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As oxidation state is often a primary determinant of the mobility of uranium in subsurface environments, a comprehensive understanding of the redox cycling of uranium is essential to predict the fate of this contaminant. The potential of iron to serve as both a reductant and an oxidant of uranium suggests that remediation strategies which primarily rely on the reduction of the uranyl ion (U(VI)) to the poorly soluble uranous ion (U(IV)) to immobilize uranium in the solid phase may be either enhanced or hindered depending on the biogeochemical transformations of iron. To identify the geochemical controls of both uranium and iron redox cycling, batch incubations with the model metal-reducing bacterium Shewanella putrefaciens were conducted with either U(VI), ferrihydrite, or both as terminal electron acceptors, and concentrations of dissolved inorganic carbon (DIC), calcium, and silica were varied to alter aqueous uranyl speciation and secondary mineralization pathways of ferrihydrite. The presence of ferrihydrite increased the pseudo-first order rate constant of initial uranium removal from solution in nearly all conditions compared to uranium-only controls due to a combination of abiotic U(VI) reduction by Fe(II) and solid-phase association of uranium with secondary mineralization products of ferrihydrite. Following an initial period of uranium reduction, U(IV) was oxidized by ferrihydrite which led to a rebound in dissolved uranium. Once ferrihydrite was completely consumed by microbial respiration and secondary mineralization, uranium was again removed from solution via reductive precipitation and association with secondary mineralization products. DIC and calcium enhanced the oxidative dissolution of U(IV) solids but did not affect the redox potential of the U(VI)/U(IV) redox couple despite the formation of aqueous uranyl carbonate and calcium-uranyl carbonate species. Instead, removal of dissolved Fe(II) from solution and the accompanied shift in the redox potential of the Fe(III)/Fe(II) redox couple was responsible for the abrupt shift in iron acting as a reductant of U(VI) to an oxidant of U(IV). The removal of dissolved Fe(II) via siderite precipitation at the highest DIC concentration and in the presence of calcium enhanced the oxidation of U(IV), and the presence of silica limited the conversion of ferrihydrite to magnetite and sustained U(IV) oxidation. A kinetic model was developed which could reproduce incubation time series of uranium and iron speciation provided the thermodynamic favorability of abiotic uranium-iron redox cycling was accounted for. The modeling exercise revealed that the non-uraninite U(IV) solids formed in the incubations have a redox potential approximately 84 mV lower than that of amorphous uraninite, highlighting the need for a thermodynamic characterization of non-uraninite U(IV) solids. The results of this study identify dissolved Fe(II) concentration as the primary geochemical control of the role of iron in uranium redox cycling at moderate to high DIC concentrations and emphasize the importance of considering this parameter when designing site-specific, in situ bioremediation strategies. [ABSTRACT FROM AUTHOR]

Managed Aquifer Recharge (MAR) is becoming an attractive water management option, with more than 223 sites operating in European countries. The quality of the produced water, available for drinking or irrigation processes is strongly depended on the aquifer's hydrogeochemical characteristics and on the MAR system design and operation. The objective of this project is the assessment of the operation efficiency of a MAR system in Cyprus. The coupling of alternative methodologies is used such as water quality monitoring, micro-scale sediment sorption experiments, simulation of groundwater flow and phosphate and copper transport in the subsurface using the FEFLOW model and evaluation of the observed change in the chemical composition of water due to mixing using the geochemical model PHREEQC. The above methodology is tested in the Ezousa MAR project in Cyprus, where treated effluent from the Paphos Waste Water Treatment Plant, is recharged into the aquifer through five sets of artificial ponds along the riverbed. Additionally, groundwater is pumped for irrigation purposes from wells located nearby. A slight attenuation of nutrients is observed, whereas copper in groundwater is overcoming the EPA standards. The FEFLOW simulations reveal no effective mixing in some intermediate infiltration ponds, which is validated by the inverse modeling simulation of the PHREEQC model. Based on the results, better control of the infiltration capacity of some of the ponds and increased travel times are some suggestions that could improve the efficiency of the system. [ABSTRACT FROM AUTHOR]

In this study, the surface electrical properties of calcium silicate hydrate (C-S-H) and their influence on ionic adsorption have been investigated to be able to predict chloride binding in slag-containing cementitious materials. The experimental data showed that the electrokinetic potential of slag cement paste (SCP) is intermediate between hydrated cement paste (HCP) and C-S-H with Ca/Si of 1.0 (C-S-H:1.0) or slag calcium hydroxide paste (SCHP). Two types of C-S-H available in the hydrated SCP for ionic adsorption are considered: one is similar to the C-S-H of HCP and other is like C-S-H:1.0. The surface complexation modeling parameters for ionic adsorption on C-S-H:1.0 were estimated by fitting the experimental data to simulation results. The estimated parameters for C-S-H:1.0 together with surface complexation modeling parameters for HCP were used to predict the adsorption of chloride on SCP. The simulation results show good agreement with experimental data and follow a Freundlich isotherm. When portland cement is partially replaced by slag, it modifies the surface electrical properties of C-S-H in addition to mineralogy and pore structure and leads to change in the chloride adsorption behavior. [ABSTRACT FROM AUTHOR]

