Chloride Transport Research Articles

Mesoscale numerical simulation of chloride ion penetration in geopolymer concrete under the externally applied electric field

This paper presents a numerical investigation on the penetration of chloride ions in a saturated geopolymer concrete under the action of externally applied electric field. The chloride ion transport model in geopolymer concrete is established, and the interactions between different ionic species such as potassium ions, sodium ions, hydroxide ions, and chloride ions are considered. By solving the modified mass conservation equation and the Poisson equation of individual ionic species, the concentration and electrostatic potential of different ions at any position and at any time are obtained. Results show that under the action of externally applied electric field, the ion electromigration plays a leading role, which is characterized by a rectangular wave in the concentration distribution curve. In addition, the average chloride ion penetration depth obtained from the numerical simulations agrees well with that measured from the experiment in the cited literature. Moreover, the transmission speed of the two positively charged ions (or the two negatively charged ions) are almost the same. While there is a striking difference in the transmission speed between the positively charged ions and the negatively charged ions, indicating that the electrostatic potential gradient is not constant, and is region related.

Structural and functional properties of the transporter SLC26A6 reveal mechanism of coupled anion exchange

Members of the SLC26 family constitute a conserved class of anion transport proteins, which encompasses uncoupled transporters with channel-like properties, coupled exchangers and motor proteins. Among the 10 functional paralogs in humans, several participate in the secretion of bicarbonate in exchange with chloride and thus play an important role in maintaining pH homeostasis. Previously, we have elucidated the structure of murine SLC26A9 and defined its function as an uncoupled chloride transporter (Walter et al., 2019). Here we have determined the structure of the closely related human transporter SLC26A6 and characterized it as a coupled exchanger of chloride with bicarbonate and presumably also oxalate. The structure defines an inward-facing conformation of the protein that generally resembles known structures of SLC26A9. The altered anion selectivity between both paralogs is a consequence of a remodeled ion binding site located in the center of a mobile unit of the membrane-inserted domain, which also accounts for differences in the coupling mechanism.

Structural and functional properties of the transporter SLC26A6 reveal mechanism of coupled anion exchange.

Members of the SLC26 family constitute a conserved class of anion transport proteins, which encompasses uncoupled transporters with channel-like properties, coupled exchangers and motor proteins. Among the 10 functional paralogs in humans, several participate in the secretion of bicarbonate in exchange with chloride and thus play an important role in maintaining pH homeostasis. Previously, we have elucidated the structure of murine SLC26A9 and defined its function as an uncoupled chloride transporter (Walter et al., 2019). Here we have determined the structure of the closely related human transporter SLC26A6 and characterized it as a coupled exchanger of chloride with bicarbonate and presumably also oxalate. The structure defines an inward-facing conformation of the protein that generally resembles known structures of SLC26A9. The altered anion selectivity between both paralogs is a consequence of a remodeled ion binding site located in the center of a mobile unit of the membrane-inserted domain, which also accounts for differences in the coupling mechanism.

Identification of chloride diffusion coefficient in concrete using physics-informed neural networks

The corrosion of steel caused by chloride is one of the important causes of the deterioration of reinforced concrete structures. In order to describe the transport behavior of chlorides in concrete and to make quantitative durability designs based on it, it is necessary to determine the chloride diffusion coefficient correctly. However, the commonly used analytical method is not suitable for the free chloride diffusion coefficient considering the nonlinear chloride binding capacity of concrete. In this study, by comparing the results of solving forward problems involving chloride transport equations using finite element method (FEM) and solving corresponding inverse problems using physics-informed neural networks (PINNs), it is verified that PINNs can be applied to identify the chloride diffusion coefficient in non-steady-state immersion tests and accelerated chloride migration tests considering no chloride binding, Langmuir binding, or Freundlich binding. The robustness of PINNs is verified by training with 5% and 10% noise data as observed data. The results of interpolation and extrapolation using PINNs trained successfully show its excellent generalization performance.

Cryo-EM structures of ClC-2 chloride channel reveal the blocking mechanism of its specific inhibitor AK-42

ClC-2 transports chloride ions across plasma membranes and plays critical roles in cellular homeostasis. Its dysfunction is involved in diseases including leukodystrophy and primary aldosteronism. AK-42 was recently reported as a specific inhibitor of ClC-2. However, experimental structures are still missing to decipher its inhibition mechanism. Here, we present cryo-EM structures of apo ClC-2 and its complex with AK-42, both at 3.5 Å resolution. Residues S162, E205 and Y553 are involved in chloride binding and contribute to the ion selectivity. The side-chain of the gating glutamate E205 occupies the putative central chloride-binding site, indicating that our structure represents a closed state. Structural analysis, molecular dynamics and electrophysiological recordings identify key residues to interact with AK-42. Several AK-42 interacting residues are present in ClC-2 but not in other ClCs, providing a possible explanation for AK-42 specificity. Taken together, our results experimentally reveal the potential inhibition mechanism of ClC-2 inhibitor AK-42.

