Charge Exclusion Research Articles

An iron-modified silica nanofiltration membrane: Effect of solution composition on salt rejection

Electrolyte rejection measurements with single salt solutions of NaCl, Na 2SO 4, MgCl 2, and MgSO 4 were performed to determine the effects of solution composition on the separation performance of an iron-modified silica nanofiltration ceramic membrane. The unsupported membrane material was characterized with respect to pore structure and zeta potential, whereas the supported membrane was utilized to evaluate gas diffusion properties, water flux and salt rejection. Charge exclusion mechanisms were determined to be the basic mode of salt rejection, and a qualitative relationship was constructed to relate salt rejection to the zeta potential of the membrane. The results obtained agree with the Donnan exclusion principle of electrically charged membranes. The data shows that the membrane zeta potential is not constant but depends extensively on the type of electrolyte in solution. For example, there is a dramatic drop in zeta potential when the membrane is exposed to magnesium salts as opposed to sodium salts. This drop in potential at the membrane surface leads to little rejection of magnesium salts. Results also show a charge reversal of the membrane zeta potential at high pH values due to specific adsorption of magnesium.

Polydivinylbenzene/Ethylvinylbenzene Composite Membranes for the Optimization of a Whole Blood Glucose Sensor

AbstractA novel ultra thin polydivinylbenzene/ethylvinylbenzene composite membrane has been developed for use as the outer covering barrier in a model amperometric glucose oxidase enzyme electrode. The composite membrane was formed via the cathodic electropolymerization of divinylbenzene/ethylvinylbenzene at the surface of gold sputter coated host alumina membranes, (serving solely as a mechanical support for the thin polymer film). Permeability coefficients were determined for the enzyme substrates, O2 and glucose, across composite membranes formed with a range of polymer thicknesses. Due to the highly substrate diffusion limiting nature of the composite membrane, it was found that anionic interferents present in blood (such as ascorbate), were effectively screened from the working electrode via a charge exclusion mechanism, in a manner similar to previous findings within our laboratory. The enzyme electrode showed an initial 32% signal drift when first exposed to whole human blood over a period of 2 hours, after which time enzyme electrode responses remained essentially stable. Whole blood patient glucose determinations yielded a correlation coefficient of r2=0.97 in comparison to standard hospital analyses.

Biosensing Properties of TitanateNanotube Films: Selective Detection of Dopamine in the Presence of Ascorbate and Uric Acid

AbstractA novel strategy based on titanate nanotubes (TNTs) for developing an electrochemical biosensor is proposed. Stable TNT films are fabricated on glassy carbon (GC) electrodes by a casting technique. Cyclic voltammetry, electrochemical impedance spectrometry, and linear‐sweep voltammetry are used to characterize the TNT membrane‐covered GC electrodes (TNT/GCs). The TNT film is shown to demonstrate selectivity by charge exclusion. The TNT film is also shown to be capable of improving the mass transport to the electrode surface and electron transfer between dopamine (DA) and the electrode. Therefore, DA exhibits a quasireversible electrochemical reaction at the TNT/GC electrode. The voltammetric signal of DA is well resolved from those of ascorbate (AA) and uric acid (UA) at the TNT/GC electrode; therefore, DA can be selectively detected in the presence of a large excess of AA and UA at physiological pH. The linear calibration curve for DA is obtained over the concentration range 0.1–30 μM in a physiological solution that contains 0.1 mM AA and 0.3 mM UA.

Aspergillus niveus Blochwitz 4128URM: new source for inulinase production

Aspergillus niveus Blochwitz 4128 URM isolated from sunflower rhizosphere demonstrated a new source of inulinase. The enzyme was produced in culture medium containing inulin as substrate in the concentrations: 10, 15 and 20g L-1. Maximum enzyme activity was obtained in medium containing 20g L-1 inulin. The enzyme was partially purified using ammonium sulphate precipitation, followed by ion charge (DE-32) and molecular exclusion (Sephadex) chromatography. The results showed the optimal pH and temperature of inulinase from crude extract were 4.0 and 4.8 and 45ºC, respectively. The enzyme was purified 34.65 fold with yield of 53.63%. A. niveus 4128URM can be used in the inulinase production with use in the food industries.

