Comparative studies on electrochemical characterization of homogeneous and heterogeneous type of ion-exchange membranes

Interpolymer films of poly-ethylene and styrene-divinyl benzene copolymer were subjected to chlorosulfonation or chloromethylation then amination for the preparation of homogeneous type of cation- or anion-exchange membranes, respectively. Heterogeneous types of ion-exchange membranes were prepared from polyvinyl chloride (PVC) as binder and ion-exchange resin powder in tetrahydrofuran solvent. Membrane potential and conductance measurements have been carried out in NaCl(aq), CuCl₂(aq) and AlCl₃(aq) solutions at different concentrations to investigate the relationship between concentration of fixed charges and electrochemical properties of these membranes. On the basis of the micro-heterogeneous model, describing the micro-structure of the membrane material, the counter-ion diffusion coefficients were estimated. Membrane conductance data, along with values of concentration of fixed ionic site in the membrane, were used for the estimation of the tortuosity factor and salt permeability employing non-equilibrium thermodynamic principles. It was concluded that electrochemical transport properties of homogeneous type of ion-exchange membranes are superior to those for heterogeneous type of ion-exchange membranes. However, both types of membranes are suitable for electrodriven separation of mono-, bi- and tri-valent electrolytes.     

Ion-exchange studies on poly-o-anisidine Sn(IV) phosphate nano composite and its application as Cd(II) ion-selective membrane electrode

An organic-inorganic composite, poly-o-anisidine Sn(IV) phosphate, was chemically synthesized by mixing ortho-anisidine into gels of Sn(IV) phosphate with different mixing volume ratios. Studies on the effect of eluant concentration, elution behavior and separation of metal ions were carried out to understand the ion-exchange capabilities. Due to Cd(II) selective nature of composite, revealed by distribution studies, Cd(II) ion selective membrane electrode was fabricated. The analytical utility of the electrode was established by employing it as an indicator electrode in electrometric titrations having fast response time, 3–5s, and long life span of six months. Some physical parameters like self-diffusion coefficient (D₀), activation energy (E_a) and entropy of activation (ΔS₀) have been evaluated under conditions favoring a particle diffusion-controlled mechanism by studying ion-exchange kinetics.      

Adsorption of Cr(VI) using activated neem leaves: kinetic studies

In the present study, adsorbent is prepared from neem leaves and used for Cr(VI) removal from aqueous solutions. Neem leaves are activated by giving heat treatment and with the use of concentrated hydrochloric acid (36.5 wt%). The activated neem leaves are further treated with 100 mmol of copper solution. Batch adsorption studies demonstrate that the adsorbent prepared from neem leaves has a significant capacity for adsorption of Cr(VI) from aqueous solution. The parameters investigated in this study include pH, contact time, initial Cr(VI) concentration and adsorbent dosage. The adsorption of Cr(VI) is found to be maximum (99%) at low values of pH in the range of 1-3. A small amount of the neem leaves adsorbent (10 g/l) could remove as much as 99% of Cr(VI) from a solution of initial concentration 50 mg/l. The adsorption process of Cr(VI) is tested with Langmuir isotherm model. Application of the Langmuir isotherm to the system yielded maximum adsorption capacity of 62.97 mg/g. The dimensionless equilibrium parameter, R _L, signifies a favorable adsorption of Cr(VI) on neem leaves adsorbent and is found to be between 0.0155 and 0.888 (0<R _L<1). The adsorption process follows second order kinetics and the corresponding rate constant is found to be 0.00137 g/(mg) (min).     