Rare earth elements (REE) can be efficiently extracted from umbers, ferromanganese metalliferous sediments of the Troodos ophiolite (Cyprus) by simple leaching and selective precipitation, without accumulation of radioactive by-products. Umbers are dominantly composed of amorphous Fe and Mn oxides with minor goethite, quartz and zeolites, and contain 350–500  mg   kg −1 of rare earth elements and yttrium (REY), 200 times lower than many of the major REY source ores. To compensate for relatively low grades, a cost-effective extraction process was developed that utilises a weak lixiviant concentration (0.1–1.5 N) and short reaction times (5 min to 11 h). Acid solutions recover 70–85% of the initial sample REY content even at 20 °C. By contrast, extraction using ionic solutions of NaCl and ammonium sulphate proved ineffective. Acid recoveries of REY increase by nearly 10% at 70 °C and the use of different acids (HCl, HNO 3 , H 2 SO 4 ) yields comparable results. The main impurities in the leachate include Ca and Na at even the weakest acid concentration (0.1 N). However, two-step leaching method greatly reduces concentrations of impurities in the REY-rich liquor, although with REY losses approaching 20%. Purification of the leach liquor via selective precipitation of REY as an oxalate is highly efficient although pH dependent. With maximum REY precipitation (96–99%) occurring between pH 1 and 2, the precise adjustment of pH allows separation of REY from other precipitating impurities (Ca). The maximum purity of the precipitate is achieved at pH 1.1 (>65%). Strong and consistent fractionation along the lanthanide series observed during the precipitation experiments has been successfully explained using a speciation modelling software (PHREEQC). The uptake of REY by oxalate in the experiments closely follows the bell-shape distribution of REY-oxalate solid complexes stability constant (−log β (RE 2 Ox 3 ·nH 2 O)) replicating the fractionation trends observed at pH < 1.1. In addition, the modelling demonstrates that at equivalent REE concentration in solution, oxalate precipitates fractionate REY in the following order: middle REE > light REE ≫ heavy REE. This ordering and the variable degrees of uptake reflects the interplay of aqueous REY-oxalate complexes (log HOx β 1 , Ox β 1 and Ox β 2 ) with the natural fractionation induced by solid REY-oxalate stability constant distribution. Overall, the combined leaching process and selective oxalate precipitation produces a total enrichment factor ranging between 1400 and 2400 for REY from the sample to the oxalate precipitate in a simple two-step process forming a high-purity end-product of mixed REY. [ABSTRACT FROM AUTHOR]

The main Mongolian water resources are lakes whose volume has significantly reduced in recent years as a consequence of global warming. This article presents research on three different types of nearly disposed Western Mongolian lakes with a wide range of brines’ composition: Shaazgai-Nuur (soda), Davsan-Nuur (chloride) and Tonhil-Nuur (sulfate). The aim of this study was to determine the differences in geochemical evolution associated with evaporation-crystallization processes of representative lakes of various chemical types. The modelling was performed by using PHREEQC and HCh software that allows application of Pitzer’s approach for saline solutions. The modelling was performed for these natural objects for the first time. The model demonstrated that the evaporation-crystallization processes are different for soda and chloride/sulfate brines. Soda type lake maintained unchanged ion composition while chemical composition of chloride and sulfate lakes went through significant alterations according to a sequence of mineral precipitation. The calculated model did not confirm the possibility of long-term transformation from SO4-Mg brines to Cl-Mg due to evaporative concentration only; additional factors including mixing with inflow water also influence sodium and chloride increase. Close agreement between the model and the actual mineralogical data was observed, testifying to the proper software and reliability of our assumptions. [ABSTRACT FROM AUTHOR]

The electrochemical atomic layer deposition (E-ALD) growth of chalcogenides materials enables the deposition of technologically interesting ultra-thin films. However, this method raises some questions about the actual growth mechanism. We addressed one of the more interesting anomalies reported lately: the occurrence of the Zn-deficiency and of the polycrystalline thread-like overgrown structures in the E-ALD growth of Cu x Zn y S. The present study was developed using a computational speciation approach under the mass balance method. Exploiting a well-established computational approach, but uncommonly applied to the electrochemical science, we calculated the predominance charts and the equilibrium speciation of the solid phases during the electrochemical process. On this basis, we obtained a deep insight into the mechanism underlying the E-ALD process from a thermodynamic standpoint. Thus, we identified the crucial steps of the Cu x Zn y S growth leading to the anomalies object of this research. [ABSTRACT FROM AUTHOR]

