Ion Concentration Profiles Research Articles

Steady-state modeling of water-splitting and multi-ionic transport of skim milk electro-acidification by bipolar membrane electrodialysis

Electrodialysis has become a relevant technology in promoting sustainability within the food industry. Bipolar membrane electrodialysis offers an efficient and eco-friendly alternative for skim milk acidification, eliminating the need for added acids that affect milk composition and properties. For the first time, this study presents a comprehensive 2-D computational model to investigate the multi-ionic transport and dynamics of skim milk electro-acidification using bipolar membrane electrodialysis. The model is based on conservation equations for mass-charge transport, coupled with the description of water-splitting through the second Wien effect. The primary focus of the analysis was on the skim milk pH evolution and the concentration profiles of the major ions. The results showed that ion concentration values varied due to concentration polarization and differences in ion mobilities. The simulations were compared with experimental data, showing reasonable agreement, particularly for Ca2+ ion concentration. Despite excluding organic components in its analysis, this model offers a novel and valuable approach to the study of skim milk electro-acidification using bipolar membrane electrodialysis, providing essential insights for process understanding and optimization.

Magnetophoresis of paramagnetic metal ions in porous media.

We report a numerical investigation of the magnetophoresis of solutions containing paramagnetic metal ions. Using a simulated magnetic field of a superconducting magnet and the convection-diffusion model, we study the transport of transition metal salts through a porous medium domain. In particular, through a detailed comparison of the numerical results of magnetophoretic velocity and ion concentration profiles with prior published experiments, we validate the model. Subsequent to model validation, we perform a systematic analysis of the model parameters on the magnetophoresis of metal ions. Magnetophoresis is quantified with a magnetic Péclet number Pem. Under a non-uniform magnetic field, Pem initially rises, exhibiting a local maximum, and subsequently declines towards a quasi-steady value. Our results show that both the initial and maximum Pem values increase with increasing magnetic susceptibility, initial concentration of metal solutes, and ion cluster size. Conversely, Pem decreases as the porosity of the medium increases. Finally, the adsorption of metal salts onto the porous media surface is modeled through a dimensionless Damkohler number Daad. Our results suggest that the adsorption significantly slows the magnetophoresis and self-diffusion of the paramagnetic metal salts, with a net magnetophoresis velocity dependent on the kinetics and equilibrium adsorption properties of the metal salts. The latter result underscores the crucial role of adsorption in future magnetophoresis research.

Entropy production during ion exchange of glass

AbstractThe thermodynamics and kinetics of interdiffusion are analyzed for the case of glass ion exchange, a process by which alkali ions from a glass inter‐diffuse with different, usually larger, alkali ions from a surrounding salt bath. The spontaneity of the ion exchange reaction is driven by entropy production. This paper details the first derivation and quantification of the entropy produced during this process. We present models for the evolution of the ion concentration profile for both common and extreme conditions, validate against experimental literature, and derive the entropy produced due to ion exchange as a function of time, temperature, composition, diffusivity, and glass and salt bath dimensions. Results reveal that when considering typical industrial parameter sets, the entropy produced in the glass accounts for <10% of the total entropy production, with the entropy produced in the salt bath being the dominant factor driving reaction spontaneity. Using the equations derived in this work, one can calculate concentration profile and entropy production for a two‐ion interdiffusion process between a low‐viscosity liquid and materials including, by not limited to, glass‐forming systems.

Neural network predicts ion concentration profiles under nanoconfinement.

Modeling the ion concentration profile in nanochannel plays an important role in understanding the electrical double layer and electro-osmotic flow. Due to the non-negligible surface interaction and the effect of discrete solvent molecules, molecular dynamics (MD) simulation is often used as an essential tool to study the behavior of ions under nanoconfinement. Despite the accuracy of MD simulation in modeling nanoconfinement systems, it is computationally expensive. In this work, we propose neural network to predict ion concentration profiles in nanochannels with different configurations, including channel widths, ion molarity, and ion types. By modeling the ion concentration profile as a probability distribution, our neural network can serve as a much faster surrogate model for MD simulation with high accuracy. We further demonstrate the superior prediction accuracy of neural network over XGBoost. Finally, we demonstrated that neural network is flexible in predicting ion concentration profiles with different bin sizes. Overall, our deep learning model is a fast, flexible, and accurate surrogate model to predict ion concentration profiles in nanoconfinement.

