Ab initio mechanism revealing for tricalcium silicate dissolution

Dissolution of mineral in water is ubiquitous in nature and industry, especially for the calcium silicate species. However, the behavior of such a complex chemical reaction is still unclear at atomic level. Here, we show that the ab initio molecular dynamics and metadynamics simulations enable quantitative analyses of reaction pathways, and the thermodynamics and kinetics of calcium ion dissolution from the tricalcium silicate (Ca3SiO5) surface. The calcium sites with different coordination environment leads to different reaction pathways and free energy barriers. The low free energy barriers lead to that the detachment of calcium ions is a ligand exchange and auto-catalytic process. Moreover, the water adsorption, proton exchange and diffusion of water into the surface layer accelerate the leaching of calcium ions from the surface step by step. The discovery in this work thus would be a landmark for revealing the mechanism of cement hydration.

Weak impact of microorganisms on Ca, Mg-bearing silicate weathering

Assessment of the microbial impact on mineral dissolution is crucial for a predictive understanding of basic (Ca, Mg bearing) silicate weathering and the associated CO2 consumption, bioerosion, and CO2 storage in basaltic rocks. However, there are controversies about the mechanism of microbial effect, which ranges from inhibiting via nil to accelerating. Here we studied diopside interaction with the heterotrophic bacterium Pseudomonas reactants and the soil fungus Chaetomium brasiliense using a combination of mixed-flow and batch reactors and in situ (AFM) and ex situ (SEM) microscopy. The results provide new nano-level insights into the degree to which microorganisms modify silicate dissolution. Taking into account negligible effects of organic ligands on diopside dissolution as reported earlier, we conclude that the microbial effect on Ca-Mg silicates is weak and the acceleration of dissolution of “basic” silicate rocks in the presence of soil biota is solely due to pH decrease in porewaters.

Hydrogeochemistry and Groundwater Quality Assessment in the High Agri Valley (Southern Italy)

The High Agri Valley (southern Italy) is one of the largest intermontane basin of the southern Apennines affected by intensive agricultural and industrial activities. The study of groundwater chemical features provides much important information useful in water resource management. In this study, hydrogeochemical investigations coupled with multivariate statistics, saturation indices, and stable isotope composition (δD and δ18O) were conducted in the High Agri Valley to determine the chemical composition of groundwater and to define the geogenic and anthropogenic influences on groundwater quality. Twenty-four sampling point ( including well and spring waters) have been examined. The isotopic data revealed that groundwater has a meteoric origin. Well waters, located on recent alluvial-lacustrine deposits in shallow porous aquifers at the valley floor, are influenced by seasonal rainfall events and show shallow circuits; conversely, spring waters from fissured and/or karstified aquifers are probably associated to deeper and longer hydrogeological circuits. The R -mode factor analysis shows that three factors explain 94% of the total variance, and F1 represents the combined effect of dolomite and silicate dissolution to explain most water chemistry. In addition, very low contents of trace elements were detected, and their distribution was principally related to natural input. Only two well waters, used for irrigation use, show critical issue for NO3- concentrations, whose values are linked to agricultural activities. Groundwater quality strongly affects the management of water resources, as well as their suitability for domestic, agricultural, and industrial uses. Overall, our results were considered fulfilling the requirements for the inorganic component of the Water Framework Directive and Italian legislation for drinking purposes. The water quality for irrigation is from “good to permissible” to “excellent to good” although salinity and relatively high content of Mg2+ can occasionally be critical.

Retrograde carbon sequestration in orogenic complexes: a case study from the Chinese Southwestern Tianshan

<p>The interaction between ascending carbonic fluids and rocks at shallow depths in orogenic systems plays an important role in carbon flux regulation. In subduction zones, most works have focused on processes related to carbon release from the subducting slab or sequestration via high-pressure (HP) carbonation of mafic or ultramafic lithologies. A significant fraction of the carbonic fluids released by deep metamorphic reactions can also reach orogenic complexes and react with crustal and exhumed metamorphic rocks. However, the amount of fluid-mediated carbonation that may take place at crustal depths in orogenic complexes is still poorly constrained.</p><p>We report the occurrence of retrograde mafic eclogites and metasomatic marbles in UHP units in the Chinese Tianshan orogenic belt. The mafic eclogites recorded two successive, superimposed metamorphic–metasomatic stages: graphite precipitation along fractures and veins at eclogite facies (Stage#1) and pervasive rock carbonation (i.e., Stage#2: silicate dissolution and carbonate precipitation) at retrograde amphibolite to greenschist facies. This work focuses on Stage#2 carbonation, which consists of the transformation of Stage#1 graphite-bearing eclogites into carbonate + paragonite (± zoisite) + quartz. We present field, microstructural, petrological, and geochemical results of carbonic fluid–rock interactions affecting exhumed mafic eclogites. These results are supported by thermodynamic modeling for low-pressure carbonation of mafic eclogite obtained by means of EQ3/6 and the Deep Earth Water model. Carbon and oxygen isotopic data and thermodynamic modeling suggest an external metasedimentary source for the Stage#2 carbonation. This deep carbon sequestration event can be referred to retrograde, greenschist-facies conditions at about 10 kbar and 450 °C, and redox conditions similar or more oxidized than the quartz–fayalite–magnetite (QFM) buffer. Our findings provide new insights into the reactivity of metastable, exhumed metamafic rocks with ascending carbonic fluids in orogenic systems. We conclude that retrograde, fluid-mediated rock carbonation can significantly impact on carbon fluxes from active collisional belts. </p>

Hydrochemical characteristics and water quality evaluation of shallow groundwater in Suxian mining area, Huaibei coalfield, China

The aim of this study is to evaluate the hydrogeochemical characteristics and water environmental quality of shallow groundwater in the Suxian mining area of Huaibei coalfield, China. The natural formation process of shallow groundwater in Suxian is explored using Piper trilinear charts and Gibbs diagrams, and by examining the ratios between the major ions. United States Salinity Laboratory (USSL) charts, Wilcox diagrams, and the water quality index (WQI) are further employed to quantify the differences in water quality. The results reveal that the main hydrochemical facies of groundwater are HCO3–Ca, and that silicate dissolution is the main factor controlling the ion content in shallow groundwater. The USSL charts and Wilcox diagrams show that most of the water samples would be acceptable for use in irrigation systems. The WQI results for each water sample are compared and analyzed, and the quality of groundwater samples around collapse ponds is found to be relatively poor.

Control of aquifer weathering degree to the groundwater chemical composition in Wates Coastal Aquifer, Yogyakarta, Indonesia

The studied coastal aquifer has been encountered a complex hydrochemical evolution. Interesting aspect is the occurrence of groundwater facies with different water chemistries in close proximity to each other. Water salinity is reported from local parts away from shore. This research was conducted for assessment of aquifer weathering and its control on groundwater chemistry. An investigation of major ions in 27 water samples was performed with geostatistics, graphical method, mineral saturation index and mass balance transport to identify the groundwater evolution. Geochemical analysis of sediments was made of 8 samples, to estimate the weathering degree using chemical index of alteration and chemical index of weathering. Silicate dissolution was promoted by low weathering, which was reflected in low ions dominated by calcium and bicarbonate in fresh water. It leaded to fine sands. Silts were dominant under moderate weathering. Sodium and calcium were higher than fresh water. Bicarbonate was still highest concentration. Main water was the mixed type which was indicated by moderate ions. Clays were formed by high weathering. Water salinity with highest dissolved solids was caused by halite dissolution and calcium adsorption on clays. Overall, groundwater chemistry in the study area is locally characterized by prevailing lithology in grouping patterns.