For the purpose of geological carbon storage, it is necessary to understand the long-term effects of introducing CO2 and sulfur-species into saline aquifers. CO2 stripped from the flue gas during the carbon capture process may contain trace SO2 and H2S and it may be economically beneficial to inject S-bearing CO2 rather than costly purified CO2. Furthermore, reactions between the S-CO2-bearing formation brines and formation minerals will increase pH and promote further dissolution and precipitation reactions. To investigate this we model reactions in a natural analog where CO2- and SO4-H2S bearing fluids have reacted with clay-rich siltstones. In the Mid-Jurassic Carmel formation in a cap rock to a natural CO2-bearing reservoir at Green River, Utah, a 3.1 mm wide bleached alteration zone is observed at the uppermost contact between a primary gypsum bed and red siltstone. Gypsum at the contact is ~1 mm thick and shows elongate fibers perpendicular to the siltstone surface, suggesting fluid flow along the contact. Mineralogical concentrations, analyzed by Quantitative Evaluation of Minerals by SCANning electron microscopy (QEMSCAN), show the altered siltstone region comprises two main zones: a 0.8 mm wide, hematite-poor, dolomite-poor, and illite-rich region adjacent to the gypsum bed; and a 2.3 mm wide, hematite-poor, dolomite-poor, and illite-poor region adjacent to the hematite alteration front. A one-component analytical solution to reactive-diffusive transport for the bleached zone implies it took less than 20 yr to form before the fluid self-sealed, and that literature hematite dissolution rates between 10–8 and 10–7 mol/m2/s are valid for likely diffusivities. Multi-component reactive-diffusive transport equilibrium modeling for the full phase assemblage, conducted with PHREEQC, suggests dissolution of hematite and dolomite and precipitation of illite over similar short timescales. Reaction progress with CO2-bearing, SO4-rich, and minor H2S-bearing fluids is shown to be much faster than with CO2-poor, SO4-rich with minor H2S-bearing fluids. The substantial buffering capacity of mineral reactions demonstrated by the S- and CO2-related alteration of hematite-bearing siltstones at the Green River CO2 accumulation implies that corrosion of such a cap rock are, at worst, comparable to the 10 000 yr timescales needed for carbon storage. [ABSTRACT FROM AUTHOR]

This work assessed both the fractionation and the seasonal mobility variations of Ga and In in systems impacted by acidic thermal waters. This was accomplished by performing thermodynamic calculations using the PHREEQC algorithm and by assessing the activity of acidophilic iron-oxidizing bacteria. The pH of the Kusatsu thermal waters in Gunma Prefecture, central Japan, is rapidly increased following the addition of a lime suspension. After an abrupt pH increase, under which conditions free ions of Ga and In and their complexes with Cl− and SO42− exist only in negligible quantities, the majority of dissolved Ga and In is removed by sorption onto suspended hydrous ferric oxides (HFOs). These HFOs are then transported to an artificial lake without significant sedimentation along the river. Subsequently, the suspended HFOs settle out and are added to sediments without significant fractionation between Ga and In. The Tamagawa thermal waters in Akita Prefecture, northeast Japan, are also treated with lime. However, complete neutralization requires mixing with some tributary streams, leading to a gradual downstream increase in pH. Dissolved Ga is, in general, sorbed by HFOs in upstream areas, leading to wide dispersal of Ga across the entire watershed. In comparison, In is transported to the lake inlet predominantly as a Cl− complex species without significant removal along the river, with the majority being precipitated in an artificial lake, where Cl− concentrations are too low to form stable complex species with In, and thus, dissolved In is sorbed by HFOs. As a result, In is effectively concentrated within downstream lakebed sediments, whereas Ga is dispersed along the river. Seasonal variations in Ga mobility within the Tamagawa field between snowmelt and low-flow seasons are primarily controlled by pH, because hydrolysis reactions of these metals, which are related to sorption reactions, tend to occur in the upstream regions in the snowmelt season. However, under warmer conditions, HFO formation preferably occurs due to the activity of acidophilic iron-oxidizing bacteria. Thus, under similar pH variations, dissolved Ga is more effectively removed by HFOs during warmer seasons. On the contrary, because HFOs are abundantly formed in low-flow season, even under colder conditions, before In hydrolysis reaction starts to occur, In mobility is less affected by water temperature and then bacterial activity. [ABSTRACT FROM AUTHOR]

Several technologies have been developed to treat acid mine drainage (AMD) and attention is shifting towards the removal of sulphate. The formation of ettringite, a hydrous calcium aluminium sulphate mineral, is an option to treat AMD and can reduce the sulphate concentration to well below the discharge specifications. The process modelled in this study includes the formation of ettringite and the recovery of gibbsite through the decomposition of recycled ettringite. The modelling of this process was done using PHREEQC and the results presented in this paper are based on the outcome of different case studies that investigated how the process is affected by a change in parameters. These include, changing the feed water pH, altering the split fractions in the hydrocyclone for the gibbsite recovery and varying the pH for ettringite formation. [ABSTRACT FROM AUTHOR]

The first version of the P-Hydroslag model for numerical simulations of steel slag filters is presented. This model main original feature is the implementation of slag exhaustion behavior, crystal growth and crystal size effect on crystal solubility, and crystal accumulation effect on slag dissolution. The model includes four mineral phases: calcite, monetite, homogeneous hydroxyapatite (constant size and solubility) and heterogeneous hydroxyapatite (increasing size and decreasing solubility). In the proposed model, slag behavior is represented by CaO dissolution kinetic rate and exhaustion equations; while slag dissolution is limited by a diffusion rate through a crystal layer. An experimental test for measurement of exhaustion equations is provided. The model was calibrated with an experimental program made of three phases. Firstly, batch tests with 300 g slag sample in synthetic solutions were conducted for the determination of exhaustion equation. Secondly, a slag filter column test fed with synthetic solution was run for 623 days, divided into 9 cells and sampled at the end of the experiment. Finally, the column was dismantled, sampled and analyzed with XRD, TEM and SEM. Experimental column curves for pH, oPO 4 , Ca and inorganic carbon were well predicted by the model. Crystal sizes measured by XRD and TEM validated the hypothesis for homogeneous precipitation while SEM observations validated the thin crystal layer hypothesis. A preliminary validation of the model resulted in successful predictions of a steel slag filter longevity fed with real wastewater. [ABSTRACT FROM AUTHOR]