Micrometer Thick Interfaces in Aqueous Biphasic Systems for Electrochemical Devices

Aqueous biphasic systems (ABSs) consist of two immiscible phases composed of only one solvent (water) with the phase separation driven by solutes such as polymers, ionic liquids and salts. Such two-phase systems have proved highly relevant in recent years for applications in electrochemical devices. Indeed, highly concentrated solutions of LiTFSI, so called “Water-in-salt” battery electrolyte, were recently found to form an ABS with LiX (with X=Cl, Br, I) aqueous solutions (Dubouis et al., ACS Cent. Sci. 2019, 5, 640-643 and Dubouis et al., J. Phys. Chem. B 2021, 125, 5365-5372). These LiTFSI-LiX ABSs enable the intercalation at high potential of halides such as Cl- or Br- into graphite, in lieu of the oxidation of water or the evolution of halogenated gas, thus enabling the assembly of efficient dual-ion batteries (Yang et al., Nature 2019, 569, 245-250). Similarly, ABS have been proposed to prevent problematic crosstalk mechanisms as observed in Li-ion/sulfur batteries (Yang et al., Proc. Natl. Acad. Sci. 2017, 114, 6197-6202) or to be used to design membraneless redox flow batteries (Navalpotro et al., Adv. Sci. 2018, 5, 1800576). However for ABS to be widely implemented in electrochemical devices, the ion transfer at liquid/liquid interface is key in obtaining good (dis)charge rate and preventing self-discharge. Thus, it is crucial to first understand the structure and chemistry of these aqueous interfaces.We studied the LiTFSI-LiCl ABS first with Fourier transform infrared spectroscopy (FT-IR) and surface tension measurement to assess ion partition and surface tension respectively. Both ion partition and surface tension are found increasing as function of increasing concentration. Such trend of the surface tension is typical of a negative adsorption of ions at the liquid/liquid interface. Using high spatial resolution Raman imaging, we were able to confirm a negative adsorption of ions by assessing the ion concentration profiles at the interface between the two aqueous phases. Indeed, we found concentration profiles of water and ions to be sigmoidal which is characteristic of a negative adsorption. Strikingly, the length of the negative adsorption is ranging from 11 to 2 μm with increasing concentrations and the Raman spectra of water and TFSI anion are continuously changing along the interface from an environment with weak hydrogen bounding network and with anion aggregate to an environment similar to diluted solutions. Moreover, when changing the cation from Li+ to H+, the temperature dependence of the phase diagram is inversed, as we could show by variable temperature nuclear magnetic resonance (VT-NMR) and micro-calorimetry, but the interface is still few microns thick. Thus, we revealed a continuous change in the chemical environment between two aqueous phases at the micrometer scale, which contrast drastically with the interface between two immiscible electrolyte solutions (ITIES) such as oil-water systems where molecularly sharp, nanometer interface are found. Such difference raise question about the impact of the thickness and the chemical composition of the interface on the dynamics of ion and electron transfer at the interface, that we are studying by electrochemical measurements. Furthermore, this work paves the way to compare liquid/liquid and solid/liquid interfaces in order to understand how ion solvation affects the interfacial ion transfer and thus enable a better engineering of the electrolyte and ABSs for better electrochemical devices.

Theory of electrolyte solutions in a slit charged pore: Effects of structural interactions and specific adsorption of ions.