Electrical conductivity and cation exchange kinetic studies on poly-o-toluidine Th(IV) phosphate nano-composite cation exchange material

An advanced organic–inorganic cation exchange material poly-o-toluidine Th(IV) phosphate nano-composite was synthesized by a modified sol–gel technique by incorporating Th(IV) phosphate precipitate with the matrix of poly-o-toluidine. The material showed good ion-exchange behavior and used successfully in separation of metal ions. The conductivity of the composite was found within the range of 10⁻² to 10⁻³ S/cm; measured by 4-in-line-probe dc electrical conductivity measuring technique. The conductivity is at the border of metallic and semiconductor region. Ion-exchange kinetics for few divalent metal ions was evaluated by particle diffusion-controlled ion-exchange phenomenon at four different temperatures. The particle diffusion mechanism is confirmed by the linear τ (dimensionless time parameter) versus t (time) plots. The exchange processes thus controlled by the diffusion within the exchanger particle for the systems studies herein. Some physical parameters like self-diffusion coefficient (D₀)_, energy of activation (E_a) and entropy of activation (ΔS⁰) have been evaluated under conditions favoring a particle diffusion-controlled mechanism.     

Development of urethane acrylate composite ion-exchange membranes and their electrochemical characterization

With the objective of introducing antifouling characteristics into interpolymer types of cation and anion exchange membranes, the surface of these membranes was coated with a 12-microm-thick urethane acrylate layer and was cured by UV radiation of wavelengths 308 and 172 nm under a complete inert atmosphere. Different urethane acrylate composite ion exchange membranes developed were characterized in NaCl solution by measuring their ion-exchange capacity, volume fraction of water, contact angle with water, membrane conductance, and membrane potential. It was found that the electrochemical transport properties of urethane acrylate composite cation-exchange membranes were increased due to resonance stabilization of the urethane group, which acts as a weak acid and dissociates as a negatively charged urethane ion and a positively charged proton. This contributes toward the net charge density of the membrane matrix responsible for enhanced selectivity and conductivity, while for urethane acrylate composite anion-exchange membranes reduction in net charge density was responsible for reduction in electrochemical transport properties. Counterion transport number, permselectivity, and counterion diffusion coefficient values for these membranes were also estimated. Experiments were also carried out in higher homologs of sodium carboxylate solutions in order to observe the fouling tendencies of these membranes. It was concluded that it is possible to obtain antifouling characteristics of ion-exchange membranes by coating and curing thin hydrophilic layers of urethane acrylate on their surfaces without sacrificing their electrochemical transport properties.     

Electrical conductivity of iodine-doped pseudo interpenetrating polymer networks of poly-(carbonate-urethane) and natural rubber

Recent DC conductivity measurements on iodine-doped full and psuedo IPNs of poly (carbonate-urethane) (PCU) and natural rubber (NR) reveal that these materials possess a modest conductivity. Both such full and pseudo IPNs (with linear NR chains) are single-phased materials if the NR is of sufficiently high molecular weight derived from Brazilian Manihot glaziovii. On iodine doping the electrical conductivity rises eight orders of magnitude of both the full IPN (to ca. 10^?5S cm^?1) and the pseudo IPN (to ca. 10^?4 S cm^?1). A simple theory based on the assumption that conduction occurs essentially along the linear NR chains, composing a percolation cluster, in the iodine-doped pseudo IPNs of PCU and NR accounts for the observed electrical conductivity dependence on temperature, iodine molality, and weight fraction of NR. As temperature decreases the DC conductivity falls and the material becomes essentially an insulator. At sufficiently low temperature (ca. 115 K) this trend reverses and the DC conductivity rises by two orders of magnitude with further decreasing temperature up to that of liquid helium. © 1994 John Wiley & Sons, Inc.     

Kinetics of ion-exchange of alkaline earth metals on antimony(V) phosphate cation exchanger

Forward and reverse ion exchange kinetics of Mg(II), Ca(II), Sr(II) and Ba(II) exchange with H(I) has been studied on antimony(V) phosphate cation exchanger applying the Nernst-Planck equations. As a result, some kinetic parameters like diffusion coefficients, activation energies, and entropies of activation have been evaluated under the conditions favoring a particle diffusion-controlled mechanism. Such kinetic parameters have also been compared with the kinetic parameters of different single and double salts.     