In worst-case leakage scenarios of CO2 geological storage, CO2 or brine may contaminate shallower drinking water aquifers. This work applies an advanced geochemical modeling methodology to predict and understand the effects of the aforementioned contamination scenarios. Several possibilities, such as equilibrium batch, kinetic batch, and 1D kinetic reactive transport simulations, were tested. These have all been implemented in the widely applied PHREEQC code. The production of figures and animations has been automated by R programming. The different modeling levels provide complementary information to each other. Both scenarios (CO2 or brine leakage) indicate the increase of ion concentrations in the freshwater, which might exceed drinking water limit values. The dissolution of CO2 changes the pH and induces mineral dissolution and precipitation in the aquifer and therefore changes in solution composition. Brine replacement of freshwater due to the pressure increase in the geological system induces mineral reactions as well. [ABSTRACT FROM AUTHOR]

Leaching tests with synthetic brines (25 g/L NaCl) between 25 °C and 90 °C were performed under oxic and anoxic conditions over 7 days on two pyrite-bearing siliciclastic rocks from the Lower Jurassic Hettangian and Sinemurian stages in the North German Basin. The release mechanisms of the mobile elements Al, As, Ba, Ca, Cu, Fe, Mn, Ni, Si, and Pb were studied and explained by means of numerical simulations of the leaching tests. The study was performed in the context of aquifer thermal energy storage (ATES) to improve the understanding of water-rock interactions during heat storage. Results showed that release patterns of Ba, Ca, Cu, Fe, Ni, and Pb were predominantly controlled by the dissolution of pyrite under oxic conditions and iron hydroxides under anoxic conditions. The mobilizations of Al and Mn could be explained by a combination of desorption and the dissolution of hydroxides. Si was mainly released from amorphous silica. The mobilization of Ca was governed by pH-sensitive desorption and calcite dissolution in one of the samples. Arsenic was immobile in both studied rocks. In general, elemental release was augmented by the presence of oxygen and the subsequent dissolution of pyrite and reduction of pH, which should therefore be avoided in ATES systems. [ABSTRACT FROM AUTHOR]

Groundwater sampling and piezometric measurements were carried out along two flow paths (corresponding to two transects) in Korba coastal plain (Northeast of Tunisia). The study aims to identify hydrochemical processes occurring when seawater and freshwater mix. Those processes can be used as indicators of seawater intrusion progression and freshwater flushing into seawater accompanying Submarine Groundwater Discharge (SGD). Seawater fractions in the groundwater were calculated using the chloride concentration. Hierarchical cluster analysis (HCA) was applied to isolate wells potentially affected by seawater. In addition, PHREEQC was used to simulate the theoretical mixing between two end members: seawater and a fresh-brackish groundwater sample. Geochemical conventional diagrams showed that the groundwater chemistry is explained by a mixing process between two end members. Results also revealed the presence of other geochemical processes, correlated to the hydrodynamic flow paths. Direct cation exchange was linked to seawater intrusion, and reverse cation exchange was associated to the freshwater flushing into seawater. The presence of these processes indicated that seawater intrusion was in progress. An excess of Ca, that could not be explained by only cation exchange processes, was observed in both transects. Dedolomitization combined to gypsum leaching is the possible explanation of the groundwater Ca enrichment. Finally, redox processes were also found to contribute to the groundwater composition along flow paths. [ABSTRACT FROM AUTHOR]

This study focuses on the investigation regarding impact of Ba2+ and Cd2+ on defluoridation by calcite because Spent Pot Lining (SPL, a byproduct generated during the aluminum production) leachate is a complex chemical cocktail. To better understand defluoridation processes and replicate the situation in the field, column tests were conducted. Results indicate that presence of neither Ba2+ nor Cd2+ has significant impact on fluoride retention when compared with the blank (no metals) because the relative concentration values ( F/Fo) as well as the transport parameters like retardation factor ( Rc) and distribution coefficient ( Kd) are almost the same. PHREEQC, a geochemical software, was then applied to simulate the column test processes ( i.e., 1D reactive/transport processes) based on the mechanism of CaF2 precipitation followed after calcium dissolution. As indicated in the batch reactor test, addition of Cd2+ may inhibit calcite dissolution; therefore, a new phase 'CalciteSupp' (a phase representing the suppressed solubility of calcite) was defined. Modeling results exhibit good predictions of observed data, and the log_k solubility of the hypothetical 'CalciteSupp' equilibrium phase implies the suppression of calcite dissolution in the metal samples. Moreover, saturation indices obtained by modeling indicate the precipitation of CdCO3 and BaCO3, revealing that addition of metals may influence fluoride removal at some point during the process, but has no big impact on overall defluoridation. The initial development of the PHREEQC geochemical model presented shows potential in being able to be used as a predictive tool for the design of fluoride remediation strategies, such as permeable reactive barriers, and transport parameters obtained on the basis of the column tests, and CXTFIT (a computer program for evaluating and estimating transport parameters) modeling can be applied in practical design. [ABSTRACT FROM AUTHOR]