Experimental and numerical analysis of chloride ion transportation in concrete under bending load with reverse seepage

The paper investigates a new technique, named as Reverse-seepage and Saturation based Active Anti-corrosion Technology (RS-AAT), for preventing chloride ion invasion within the marine concrete under the loading effect. By the indoor model experiments, the effect of RS-AAT on chloride ion invasion within the marine concrete under 10%, 20%, and 30% ultimate bending loads are explored and the results indicate that RS-AAT can effectively reduce 24.36% and 27.29% in chloride ion concentration in the compression and tension regions, respectively. Otherwise, the use of RS-AAT can effectively decrease the surface chloride ion concentration. Furthermore, the chloride ions convection-diffusion model with loading effect is established based on the mesoscopic structure theory of concrete pores and the K-C model, of which variability is verified by comparing with the experimental results.

Study of groundwater recharge using combined unsaturated‐and saturated‐zone chloride mass balance methods

AbstractUnderstanding groundwater recharge mechanisms is essential for reliable groundwater recharge estimation for sustainable groundwater resource assessment and management, especially in arid and semiarid areas. In this study, chloride, together with stable isotopes (18O and 2H), in 1‐year rainfall, soil water from representative profiles, and groundwater samples, collected in the Ordos Plateau, northern China were analysed. Groundwater recharge was estimated using unsaturated‐ and saturated‐zone chloride mass balance (CMB). An equation that describes the relationship between chloride concentrations in saturated zone water (Csz) and soil water residence time (τ) was derived. With the help of the relationship, chloride concentrations in unsaturated zone water (Cuz) and in groundwater (Csz) can be used to uncover water flow and chloride transport in the unsaturated zone. The relationship between Csz and Cuz indicates that, the groundwater was recharged mainly by intense rainfall events. Small rainfall events did not lead to groundwater recharge but contributed to the flux of chloride to soil surface. Stable isotopic compositions of 18O and 2H provided corroborative evidence of the recharge processes. The relationship between Csz and τ indicates further that there is no source of chloride in the groundwater other than that from precipitation. Thus, Csz can give more reliable recharge estimates. Cuz was influenced by heavy rainfall‐induced runoff, run‐on and bypass flow events and cannot give reliable recharge estimates. However, if used jointly with Csz, Cuz can help to gain insights into recharge processes and yield groundwater recharge estimates with higher certainty. The outcomes of this study can assist for groundwater recharge investigation and assessment in regions where the assumptions and boundary conditions necessary for the correct application of the CMB method may not be met.

Chloride Transport and Related Influencing Factors of Alkali-Activated Materials: A Review.

Chloride transport is a vital issue in the research on the durability of alkali-activated materials (AAMs). Nevertheless, due to its miscellaneous types, complex mix proportions, and limitations in testing methods, the reports of different studies are numerous and vary greatly. Therefore, in order to promote the application and development of AAMs in chloride environments, this work systematically reviews the chloride transport behavior and mechanism, solidification of chloride, influencing factors, and test method of chloride transport of AAMs, along with conclusions regarding instructive insights to the chloride transport problem of AAMs in future work.

Effect of Metakaolin on the Diffusion Properties of Chloride Ions in Cement Mortar under the Coupling Effect of Multiple Factors in Marine Environment

To address the problem of chloride ion transport in cement concrete in marine environment, this study investigates the effect of metakaolin dosage on the chloride ion diffusion resistance of mortar and its mechanism by testing the chloride ion binding capacity and microstructure of mortar under the coupling effect of chlorine salt-sulfate-carbonation multiple factors. The results show that the coupling of sulfate or carbonation reduces chloride ion transport to some extent compared with single chlorine salt attack, while the three-factor coupled environment promotes free chloride ion diffusion. This is because the products of calcium alumina, gypsum, and calcium carbonate grow together and compete with each other to form more large capillaries; thus, accelerating the diffusion of chloride ions in cement mortar. Metakaolin, due to its higher pozzolanic activity, increases the monocarbon aluminate content in the erosion products, promotes F-salt generation, and increases the Al/Si ratio, which strengthens the binding ability of C-S-H gel to chloride ions, so the free chloride ion concentration inside the specimens doped with metakaolin is lower. In particular, the three-factor coupled environment has less 0.05–10 μm capillary pore content and higher F-salt stability in the specimens, which has the strongest effect on chloride ion curing, and the free chloride ion concentration integral in M-SCCl is reduced by nearly 30% compared with MF-SCCl and F-SCCl.

