External Electrolyte Concentration Research Articles

Optimized microbial fuel cell-powered electro-Fenton processes to enhance electricity and bisphenol A removal by varying external resistance and electrolyte concentrations

This study is the first to investigate the effects of external resistance and electrolyte concentration on the performance of a bioelectro-Fenton (BEF) system, involving measurements of power density, H2O2 generation, and bisphenol A (BPA) removal efficiency. With optimized operating conditions (external resistance of 1.12 kΩ and cathodic NaCl concentration of 1,657 mg/L), the BEF system achieved a maximum power density of 38.59 mW/m2, which is about 3.5 times higher than with 1 kΩ external resistance and no NaCl. This system featured a 71.7 % reduction in total internal resistance. The optimized BEF also accelerated the oxygen reduction reaction rate, increasing H2O2 generation by 4.4 times compared to the unoptimized system. Moreover, it exhibited superior BPA degradation performance, removing over 99 % of BPA within 14 hs, representing a 1.1 to 3.3-fold improvement over the unoptimized BEF. By the fifth cycle (70 h), the optimized BEF still removed 70 % of BPA. Optimizing the operating conditions significantly increased the abundance of electrochemically active bacteria (Pseudomonadaceae) from 2.2 % to 20 %, facilitating rapid acclimation. The study demonstrates the strong potential of an optimized BEF system for removing persistent pollutants.

Response of current distribution in a liter-scale microbial fuel cell to variable operating conditions

Microbial fuel cells (MFCs) are an emerging technology in renewable energy and waste treatment and the scale-up is crucial for practical applications. Undoubtedly, the analysis and comprehension of MFC operation necessitate essential information regarding the response of the current distribution to variable operating conditions, which stands as one of its significant dynamic characteristics. In this study, the dynamic responses of current distribution to external stimuli (external load, temperature, pH, and electrolyte concentration) were investigated by employing a segmented anode current collector in a liter-scale MFC. The results demonstrated that, with respect to the anodic segment close to the cathode, a major response of the segment current to changes in load, temperature and pH was observed while minor response to changes in ion concentration. It was also found that external stimuli-induced high current usually led to a worse current distribution while increasing electrolyte ion concentration could simultaneously improve the maximal power generation and current distribution. In addition, the response time of segment current to input stimulus followed the pattern of temperature ˃ pH ˃ ion concentration ˃ external load. The results and implication of this study would be helpful in enhancing the operational stability of scale-up MFCs in future practical application.

Preparation and Characterization of 18-Crown-6 Modified Cellulose Acetate Membrane in Uni-Univalent Aqueous Electrolyte Solutions

In present study, a cellulose acetate/18-crown-6 blend membrane (selective for K+ ions) was prepared by the solvent vapourization method. The surface morphology of the modified membrane was characterized by scanning electron microscopy (SEM) analysis, which indicate the smooth and homogeneous membrane surface. The effect of water and electrolyte (KCl and NaCl) concentration on the rate of water sorption and porosity was explained on the basis of physico-chemical parameters i.e. water content and water activity of the membrane phase. The variation of ionic transport number with respect to water activity, i.e. [dtmK+/dam w > dtmNa+/dam w] has been observed in the present system. The transport of ions under concentration gradient in term of permeability, flow and flux was estimated by membrane/solution conductance-time data. The variations of permeability, flux and flow values with respect to time for K+ and Na+ ions were found to be different. Membrane water activity (am w) decreases with the increase of external electrolyte concentration. The effective fixed charge density of the membrane increases with increase of external electrolyte concentration. Permselectivity parameter of the membrane in both cases decline with increase of external electrolyte concentration.

The Correlation between the Water Content and Electrolyte Permeability of Cation-Exchange Membranes.

