1,721,027 research outputs found
Estimation of nucleation and growth parameters from in situ Raman spectroscopy in carbonate systems
No full textThis paper presents a mathematical model to describe precipitation in carbonate systems. The model combines the population balance equation with geochemistry, a multivariate kinetics model, and the particle size distribution reconstruction technique. In situ Raman spectroscopy data recorded during unseeded magnesite precipitation were used to validate the model. Nucleation and growth rates were described by empirical equations based on classical nucleation theory and the birth-and-spread growth mechanism. The parameters were estimated through the inversion of the model on Raman spectroscopy data without calibration. The model agrees very well with the experiments and upon optimization the particle size distribution of the produced crystals was determined through Laplace transformation technique. In situ Raman spectroscopy is a fundamental tool to monitor precipitation and solution composition. The model presented here allows to extract quantitative information from a Raman spectra recorded in a chemically complex solution
Transport of produced water through reactive porous media
No full textDuring hydraulic fracturing (or fracking) large volumes of wastewater (flow-back and produced water) are generated, which are naturally rich in heavy metals and radionuclides, such as radium. Spills may occur during operations and contaminate the groundwater. Therefore, there is an urgent need to identify a practice that can mitigate the negative impact of accidental leaks on water resources. Here, we present an experimental and modeling work on the transport of alkaline earth elements in produced water, which are congeners of radium, namely, barium (Ba2+), strontium (Sr2+), calcium (Ca2+), and magnesium (Mg2+) in addition to sodium (Na+). Column-flood tests were conducted using produced water from a shale-gas site and reactive porous media made of ubiquitous minerals such as sand, hydrous ferric oxide, activated alumina, and manganese oxide. In all cases, no retardation of the ions was observed at the salinity conditions of the produced water, but strong retardation in the pH front was measured, indicating that adsorption indeed occurred. When using manganese oxide and upon dilution of produced water, the concentration fronts of all major divalent cations were retarded. However, a fast wave of solute, traveling at the average flow velocity, emerged. This phenomenon confirmed that significant adsorption occurred under those conditions. But, pH-dependent adsorption and hydrodynamic dispersion favored fast solute transport. Overall, these results suggest that manganese oxide could be used as a reactive material in the lining of temporary storage tanks and in the well cases in order to retard the migration of the major toxic elements in produced water. However, mixing must be controlled to avoid the emergence of an instability at the concentration fronts favoring the formation of fast waves
Measuring and modeling nanoparticle transport by foam in porous media
No full textIn this paper, an experimental study of nanoparticle transport by foam is presented. Bubbles made of N2-gas were stabilized with either a cationic surfactant (Cetyl Trimethyl Ammonium Bromide, CTAB), silica nanoparticles, or a combination of them. The concentrations of the surface active materials were selected upon foamability and stability tests. Column-flood tests were run until steady-state changing nanoparticle concentration, foam quality (fg), and flow rate. A synergistic behaviour of surfactant and nanoparticles help the formation of a strong foam. The measurements were used to validate a mechanistic model, presented in our earlier work (Li and Prigiobbe, 2020), which couples foam and nanoparticles transport with agglomeration and extended-DLVO theory. The model agrees well with the measurements and results show that an high-quality (ca. 90% gas fraction) can be used to carry nanoparticles and the efficient increases with flow velocity. This opens the opportunity for the application of foam as a carrier of nanoparticles in subsurface applications such as the remediation of contaminated sites and makes the model a valuable tool to design and predict such operations
Modeling Nanoparticle Transport in Porous Media in the Presence of a Foam
No full textNano-remediation is a promising in situ remediation technology. It consists in injecting reactive nanoparticles (NPs) into the subsurface for the displacement or the degradation of contaminants. However, due to the poor mobility control of the reactive nanoparticle suspension, the application of nano-remediation has some major challenges, such as high mobility of the particles, which may favor override of the contamination, and particle aggregation, which can lead to a limited distance of influence. Previous experimental studies show the potential of combining nano-remediation with foam flooding to overcome these issues. However, in order to design and optimize the process, a model which couples nanoparticle and foam transport is necessary. In this paper, a mechanistic model to describe the transport of NPs with and by a foam is presented. The model considers the delivery of nanoscale zero-valent iron (nZVI) and accounts for the processes of aggregation, attachment/detachment, and generation/destruction. Simulations show that when NPs are dispersed in the liquid phase, even in the presence of a foam, they may travel much slower than the NPs carried by the foam bubbles. This is because the nanoparticles in suspension are affected by the attachment onto the rock walls and straining at the pore throats. When the nanoparticle surface is, instead, modified in order to favor their adsorption onto the gas bubbles, NPs are carried by the foam without retardation, except for the small fraction suspended in the liquid phase. Moreover, very stable high-quality foam (fg), i.e., 80–90 vol% of gas, can be attained using properly surface-modified nZVI (i.e., a nanoparticle-stabilized foam), allowing a significant reduction of water for the operation, while increasing the efficiency of nZVI delivery, even in a low-permeability medium within the shallow subsurface