Molecular dynamics study on the transport of water molecules and chloride ions in graphene oxide-modified cement composites

ABSTRACT The ability of composite cement materials to resist erosion by chloride ions is a critical factor in evaluating their dependability. This study aims to examine the influence of graphene oxide on the transportation of water molecules and chloride ions in modified cement composites. Molecular dynamics analysis suggests that graphene oxide can effectively bond to the substrate of hydrated calcium silicate gel pores, which forms a stronger confined fluid zone under the action of electrostatic interactions and van der Waals forces. Graphene oxide has negatively charged oxygen functional groups on its surface, and within a certain size range, it becomes more effective at restricting the penetration of water molecules and chloride ions. In addition, chloride solution immersion experiments were performed on graphene oxide modified cement mortar. The results demonstrated that a small quantity of graphene oxide can significantly improve the resistance of modified cement mortar to chloride ion erosion, whereas excessive amounts are detrimental, which aligns with the simulation results. It is hoped that this study will provide valuable insights into the use of graphene oxide nanoparticles in the corrosion protection of cement composites.

Understanding the acceleration impact of load and flowing water on the chloride ion transport properties of fly ash-based geopolymer concrete

In this study, the acceleration impact of load and flowing water on the Cl− transport properties of fly ash-based geopolymer concrete (FABGC) was studied for concrete in a complex environment. The results showed that when the erosion age remained unchanged, the concentration of Cl− gradually decreased, and the proportion of bound Cl− increased with an increase in depth from the FABGC surface. At a constant depth from the FABGC surface, Cl− mainly exists in the form of free Cl−. Both flowing water erosion and loading promoted the transport of Cl−, and their combined effect was the primary factor influencing ion transport in the tension zone. When its content in FABGC is high, Al will change the gel structure in FABGC, thus inhibiting the transport of Cl−. A Cl− transport model considering multiple factors (such as the bound Cl−, load, porosity, and flowing water) was established, and its rationality was verified.

Structure and Heterogeneity of Retinal Chromophore in Chloride Pump Rhodopsins Revealed by Raman Optical Activity.

Chloride transport by microbial rhodopsins is actively being researched to understand how light energy is converted to drive ion pumping across cell membranes. Chloride pumps have been identified in archaea and eubacteria, and there are similarities and differences in the active site structures between these groups. Thus, it has not been clarified whether a common mechanism underlies the ion pump processes for all chloride-pumping rhodopsins. Here, we applied Raman optical activity (ROA) spectroscopy to two chloride pumps, Nonlabens marinus rhodopsin-3 (NM-R3) and halorhodopsin from the cyanobacterium Mastigocladopsis repens (MrHR). ROA is a vibrational spectroscopy that provides chiral sensitivity, and the sign of ROA signals can reveal twisting of cofactor molecules within proteins. Our ROA analysis revealed that the retinal Schiff base NH group orients toward the C helix and forms a direct hydrogen bond with a nearby chloride ion in NM-R3. In contrast, MrHR is suggested to contain two retinal conformations twisted in opposite directions; one conformation has a hydrogen bond with a chloride ion like NM-R3, while the other forms a hydrogen bond with a water molecule anchored by a G helix residue. These results suggest a general pump mechanism in which the chloride ion is "dragged" by the flipping Schiff base NH group upon photoisomerization.

Time-dependent reliability analysis of reinforced concrete beams considering marine environmental actions

Under marine atmospheric environments, reinforced concrete (RC) structures may suffer from structural deterioration induced by environmental actions, affecting structural performance and safety. Thus, it is essential to evaluate and predict the deterioration process of an RC structure subject to environmental actions. This paper proposes a novel and generic computational framework to assess the time-dependent reliability of an RC beam under environmental actions. This framework includes a comprehensive life-cycle performance assessment model that considers climate change, two-dimensional chloride transport, the coupling effects and corrosion non-uniformity of reinforcement, and the nonlinear behavior of RC beams, which is validated by experimental studies. Additionally, performance functions for an RC beam are established for different failure modes, and their time-dependent probabilistic information and reliability index are captured using the probability density function-informed method (PDFM). To demonstrate the computational processes of the developed framework under practical marine atmospheric conditions, illustrative cases are presented. The accuracy and efficiency of the proposed method are validated by traditional Monte Carlo Simulation (MCS). Results indicate that the proposed method is feasible and efficient, aiding the life-cycle design and maintenance of RC structures.