The storage of natural gas in salt caverns can entail the risk of H 2 S generation, which in turn leads to gas pollution. H 2 S is generated by bacterial sulfate reduction. The bacteria use aqueous sulfate (aq) as an electron acceptor to oxidize the dissolved hydrocarbons and generate sulfide. Anhydrite is available in the rock salt surrounding the cavern and acts as a sulfate (aq) source. The stored natural gas, with its main component, methane, is in solubility equilibrium with the brine and is additionally delivered by diffusion into the brine. The generated H 2 S reaches the stored gas by outgassing from the brine. In this study, these processes are simulated by one- and three-dimensional hydrogeochemical diffusive mass transport models, which are based on equilibrium reactions for gas-water-rock interactions and kinetic reactions for sulfate reduction. Modelling results show that the greatest amount of H 2 S is generated in the brine. The amount of generated H 2 S (g) is mainly controlled by the amount of available sulfate (aq) as well as the rate of diffusion, which is coupled with the maximum operating live time of salt caverns. Additionally, the amount of generated and released H 2 S (g) is sensitive to the chosen kinetic rate constant. To ensure constant gas quality over time, the gas and the brine must be analyzed continuously and technical methods must be applied when the H 2 S (g) concentration increases. According to the modelling results, H 2 S (g) generation is inhibited by addition of dissolved ferrous iron to the brine. Dissolved ferrous iron reacts with sulfide-sulfur to form mackinawite (FeS (s) ) so that aqueous sulfide is no longer available for H 2 S (g) generation. Another method is the addition of NaOH to increase the pH of the brine. Then, higher fractions of generated sulfide-sulfur are transformed to free S 2− (aq) instead of H 2 S (g) and H 2 S (aq) . [ABSTRACT FROM AUTHOR]

Based on both chemical leaching and adsorption tests and a simple modelling using PHREEQC, geochemical behaviours of uranium during the ignimbrite-water interaction were evaluated mainly as a function of temperature, pH, and solution chemistry (esp., alkalinity). The main results of this work are: (1) uranium is more easily mobilized by slightly basic solution (pH 7.5) than by acidified water (pH 4.5) when relative concentrations of the main uranyl ion-calcium-carbonate species, CaUO(CO), increase from 0.6 to 90%; (2) the greatest leaching of uranium occurs at 50 °C (not at higher temperature) because the first dissociation constant of HCO is directly correlated with temperature up to about 50 °C, but decreases from 50 to 80 °C. This directly influences the concentration of HCO which is mirrored by dissolved CO variations; (3) the presence of alkalinity, total C-species and calcium controls the saturation index of sorbate solution with respect to calcite, influencing also speciation, solubility, and sorption of dissolved U; and (4) higher adsorption of uranium is obtained in tests with deionized water (90.0 ± 0.7 mg/kg, at equilibrium) compared to those performed with the natural water (26.0 ± 1.5 mg/kg, at equilibrium), strengthening the role of complexes between uranyl, HCO , and Ca ions in solution to explain the corresponding decrease in uranium adsorption. [ABSTRACT FROM AUTHOR]

Storage of CO 2 into geological formations, such as deep saline aquifers and depleted oil/gas reservoirs, is being investigated to help reduce releases to the atmosphere. A critical issue is the long-term efficacy of engineered (man-made) hydrated cement seals that surround or plug boreholes. In order to investigate the impact of CO 2 interactions with hydrated cement, the present study focuses on the alteration processes that were observed in a hydrated cement core (based on Type-G Portland cement) after laboratory experimental reaction with a CO 2 -saturated brine for 6 months. The carbonated cement block consisted of an orange outer zone, an alteration rim, and the intact cement core. The PHREEQC geochemical code was used to simulate the experimental conditions and data using 1D reactive transport modeling. The first phases to dissolve were portlandite, CS and ettringite providing calcium to solution. Three CaCO 3 polymorphs were predicted to precipitate, but the less stable of these (aragonite, vaterite) were transformed to calcite over time. According to simulations the final product after 1000 years of reaction will be 68% calcite, 18% Friedel's salt and 14% Al-Fe-Si hydro/oxides. As the reaction front moved within the sample, porosity decreased due to the formation of amorphous phases and carbonates. The proposed model is a useful tool in describing longer-term hydrated cement alteration behavior during CO 2 geological storage and to compare their results with in situ observations from field experiments. [ABSTRACT FROM AUTHOR]

We showcase a flexible, extensible yet efficient framework for reactive transport modelling, including the ability to replace “full physics” geochemical simulations with surrogate models for speedup. Surrogates are data-driven models trained on a set of pre-calculated simulations by means of machine-learning methods. We offer also an input-output-error visualization component for interactive assessment and tuning of their accuracy. Our framework, based on open source or freely available software, makes possible complex reactive transport simulations and ease further research on optimized algorithms to tackle many geoscientific problems. [ABSTRACT FROM AUTHOR]