Surface pKa of Self-Assembled Monolayers

The concept of surface pKa is introduced and developed for self-assembled monolayers (SAMs). Students use a titration method to determine the pKa of 3-mercaptopropionic acid (MPA) in solution. At the same time they determine the surface pKaof a SAM of MPA on a gold electrode by running a series of cyclic voltammograms of ferricyanide in buffers of differing pH. At different pH the extent of ionization of the SAM changes. Therefore, as ferricyanide is anionic, the charge exclusion of the ferricyanide, as ascertained from cyclic voltammetry peak currents, allows the inference of the charge of the MPA SAM. A plot of the peak current versus pH yields surface pKa values consistent with the literature. The dual aims of elucidating pKa and gaining experience in cyclic voltammetry are complementary. This practical exercise highlights the differences between surface layers and solution components with applications across a range of disciplines.

Influence of surface charge and solution pH on the performance characteristics of a nanofiltration membrane

Acetyl cellulose nanofiltration membrane (NF) was manufactured by a dry/wet phase inversion process and studied on its performance characteristics for the electrolyte solutions NaCl, MgCl2,Na2SO4 with a concentration range of 10-3–10-2 N at various pH values. It was shown that the multivalent cation Mg2+ or sulfate anion SO2−4 present in non-symmetrical electrolytes makes the membrane more positively or negatively charged as a result of their adsorption on the amphoteric pore surface, which greatly increases the membrane selectivity up to 85–95% for these electrolyte solutions. Meanwhile,with respect to 1:1 electrolyte solution NaCl the rejection capacity is low (R=30–40%) because there is essentially no adsorption on the membrane surface. For a feed source made of the RO retentate the experimental data showed that the fabricated membrane possesses high rejection capacity for multivalent ions such as SO2−4 (R=85.7%) and Mg2+ (R=93.5%) and low selectivity for monovalent salts such as NaCl (R=33.7%). The following salts rejection sequence with respect to symmetric and nonsymmetric electrolyte solutions has been set up: RNa2SO4>RNaCl>RMgCl2 and RMgCl2>RNa2SO4>RNaCl at pH≥7 and pH≤7, respectively.The fact that at pH values above 7 rejection of sodium chloride is higher than that of magnesium chloride, although its ion size is smaller than that of the latter, can be explained only by charge exclusion effect. From these results it can be concluded that Donnan exclusion effect is the predominant mechanism for salt removal by NF membranes.

Electrochemical investigation of the permselectivity of a novel positively-charged sol–gel silicate prepared from tetraethyloxysilane and N-octadecyldimethyl[3-(trimethoxysilyl)propyl]ammonium chloride

A positively-charged sol–gel silicate was simply prepared by hydrolyzed and co-polymmerized N-octadecyldimethyl[3-(trimethoxysilyl)propyl]ammonium chloride (C 18-TMS) and tetraethyloxysilane (TEOS). The permeation of the prepared silica film was investigated by cyclic voltammetry (CV). The film shows selectivity by charge exclusion, that is, favorably selective permeation of anionic redox probe and rejection of cationic one. Furthermore, the film shows a little higher relative response to inorganic anionic redox probe than its organic counterpart.

U, Th, Eu and colloid mobility in a granite fracture under near-natural flow conditions

Summary Laboratory core migration experiments were performed in a granite fracture from the Grimsel Test Site (GTS, central Swiss Alps). The flow velocity was varied (46 m yr-1, 94 m yr-1, 187 m yr-1) and solutions with 10-6 mol L-1 U(VI), 10-8 mol L-1 Th(IV) and 10-8 mol L-1 Eu(III) without (cocktail I) and with addition of 2 mgL-1 bentonite colloids (cocktail II) have been injected. Results are compared with those obtained in a field study at the GTS. Flow field-flow fractionation and ultrafiltration measurements show that U(VI) is not colloid borne, different from Th(IV) and Eu(III) which are associated with colloids in both spiked natural groundwaters with and without bentonite colloid addition. The partly unretarded U(VI) migration observed at short contact times (1-2 h) diminished under near-natural groundwater velocities (∼46 m yr-1) and only a weak retardation (R f=19.5) could be observed. Furthermore, the U(VI) mass recovery decreased with increasing contact time and was as expected unaffected by bentonite colloid addition. A groundwater colloid mediated Th(IV)/Eu(III) migration without bentonite colloid addition could be confirmed in the core experiments but only at fast groundwater flow rates. Experiments at a water flow rate of 94 m yr-1 with cocktail II demonstrated a partially bentonite colloid facilitated transport of Th(IV) and Eu(III) with 27% and 37% mass recovery, respectively. At long tracer residence times in the flow field, no breakthrough of colloidal Th(IV)/Eu(III) could be observed in both runs, indicating the strong dependence of reaction kinetics on their mobility. Reference colloid experiments using fluorescence dye labeled carboxylated polystyrene microspheres (25 nm, 50 nm and 100 nm) revealed in general a higher recovery of smaller colloid size classes increasing with groundwater velocity. Additionally, an earlier peak arrival time of colloids and colloid-associated Th(IV) and Eu(III) was observed with respect to the conservative tracer 3H in all experiments due to pore-size or charge exclusion effects. In general, the colloid recoveries found in the laboratory core experiments are lower than those obtained in the field studies. The decrease of colloid mobility with increasing residence time suggests the occurrence of colloid attachment to the rock surfaces even under the colloid stabilizing groundwater conditions.