The salt permeability through three commercial cation-exchange membranes with different morphologies is investigated in aqueous NaCl solutions. Ion-exchange membranes (IEMs) find application in different processes such as electrodialysis, reverse osmosis, diffusion dialysis, membrane electrolysis, membrane fuel cells and ion exchange bioreactors. The aim of this paper is the experimental determination of the electrolyte permeability in the following membranes: MK-40 membrane, Nafion N324 membrane and Nafion 117 membrane. The latter is selected as being a reference membrane. The effect of an increase in the NaCl concentration in the solutions on membranes transport properties is analyzed. With regard to membranes sorption, a decrease in the water content was observed when the external electrolyte concentration is increased. Concerning permeation through the membranes, the salt permeability increased with concentration for the Nafion 117 membrane and remained nearly constant for the other two membranes. A close relation between the degree of liquid sorption by the membranes and the electrolyte permeability was observed.

Construction of Ag3PO4/Ag4P2O7 nanospheres sensitized hierarchical titanium dioxide nanotube mesh for photoelectrocatalytic degradation of methylene blue

Ag3PO4/Ag4P2O7 nanospheres were deposited on the surface of TiO2 nanotube arrays/Ti mesh by the hydrothermal method, and the cetyltriethylammnonium bromide (CTAB) addition amount significantly influenced the morphology, phase composition, solar absorption, photoelectric conversion and photoelectrocatalytic (PEC) degradation of methylene blue (MB). The results indicated that the sample prepared with the CTAB addition of 0.025 g showed the optimal visible light photocurrent density (103.51 μA/cm2), photovoltage (−0.19 V/cm2) and PEC efficiency (99.39%) for MB decomposition. The influence factors of PEC performances including external voltage and electrolyte concentration were further explored. Beside, the PEC mechanism of TiO2 NTs/Ag3PO4/Ag4P2O7 was tentatively proposed. The excellent photoelectrochemical activity of the three dimensional (3D) TiO2 NTs/Ag3PO4/Ag4P2O7 photoelectrode would drive it prospective applications in solar cells and waste-water treatment.

Extraction of hydrophobic analytes from organic solution into a titanate 2D-nanosheet host: Electroanalytical perspectives

Titanate nanosheets (single layer, typically 200 nm lateral size) deposited from aqueous colloidal solution onto electrode surfaces form lamellar hosts that bind redox active molecular redox probes. Here, hydrophobic redox systems such as anthraquinone, 1-amino-anthraquinone, deca-methylferrocene, 5,10,15,20-tetraphenyl-21H,23H-porphine manganese (III) chloride (TPPMnCl), and α-tocopherol are shown to bind directly from cyclopentanone solution (and from other types of organic solvents) into the titanate nanosheet film. For anthraquinone derivatives, stable voltammetric responses are observed in aqueous media consistent with 2-electron 2-proton reduction, however, independent of the pH of the outside solution phase environments. For decamethylferrocene a gradual decay of the voltammetric response is observed, but for TPPMnCl a more stable voltammetric signal is seen when immersed in chloride containing (NaCl) electrolyte. α-Tocopherol exhibits chemically irreversible oxidation and is detected with 1 mM–20 mM linear range and approximately 10−3 M concentration limit of detection. All redox processes exhibit an increase in current with increasing titanate film thickness and with increasing external electrolyte concentration. This and other observations suggest that important factors are analyte concentration and mobility within the titanate host, as well as ion exchange between titanate nanosheets and the outside electrolyte phase to maintain electroneutrality during voltammetric experiments. The lamellar titanate (with embedded tetrabutyl-ammonium cations) behaves like a hydrophobic host (for hydrophobic redox systems) similar to hydrophobic organic microphase systems. Potential for analytical applications is discussed.

Construction of Er3+ :YAlO3 /RGO/TiO2 Hybrid Electrode with Enhanced Photoelectrocatalytic Performance in Methylene Blue Degradation Under Visible Light.