In this paper, we present a continuation of our research on modeling electrolyte solutions within charged pores. We make use of the model developed by Blossey et al. [Phys. Rev. E 95, 060602 (2017)], which takes into account the structural interactions between ions through a bilinear form over the gradients of local ionic concentrations in the grand thermodynamic potential, as well as their steric interactions through the lattice gas model. The structural interactions may describe the effects of the molecular structure of ions at a phenomenological level. For example, these effects include steric effects due to non-spherical shapes of ions, their conformational lability, and solvent effects. In addition, we explore their specific interactions with the pore walls by incorporating external attractive potentials. Our primary focus is on observing the behavior of ionic concentration profiles and the disjoining pressure as the pore width changes. By starting with the local mechanical equilibrium condition, we derive a general expression for the disjoining pressure. Our findings indicate that considering the structural interactions of ions leads to a pronounced minimum on the disjoining pressure profiles at small pore widths. We attribute this minimum to the formation of electric double layers on the electrified surfaces of the pore. In addition, our results demonstrate that the inclusion of the attractive interactions of ions with the pore walls enhances this minimum and shifts it to smaller pore thicknesses. Our theoretical discoveries may be useful for those involved in supercapacitor electrochemical engineering, particularly when working with porous electrodes that have been infused with concentrated electrolyte solutions.

Depth profile analysis of 100 keV Ni ions in Si 〈100〉 substrate

Nickel implanted silicon substrate shows potential applications for the fabrication of infrared detectors, solar cells, spintronic devices as well as for the synthesis of an embedded nickel silicide layer inside Si substrates. Emerging applications of the transition metal implanted silicon structures for the fabrication of intermediate band materials motivated us to study the in-depth distribution profile of Ni ions as well as the structural changes that occur at the top of silicon substrate surface before and after annealing process. In the present work, we have carried out detailed study on the crystalline silicon substrates, implanted with Ni− ions comprising ion dose value of 5 × 1016 ions/cm2 at 100 keV implantation energy followed with the post implantation annealing at 800 °C temperature for a time duration of 2 h. The distribution profile of nickel ions inside the silicon substrate was estimated using the non-destructive simultaneous XRR-GIXRF measurements and were also compared with results obtained from the RBS and SIMS measurements. The in-depth concentration profile of Ni ions inside Si substrate derived from XRR-GIXRF measurements was found to be in close agreement with that of obtained from the RBS measurements. Furthermore, our investigations clearly showed that the annealing process causes the inward and outward diffusion of the Ni atoms inside the Si substrate, thus significantly changing the in-depth concentration profile of Ni ions in the Si matrix. The XANES and EXAFS results also demonstrate the formation of NiSi2 phase around Ni atoms in the implanted region of Si 〈100〉 substrate.

Numerical Model to Simulate Electrochemical Charging of Nanocrystal Films.

Electrochemical charging of nanocrystal films opens up new possibilities for designing quantum dot-based device structures, but a solid theoretical framework of this process and its limitations is lacking. In this work, drift-diffusion simulations are employed to model the charging of nanocrystal films and gain insight into the electrochemical doping process. Through steady state simulations it is shown that the Fermi level and doping density in the nanocrystal film depend on the concentration of the electrolyte in addition to the value of the applied potential. Time-resolved simulations reveal that charging is often limited by transport of electrolyte ions. However, ion transport in the film is dominated by drift, rather than diffusion, and the concentration profile of ions differs substantially from concentration profiles of diffusing redox species at flat electrodes. Classical electrochemical theory cannot be used to model this type of mass transport limited behavior in films of nanocrystals, so a new model is developed. We show that the Randles-Ševčík equation, which is derived for electrochemical species diffusing in solution, but is often applied to films as well, results in a significant underestimation of the diffusion coefficients of the charge compensating electrolyte ions.

Correlative Analysis of Ion-Concentration Profile and Surface Nanoscale Topography Changes Using Operando Scanning Ion Conductance Microscopy.

Although various spectroscopic methods have been developed to capture ion-concentration profile changes, it is still difficult to visualize the ion-concentration profile and surface topographical changes simultaneously during the charging/discharging of lithium-ion batteries (LIBs). To tackle this issue, we have developed an operando scanning ion conductance microscopy (SICM) method that can directly visualize an ion-concentration profile and surface topography using a SICM nanopipette while controlling the sample potential or current with a potentiostat for characterizing the polarization state during charging/discharging. Using operando SICM on the negative electrode (anode) of LIBs, we have characterized ion-concentration profile changes and the reversible volume changes related to the phase transition during cyclic voltammetry (CV) and charge/discharge of the graphite anode. Operando SICM is a versatile technique that is likely to be of major value for evaluating the correlation between the electrolyte concentration profile and nanoscale surface topography changes.

