Sodium Ion Exchange Research Articles

Enhanced removal of ammonia nitrogen from rare earth wastewater by NaCl modified vermiculite: Performance and mechanism.

Wastewater from rare earth mining (WREM) is very harmful to environment and human health due to its high concentration of ammonia nitrogen (NH3-N). It is therefore necessary and urgent to find a low-cost and convenient technique to remove high concentration of NH3-N from WREM. In this study, Natural powdered vermiculite (NV) was modified with seven sodium chloride (NaCl) solutions, and seven kinds of sodium chloride modified vermiculite (Na-V) were obtained. The NH3-N adsorption performance of Na-V is greatly improved compared with NV. Among them, vermiculite modified with 180g/L NaCl yielded the highest ammonium adsorption capacity (Qm, 11.569mg/g), which was 63.43% higher than NZ (Qm, 7.079mg/g). The characterizations of 180-Na-V confirmed the removal mechanism of NH3-N that the improved capacity of modified vermiculite was attributed to its higher mesoporous volume and ion-exchange capacity, which are the result of sodium-ion exchange and Interlayer effect from high concentration of NaCl. The adsorption isotherms and kinetics were respectively best consistent with Langmuir model and the pseudo-second-order (PSO) model. The adsorption capacity (3.808mg/g) of vermiculite after 5 cycles could still maintain 75.09% of the initial adsorption capacity (5.071mg/g). A large amount of Na-V had little effect on pH of water, which meet the requirements of practical application. Including pH, dosage, coexisting ions, the effects of other factors on ammonium adsorption were also determined. This study provides a new method for vermiculite to remove high concentration of NH3-N.

Hydrothermal synthesis of zeolites-calcium silicate hydrate composite from coal fly ash with co-activation of Ca(OH)2-NaOH for aqueous heavy metals removal

Coal fly ash is a typical secondary aluminum/silicon resource. The preparation of zeolite-type absorbent is a potential way for its value-added utilization, while the purity and adsorption property of zeolite are limited due to the occurrence of side reactions in the synthesis process. In this study, a designated composite consisted of crystalline zeolites and amorphous calcium silicate hydrate was selected, which was direct synthesized from fly ash under conditions of a Ca/Si molar ratio of 0.8, an initial NaOH concentration of 0.5 mol/L, a hydrothermal temperature of 170 ℃ and a liquid–solid ratio of 15 mL/g. The results indicated that this composite had superior adsorption property for a variety of heavy metals, which was based on the exchange of calcium and sodium ions in zeolites and calcium silicate hydrate. Its adsorption capacities for Pb 2+ , Ni 2+ , Cd 2+ , Zn 2+ , Cu 2+ and Cr 3+ attained 409.4, 222.4, 147.5, 93.2, 101.1 and 157.0 mg/g, respectively, in single solution with a pH of 4.5. After regulating the synthesis conditions, the transformation of amorphous calcium silicate hydrate into crystallized tobermorite weakened the adsorption capacity of the composite. Besides, due to the competitive adsorption in a multiple ions solution, the adsorption capacities for these heavy metals had a reduction.

Syngas evolution and energy efficiency in CO2 assisted gasification of ion-exchanged pine wood

In this study, the effect of ion-exchange sodium concentration on gasification process was first studied in a semi batch rector at 1073 K. In addition, four specific temperatures from 973 to 1223 K were further examined during gasification, with focus on the amounts of syngas components’ yield, energy yield, energy efficiency as well as the CO2 consumption for the raw and ion-exchange samples. Results showed that pine wood with higher ion-exchange sodium resulted in higher CO and syngas yield, while the acid washed sample provided the lowest syngas yield. Presence of ion-exchange sodium showed a strong catalytic effect on gasification, which enhanced the syngas yield and energy generation. Besides, the increase in gasification temperature led to enhanced syngas yield, cold gas efficiency and CO2 consumption, mainly due to its promotional effect on Boudouard reaction. Highest energy yield of 7.18 and 19.88 kJ/g, cold gas efficiency of 40.66% and 112.65%, and CO2 consumption of 0.74 and 1.75 g/g were found for the ion-exchange sample at 973 K and 1223 K, respectively. Overall energy efficiency showed a trend of first an increase followed by a decrease with time. The peak value of the ion-exchange sample was shifted to longer reaction time as compared to the raw sample. A maximum overall energy efficiency of 50.52% was obtained at 26.3 min for the ion-exchange sample at 1223 K. Therefore, temperature also offers a promotional effect on energy efficiency, which is of significant value in practical applications. This research provides a new insight to achieve maximum energy yield and efficiency with the help of ion-exchange species, in addition to the advantages of effective CO2 utilization.