Much attention has been paid on doping TiO2 to narrow its band gap to promote the absorption of visible light and restrain the recombination of electron-hole pairs to improve its efficiency in photoelectrocatalysis (PEC) under visible-light irradiation. However, the oxidation potential energy of photo-induced holes for the modified catalysts by visible-light excitation is lower than that without modification by UV excitation. In this work, we synthesized a co-coupled TiO2 electrode (denoted ERT) with the Er3+ :YAlO3 and reduced graphene oxide (RGO), achieving the synergetic effect of visible-light-to-UV up-conversion and response and great electron transfer ability. The effects of external bias voltage, electrolyte concentration and pH on the PEC activity were studied with the methylene blue (MB) as the target pollutant. The results indicated that PEC by the ERT electrode showed the highest MB removal compared with those by the electrodes coupled with RGO or Er3+ :YAlO3 alone. In addition, the kinetic rate constant of the PEC process using the ERT electrode was higher than the sum of those of the photocatalytic and electrocatalytic processes. The optimal conditions for PEC by the ERT electrode were an external bias voltage of 1.0 V, 0.1 mol L-1 Na2 SO4 and pH = 10.

Photoelectrocatalytic degradation of refractory organic compounds enhanced by a photocatalytic fuel cell

A novel composite photoelectrocatalytic system, composed of a photoelectrocatalytic reactor and a photocatalytic fuel cell (PFC), was established with the aim to efficiently degrade refractory organic compounds. The PFC is device, in which the organic “substrates” were photodecomposed and the chemical energy of organic “substrates” was converted into electrical energy simultaneously. Within the composite system, the electricity produced by PFC, rather than external power supply, was exerted on the photoelectrocatalytic reactor to serve as potential bias. The organic compound degradation performance and affecting factors of the composite system were also studied. The experimental results demonstrate that the removal efficiency of organic compound in the photoelectrocatalytic reactor was evidently enhanced by the PFC system. The kinetic constant of the photoelectrocatalytic process was found to be ∼1.75 times that of the pure photocatalytic process. Furthermore, the performance of the photoelectrocatalytic reactor was found to increase with the illumination area (of the anode) of PFC and decrease with the external resistor and electrolyte concentration. The category and concentration of the substrate in PFC system were also key factors influencing the efficiency of organic compound removal in the photoelectrocatalytic reactor. The composite system provides a novel way for utilizing of the electrical energy generated from the PFC system and has potential applications for the treatment of various refractory organic pollutants.

Degradation of Organic Pollutants in a Photoelectrocatalytic System Enhanced by a Microbial Fuel Cell

Photocatalytic oxidation mediated by TiO(2) is a promising oxidation process for degradation of organic pollutants, but suffers from the decreased photocatalytic efficiency attributed to the recombination of photogenerated electrons and holes. Thus, a cost-effective supply of external electrons is an effective way to elevate the photocatalytic efficiency. Here we report a novel bioelectrochemical system to effectively reduce p-nitrophenol as a model organic pollutant with utilization of the energy derived from a microbial fuel cell. In such a system, there is a synergetic effect between the electrochemical and photocatalytic oxidation processes. Kinetic analysis shows that the system exhibits a more rapid p-nitrophenol degradation at a rate two times the sum of rates by the individual photocatalytic and electrochemical methods. The system performance is influenced by both external resistor and electrolyte concentration. Either a lower external resistor or a lower electrolyte concentration results in a higher p-nitrophenol degradation rate. This system has a potential for the effective degradation of refractory organic pollutants and provides a new way for utilization of the energy generated from conversion of organic wastes by microbial fuel cells.

Optimization of electrochemical treatment of industrial paint wastewater with response surface methodology

The electrochemical oxidation of water-based paint wastewater was investigated batch-wise in the presence of NaCl electrolyte with carbon electrodes for the first time in literature. The electrochemical treatment conditions were optimized using response surface methodology where potential difference, reaction temperature and electrolyte concentration were to be minimized while chemical oxygen demand (COD), color and turbidity removal percents and initial COD removal rate were maximized at 100% pollution load. The optimum conditions were satisfied at 35 g/L external electrolyte concentration, 30 °C reaction temperature and 8 V potential difference (64.37 mA/cm 2 current density) realizing 51.8% COD and complete color and turbidity removals, and 3010.74 mg/L h initial COD removal rate. According to these results, the electrochemical method could be a strong alterative to conventional physicochemical methods for the treatment of water-based paint wastewater.