Polymerized ionic liquids on charged electrodes: New prospects for electrochemistry

The number of electrochemical applications of ionic liquids and their polymerized forms is steadily growing. Nevertheless, polymeric ionic liquid-based electrochemical systems have hardly ever been described theoretically in the same way as monomeric ionic liquids-based ones. In this opinion, we briefly outline recent studies addressing basic and applied research of polymeric ionic liquids and shortly discuss our recently developed theoretical model, which describes the behavior of a polymeric ionic liquid and its solution on a charged electrode. We summarize the results of the obtained capacitance, electrostatic potential, and ionic concentration profiles and outline further development of the theoretical description of polymeric ionic liquids.

Influence of chloride salt type on chloride ion diffusion performance of alkali-activated slag mortar

• Alkali-activated slag (AAS) mortar has better resistance to chloride ion corrosion. • Different cation types affect the D app of AAS mortar. • MIP and XRD was performed to show a decrease in porosity. • EIS was performed to show an inverse exponential relationship between R c,d and D app . This work focuses on evaluating the influence of four cationic chloride salts (NaCl, KCl, MgCl 2 and CaCl 2 ) on chloride ion diffusion characteristics of alkali-activated slag mortar. The chloride ion concentration profiles of alkali-activated slag mortar were tested in single cation and combined cation source solutions. The hydration products and mechanism of action were analyzed by XRD and MIP, and the chloride ion transmission characteristics of alkali-activated slag mortar were further analyzed in combination with EIS. The chloride ion apparent diffusion coefficient D app related to the chloride salt solutions with the four cation types decreases in the order of D app (KCl) > D app (NaCl) > D app (MgCl 2 ) > D app (CaCl 2 ). The combination of monovalent cations can promote the chloride ions diffusion of alkali-activated slag mortar, and the results of the combination of monovalent cations and divalent cations are different. The hydration products of slag are mainly C-S-H gels with low calcium to silicon ratio, which reacts with chloride salt to form Friedel's salt, which can fill the internal pores, optimize pore size distribution, and improve the resistance to chloride ion corrosion.

Chemical oscillation at the grain boundary of aliovalently-doped solid-oxide electrolytes

The ionic conductivity of polycrystalline aliovalently-doped solid oxide electrolytes is generally lower than that of the corresponding single-crystal material by a few orders of magnitude. This behavior arises due to slow ionic movement in the space-charge region at the grain boundary (GB). Although traditionally the Guoy-Chapman (G-C) and Mott-Shottky (M-S) models have been used to explain the nature of the space charge layer and ionic conduction at GB, these models tend to ignore the chemical interactions and the molecular scale-atomic structure at the GB. To highlight the importance of chemical interactions and GB misorientation, we systematically probe the space-charge layers at Σ5 and Σ13 GBs in two commonly-used solid electrolyte materials, namely, yttria stabilized zirconia (YSZ) and gadolinium doped ceria (GDC). Using molecular dynamics (MD), we provide first evidence of chemical oscillation in the O2− ion concentration profile in the space charge region. These oscillations arise from the competing chemical, electrostatic and elastic interactions, which is missed by the G-C and M-S models. Space charge layer characteristics in each of the GBs studied is found to be different. An alternate theory which explains the chemical oscillations in the space-charge layer is presented. Implications on the ionic conduction are discussed.

Monitoring the Early Stages of Formation of Oil-Water Emulsions Using Flow Cytometry.