Removal of arsenic and pesticides from drinking water by nanofiltration membranes

The removal of arsenic and pesticides from natural ground water from the Slavonia region, Croatia, by two commercial nanofiltration membranes (NF270 and NFc) was investigated. The nanofiltration membranes and a comparing reverse osmosis membrane were first examined using sodium dibasic arsenate solution and judged against their performances using sodium chloride and sodium sulfate solutions. The rejections of the sodium dibasic arsenate as well as the arsenate anion from the natural groundwater by the nanofiltration membranes are satisfactory high, and one of the nanofiltration membranes exhibit the superior permeation rate values. The outstanding permeation rate values of these membranes in relation to the reverse osmosis membrane promise a noteworthy decrease of energy consumption and energy costs for the process when using these membranes. The prevailing mechanism of ion retention by the negatively charged nanofiltration membranes is the charge exclusion. The size exclusion mechanism is also important but not sufficient for the uncharged organic molecules rejection. The membrane material and the membrane pore size distribution (PSD) also influence the uncharged organic molecules rejections. The rejections of pesticides are reasonably high, and besides the size effect, the specific physicochemical phenomena should be considered in order to understand their rejection by the nanofiltration membranes.

Un-cooled infrared (8–12 μm) emitters of positive and negative contrast

We report studies of infrared emitters with positive and negative contrast for the 8–12 μm spectral range. The operating principle is based on modulation of semiconductor thermal emission in charge carrier injection and exclusion modes. The sources, made of Ge, have semitransparent n–p or p +–p contacts on the radiating face. This makes it possible to realise vertical output of radiation and uniform emission from the working area of emitter. The results of investigations of the principal characteristics of the sources are presented, and practical aspects of their application in infrared engineering are discussed.

Pulsed amperometric detection of underivatized amino acids using polypyrrole modified copper electrode in acidic solution

The successful pulsed amperometric detection of underivatized amino acids have been carried out in an acidic media on a polypyrrole (PPy) modified Cu electrode. The formation of PPy film doped with glutamate (glu) on a Cu electrode surface changes the mechanism of Cu dissolution. After application of multistep potential waveform, the PPy film was glu free due to the electro-reduction and overoxidation. High anodic potential polarization treatment yielded partially overoxidized PPy film as long as the Cu surface dissolution and amino acid permeation through the film was well controlled. This overoxidized PPy film acted as a charge and size exclusion barrier in order to improve the selectivity and stability of a Cu electrode. Various process parameters such as film modification time, detection and cleaning potential and pH of solution have been optimized to maximize the beneficial electrocatalytic properties of the electrode surface. At an optimized condition, detection limits for positively charged histidine and arginine are 19 and 22 pg respectively, whereas the neutral amino acids detected in amounts of 0.9–2.3 ng. Furthermore, the PPy coated Cu electrode response was long lived, stable and reproducible.

Role of Charge (Donnan) Exclusion in Removal of Arsenic from Water by a Negatively Charged Porous Nanofiltration Membrane