Studies on electrochemical characterization and performance prediction of cellulose acetate and Zeocarb-225 composite membranes in aqueous NaCl solutions

We have mixed cellulose acetate and Zeocarb-225 in different ratios, leading to the preparations of Membrane-1 and Membrane-2. Membrane potential, water content, and conductance measurements have been carried out to estimate and analyze the data in terms of equilibria and important electrochemical parameters. The Donnan equilibrium has been incorporated to estimate the activity coefficient of counterions, ypM, and solute, y±M in the membrane phase along with the parameter, so called ϕ expressing non-ideality. Dependence of the extent of hydrophilicity of both membranes on mean electrolyte concentrations has been examined. Selectivity in membranes is discussed in terms of dissociation equilibria, Kds and Kdf. It has been found that membrane surface charge density σs increases with increasing of external NaCl concentration. Dependence of water transport number and cationic transport number on electrolyte concentration shows a similar trend of variation. At higher mean concentration of electrolyte, water transport number in Membrane-2 has a negative value. Membrane-2 has a higher value of water transport number than Membrane-1. The entropy production due to solute and water transport has been quantified for both the membranes in the light of nonequilibrium thermodynamics. The various type of interactions such as solute–membrane, solute–water, and water–membrane are analyzed in terms of friction coefficients (fij) of Spiegler's frictional pore model. In our case, an fwm<fsm<fsw-like trend is observed in both membranes. These frictional coefficients show close dependence on external electrolyte concentrations. Pore potential values of Membrane-1 and Membrane-2 have been worked out using the Poisson–Boltzmann equation. In both systems pore potential values increase with increasing mean electrolyte concentrations. The transport through Membrane-1 and Membrane-2 tends to follow diffusion-control criteria, i.e., (D±⋅C⋅d/D±MCM⋅δ)≫2. A slightly higher value of solute rejection is found in Membrane-2.

Surface resistance measurements on thin gold film electrodes coated with poly( o-aminophenol) films

The relative resistance change (Δ R/ R) of a thin gold film electrode modified by a poly( o-aminophenol) (POAP) film was monitored during the oxidation–reduction process of the polymer. These resistance changes were analysed in terms of the specularity ( r) of the metal ∣ polymer interface for the reflection of the conduction electrons from inside the gold film on its surface. When the polymer is reduced (amine sites) the metal ∣ polymer interface exhibits a higher degree of specularity than when it is oxidised (imine sites). The observed resistance changes as a function of the degree of oxidation ( θ) of the polymer allows one to differentiate between different distributions of oxidised sites at the metal ∣ polymer interface as θ increases. The distribution of oxidised sites at low degrees of oxidation ( θ<0.2) of the polymer film presents a less reflecting interface for the conduction electrons of the gold film than the corresponding distribution at higher degrees of oxidation ( θ>0.3). This was interpreted on the basis of an initial distribution of oxidised sites with neighbour distances larger than those corresponding to the distribution at higher θ values. However, for thin POAP films, the Δ R/ R change at high θ values also depends on the polymer film thickness and external electrolyte concentration. Thus, for different θ values a ratio of resistance changes (Δ R θ=0.9 /Δ R θ=0.1 ) was used to evaluate the effect of the external electrolyte on the resistometric changes due to the proper redox transformation of the polymer. The influence of the polymer film thickness and external electrolyte concentration on the specularity of the metal ∣ polymer interface was attributed to changes in the polymer morphology as the film thickness varies. Decrease in the specularity of the metal ∣ polymer interface at high θ values with decreasing polymer film thickness and increasing external electrolyte concentration was associated with open structures at low film thickness which allow incorporation of electrolyte into the polymer matrix. Also, the presence of redox active species in the electrolyte solution, such as hydroquinone–benzoquinone, which are able to reach the metal ∣ polymer interface affect the reflection of the conduction electrons on the gold film surface. Also, this last effect was more pronounced for thin polymer films.