Characterization of complex oil emulsions is critical yet challenging both in science and in many industrial applications. Here we demonstrate for the first time the use of flow cytometry as a fast method for characterizing complex, polydisperse oil-water emulsions. Owing to our interest in understanding how the presence of specific ions might affect the properties of oil-water emulsions including size, polydispersity, and complexity, we present a systematic study of oil emulsions in deionized water and various brines of different ionic strength. Forward scatter (FSC) and side scatter (SSC) intensities associated with detailed statistics were judiciously combined to provide a better understanding of these complex systems. We find that the type and concentration profiles of ions around the oil droplets affect significantly the properties of the emulsion. Weakly hydrated cations NH4+ and Ca2+ appear to be more effective in screening the charge of oil droplets compared to the monovalent Na+ and divalent Mg2+ ions, respectively. As a result, coalescence and formation of larger droplets are seen in the case of NH4Cl and CaCl2 compared to NaCl and MgCl2, respectively. In addition, weakly hydrated anions such as Cl- can come closer to the oil surface and, thus, decrease the effective screening that the Na+ ions provide as compared to SO42- ions, which leads to more stable emulsions in NaCl compared to Na2SO4. In addition to these specific findings, the work demonstrates the utility of the technique as a new tool for characterizing oil emulsions in a wide spectrum of fields ranging from food to oil and gas applications.

Contact pairs of RNA with magnesium ions-electrostatics beyond the Poisson-Boltzmann equation

The electrostatic interaction of RNA with its aqueous environment is most relevant for defining macromolecular structure and biological function. The attractive interaction of phosphate groups in the RNA backbone with ions in the water environment leads to the accumulation of positively charged ions in the first few hydration layers around RNA. Electrostatics of this ion atmosphere and the resulting ion concentration profiles have been described by solutions of the nonlinear Poisson-Boltzmann equation and atomistic molecular dynamics (MD) simulations. Much less is known on contact pairs of RNA phosphate groups with ions at the RNA surface, regarding their abundance, molecular geometry, and role in defining RNA structure. Here, we present a combined theoretical and experimental study of interactions of a short RNA duplex with magnesium (Mg2+) ions. MD simulations covering a microsecond time range give detailed hydration geometries as well as electrostatics and spatial arrangements of phosphate-Mg2+ pairs, including both pairs in direct contact and separated by a single water layer. The theoretical predictions are benchmarked by linear infrared absorption and nonlinear two-dimensional infrared spectra of the asymmetric phosphate stretch vibration which probes both local interaction geometries and electric fields. Contact pairs of phosphate groups and Mg2+ ions are identified via their impact on the vibrational frequency position and line shape. A quantitative analysis of infrared spectra for a range of Mg2+-excess concentrations and comparison with fluorescence titration measurements shows that on average 20–30% of the Mg2+ ions interacting with the RNA duplex form contact pairs. The experimental and MD results are in good agreement. In contrast, calculations based on the nonlinear Poisson-Boltzmann equation fail in describing the ion arrangement, molecular electrostatic potential, and local electric field strengths correctly. Our results underline the importance of local electric field mapping and molecular-level simulations to correctly account for the electrostatics at the RNA-water interface.

(Invited) Model-based Analysis of Carbonation Effects in Anion Exchange Membrane Fuel Cells