The removal of arsenic from water by a negatively charged "loose" (porous) nanofiltration (NF) membrane was investigated. To better understand the mechanisms of arsenic removal by the polymeric membrane, its surface charge, pore size, and separation behavior for several salt solutions (NaCl, CaCl2, and Na2SO4) were first investigated. The ability of the membrane to remove inorganic arsenic species - As(III) and As(V) - from water was further evaluated. The removal of As(V) increased from 60 to 90% as the arsenic feed water concentration was increased from 10 to 316 μg/L. Compared to As(V), the rejection of the uncharged As(III) species was significantly lower; the rejection of As(III) decreased from 28 to 5% as the arsenic feed concentration was increased from 10 to 316 μg/L. In addition, rejection of As(V) decreased sharply from 85% at pH 8.5 to only 8% at pH 4.5. This behavior is mainly attributed to changes in As(V) speciation with pH (HAsO42- at pH >6.8, H2AsO4- at pH <6.8), and to a lesser extent to a decrease in membrane charge with decreasing pH. The separation of the uncharged As(III), on the other hand, was independent of pH over the studied pH range (removal less than 10%). The results were analyzed in light of the membrane properties (charge and pore size) and arsenic speciation and molecular weight, emphasizing the paramount role of charge (Donnan) exclusion in achieving high rejection of As(V) species, despite the relatively large membrane pore size.

Polyacrylonitrile Thin-Film Composite Membranes for the Optimization of a Whole Blood Glucose Sensor

A polyacrylonitrile thin film composite membrane has been used as the outer covering barrier in an amperometric glucose oxidase enzyme electrode. The composite membrane was formed via the cathodic electropolymerization of acrylonitrile at gold sputter coated highly porous host alumina membranes. Membranes of this type served as diffusion limiting barriers to the enzyme substrates. Anionic interferents such as ascorbate were also excluded via the use of the membrane by means of a charge exclusion mechanism. Oxygen and glucose permeability coefficients were determined using diffusion chamber apparatus. On exposure to blood, a 25% signal drift was observed over a period of 1 h. Blood glucose determinations were performed following this initial exposure to blood since after this time the performance of the electrode remained almost stable. Glucose determinations within whole blood patient samples performed using the enzyme electrode sensor developed here compared favorably to standard hospital analyses with a correlation coefficient of r2=0.94.

Arsenic removal from drinking water by a “loose” nanofiltration membrane

The removal of arsenic from water by a “loose” nanofiltration (NF) membrane was investigated. Prior to the arsenic removal studies, the loose NF membrane was characterized for molecular weight cut-off and pore size by saccharide retention measurements, and electrokinetic charge by streaming potential measurements. In addition, separation of both single salt and mixed salt electrolyte solutions was studied to investigate the ion transport properties of the membrane. Arsenic rejection experiments included variation of pH, arsenic feed concentration, and presence of background electrolyte. In general, arsenic rejection increased with increasing pH and arsenic feed concentration, and was enhanced in the presence of 0.01 M NaCl. Arsenic was removed 60–90% from synthetic feed waters containing 10, 32, 100, and 316 μg/L As(V), resulting in permeate arsenic concentrations of 4, 6, 10, and 25μg/L, respectively. The behavior of the membrane is consistent with the extended Nernst-Planck equation model predictions for an uncharged membrane where size exclusion controls ion retention. However, separation of Arsenic species was a due to a combination of size exclusion, preferential passage of more mobile ions, and charge exclusion.

Administrative Discretion Gone Awry: The Reintroduction of the Public Charge Exclusion for HIV-Positive Refugees and Asylees

Administrative Discretion Gone Awry: The Reintroduction of the Public Charge Exclusion for HIV-Positive Refugees and Asylees

Development of amperometric sensors for choline, acetylcholine and arsenocholine

Immobilisation of acetylcholine esterase (ACE) and choline oxidase (COx) within a polymeric coating on a glassy carbon electrode resulted in the development of a sensor for the important transmitter acetylcholine and its metabolite choline. Anodic detection at +0.25 V vs. SCE is facilitated by the addition of the redox-mediating hexacyanoferrate(III) ion. The influence of various experimental parameters is described. Linearity for acetylcholine and choline responses extended up to 5 mM and 13 mM, with limits of detection of 43 μM and 50 μM, respectively. The simple immobilisation procedure and the rapid reaction kinetics between the mediator and the enzyme, coupled to a fast heterogeneous rate constant, resulted in rapid response times and good precision. An important application of the biosensor was shown for the determination of arsenocholine, using the choline oxidase immobilised electrode, resulting in a response ten times that obtained for choline, with linearity extending up to 0.6 mM and a limit of detection of 10 μM, The high selectivity offered by this sensor was indicated by the lack of interference offered by easily oxidised compounds normally found in serum, because of both the charge exclusion properties of the Nafion polymeric coating and the low operating potential required to re-oxidise the mediator.