Precipitation of Mg-carbonates at elevated temperature and partial pressure of CO2
No full textIn this paper, we present an experimental study on the precipitation of magnesium carbonates (Mg-carbonates) from a solution containing magnesium chloride and sodium carbonate and in the presence of supercritical carbon dioxide. We performed homogeneous (unseeded) batch precipitation experiments at 100bar of CO2 and at 90°C, 120°C, and 150°C. The system was monitored with online temperature and pressure sensors and an online Raman spectroscopy probe. We investigated the effect of temperature and solution composition, i.e., supersaturation ratio, pH, speciation, ionic strength, and water activity on the mechanism and the kinetics of the precipitation of Mg-carbonates. Raman spectroscopy measurements allowed us to follow the temporal evolution of the solution and suspension composition and showed two Mg-carbonates can form under the investigated conditions, i.e., magnesite and hydromagnesite. The precipitation of these two phases occurred between pH 5.5 and 6.5 and was influenced by temperature, supersaturation ratio with respect to magnesite (SM) and hydromagnesite (SH), and the initial concentration of magnesium.At all investigated temperatures, we observed direct precipitation of magnesite while at 120°C and 150°C also simultaneous precipitation of magnesite and hydromagnesite followed by the transformation of the latter into the former. Under highly supersaturated conditions with respect to magnesite, SM as large as 20, magnesite nucleated rapidly when the system was also supersaturated with respect to hydromagnesite, SH as large as 1.5; whereas no nuclei formed within 20h, otherwise. The analysis of the data collected at 120°C and at 150°C highlighted that the change in the type of mechanism was associated with the initial supersaturation ratios and the initial concentration of magnesium in solution. At 120°C, the transformation process lasted for 2h, slowing down the formation of magnesite, despite of the large SM; whereas at 150°C, the transformation process was only 5min long, without affecting magnesite precipitation. © 2013 Elsevier B.V
Measuring and modeling the influence of salinity change on the transport behaviour of Escherichia coli through quartz sand
No full textPathogenic bacteria can be discharged in the environment through natural as well as anthropogenic activities. Once in the environment, they may contaminate soil and sediments and migrate towards water bodies. Transient chemical conditions may occur in soil/sediments and favor mobilization of bacteria, e.g., upon the reduction of salinity (or ionic strength). However, the magnitude of this phenomenon and its relationship with particle size is not well understood, yet. In this work, we investigated the transport of Escherichia coli under variable salinity conditions (between 1 and 20 part per thousand, ppt) and for different soil grain sizes (between 150 and 710 μm). A model developed in our group was applied in this work. It couples bacteria and salinity transport equations in order to account for transient water composition in the description of bacteria migration. The model was calibrated and validated with laboratory experiments. The tests were monitored continuously with UV–Vis spectroscopy, which allowed to record highly resolved concentration fronts. The results show that salinity increases the retardation of the bacteria. Upon salinity drop, a release of bacteria occurs forming a peak whose magnitude increases with salinity change. This effect becomes more important as the grain size decreases. Simulations suggest that the dominant retention mechanism is attachment for coarse sand and straining for fine sand. The retention can be reversed as the salinity is reduced causing a sudden bacteria mobilization. Such a behaviour may have important implications on microbial contamination of water bodies when soil/sediments undergo transient chemical conditions
Quantification of sewer system infiltration using δ18O hydrograph separation
No full textThe infiltration of parasitical water into two sewer systems in Rome (Italy) was quantified during a dry weather period. Infiltration was estimated using the hydrograph separation method with two water components and δ18O as a conservative tracer. The two water components were groundwater, the possible source of parasitical water within the sewer, and drinking water discharged into the sewer system. This method was applied at an urban catchment scale in order to test the effective water-tightness of two different sewer networks. The sampling strategy was based on an uncertainty analysis and the errors have been propagated using Monte Carlo random sampling. Our field applications showed that the method can be applied easily and quickly, but the error in the estimated infiltration rate can be up to 20%. The estimated infiltration into the recent sewer in Torraccia is 14% and can be considered negligible given the precision of the method, while the old sewer in Infernetto has an estimated infiltration of 50%. © IWA Publishing 2009
Mechanistic Study of Radium Adsorption onto Goethite