Anion exchange membrane fuel cell (AEMFC) is a promising next generation (Pt-free) fuel cell technology. However, AEMFC do not yet demonstrate high performance when running on ambient air: CO2 in air results in a carbonation reaction that impacts membrane conductivity and electrode performance. This contribution analyses and quantifies the effect of CO2 from ambient air on the concentration profiles in the membrane and the anode and assesses the CO2 impact on fuel cell performance via modeling.[1] The physico-chemical model contains chemical and electrochemical reactions, liquid-gas phase equilibria as well as the transport processes in the cell. Results imply that a significant part of the CO2 that enters the cell is absorbed in the cathode and is transported as carbonate ions to the anode. Concentration profiles in the membrane reveal an enrichment zone of CO2 in the membrane close to the anode, negligible hydrogencarbonate and a wide distribution of carbonate across the membrane. The carbonate distribution affects overall anion exchange membrane conductivity. For practically relevant current densities of i > 500 mA cm-2 and typical excess ratios of 1.5 for the hydrogen feed, less than 10% of the anions in the membrane are carbonates. We show that while increasing cell temperature has an ambiguous effect on the carbonation process and on the total effect of CO2 on the cell, current density has a significant effect. The impact of CO2 on AEMFC performance can be significantly decreased when operating the cell at high current densities. A closer look at the transport processes in the membrane reveals that carbonate ions are purged out of the membrane when increasing current density. However, this process is unexpectedly slow. In the talk we will compare simulated and experimental temporal evolution of concentration profiles and conductivity. For validation, data obtained during decarbonation experiments in a setup to determine the true hydroxide conductivity of AEM [2] is used. Ion concentration profiles in the membrane are described quantitatively and it is shown how the differences in ion mobility delay the complete decarbonation process. The model-based insights of the talk aid experimentalists to build and operate well-performing air-fed AEMFCs. [1] U. Krewer, C. Weinzierl, N. Ziv, D. Dekel, Electrochimica Acta 263 (2018) 433-446 [2] A. Zhegur-Khais, F. Kubannek, U. Krewer, D.R. Dekel, J. Memb. Sci. 612, (2020) 118461

Elucidating cycling rate-dependent electrochemical strains in sodium iron phosphate cathodes for Na-ion batteries

Battery electrodes materials undergo significant mechanical instabilities which affect their longevity and exert rate-limitations during the cycling process. In this study, we investigate the rate-dependent mechanical response of sodium iron phosphate (NaFePO4, NFP) cathodes during Na intercalation via galvanostatic cycling at different rates by employing digital image correlation, electrochemical methods, and mathematical model. The mechanical behavior of the electrode shows strong dependance on the applied scan rate. At slower rates, electrode shows asymmetrical strain generation between anodic and cathodic cycles, which is attributed to the formation of cathode-electrolyte interface layers. The electrode undergoes smaller strain generation when cycled at slower rates when the same amount of Na ions is removed or inserted into the electrode. A mathematical model was developed to predict strain evolution in the composite electrode as well as the concentration profile of the Na ions in the electrode particles. Rate-dependent and time-dependent factors on the strain generation in the electrode are attributed to the capacity-dependent intercalation strains, rate-dependent mismatch strains, and time-dependent irreversible strains. The combination of in situ strain measurements with the analytical model provided new insight into the electrochemically induced mechanical deformations in Na-ion cathode electrodes.

Scale-up and Modelling of Flow-electrode CDI Using Tubular Electrodes

A novel design for a flow-electrode capacitive deionization (FCDI) system consisting of tubular electrodes in a shell and tube heat exchanger configuration is proposed. Each electrode consists of a metallic mesh current collector along the inner circumference of a tubular ion-exchange membrane. This tubular FCDI design is suitable for scale-up as it consists of easily manufactured components which can be assembled in an array. An apparatus with 4 tubular electrodes with a large effective area (202.3 cm2) was constructed and shown to provide a high net salt (NaCl) removal rate (0.15 mg s−1 at 1.2 V applied voltage and ∼2000 mg L−1 influent total dissolved solids concentration). A computational fluid dynamics (CFD) model incorporating ion migration and transport mechanisms was developed to simulate the ion concentration and electrical potential profiles in the water channel. The results of CFD modelling highlighted the need to maximize regions of both high potential gradient and high hydraulic flow in order to achieve optimal salt removal. In brief, this study presents a new design approach for FCDI scale-up and provides a computational tool for optimization of this design and future innovative FCDI designs.