Voltammetric study of the reduction and relaxation of poly(o-toluidine). Effect of the polymer thickness and the external electrolyte nature and concentration

The reduction and relaxation of poly(o-toluidine) (POT) was studied as a function of the wait time at different waiting potentials near the reduction potential of the polymer. The influence of the film thickness, the acid concentration, and the ionic strength of the external electrolytic solution on these processes were also studied. Two types of electrolytes were employed: perchloric and sulfuric acid. Both the reduction and the relaxation times depend on the proton concentration of the external electrolyte media and on the film thickness. They are independent of the ionic strength and, in a limited range, of the waiting potential. The voltammetric response of fully reduced and relaxed polymers shows that, at low sweep rates, the kinetics are controlled by slow ionic movements within the polymer. Experiments with medium exchange show that, once the polymer is fully reduced and relaxed, its state is independent of the composition and concentration of the electrolyte in which this particular state was obtained. Furthermore, they also show that the shape of the voltammetric oxidation profile depends exclusively on the composition and concentration of the electrolyte in which the polymer is being oxidized. This means that the effect of the solution composition and concentration is manifested only through the participation of protons and anions in the mechanism of oxidation of the polymer.

Membrane Potential across a High Water Content Anion-Exchange Membrane Separating Two Solutions with a Common Counterion but Two Different Co-ions

A theory for bi-ionic potential, especially for bi-co-ionic potential, which is a potential difference across a membrane due to the difference in co-ions, is proposed. A high water content interpolymer membrane of poly(ethyleneimine) and poly(vinyl alcohol) was prepared as a low-charged anion-exchange membrane. The potential difference Δψ for the system of KCl/membrane/1/2MgCl2 or 1/2CaCl2 was measured by changing the external electrolyte concentrations cs and analyzed according to the theory. The bi-co-ionic potential Δψ increased with increasing cs at low concentrations, and after the potential Δψ reached a maximum value, the potential Δψ decreased with increasing cs at high concentrations; that is, a bell-shaped dependence of Δψ was observed against cs. This is because the low mobility of co-ions with charge number +2 in the charged membranes increases with increasing cs and finally reaches the mobility in the free electrolyte solutions.

Electrochemical properties of cellulosic ion-exchange membranes II. Transport numbers of ions and electro-osmotic flow

Transport numbers of counter-ions inn cellulosic cation- and anion-exchangemembranes have been determined by both emf and Hittorf's methods for various (1:1)-electrolytes at different concentrations. The transport number derived from the emf data, in general, is lower than the value obtained by the Hittorf method, for any given external electrolyte concentration. This difference is attributed to transport of water occurring across the membrane. Electro-osmotic water transport across the membranes in equilibrium with several electrolytes has been determined at different concentrations. The data obtained have been interpreted in terms of the hydrated ion size of the counter-ions. Volume flow and electrical conductance across the membranes in the presence of various electrolytes have been measured at several electrical potential differences. The experimental results have been interpreted in terms of linear phenomenological equations. These relationships have been used to estimate the magnitudes of the phenomenological coefficients for volume flow and conductance across the cationic and anionic membranes. The validity of the phenomenological equation in volume flow across the membranes has been tested by comparison with the data obtained by direct measurements.

Quantitative Presentation of Potentiometric Titration Curves of Ion Exchangers

Neutralization of ion exchangers in H + or OH - forms by alkali M(OH) z or acid H z A was considered as a specific case of ion exchange H + -M z+ or OH - -A z- accompanied by the neutralization reaction. An equation connecting pH and the external electrolyte concentration with the ion exchanger neutralization degree was derived. It can be used for presentation of the potentiometric titration curves in mathematical form or their calculation at different (not necessarily constant) concentrations of external electrolyte. Applicability of the modified Henderson-Hasselbach equation as one of the possible means for description of potentiometric titration curves of ion exchangers was also discussed. The range of its applicability and limitations were established. The physical meaning of its constants was clarified, and the equations of their relations with the apparent ion exchange equilibrium constant have been obtained. The theoretical considerations are illustrated with the example of potentiometric titration curves of a carboxylic acid ion exchanger obtained in a wide range of degrees of neutralization and external electrolyte concentrations.

