 40 weeks. Carbonation in basalt fractures was observed as early as 6 weeks, with Mg- and Ca-bearing siderite formed in both basalts reacted at 100°C and Mg-Fe-Ca carbonate minerals formed in the flood basalt reacted at 150°C. X-ray μCT segmentation revealed that precipitates filled 5.4% and 15% (by volume) of the flood basalt fracture after 40 weeks of reaction at 100°C and 150°C, respectively. Zones of elevated carbonate abundance did not completely seal the fracture. Limited siderite clusters (<1% volume fraction) were found in localized areas in the serpentinized basalt fracture. A 1-dimensional reactive transport model developed in CrunchTope examined how geochemical gradients drive silicate mineral dissolution and carbonate precipitation in the fracture. The model predicts that siderite will form as early as 1day after the addition of CO2. The predicted location of maximum siderite abundance is consistent with experimental observations, and the predicted total carbonate volumes are comparable to estimates derived from CT segmentation.

Investigation of Geochemical Characteristics and Controlling Processes of Groundwater in a Typical Long-Term Reclaimed Water Use Area

The usage of reclaimed water can efficiently mitigate water crises, but it may cause groundwater pollution. To clearly understand the potential influences of long-term reclaimed water usage, a total of 91 samples of shallow and deep groundwater were collected from a typical reclaimed water use area during the dry and rainy seasons. The results suggest both shallow and deep groundwater are mainly naturally alkaline freshwater, which are composed mainly of Ca-HCO3, followed by mixed types such as Ca-Na-HCO3 and Ca-Mg-HCO3. A seasonal desalination trend was observed in both shallow and deep aquifers due to dilution effects in the rainy season. Groundwater chemical compositions in both shallow and deep aquifers are still dominantly controlled by natural processes such as silicate weathering, minerals dissolution and cation exchange. Human activities are also the factors influencing groundwater chemistry. Urbanization has been found responsible for the deterioration of groundwater quality, especially in shallow aquifers, because of the relative thin aquitard. Reclaimed water usage for agricultural irrigation and landscape purposes has nearly no influences on groundwater quality in rural areas due to thick aquitards. Therefore, reclaimed water usage should be encouraged in arid and semiarid areas with proper hydrogeological condition.

Distributions, Sources, and Species of Heavy Metals/Trace Elements in Shallow Groundwater Around the Poyang Lake, East China

The shallow groundwater in the Poyang Lake area is one of the important sources of drinking water. However, there has been no systematic assessment on the groundwater quality associated with human consumptions. Here, we reported the concentrations and distributions of heavy metals/trace elements in the groundwater. Geographic Information System, ions relationship, and geochemical modeling program PHREEQC were used to investigate the distributions, sources, and chemical species of heavy metals/trace elements. The results showed that most of the shallow groundwater samples in the Poyang Lake area were in neutral or slightly acidic and oxidizing environment. The concentrations of Fe, Cr, Se, Cu, Zn, As, and Cd in most of the shallow groundwater samples were not significant in the area. However, the concentrations of Mn, Al in a few of samples and Pb in half of samples exceeded the limits, which makes the water unsuitable for drinking. The chemical components of shallow groundwater originated mainly from the dissolution of minerals and the input of anthropogenic activities. The concentrations and distributions of Pb, Mn, Al in groundwater largely depended on the dissolution of silicate and carbonate minerals, as well as the influence of human activities, as some of samples with high concentrations were found near industrial and domestic sites. The dissolution of minerals dominated the concentration distributions of Sr, Si, and Li. The concentrations of Al and Si in groundwater were also affected by pH. Chemical species of heavy metals/trace elements were also analyzed in this study. The data evaluation methods and results of this study could be useful to the water resource management and utilization in the area and other similar areas.

Response of microbial communities to supercritical CO2 and biogeochemical influences on microbially mediated CO2-saline-sandstone interactions

In this study, laboratory experiments were conducted to investigate the influences of supercritical CO2 (scCO2) on the indigenous microbial communities. Results revealed that the abundance of gene copies (mL−1) in samples and the shifts in the microbial community were strongly affected by scCO2 injection. Furthermore, Proteobacteria showed better ability of tolerance to the scCO2 injection than Firmicutes in initial period and both of them were the predominant phyla after 180days' experiments. In addition, the acidogenic mineral-attached biofilms formed by Pseudomonas stutzeri, Acinetobacter soli etc. surrounding the feldspars and clays could lower the pH in solution and accelerate the dissolution of silicate minerals. As the secondary carbon sink minerals, amount of CaCO3 and FeCO2 were observed in the microbially mediated CO2-saline-sandstone interactions, demonstrated the Fe(III)-reducing microbes (Shewanella putrefaciens, Citrobacter sp. LAR-1 and Pseudomonas fluorescens) could reduce the Fe(III) released from clays to Fe(II) and induce siderite biomineralization through metabolic processes.

Rates of CO2 Mineralization in Geological Carbon Storage.

Geologic carbon storage (GCS) involves capture and purification of CO2 at industrial emission sources, compression into a supercritical state, and subsequent injection into geologic formations. This process reverses the flow of carbon to the atmosphere with the intention of returning the carbon to long-term geologic storage. Models suggest that most of the injected CO2 will be "trapped" in the subsurface by physical means, but the most risk-free and permanent form of carbon storage is as carbonate minerals (Ca,Mg,Fe)CO3. The transformation of CO2 to carbonate minerals requires supply of the necessary divalent cations by dissolution of silicate minerals. Available data suggest that rates of transformation are highly uncertain and difficult to predict by standard approaches. Here we show that the chemical kinetic observations and experimental results, when they can be reduced to a single cation-release time scale that describes the fractional rate at which cations are released to solution by mineral dissolution, show sufficiently systematic behavior as a function of pH, fluid flow rate, and time that the rates of mineralization can be estimated with reasonable certainty. The rate of mineralization depends on both the abundance (determined by the reservoir rock mineralogy) and the rate at which cations are released from silicate minerals by dissolution into pore fluid that has been acidified with dissolved CO2. Laboratory-measured rates and field observations give values spanning 8 to 10 orders of magnitude, but when they are evaluated in the context of a reservoir-scale reactive transport simulation, this range becomes much smaller. The reservoir scale simulations provide limits on the applicable conditions under which silicate mineral dissolution and subsequent carbonate mineral precipitation are likely to occur (pH 4.5 to 6, fluid flow velocity less than 5 m/year, and 50-100 years or more after the start of injection). These constraints lead to estimates of 200 to 2000 years for conversion of 60-90% of injected CO2 when the reservoir rock has a sufficient volume fraction of divalent cation-bearing silicate minerals and confirms that when reservoir rock mineralogy is not favorable the fraction of CO2 converted to carbonate minerals is minimal over 104 years. A sufficient amount of reactive minerals is typically about 20% by volume. Our approach may allow for rapid evaluation of mineralization potential of subsurface storage reservoirs and illustrates how reservoir scale modeling can be integrated with other observations to address key issues relating to engineering of geologic systems.