Оценка пригодности очищенных хозяйственно-бытовых сточных вод КОС «Кадамовские» для орошения

The purpose of the research is to study the chemical composition of treated domestic wastewater and to assess the potential capability of their use for irrigation purposes. The object of the study is domestic wastewater from the «Kadamovskie» WWTP, Novocherkassk, after mechanical and biological treatment. Research methods included the determination of the wastewater suitability according to the following indicators: the concentration of toxic salts, the ratio of cations, the content of the main biogenic elements (NPK), the irrigation coefficient according to Stebler, the ion exchange coeffi cient by I.N. Antipov-Karataev and G.M. Kader, sodium-adsorption ratio (SAR), risk of magnesium alkalinization (Sabolch and Darab). For studying the chemical composition of treated domestic wastewater, wastewater samples were taken from the «Kadamovskie» WWTP in Novocherkassk and their chemical composition was determined in an accredited environmental analytical laboratory of the Federal State Budget Scientifi c Institution RosNIIPM. Based on laboratory results, wastewater is classifi ed as lowmineralized (1.0…3.0 g/dm3), according to the degree of mineralization. Wastewater (after mechanical and biological treatment) belongs to class III and is characterized as satisfactory. In accordance with the soil-reclamation classifi cation, its suitability for irrigation is limited due to the possibility of chloride salinization processes development. According to calculations based on irrigation coefficients, ion exchange coefficients and sodium adsorption ratio, treated domestic wastewater is suitable for irrigation and it doesn’t cause soil salinity hazard (soil alkalization). Thus, it is recommended to consider treated domestic wastewater from the «Kadamovskie» WWTP in Novocherkassk as an alternative source of irrigation water.

Sustainable oxidation catalysis supported by light: Fe-poly (heptazine imide) as a heterogeneous single-atom photocatalyst

Fe-N-C materials, when prepared as single-atom catalysts (SAC), display excellent activities in oxidation reactions. The systematic investigation of the iron coordination mode revealed that Fe-N4C catalysts are the most active for C-H bond oxidation. However, many of these catalysts are synthesized through pyrolysis, which is characterized by a lack of control and structures with heterogeneous composition, rarely presenting only atomically dispersed Fe−N − C as active sites. Herein, an alternative, reliable and easily reproducible method to obtain highly active Fe SACs (atomically dispersed) with Fe-N4 sites is presented, which is based on ion exchange of sodium from high crystalline sodium poly(heptazine imide) (Na-PHI) by other ions. The obtained catalyst can photocatalytically oxidize C-H bonds selectively toward ketones using only dioxygen. Detailed mechanism investigations indicate that the active species in the C-H bond oxidation are highly valent Fe(IV)/Fe(V)-oxo species, which are further activated by the holes generated at the PHI support under light irradiation.

Experimental Study on the Comprehensive Identification and Improvement of Dispersive Soil in Western Jilin Province, China

In this study, physical experiments, clay mineral determination, and pH testing were performed to examine the basic properties of soil samples from a soil material yard selected for dam construction at Hua’ao Lake, Qian’an County, Jilin Province, China. The results show that the soil in the study area is cohesive, the mineral content of illite in the illite/montmorillonite mixed layer is approximately 50%, and the pH value of the environment from which the soil samples were taken is 8.43-8.91. These factors enable the soil in this area to be dispersed. The dispersibility of the soil sampled from this area was evaluated by a double hydrometer test, a pinhole test, a fragmentation test, a sodium adsorption ratio test, and determination of the percentage of exchangeable sodium ions. Because these test methods had inconsistent results, the test methods in combination with the typical geomorphic conditions of the sampling points were ultimately used to comprehensively evaluate the soil dispersion. The results demonstrate that the cohesive soil sampled from the soil material yard is dispersible and must be treated with improvement measures before it can be used as a filling material for the dam. To improve the dispersive and transitional soil, 2%, 3%, 4%, 5%, and 6% L1Fa2 (a 1 : 2 ratio of lime and fly ash) and C1L1Fa4 (a 1 : 1 : 4 ratio of cement, lime, and fly ash) were used to perform improvement tests on 10 groups of dispersive soil samples and 10 groups of transitional soil samples. The results reveal that the addition of 4% L1Fa2 best improves the dispersive soil in this area. Therefore, the soil intended for this project should be used as a dam-building material after improvement with the 4% addition of L1Fa2.