Forward ion-exchange kinetics of heavy metal ions on the surface of carboxymethyl cellulose Sn(IV) phosphate composite nano-rod-like cation exchanger

The Nernst?CPlanck equations with some additional assumptions was used in this study to investigate the forward kinetics and ion-exchange mechanism of heavy metal ions viz. Ni²⁺?CH⁺, Cu²⁺?CH⁺, Mn²⁺?CH⁺ and Zn²⁺?CH⁺ on the surface of carboxymethyl cellulose Sn(IV) phosphate composite nano-rod-like cation-exchanger. It was observed that heavy metals' exchange processes were imparted by the particle diffusion-controlled phenomenon. Some physical parameters i.e., fractional attainment of equilibrium U(??), self-diffusion coefficients (D _o), energy of activation (E _a), and entropy of activation (??S*) were estimated. These investigations revealed that the equilibrium is attained faster at higher temperature probably because of availability of thermally enlarged matrix of carboxymethyl cellulose Sn(IV) phosphate composite nano-rod-like cation-exchange material. The physical parameters observed for this composite cation exchanger were also compared with other composite ion exchangers. The results showed that the ion-exchange phenomenon is more feasible on the surface of this composite cation exchanger as compared with the other ion exchangers which indicated the usefulness of this composite ion exchanger in various applications.     

Distribution studies of metal ions on arsenophosphates of Sn(IV) and Cr(III) and on amine Sn(II) hexacyanoferrates (II) using radio tracers: Separation of Sr2+−Cs+, Hg2+−Ag+ and Hg2+−Zn2+

Summary Distribution studies of some metal ions have been made on Sn(IV) and Cr(III) arsenophosphates and on some samples of Sn(II) amine hexacyanoferrates(II), using radiotracers. The K_d values of Cs⁺ and Rb⁺ have been followed at varying HNO₃ concentrations also. As a result 3 useful binary separations have been achieved on Sn(IV) and Cr(III) arsenophosphates, such as Sr²⁺–Cs⁺, Hg²⁺–Ag⁺ and Hg²⁺–Zn²⁺.     

Synthesis,structure, and kinetic studies on [RuCl2(NCCH3)2(cod)]

[RuCl₂(NCCH₃)₂(cod)], an alternative starting material to [RuCl₂(cod)] _n for the preparation of ruthenium(II) complexes, has been prepared from the polymer compound and isolated in yields up to 87% using a new work-up procedure. The compound has been obtained as a yellow solid without water of crystallization. The complexes [RuCl₂(NCR)₂(cod)] spontaneously transform into dimers [Ru₂Cl(μ-Cl)₃(cod)₂(NCR)] (R?=?Me, Ph). ¹H NMR kinetic experiments for these transformations evidenced first-order behavior. [RuCl₂(NCPh)₂(cod)] dimerizes slower by a factor of ten than [RuCl₂(NCCH₃)₂(cod)]. The following activation parameters, ΔH ^#?=?114?±?3?kJ?mol^?1 and ΔS ^#?=?66?±?9?J?K^?1?mol^?1 for R?=?CH₃CN (ΔG ^#?=?94?±?5?kJ?mol^?1, 298.15?K) and ΔH ^#?=?122?±?2?kJ?mol^?1 and ΔS ^#?=?75?±?6?J?K^?1?mol^?1 for R?=?Ph (ΔG ^#?=?100?±?4?kJ?mol^?1, 298.15?K), have been calculated from the first-order rate constants in the temperature range 294–323?K. The kinetic parameters are in agreement with a two-step mechanism with dissociation of acetonitrile as the rate-determining step. The molecular structures of [Ru₂Cl(μ-Cl)₃(cod)₂(NCR)] (R?=?Me, Ph) have been determined by X-ray diffraction.     

Uranium(VI) recovery from its leach liquor using zirconium molybdophosphate composite: kinetic,equilibrium and thermodynamic studies

Journal of Radioanalytical and Nuclear Chemistry - A zirconium molybdophosphate composite was designed for the selective recovery of uranium ions. The synthesized composite was well-characterized...     