Lacustrine carbonate deposits with spherulitic facies are poorly understood, but are key to understanding the economically important “Pre-Salt” Mesozoic strata of the South Atlantic. A major barrier to research into these unique and spectacular facies is the lack of good lacustrine spherulite-dominated deposits which are known in outcrop. Stratigraphy and petrography suggest one of the best analogue systems is found in the Carboniferous of Scotland: the East Kirkton Limestone. Here we propose a hydrogeochemical model that explains why the CaCO 3 , SiO 2 , Mg-Si-Al mineral suite associated with spherular radial calcite facies forms in alkaline lakes above basaltic bedrock. Demonstrating links between igneous bedrock chemistry, lake and spring water chemistry and mineral precipitation, this model has implications for studies of lacustrine sediments in rift basins of all ages. Using empirical and theoretical approaches, we analyze the relationship between metal mobilization from sub-surface volcaniclastic rocks and the potential for precipitation of carbonate minerals, various Mg-bearing minerals and chalcedony in a lacustrine spherulitic carbonate setting. This suite of minerals is most likely formed by in-gassing of CO 2 to a carbon-limited alkaline spring water, consistent with the reaction of alkali igneous rocks in the subsurface with meteoric groundwater. We suggest that an analogous system to that at East Kirkton caused development of the ‘Pre-Salt’ spherulitic carbonate deposits. [ABSTRACT FROM AUTHOR]

In this research, montmorillonite nanoclay (MNC) and vermiculite were used to adsorb ammonium (NH ) from simulated wastewater. The effect of organic acids, cations, and anions on adsorption of NH was also studied using batch experiments. The presence of organic acids significantly decreased the NH adsorption using both adsorbents and the reduction followed the order of citric acid > malic acid > oxalic acid. The presence of cations in wastewater could decrease the adsorption of NH and the ion exchange selectivity on the MNC and vermiculite followed the orders Mg > Ca ≥ K > Na and Mg > > Ca > Na > K, respectively. Adsorption of NH by adsorbents in the presence of sulfate (SO) was higher than those in the presence of phosphate (PO) and chloride (Cl) anions. Results indicated that MNC and vermiculite had good potential for NH removal depending on adsorbent dosage, pH, contact time, and initial NH concentration. The effect of pH on removal of NH indicated that MNC would be more appropriate as the adsorbent than vermiculite at low pH values. Kinetic analysis demonstrated that the rate-controlling step adsorption for NH by MNC and vermiculite was heterogeneous chemisorption and followed the pseudo-second-order model. The desorption experiments indicated that the adsorption of NH by adsorbents was not fully reversible, and the total recovery of adsorbed NH for MNC and vermiculite varied in the range of 72 to 94.6% and 11.5 to 45.7%, respectively. Cation exchange model (CEM) in PHREEQC program was used to simulate NH adsorption. Agreement between measured and simulated data suggested that CEM was favored in simulating adsorption of NH by clay minerals. The results indicated that MNC and vermiculite have good performance as economic and nature-friendly adsorbents that can ameliorate the water and environment quality. [ABSTRACT FROM AUTHOR]

Arsenic contamination of groundwaters in South and Southeast Asia is a major threat to public health. In order to better understand the geochemical controls on the mobility of arsenic in a heavily arsenic-affected aquifer in northern Kandal Province, Cambodia, key changes in inorganic aqueous geochemistry have been monitored at high vertical and lateral resolution along dominant groundwater flow paths along two distinct transects. The two transects are characterized by differing geochemical, hydrological and lithological conditions. Arsenic concentrations in groundwater are highly heterogenous, and are broadly positively associated with iron and negatively associated with sulfate and dissolved oxygen. The observed correlations are generally consistent with arsenic mobilization by reductive-dissolution of iron (hydr)oxides. Key redox zones, as identified using groupings of the PHREEQC model equilibrium electron activity of major redox couples (notably ammonium/nitrite; ammonium/nitrate; nitrite/nitrate; dissolved oxygen/water) have been identified and vary with depth, site and season. Mineral saturation is also characterized. Seasonal changes in groundwater chemistry were observed in areas which were (i) sandy and of high permeability; (ii) in close proximity to rivers; and/or (iii) in close proximity to ponds. Such changes are attributed to monsoonal-driven surface-groundwater interactions and are consistent with the separate provenance of recharge sources as identified using stable isotope mixing models. [ABSTRACT FROM AUTHOR]

This study focused on acid neutralization reactions and the effects of water composition on the release and mobility of metals from mine tailings. The aims of this study were to: investigate leaching of metals from neutral mine tailings, determine the factors responsible for metal leaching, and investigate potential metal filtering by the soil. Tailings and soil samples were collected from an iron mine and analyzed. Equilibrium thermodynamic data and metal fractionation were then used to predict precipitation/dissolution of minerals and ion adsorption/desorption. Three column experiments were designed. The first column was filled with tailings, while the second column contained tailings above a layer of soil; both were leached with distilled water as rainfall. The third column was packed with soil and percolated with synthetic groundwater. The results indicated that iron (Fe) and zinc (Zn) mobility are mainly controlled by precipitation-dissolution mechanisms, while sorption onto oxides and carbonates limit the mobility of copper (Cu) and nickel (Ni). Cadmium (Cd) and manganese (Mn) mobility are affected by both mechanisms. Water discharging from column 3 (soil washed with groundwater) contained high concentrations of dissolved metals, indicating that water composition played an important role in metal mobility. Buffering minerals like carbonates and hornblende, chlorite, and albite decreased acid generation. [ABSTRACT FROM AUTHOR]

This project aims at assessing the feasibility of the CO 2 -DISSOLVED concept, combining injection of dissolved CO 2 and recovery of the geothermal heat. The objective here was to identify and to quantify the thermo-hydro-geochemical processes induced by a massive injection of dissolved CO 2 into a carbonated aquifer and a clastic reservoir. For that purpose, several simulations were performed using the MARTHE-PHREEQC and MARTHE-REACT reactive transport codes. Simulation results corroborate the expected great reactivity of the carbonated reservoir under the injection of CO 2 -rich brine. Numerical results provide new arguments confirming the feasibility of the CO 2 -DISSOLVED approach. [ABSTRACT FROM AUTHOR]