A Study on the Durability of Dispersive Soils Improved by Alum in Western Jilin Province of China

Dispersive soil, which has the characteristics of low erosion resistance and high dispersibility in water, is the main reason for the channel slope failure that happened in the planning area of the Western Alkaline Treatment project in Jilin Province. Therefore, the study focused on the improvement of dispersive soil. In this research, pinhole test and crumb test were conducted on the soil under varying percentages of alum (1%, 1.5%, 2%, 2.5%, and 3%). Results indicated that alum can reduce the dispersivity of soil distinctly, and the optimal content of alum was 2.5%. This research also investigated the durability of 2.5%-alum-improved dispersive soil for dispersibility under the condition of freeze-thaw cycle. The soil samples with 2.5% alum content were subjected to pinhole test, crumb test, double hydrometer test, and percentage of exchangeable sodium ion test under the different number of freeze-thaw cycles. The results showed that the 2.5%-alum-improved soil was unaffected by the number of freeze-thaw cycles, which illustrated that alum can be used to improve soil dispersivity in engineering practice.

Utilization of bioethanol industry recycled waste for sustainable soil improvement: A win-win application

Dispersive soils are a common problematic soil that may pose engineering distress if not adequately taken care of. On the other hand, the stabilization of problematic soils with waste materials has recently gained more interest from an environmental and sustainable point of view. Hence, this study utilized sulfur-free lignin (SFL), which is a bioethanol-based by-product, to improve dispersive silt clay and investigated the stabilization effectiveness considering combine impacts of water and freeze-thaw (F-T). The SFL was added to five sample groups ranging from 3 to 15%. Then, a series of experiments was performed to study the water stability of the SFL stabilized soil from a mechanical, chemical and micro-structural point of view. The results reveal that the dispersive soil moisture resistance can be significantly improved with addition of SFL and low content SFL (3%) is slightly better than high content (15%) under no cyclic F-T. This is attributed to the increased cation exchange capacity (CEC) owing to the exchangeable sodium ions used during the pretreatment process of bioethanol. Nevertheless, the high content SFL stabilized samples exhibit superior water stability characteristics when considering the adverse effects of F-T cycles. According to X-ray fluorescence analysis (XRF), no new elemental composition was found indicating that SFL does not bring chemical contamination risk to the soil. However, the organic matter content increases with SFL content. Scanning electron microscopy (SEM) provided visual evidence that the formation of physical bonds and denser soil structure due to SFL, is responsible for the improvements observed. In addition, cracks in the soil matrix caused by F-T cycles were also observed. Overall, utilization of the studied SFL shows good potential to improve the engineering performance of dispersive soils.

Deoxygenation of methyl laurate: influence of cation and mesoporosity in fau zeolites

Catalytic activity of materials prepared by means of sodium ion-exchange by potassium and cesium in NaY zeolite, was studied in the deoxygenation of methyl laurate, as biodiesel model compound. The increase of the Ion-exchange produced a basicity increase that leads to a lower deactivation in the deoxygenation of methyl laurate, reaching a steady conversion of about 92% for 4 h (time on stream, TOS) with the CsKY zeolite. However, the potassium ion exchange leaded to higher production of bio-hydrocarbons, especially unsaturated (96% of the C10, C11 and C12 production), with higher production of C11 by means of decarbonylation. Furthermore, mesoporosity on the KY zeolite was generated, using SDBS (sodium dodecylbenzene sulphonate) during the synthesis, with the aim of increasing its capacity to process bulky molecules. The presence of mesoporosity in the potassium zeolite allowed a higher tolerance to deactivation maintaining a steady conversion of about 90% for 4 h TOS. The yield to desirable products (C10 to C12 hydrocarbons) was about 37%. Therefore, biofuel upgrading can be carried out using potassium Y zeolite with mesoporosity.

Controlled-release urea improved rice yields by providing nitrogen in synchrony with the nitrogen requirements of plants.