The double layer of activated carbon electrodes: Part 1. The contribution of ions in the pores

The changes produced in the electrolyte concentration upon introducing well-defined activated carbon particles to the solution and upon polarizing the carbon material have been studied in order to obtain quantitative information about the double layer. Immobilization of a certain amount of water, corresponding to a monomolecular layer on the pore surface, results in an increase in the external electrolyte concentration in the case of KF and K 2SO 4, while the adsorption of anions (Cl −, Br −, NO 3 −, I −, ClO 4 −, SCN −) or cations (H +, tetra-alkyl ammonium ions) produces a substantial accumulation of electrolyte in the pores. Owing to the narrowness of the pores, there is a membrane potential at the pore openings instead of a Gouy-Chapman layer near the phase boundary. In the presence of adsorbed anions, the charge on the polarized carbon material is balanced by a variation of the amount adsorbed, with the cation concentration in the pores remaining almost constant. In the variation of which is balanced by the SO 4 2− ion. This behaviour suggests the formation of COH groups on adsorption of H + at CO surface groups.

Electrochemical properties of cellulosic ion-exchange membranes I. Equilibrium and permselectivity

The emf values of the cell Ag,AgCl|KCl( a′)|membrane|KCl( a″)|Ag,AgCl have been measured. Two types of cellulosic strong ion-exchange membranes were tested, containing either cationic or anionic fixed groups. The membranes were prepared by grafting acrylic monomers onto paper followed by opening of epoxy groups. The membrane phase was analysed for its electrolyte and water contents, and intramembrane activity coefficients were derived as a function of external electrolyte concentration. The data were used for calculation of the emf of membrane cells using the TMS theory reformulated by Hills et al. The main finding of this investigation is that, for dilute external solutions, the membrane potentials are close to the maximum theoretical value and can be calculated with reasonable accuracy using the fixed charge theory. With a more concentrated solution this is not so; the membrane potentials are progressively smaller than the maximum value and the theory did not give satisfactory agreement between the observed and the calculated emf of the membrane cells. This is readily explained by the decrease in the selectivity of the membranes, the increasing concentrations of co-ions, and the diffusion of electrolyte through the membrane.

Effects of average molecular charge on amino acid interfacial partitioning in reversed micelles

The effect of pH on amino acid interfacial partitioning in reversed micelles is reported. A simple thermodynamic analysis of the data allows estimates of the interfacial electrostatic potential in these reversed micelles to be made, from which the effects of external electrolyte type and concentration may be inferred. Good agreement with the predictions of the Poisson-Boltzmann equation is obtained. Inferences can be drawn as to the orientation of the amino acids within the structured interfaces, and it is argued that the amino acid headgroup dipole must be aligned almost normal to the direction of the electric field. Changes in amino acid speciation in reversed micellar systems, coupled with an apparent shift in pH of the external aqueous phase, can be attributed to a concentration of the hydroxonium ion in the reversed micellar water pools, and not to changes in intrinsic dissociation constants. It is demonstrated that species distribution in the AOT Winsor II system closely resembles various chromatographic processes, there being a direct analogy between the chromatographic retention coefficient and the reversed micellar interfacial partition coefficient.

Direct measurement of interaction forces in free thin liquid films stabilized with phosphatidylcholine

Microscopic foam films from suspensions of small unilamellar dimyristoylphosphatidylcholine (DMPC) vesicles have been obtained. The film formation is facilitated at temperatures above the gel-liquid crystalline transition of DMPC. A detailed study of the dependence of equilibrium thickness of DMPC foam films on electrolyte concentration at constant capillary pressure and a direct measurement of the disjoining pressure isotherm has been carried out. Formation of thick equilibrium horizontal microscopic films stabilized with DMPC at low external pressures and low electrolyte concentrations was found and interpreted as being due to the existence of long-range electrostatic interactions in these films. A diffuse electric layer potential of 36 mV has been calculated. The DMPC films have been compared to films obtained from non-ionic surfactant solutions where the long range electrostatic repulsion is explained as being due to specific adsorption of OH− at the film interfaces. However, unlike the results obtained for surfactant films, in this study formation of common black films and thinning of the DMPC Newton film with pressure have been observed.