Significance of chloride salt type and sulfate salt on chloride transport mechanism of concrete in the presence of corrosion inhibiting admixtures

In this study, the effect of chloride and their cations on free chloride (Clf), chloride diffusion coefficient (Dc), and chloride binding capability (CBC) of concrete treated with inhibitors, subjected to chloride and composite chloride-sulfate environments was investigated. Results revealed that in absence of sulfate, the Clf and Dc were higher for NaCl, while in presence of sulfate they were higher in CaCl2. The Dc increased and CBC decreased in presence of sulfate. Inhibitors were effective in reducing the Clf and Dc of concrete subjected to various environments. Na2HPO4 showed higher CBC in chloride and lower in composite environments than NaNO2.

Effect of loading-induced damage on chloride ingress behavior of recycled aggregate concrete: A comprehensive review

The recycling technology of construction and demolition wastes is an inevitable choice towards the sustainable development of the construction industry, aiming at addressing the problems of raw materials shortage and waste disposal. The utilization of recycled coarse aggregate (RCA) in new concrete production has been identified as an effective way to reasonable application of natural resources replacement and environmental preservation. In service, external mechanical loading can easily cause the initiation and interconnectivity of microcracks inside concrete, specifically the complex multiple interface structures of recycled aggregate concrete (RAC) due to the old adhered mortar at the surface of RCAs, which can remarkably accelerate the process of water penetration and aggressive ions penetration into concrete. Thus, knowledge of mass transport properties in the loading-induced damaged RAC contributes to the durability evaluation of RAC and its extensive application in the practical engineering. This paper reviews the previous findings on the mechanical loading effects on the chloride transport properties of RAC. The effects of loading-induced damage on microstructure evolution of RAC as well as the damage characteristics were discussed. The research progress on the chloride ingress behavior of RAC subjected to mechanical loading was highlighted from the viewpoint of testing method, loading devices and numerical calculation, and the modified effects on the transport properties were further analyzed. On this basis, the future research on the durability-related properties of RAC would be finally proposed, which is helpful to further perfect theoretical development and technological innovation of RAC durability in the aggressive environment.

An Artificial Single Molecular Channel Showing High Chloride Transport Selectivity and pH-Responsive Conductance.

Inspired by the unique structure and function of the natural chloride channel (ClC) selectivity filter, we present herein the design of a ClC-type single channel molecule. This channel displays high ion transport activity with half-maximal effective concentration, EC50 , of 0.10 μM, or 0.075 mol % (channel molecule to lipid ratio), as determined by fluorescent analysis using lucigenin-encapsulated vesicles. Planar bilayer lipid membrane conductance measurements indicated an excellent Cl- /K+ selectivity with a permeability ratio P /P up to 12.31, which is comparable with the chloride selectivity of natural ClC proteins. Moreover, high anion/anion selectivity (P /P =66.21) and pH-dependent conductance and ion selectivity of the channel molecule were revealed. The ClC-like transport behavior is contributed by the cooperation of hydrogen bonding and anion-π interactions in the central macrocyclic skeleton, and by the existence of pH-responsive terminal phenylalanine residues.

An Artificial Single Molecular Channel Showing High Chloride Transport Selectivity and pH‐Responsive Conductance

AbstractInspired by the unique structure and function of the natural chloride channel (ClC) selectivity filter, we present herein the design of a ClC‐type single channel molecule. This channel displays high ion transport activity with half‐maximal effective concentration, EC50, of 0.10 μM, or 0.075 mol % (channel molecule to lipid ratio), as determined by fluorescent analysis using lucigenin‐encapsulated vesicles. Planar bilayer lipid membrane conductance measurements indicated an excellent Cl−/K+ selectivity with a permeability ratio P /P up to 12.31, which is comparable with the chloride selectivity of natural ClC proteins. Moreover, high anion/anion selectivity (P /P =66.21) and pH‐dependent conductance and ion selectivity of the channel molecule were revealed. The ClC‐like transport behavior is contributed by the cooperation of hydrogen bonding and anion–π interactions in the central macrocyclic skeleton, and by the existence of pH‐responsive terminal phenylalanine residues.