Lattice Boltzmann method to simulate three-dimensional ion channel flow using fourth order Poisson–Nernst–Planck–Bikerman model

Over the past three decades, the lattice Boltzmann method (LBM) has been applied to a vast range of hydrodynamic and non-hydrodynamic (e.g., ion transport) systems. In conjunction with the immersed boundary method (IBM), the LBM has been successfully implemented to solve systems with complex geometries. In this study, the immersed boundary–lattice Boltzmann method (IB-LBM) is implemented to simulate nanoscale ion transport. Traditionally, ion transport is described through the Poisson–Nernst–Planck (PNP) equations where ionic interactions are included. In the current paper, the fourth order Poisson–Nernst–Planck–Bikerman (4PNPBik) model has been used. In addition to ionic interactions, the 4PNPBik model includes the effects of the finite size of particles (ions and water) and interactions between ions and its surrounding medium. Applicability of the 4PNPBik model is demonstrated through comparison of the experimental and predicted ion activity. Implementation of the 4PNPBik model has been validated by comparing the predicted current–voltage curve with the analytical result. The transient receptor potential (TRP) ion channel of the vanilloid group (TRPV4) is used to demonstrate the applicability of this approach. The TRPV4 is a nonselective cation channel that prefers divalent cationic species over monovalent cations. In this study, this selectivity is demonstrated by comparing the concentration profiles of calcium, sodium, and chloride ions. Further, the role of the finite size of particles and nonlocal electrostatics is discussed by comparing the results obtained from the PNP and 4PNPBik models under identical initial and boundary conditions.

A novel retarded HCl acid system for HPHT carbonate acidizing applications: Experimental study

AbstractRetarded acid systems for high‐pressure and high‐temperature (HPHT) carbonate reservoirs are the key to provide a deep acid penetration and higher acid efficiency. The current study experimentally evaluates a new retarded HCl acid for HPHT carbonate acidizing applications. The acid retardation mechanism is attained by using a polymeric resin‐based composition that restricts the proton mobility of the acid species. Ten Indiana limestone cores were used with the dimensions of 6 in (0.15 m) length and 1.5 in (0.038 m) diameter and average permeability of 9 mD (0.009 ). Two sets of experiments were conducted, one at 250°F (394 K) and another at 300°F (422 K), with acid injection rates from 0.5–10 cm3/min (8.3E−9 to 1.67E−7 m3/s). The pore volume of acid required to breakthrough (PVBT) was monitored. Effluent samples were collected to track the pH and the dissolved material. Computed tomography (CT) was used to scan the core before and after the experiment to determine the wormhole propagation along with the core. The optimum injection rate at 250 and 300°F (394 and 422 K) were found to be 2.5 and 6 cm3/min (1.67E−8 and 1.25E−7 m3/s) with PVBT of 0.41 and 0.43 PV, respectively. The 3D CT scan showed an effective wormhole profile featuring a single dominant trajectory with few branches. These results are confirmed by calcium ion concentration profiles through inductively coupled plasma (ICP) analysis. The use of the new acid system will provide deep penetration with low injected acid volume. As a result, the stimulation job efficiency will improve.

Effect of solution pH on removal of anionic surfactant sodium dodecylbenzenesulfonate (SDBS) from model wastewater using nanoscale zero-valent iron (nZVI)

Batch experiments for removal of anionic surfactant sodium dodecylbenzenesulfonate (SDBS) from model wastewater using nanoscale zero-valent iron (nZVI) were conducted to elucidate mechanisms of SDBS removal by nZVI. To clarify the effects of solution pH on removal of SDBS by nZVI, the concentration profiles of Fe ions and dissolved oxygen (DO) were measured besides the concentrations of SDBS and total organic carbon (TOC) at pH of 3, 5, 7 and 9. The SDBS concentration rapidly decreased to less than 4% of the SDBS loadings within 40 min at pH 3, 7 and 9. However, TOC decreases were rather slow and the maximum TOC removal of 62% after 240 min was obtained at pH 7. The XRD patterns confirmed that the main corrosion product was lepidocrocite at pH 5 and 7 while magnetite was dominant at pH 9. The maximum removal of SDBS by nZVI at pH 7 was attributed mainly to the ionic adsorption of anionic SDBS onto the positively charged nZVI surface. The novel kinetic model for dynamic linkage of SDBS removal with concentrations of total eluted iron ions and DO via the formation of iron oxide/hydroxide layer on nZVI surface could simulate the experimental results with the average correlation coefficient of 0.952. The mechanisms of SDBS removal by nZVI at different pH values were clarified and quantified by the proposed kinetic model.