Tracers in recharge — Effects of partitioning in soils

Partitioning among mobile and stationary water phases in a soil measurably reduces movement of a tracer relative to the mobile water phase. The quantity of water infiltrating the soil, however, still follows from the product of the distance travelled by the tracer and average moisture content of the soil over that distance. Partitioning between surface and water phases in a soil by proton and protium exchange has only a small effect on the retention of isotopically labelled water. Partitioning between surface, water and gas phases in a soil, however, strongly decreases diffusive movement of isotopically labelled water in the gas phase. In sandy soils with moisture contents less than 5%, the effective diffusion coefficient in the gas phase can be reduced by a factor between 50 and 5000. As a soil dries out, diffusive movement of 18O labelled water will become larger than of HDO and HTO. Negative charges on soil surfaces repel chloride and bromide. The effect of charge exclusion on movement of Cl − and Br − relative to HDO was measured in small columns for a number of soils from the southwest of Western Australia. Data from these experiments show that in the unsaturated zone of sandy soils, charge exclusion can reduce the pore volume available to Cl − and Br − in the water phase at field capacity by > 20%. Recharge to ground water, if calculated from the position of Cl − or Br − tracers and average moisture in the soil profile, must then be reduced by the same percentage. The effect of charge exclusion in soils on the calculation of recharge from profile data, was evaluated for a practical situation. Significant amounts of bromide from car exhaust gases accumulate in soils in urban areas and can be used to trace water movement. A site was chosen in metropolitan Perth on a sparsely vegetated calcareous sand near the junction of two busy roads. A seasonal effect on bromide accumulation in the soil profile at this site was clearly visible from data on the soil solution. The distribution of bromide in the soil profile appears to correspond to 93% of rainfall recharging ground water. Correcting the data for charge exclusion, results in a calculated recharge of ≈ 70% of annual rainfall.

The relationship between pore water velocity and longitudinal dispersivity of Cl −, Br −, and D 2O in soils

Dispersivity was measured as a function of pore water velocity for saturated columns of sandy soil from breakthrough curves of pulse injections of bromide, chloride and deuterated water. Organic carbon contents of the soils ranged from 1 to 10%. Pore water velocities ranged from 10 to 4500 cm d −1 and mechanical dispersivities from 0.02 to 0.7 cm. The halides and D 2O showed identical elution behaviour in columns packed with sandy soils of low organic carbon content. At higher organic carbon and clay contents, charge exclusion increased the dispersion of D 2O in soil relative to that of the halides. Data were fitted to Hiby's equation (Hiby, 1962), describing the nonlinearity between the longitudinal hydrodynamic dispersivity and pore water velocity in columns packed with glass spheres. Below 2 cm 2d −1 the longitudinal hydrodynamic dispersion coefficient was dominated by molecular diffusion and dispersivity decreased with increasing pore water velocity. In the region above 2 cm 2d −1 mechanical dispersion was dominant and dispersivity increased with increasing pore water velocity. From extrapolation of the data obtained for this region it follows that effects of flow rate on dispersivity, though small, are still measurable for dispersion coefficients up to 1–10m 2d −1.

Poly(4-vinylpyridine)-coated glassy carbon flow detectors.

The performance of a thin-layer flow detector with a glassy carbon electrode coated with a film of protonated poly(4-vinylpyridine) is described. Substantial improvement in the selectivity of amperometric detection for liquid chromatography and flow injection systems is observed as a result of excluding cationic species from the surface. The detector response was evaluated with respect to flow rate, solute concentration, coating scheme, film-to-film reproducibility, and other variables. Despite the increase in diffusional resistance, low detection limits of ca. 0.04 and 0.10 ng of ascorbic acid and uric acid, respectively, are maintained. Protection from organic surfactants can be coupled to the charge exclusion effect by using a bilayer coating, with a cellulose acetate film atop the poly(4-vinylpyridine) layer. Applicability to urine sample is demonstrated.

Evaluation of the Use of Lithium Nitrate as a Test Substance for the Determination of the Hold-Up Time of a Reversed-Phase Packing

Abstract The use of lithium nitrate as a test substance for the determination of the hold-up time on μ Bondapak C18 columns has been evaluated by comparison with the hold-up time calculated with the homologous series of n-alcohols. The influence of charge exclusion effects on the retention time of lithium nitrate is investigated. The addition of phosphoric acid to the eluent appears to be an effective method to reduce charge exclusion effects. The existence of deviations from linearity are demonstrated for the homologous series of n-alcohols in eluents with a low methanol content.