REPP-CO 2 is a Czech-Norwegian project representing a major step towards CO 2 storage in the Czech Republic. The project is an advancement towards a pilot project in geological settings with large CO 2 storage potential in the future. The presented work is focused mainly on static long-term laboratory experiments in CO 2 -rock-brine systems. Both equilibrium and kinetic geochemical modelling studies on CO 2 -rock-brine interaction were consequently carried out using geochemical program PHREEQC. The results from experiments and simulations could support the risk assessment argumentation of the storage site performance. [ABSTRACT FROM AUTHOR]

Geologic storage of CO 2 is one option for avoiding CO 2 emissions from a large-scale point source such as a thermal power plant and a gas refinery. The alteration of well materials by CO 2 under reservoir conditions requires characterization because the wells are the main possible leakage pathways for CO 2 from a geological reservoir. This paper presents a numerical modeling of interaction experiments involving a composite well sample formed from steel casing surrounded by Portland cement, itself surrounded by sandstone and CO 2 -saturated brine at 10 MPa and 50 °C during a period of up to 8 weeks, as reported by Mito et al. (2015). A reactive-transport model was developed to simulate diffusion of the CO 2 -saturated brine in the well sample and the resulting successive dissolution/precipitation reactions in the sandstone, cement and steel. The observed changes in mineralogy (which primarily consist of dissolution of portlandite and Ca-rich CSH phases and precipitation of calcite, amorphous silica and zeolite) and the associated evolution in brine composition were reproduced by the model. A buffering role of sandstone on the cement degradation was evidenced, thus avoiding the re-dissolution of calcite usually observed in experiments with direct interaction between cement and CO 2 -saturated brine. Interestingly, the model results also noted a possible perturbation in the measured pH and Ca content due to CO 2 outgassing during solution sampling. The Si behavior control linked with the uncertainty in zeolite stability is also discussed. [ABSTRACT FROM AUTHOR]

The Lower Cretaceous presalt section of the Kwanza Basin (Angola) is in the spotlight following the discoveries of petroleum systems in this basin, and more generally in the South Atlantic. These systems are mostly composed of continental carbonates in close association with volcanic rocks. This work is focused on the study of an offshore Kwanza presalt volcanic sequence characterized as Valanginian trachytic subaerial lava flows. A detailed petrological analysis of the altered trachyte in association with fluid inclusion microthermometry was conducted in order to depict the initial mineralogy (albite, sanidine, titanomagnetite) and obtain a paragenetic sequence (quartz, siderite, kaolinite, calcite). Thermodynamic equilibrium modelling of the trachytes alteration by meteoric fluids, over a range of temperatures (25 °C–200 °C) and CO 2 partial pressure (pCO 2 : 0.01 mbar to 100 bar), were performed with PHREEQC, and compared to the observed paragenetic sequence. Some numerical simulations reflect the observed paragenesis. As a result, the pCO 2 is constrained by the occurrence of siderite (from 0.1 bar at 50 °C to 30 bar at 125 °C) and kaolinite (from 0.2 bar at 50 °C to 1.2 bar at 125 °C). The simulations emphasize the need for a high pCO 2 in the hydrothermal system, to achieve the observed trachyte transformation. After reaching equilibrium with the trachytes, the simulated fluids highlight a mid-alkaline to near neutral pH with high Fe, HCO 3 +CO 3 , and alkali concentrations. The palaeofluids could have evolved from Ca- and Mg-rich to Ca- and Mg-poor with increasing temperature. Inversely, Si concentrations are positively correlated with increasing temperatures. This methodology, integrating a petrological approach and numerical simulations, proves to be a powerful tool leading to better understanding of the proxies (pCO 2 , temperature, redox conditions) controlling paragenesis. To push further, these simulations are also a step toward improved understanding of palaeofluid evolutions in presalt systems and better prediction of reservoir quality. [ABSTRACT FROM AUTHOR]

AOD (argon oxygen decarburization) slag, which is the by-product of the stainless steel refining process, is a recyclable slag because of its high content of calcium and silicon. The leaching toxicity cannot be ignored in the recycling process because the slag contains a certain amount of Cr. In this study, the mineral analysis, batch leaching tests and thermodynamic and kinetic modeling by PHREEQC combined with FactSage software were performed to explore the influence of the dissolution of primary minerals and the precipitation of secondary minerals on the elution of Cr from AOD slag. The results indicated that the main minerals in the original AOD slag are larnite, merwinite, pyroxene and periclase. Cr was dispersed in the mineral phases mentioned above. The simulation of Cr leaching controlled by Cr(III)-hydroxide corresponded better to the batch leaching tests, while the Cr leaching controlled by chromite or double control was underestimated. Increasing the L/S ratio enhances the pH of the leachate and restrains the elution of Cr from the AOD slag. [ABSTRACT FROM AUTHOR]