Excessive application of nitrogen (N) fertilizer and low nitrogen-use efficiency (NUE) are prevalent problems in rice production. Controlled-release urea (CRU) is widely adopted to increase rice yields, but the synchronicity of N release from CRU with uptake of N by plants has rarely been studied. A 2-year field experiment involving CRU and urea applications at three different N rates (240, 192 and 144 kg N ha-1 , equal to 100%, 80% and 60% of the recommended rate, respectively) was performed to compare their effects on N uptake, soil N content and rice yields. The successive release curves of CRU in the soil matched the corresponding N uptake curves of rice plants, and significant linear correlations were observed. Grain yield and N uptake under the CRU treatment increased by 5.25-7.88% and 7.13-17.94% than urea treatments, at the same N rate, and no obvious difference was found between CRU60% and Urea100%. CRU80% and CRU60% presented the highest NUE. The contents of ammonium-nitrogen (NH4 + -N), nitrate-nitrogen (NO3 - -N), and total N and the chlorophyll relative value - SPAD (Soil Plant Analysis Development) values - of the leaves under the CRU treatments were significantly higher than those under the urea treatments from heading to harvest. The contents of exchangeable sodium ion (Na+ ) and calcium ion (Ca2+ ) and the cation exchange capacity increased in response to CRU. CRU increased rice yields by providing N strongly in synchrony with the N requirements of the plants, and applying CRU at 192 kg N ha-1 was an effective strategy to conserve N fertilizer, increase soil N contents and enhance NUE. © 2021 Society of Chemical Industry.

Pattern selection in coupled neurons under high-low frequency electric field

Biological neurons exposed to an external electric field can induce polarization and charge fluctuation. Indeed, the exchange of calcium, sodium and potassium ions across the cell membrane can induce an electric field as a result of a time-varying electromagnetic field set up. This field could further modulate the cell electrical activity by inducing multiple firing modes. Based on the physical law of electric field, an improved model which includes an additive electrical field variable is constructed for a network of neurons to study wave propagation and mode transition by exploring the longtime dynamics of slightly perturbed plane waves in the network. Wave pattern and mode transition dependence on the different parameters of external electric field are discussed. It is found that the plane wave propagating in the coupled system breaks down to localized structures under the activation of modulational instability. The network under high-low external electric field supports bursting synchronization. This could be a fruitful avenue to discern the occurrence of paroxysmal epilepsy.

Simple synthesis of layered H-octosilicate in situ surface-modified with lecithin

A simple process to synthesize H-octosilicate that is coated with a biocompatible natural surfactant lecithin has been developed, where the hydrothermal synthesis of Na-octosilicate, ion exchange of sodium ions in Na-octosilicate with protons, and surface modification with hydrogenated soy lecithin are sequentially performed in an identical reactor vessel. In the hydrothermal process, the solution pH was appropriately controlled according to a specific pH adjustment procedure, which successfully achieved a high yield (91% in 72 h) of Na-octosilicate. This high yield enabled the conversion of Na-octosilicate to H-octosilicate in the slurry through an acid treatment with no additional operations to remove the unreacted silicate species, such as washing and solid-liquid separation. Subsequently, the addition of a lecithin solution to the H-octosilicate slurry resulted in the formation of lecithin-coated H-octosilicate in situ. The final product (organo-octosilicate) was characterized through XRD, FT-IR, TG-DTA and SEM analyses, which confirmed that it maintained original shapes after the treatments, and lecithin was adsorbed on the surface of H-octosilicate with a mass ratio of approximately 0.24 g-lecithin/g-H-octosilicate. An aqueous suspension of lecithin-coated H-octosilicate showed good dispersion under neutral conditions according to the pH dependence of the zeta potential. In addition, the surface modification inhibited their aggregation even under dry conditions. The results suggest that the developed process contributes to an industrial production of surfactant-coated H-octosilicate.

Novel Approach Through the Harmonized Sulfur in Disordered Carbon Structure for High-Efficiency Sodium-Ion Exchange.

Sodium-ion batteries (SiBs) have recently attracted considerable interest due to the plentiful supply of raw materials for their production and their electrochemical behavior, which is similar to that of lithium-ion batteries (LiBs). However, the relatively larger radius of sodium ions than that of lithium ions is not suitable for storage in conventional graphite, which is widely used as the anode. To resolve this issue, in this study, we developed a new harmonized carbon material with a three-dimensional (3D) grapevine-like structure and a sulfur component using an efficient synthesis process. On the basis of these advantages, the harmonized sulfur-carbon material exhibited a highly reversible capacity of 146 mA h g-1 at an extremely high specific current of 100 A g-1 and long-term galvanostatic cycling stability at 10 and 100 A g-1 with superior electrochemical performance. Our results are anticipated to provide new insights into SiB anode materials that would advance their production.