Chloride penetration in reinforced slag-based concrete beams under combined effects of loads and drying–wetting cycles

The use of concrete incorporating slag is necessary due to global environmental concerns, but the decrease in concrete durability caused by environments rich in chloride ions requires more advanced concrete mix designs, especially in coastal areas. The aim of this study was thus to assess the influence of slag addition on the transport of chloride ions in reinforced concrete (RC) beams under sustained bending moments, considering two conditions of environmental chlorine exposure: drying–wetting cycles and soaking. The results showed that the chloride diffusion coefficient (CDC) increased in tension zones and decreased in compression zones when the sustained bending load in the concrete beam was increased. However, it was also found that the addition of slag can significantly reduce the chloride diffusion depth and CDC for both environmental conditions, with a more significant effect under drying–wetting cycles. The chloride concentration on the surface of slag concrete in the compression zone was greater than that in the tension zone and greater than that in both the tension and compression zones of the ordinary concrete. A prediction model was established for describing the chloride penetration characteristics of concrete beams subjected to sustained bending moment. The proposed model is a reasonable approach for predicting chloride distribution in RC beams with slag addition subjected to bending moments.

The effect of vitamin D supplementation on human airway epithelial antimicrobial activity, ion transport, and liquid absorption

Vitamin D deficiency is implicated in chronic lung disease. Given its popularity as a respiratory season supplement, we sought to characterize the effect of active vitamin D (calcitriol) on human airway epithelial cell innate immune properties, including ion transport in vitro. We hypothesized calcitriol supplementation would enhance epithelial antimicrobial properties and alter airway surface liquid (ASL) composition and volume.We assayed primary human airway epithelial cells (hAECs) pretreated with calcitriol (10 -7 M) for 22h vs. paired controls. ASL bactericidal activity was tested by quantifying bioluminescent Staphylococcus aureus incubated with ASL from hAECs ±calcitriol. Epithelial transcriptional changes were measured by RNA sequencing and RT-qPCR. We measured the final volume of an apical 60 μL bicarbonate solution after 4h to assess the rate of epithelial fluid absorption ( J v ). To evaluate ion transport properties, epithelia were mounted in Ussing chambers and short-circuit current ( I sc ) and transepithelial conductance ( G t ) recorded. We quantified the half-life of ENaC by blocking protein translation with cycloheximide, then measured amiloride-sensitive I sc or G t in the presence of anion channels inhibitors.ASL from epithelia supplemented with calcitriol exhibited enhanced bactericidal activity against S. aureus, due to enhanced cathelicidin expression. RNA sequencing analysis revealed calcitriol decreased mRNA transcripts of proteins involved in airway epithelial sodium and chloride transport ( SCNN1G and ATP1B1). Consistent with RNA sequencing data, calcitriol-treated epithelia had decreased J v , amiloride-sensitive I sc , and amiloride-sensitive G t . Electrophysiological responses from anion channel blockers were unaffected. We obtained similar results with prolonged calcitriol incubation (7-day), while acute treatment (≤20 min.) did not affect I sc or G t . After apically treating epithelia with nystatin, no differences were observed in ouabain-sensitive I sc ±calcitriol, indicating calcitriol did not alter Na-K pump activity. Supplementation also did not alter the half-life of amiloride-sensitive I sc . RT-PCR experiments demonstrated both 3h and 22h calcitriol treatment increased CYP24A1 and CAMP expression (Vitamin D response element controls). However, ENaC genes only trended down for both timepoints, indicating the VDR likely indirectly decreased ENaC activity.In summation, calcitriol increased hAEC cathelicidin expression and ASL bacterial killing, while reducing sodium and liquid absorption across the epithelia via decreased ENaC activity. Calcitriol supplementation may be useful in airway diseases involving chronic bacterial infection and mucus dehydration. The Bowers Emphysema Fund and P01 HL091842/HL/NHLBI NIH HHS/United States This is the full abstract presented at the American Physiology Summit 2023 meeting and is only available in HTML format. There are no additional versions or additional content available for this abstract. Physiology was not involved in the peer review process.

Electrochemical characterization of chloride ion transport in rubber concrete

Chloride-induced corrosion of steel in concrete structures is a significant issue. As an environmentally-friendly building material, rubber concrete has been proven to have the potential to resist chloride-induced permeation. In this paper, the transport behavior of chloride ion in rubber concrete was investigated by electrochemical impedance spectroscopy (EIS) approach. A novel electrochemical equivalent circuit Rs(Q1(Rct1W1)) (Q2(Rct2W2)) was used to study the electrochemical properties of rubber concrete systems and compare with other equivalent circuits by evaluation indicator. The results demonstrate that Rs(Q1(Rct1W1)) (Q2(Rct2W2)) is more suitable for the study of rubber concrete system and the increase of rubber will enhance the resistance to chloride ion transmission. The equation was established by the relation between the equivalent circuit parameter (Rct1) and the migration depth of chloride ion. On this basis, the EIS method can be utilized to predict the depth of chloride ion penetration in rubber concrete.