Karst collapse columns, as a potential water passageway for mine water inrush, are always considered a critical problem for the development of deep mining techniques. This study aims to identify the mixing process of groundwater deriving two different limestone karst-fissure aquifer systems. Based on analysis of mining groundwater hydrogeochemical properties, hydraulic connection between the karst-fissure objective aquifer systems was revealed. In this paper, piper diagram was used to calculate the mixing ratios at different sampling points in the aquifer systems, and PHREEQC Interactive model (Version 2.5, USGS, Reston, VA, USA, 2001) was applied to modify the mixing ratios and model the water-rock interactions during the mixing processes. The analysis results show that the highest mixing ratio is 0.905 in the C12 borehole that is located nearest to the #2 karst collapse column, and the mixing ratio decreases with the increase of the distance from the #2 karst collapse column. It demonstrated that groundwater of the two aquifers mixed through the passage of #2 karst collapse column. As a result, the proposed Piper-PHREEQC based method can provide accurate identification of karst collapse columns' water conductivity, and can be applied to practical applications. [ABSTRACT FROM AUTHOR]

The disposal of high-level radioactive waste (HLW) in deep geological repository requires reliable data including the solubility and speciation of americium and other actinides in safety assessment. In this work, a generic thermodynamic equilibrium model using the geochemical codes PHREEQC-3 and PhreePlot was used to investigate the solubility of americium in the groundwaters of Beishan in the possible ranges of pH (6–11) and P CO2 (10−5.5 - 10−2.5 bar). The pH-logP CO2 diagram of solid phases and aqueous species of Am was computed and plotted with Am solubility contour lines. The results indicate that the solubility of Am under the repository far-field conditions is dependent on pH and under the near-field conditions on pH and P CO2. Based on our calculations, we have determined that AmCO 3 OH·0.5H 2 O(cr) is thermodynamically stable in both the near- and far-field conditions, leading to an Am solubility of 4.0 × 10−9 mol/kg and 1.5 × 10−8 mol/kg under, respectively, the near- and far-field conditions of the proposed Beishan repository. However, the amorphous AmCO 3 OH(am, hyd) phase, while thermodynamically metastable, is expected to persist for an extended period of time, possibly hundreds of years or more after the closure of a repository. As such, it is assumed to be the solid phase that limits the solubility of americium, leading to much enhanced Am solubilities of 6.3 × 10−7 and 2.4 × 10−6 mol/kg for the Beishan near- and far-field conditions, respectively. Therefore, solely based on the solubility data, Am could be more toxic than the natural uranium in the Beishan granitic groundwater within ∼5000 years after the proposed repository's closure. It is thus important to ensure that Am is well contained by the engineered barriers within this timeframe. Since Am solubilities in Beishan groundwater are rather low, the effect of sorption and colloidal formation could be potentially more important for Am migration in groundwaters, however further studies are required before these processes can be properly evaluated. • PHREEQC-3 was used with evaluated thermodynamic data to establish the solubility and speciation of Am in Beishan groundwaters. • The solid phase AmCO 3 OH·0.5H 2 O(cr) was found to be stable under both near- and far-field conditions in Beishan. • The solubility limiting solid phase in Beishan groundwater was assumed to be the metastable AmCO 3 OH(am, hyd). • Am solubility was calculated to be 6.3 × 10−7 mol/kg and 2.4 × 10−6 mol/kg under near- and far-field conditions, respectively. • If not contained, Am could pose a health hazard in Beishan groundwater within ∼5000 y after the proposed repository's closure. [ABSTRACT FROM AUTHOR]

This study focuses on determining significant controlling factors of chemical consequences, inverse geochemical modeling, water quality, and human health risk in the Varuna River basin of India. The study interprets that according to pH, total dissolved solids, and total hardness, the maximum number of groundwater samples are alkaline, fresh, and have substantial hardness. The abundance of major ions follows a pattern: Na > Ca > Mg > K, and HCO 3  > Cl > SO 4  > NO 3  > F. Piper diagram shows that Ca-Mg-HCO 3 facies are predominant during both seasons. Na-normalized molar ratios of HCO 3 /Na, Mg/Na, and Ca/Na are 0.62, 0.95, and 1.82 (pre-monsoon) and 0.69, 0.91, and 1.71 (post-monsoon), respectively, elucidating the coupled silicate and carbonate weathering (dolomite dissolution) sources. The Na/Cl molar ratio is 5.3 (pre-monsoon) and 3.2 (post-monsoon), indicating silicate alteration as the primary process rather than halite dissolution. The chloro-alkaline indices confirm the presence of reverse ion- exchange. Geochemical modeling using PHREEQC identifies the formation of secondary kaolinite minerals. The inverse geochemical modeling categorizes the groundwaters along the flow path from recharge area waters (Group I: Na-HCO 3 -Cl), transitional area waters (Group II: Na-Ca-HCO 3 ), and discharge area waters (Group III: Na-Mg-HCO 3 ). The model demonstrates the prepotency of water-rock interactions in pre-monsoon justified by the precipitation of Chalcedony and Ca-montmorillonite. The mixing analysis shows that in the alluvial plains, groundwater mixing is a significant hydrogeochemical process that affects groundwater quality. The Entropy Water Quality Index ranks 45% (pre-monsoon) and 50% (post-monsoon) of samples as an excellent category. However, the non-carcinogenic health risk assessment shows that children are more susceptible to fluoride and nitrate contamination., (© 2023. The Author(s), under exclusive licence to Springer Nature B.V.)