No full textRadium (Ra2+) is a radioactive element with a long half-life. It is used in industry and is often found in shallow aquifers due to natural or anthropogenic leakage or spill of brine from deep subsurface. A major factor influencing the transport and fate of Ra2+ in water is the adsorption/desorption process onto soil particles. Goethite, a ubiquitous natural mineral generally present as a coating of soil particles, contributes significantly to the adsorption of Ra2+ in the subsurface. However, the chemical reactions of adsorption of Ra2+ onto goethite are not well-established, yet. Previous studies reported that Ra2+ creates either tetradentate or monodentate complexes on goethite and no distinction between outer-sphere and inner-sphere types of complexes were made. Knowledge of the type and structure of Ra-goethite surface complexes is important to predict the behavior and the fate of Ra2+ in the subsurface, e.g., upon a spill and during the remediation of a contaminated site. Here, the adsorption of Ra2+ onto goethite is investigated by density functional theory (DFT) calculations. By using a combination of geometric structure, adsorption energy, and electronic state (i.e., density of states and magnetic moment) analyses, the outer-sphere adsorption was found to dominate Ra2+ complexation on goethite, suggesting a significant effect of salinity on Ra2+ transport in the subsurface. Inner-sphere adsorption configuration was not observed to be thermodynamically favorable, resulting from ionic rather than covalent interaction between Ra2+ and goethite. Based on these results, a surface complexation model was developed and validated successfully with literature data. This study provides insights into the mechanism of Ra2+ adsorption on soils containing goethite and provides chemical reactions of Ra-goethite surface interaction that can be coupled with a transport model to predict Ra2+ migration in the subsurface
Dissolution of olivine in the presence of oxalate, citrate, and CO2 at 90 °C and 120 °C
No full textIn this article, we report the results from a study of olivine dissolution kinetics under operating conditions suitable for ex situ aqueous mineral carbonation for CO2 storage. We studied the effect of oxalate and citrate ions on the dissolution of gem-quality San Carlos olivine (Mg1.82Fe0.18SiO4). Flow-through experiments were performed at 90°C and 120°C, at fCO2 between 4 and 81bar, with a solution containing either sodium oxalate or sodium citrate in a molality range between 10-3 and 10-1. The pH was varied between 2 and 7 by adding HCl, LiOH, and adjusting fCO2. At all investigated temperatures and for pH values in a broad range, both sodium oxalate and sodium citrate increased dissolution rate with the strongest effect up to one order of magnitude in presence of 0.1m of oxalate, at 120°C, and above pH 5. The enhancement effect was primarily ascribed to the oxalate or citrate ions that are the dominant species in this pH range. The overall dissolution process was described using the population balance equation (PBE) coupled with a mass balance equation to account for the evolution of the particle size distribution (PSD) of olivine. Far from equilibrium conditions for dissolution were established in all the experiments in order to achieve a surface-reaction controlled mechanism. We described the reaction with a surface complexation model, which assumes adsorption of a proton and of an oxalate (citrate) ions (proton and oxalate) on adjacent sites in order to enhance dissolution, and we derived a dissolution rate equation in presence of oxalate:r=r*1+KHaHn1+ΒKXaX1+KXaX,where r* is the specific dissolution rate commonly used in absence of organic compounds, and KH, KX, and Β are thermodynamic and kinetic parameters. The values of these parameters have been estimated from the experimental data and the agreement between the model results and the experiments is very good. © 2011 Elsevier Ltd
Effect of ionic strength on barium transport in porous media
No full textHydraulic fracturing (or fracking) is a well stimulation technique used to extract resources from a low permeability formation. Currently, the most common application of fracking is for the extraction of oil and gas from shale. During the operation, a large volume of brine, rich in hazardous chemicals, is produced. Spills of brine from wells or pits might negatively impact underground water resources and, in particular, one of the major concerns is the migration of radionuclides, such as radium (Ra2+), into the shallow subsurface. However, the transport behaviour of Ra2+ through a reactive porous medium under conditions typical of a brine, i.e., high salinity, is not well understood, yet. Here, a study on the transport behaviour of barium (Ba2+, congener of radium) through a porous medium containing a common mineral such as goethite (FeO(OH)) is presented. Batch and column flood tests were carried out at conditions resembling the produced brine, i.e., large values of ionic strength (I), namely, 1 to 3 mol/kg. The measurements were described with the triple layer surface complexation model coupled with the Pitzer activity coefficient method and a reactive transport model, in the case of the transport tests. The experimental results show that the adsorption of Ba2+ onto FeO(OH) increases with pH but decreases with I and it becomes negligible at the brine conditions. Moreover, even if isotherms show adsorption at large I, at the same conditions during transport, Ba2+ travels without retardation through the FeO(OH) porous medium. The triple layer model agrees very well with all batch data but it does not describe well the transport tests in all cases. In particular, the model cannot match the pH measurements at large I values. This suggests that the chemical reactions at the solid-liquid interface do not capture the mechanism of Ba2+ adsorption onto FeO(OH) at large salinity. Finally, this study suggests that barium, and potentially its congeners, namely, radium, calcium, magnesium, and strontium, may travel at the average flow velocity through a soil where the dominant reactive mineral is goethite
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