Synthesis of silica-composited biochars from alkali-fused fly ash and agricultural wastes for enhanced adsorption of methylene blue

Two types of silica-composited biochars were prepared by mixing swine manure or rice straw with alkali-fused fly ash (AFFA) followed by pyrolysis. A 10% (w/w) AFFA modification improved the specific surface area, pore volume, and average pore size of the biochars. Certain surface oxygen-containing functional groups (i.e., –OH and CO) in the biochars were protected, and silicon–oxygen bonds (i.e., O–Si–O and OSi) were strengthened considerably by AFFA modifications during high-temperature pyrolysis. The adsorption capacity of biochar for methylene blue (MB) was enhanced after AFFA modification, and a modified biochar with the highest adsorption capacity was prepared at a pyrolysis temperature of 700 °C, pyrolysis holding time of 2 h, and an AFFA proportion of 10%. The MB adsorption capacity of the modified biochars significantly increased when the pH of the solution increased (from 3 to 13). The adsorption data were well described by a pseudo-second-order model and Langmuir isotherms. The maximum MB adsorption capacities of the modified swine manure and rice straw biochars were 143.76 mg/g and 131.58 mg/g, respectively. The adsorption capacities of the AFFA-modified biochars were 10.7–112.3% higher than those of the unmodified biochars. The enhanced MB adsorption capacities of the former appear to be attributed to their increased specific surface areas, increased porosities, strong oxygen-containing functional groups, and high contents of exchangeable sodium ions. These results indicate that industrial and agricultural wastes can be reused to produce novel silica-composited biochars with high MB removal capacity. Accordingly, these biochars could be effectively used to treat wastewater and thus to mitigate solid waste disposal-related problems.

Ammonium removal using a calcined natural zeolite modified with sodium nitrate

Ammonium is one of the key factors responsible for the eutrophication of water bodies. The purpose of this study was to remove ammonium from water using a natural zeolite (NZ) modified with sodium nitrate (NaNO3) by impregnation and calcination. The ability of the NZ to remove ammonium from water was determined by single calcination; however, its efficiency was significantly enhanced by impregnation with a NaNO3 solution. Zeolite modified with 3.00 M NaNO3 and calcination at 673 K yielded the best ammonium removal efficiency, which was 39.88 % higher than the NZ alone. The zeolites that were regenerated over six times maintained a removal rate of 79.35–84.79 % by mixing 25.0 mg of the NZ into 50 mL of a 5.0 mg/L ammonium solution. The improved performance of the modified zeolite (qm, 16.96 mg/g) was mainly attributed to its relatively elevated mesopore volumes and higher ion-exchange capacity that results from nitrate decomposition, oxygen release, and sodium-ion exchange. The adsorption kinetics and isotherms are best described by the pseudo-first-order (PFO) and Freundlich model, respectively, and the process was endothermic. The effects of other factors, including coexisting ions, pH, and dosage, on ammonium adsorption were also determined.

Organic Montmorillonite Intercalated Nano-composites Prevent Post-Surgical Associated Infections

In this study, organic montmorillonite (OMMT) is a modified form of montmorillonite (MMT) in which chitosan (CS) intercalated MMT by ion exchange of sodium ions from Na/MMT with –NH3 + of CS. The structural analysis confirmed intercalation of CS into MMT layers, indicating that CS molecular chains incorporated into the MMT layers. The interlayer distance of the MMT layered was 1.128nm and in the OMMT layers enlarged to 2.365 nm. Antibacterial activity analysis showed that unmodified MMT could not inhibit the growth of bacteria. Nevertheless, after addition of the CS molecules, an increase in the interlayer distance of MMT was observed. No difference was observed between the viability of the human dental pulp stem cells (hDPSCs) contacted to different concentrations (ranging from 0.5 to 2mg/ml) of MMT and OMMT in all time intervals, when compared with the control samples. Furthermore, neither MMT nor OMMT showed apoptosis and cytotoxicity effect on the cells. The strong antibacterial activity of the synthesized OMMT nanocomposite was also confirmed against E. coli, S. aureus, K. pneumonia and P. aeruginosa, suggesting its high potential for the prevention of post-surgical infections.

High Capacity and High‐Rate NASICON‐Na3.75V1.25Mn0.75(PO4)3 Cathode for Na‐Ion Batteries via Modulating Electronic and Crystal Structures

AbstractSodium superionic conductor (NASICON) cathodes are attractive for Na‐ion battery applications as they exhibit both high structural stability and high sodium ion mobility. Herein, a comprehensive study is presented on the structural and electrochemical properties of the NASICON‐Na3+yV2−yMny(PO4)3 (0 ≤ y ≤ 1) series. A phase miscibility gap is observed at y = 0.5, defining two solid solution domains with low and high Mn contents. Although, members of each of these domains Na3.25V1.75Mn0.25(PO4)3 and Na3.75V1.25Mn0.75(PO4)3 reversibly exchange sodium ions with high structural integrity, the activity of the Mn3+/Mn2+ redox couple is found to be absent and present in the former and latter candidate, respectively. Galvanostatic cycling and rate studies reveal higher capacity and rate capability for the Na3.75V1.25Mn0.75(PO4)3 cathode (100 and 89 mA h g−1 at 1C and 5C rate, respectively) in the Na3+yV2−yMny(PO4)3 series. Such a remarkable performance is attributed to optimum bottleneck size (≈5 Å2) and modulated V‐ and Mn‐redox centers as deduced from Rietveld analysis and DFT calculations, respectively. This study shows how important it is to manipulate electronic and crystal structures to achieve high‐performance NASICON cathodes.

Ion Binding and Selectivity of the Na+/H+ Antiporter MjNhaP1 from Experiment and Simulation.

Cells employ membrane-embedded antiporter proteins to control their pH, salt concentration, and volume. The large family of cation/proton antiporters is dominated by Na+/H+ antiporters that exchange sodium ions against protons, but homologous K+/H+ exchangers have recently been characterized. We show experimentally that the electroneutral antiporter NhaP1 of Methanocaldococcus jannaschii (MjNhaP1) is highly selective for Na+ ions. We then characterize the ion selectivity in both the inward-open and outward-open states of MjNhaP1 using classical molecular dynamics simulations, free energy calculations, and hybrid quantum/classical (QM/MM) simulations. We show that MjNhaP1 is highly selective for binding of Na+ over K+ in the inward-open state, yet it is only weakly selective in the outward-open state. These findings are consistent with the function of MjNhaP1 as a sodium-driven deacidifier of the cytosol that maintains a high cytosolic K+ concentration in environments of high salinity. By combining experiment and computation, we gain mechanistic insight into the Na+/H+ transport mechanism and help elucidate the molecular basis for ion selectivity in cation/proton exchangers.

High expression of TRPV-1 is associated with poor prognosis in cervical cancer

Objective: The Transient Receptor Potential Vanilloid type 1 (TRPV-1) is one of the well-investigated transient receptor potential (TRP) channels, nonselective membranous ion channels associated with calcium and sodium ion exchange. Its role has been pronounced in malignancies. However, the role of TRPV-1 in cervical cancer has not yet been elucidated. Here, we investigated the expression and clinical significance of TRPV-1 in cervical cancer.

Mechanism of the electroneutral sodium/proton antiporter PaNhaP from transition-path shooting

Na+/H+ antiporters exchange sodium ions and protons on opposite sides of lipid membranes. The electroneutral Na+/H+ antiporter NhaP from archaea Pyrococcus abyssi (PaNhaP) is a functional homolog of the human Na+/H+ exchanger NHE1, which is an important drug target. Here we resolve the Na+ and H+ transport cycle of PaNhaP by transition-path sampling. The resulting molecular dynamics trajectories of repeated ion transport events proceed without bias force, and overcome the enormous time-scale gap between seconds-scale ion exchange and microseconds simulations. The simulations reveal a hydrophobic gate to the extracellular side that opens and closes in response to the transporter domain motion. Weakening the gate by mutagenesis makes the transporter faster, suggesting that the gate balances competing demands of fidelity and efficiency. Transition-path sampling and a committor-based reaction coordinate optimization identify the essential motions and interactions that realize conformational alternation between the two access states in transporter function.