Synthesis, characterization and ion-exchange properties of a new and novel ‘organic-inorganic’ hybrid cation-exchanger: Nylon-6,6, Zr(IV) phosphate

Organic-inorganic hybrid materials enable the integration of useful organic and inorganic characteristics within a single molecular-scale composite. Unique ion-exchange properties of these types of materials have been observed, and many others can be envisioned for this promising class of materials. In this paper, we describe the ion-exchange and physico-chemical properties of one family of self-assembling organic-inorganic hybrid based on nylon-6,6, framework with Zr(IV) phosphate an inorganic ion-exchanger. The physico-chemical properties of this hybrid material were determined using atomic absorption spectrophotometry (AAS), CHN elemental analysis, ICP-MS, UV-vis spectrophotometry, FTIR, TGA-DTA and scanning electron microscope (SEM) studies. Ion-exchange capacity (IEC), thermal stability and distribution behavior, etc. were also carried out to understand the cation-exchange behavior of the material. On the basis of distribution studies, the material was found to be highly selective for Hg(II), a highly toxic environmental pollutant. Its selectivity was examined by achieving some important binary separations like Hg(II)-Mg(II), Hg(II)-Zn(II), Hg(II)-Fe(III), Hg(II)-Bi(III), etc. Thus, the relatively new field of “organic-inorganic” hybrids offers a variety of exciting technological opportunities to decrease the environmental pollution.     

Comparative studies on hydrolysis of bis(p-nitrophenyl) phosphate catalyzed by short- and long-alkyl-multiamine metallomicelles

Four short- and long-alkyl-multiamine ligands L¹–L⁴ have been synthesized and characterized. The catalytic efficiency of complex CuL¹ and functional metallomicelles CuL²–CuL⁴ were comparatively investigated for the hydrolysis of bis(p-nitrophenyl) phosphate (BNPP) in buffered solution at 30 °C. The ternary kinetic model for metallomicellar catalysis was suggested to analyze the experimental data. The kinetic and thermodynamic parameters k^′_N, K_T and pK_a were obtained. The results indicated that the complexes with 1:1 ratio of ligands L²–L⁴ to copper(II) ion were the kinetic active catalysts, and the deprotonized Cu(II) complex formed by activated water molecule was the real active species for BNPP catalytic hydrolysis. The real rate constant of the reaction catalyzed by CuL¹–CuL⁴ was 4.00 × 10⁻⁶, 7.44 × 10⁻⁵, 1.42 × 10⁻⁴ and 4.10 × 10⁻⁴ s⁻¹, respectively. The effects of ligand and microenvironment on the hydrolytic reaction of BNPP have been discussed in detail.     

Statistical optimizing of electrocoagulation process for the removal of Cr(VI) using response surface methodology and kinetic study

Optimization of electrocoagulation (EC) using copper electrode in terms of Cr(VI) removal from simulated waste water was executed by applying surface methodology and kinetic study. In this research, electrocoagulation process was applied to evaluate the outcome of operational parameters such as initial Cr(VI) concentration, pH, electrode distance, current density and supporting electrolyte (NaCl) concentration for the removal of Cr(VI). The experimental results showed that current density of 41.32 A/m², electrode distance of 1.4 cm, initial pH of 5.65, time of electrocoagulation of 40 min and initial conductivity 0.21 ms are the optimal operating parameters to attain 93.33% removal efficiency of Cr(VI) ions from simulated waste water. The high value of R² = 98.15 and R²_adj = 96.49 show that fitted model confirms a good agreement with the real and predicted Cr(VI) removal percentage. It was concluded that Cr(VI) ion removal follows the first-order kinetic model by kinetic study of EC process.     

导电聚合物的现场电导/电化学研究——聚(3-甲基噻吩)和聚噻吩

本文提出一种研究导电聚合物的现场电位、电导测量/电化学方法。该实验方法基于一种可重复使用的玻璃碳-碳纤维组双电极。用该方法研究了聚(3-甲基噻吩)和聚噻吩的现场电位、电导/电化学行为。     

Combining Q2MM modeling and kinetic studies for refinement of the osmium-catalyzed asymmetric dihydroxylation (AD) mnemonic

The interactions between the substrate and the ligand in the Sharpless AD reaction have been examined in detail, using a combination of substrate competition experiments and molecular modeling of transition states. There is a good agreement between computational and experimental results, in particular for the stereoselectivity of the reaction. The influence of each moiety in the second-generation ligand (DHQD)₂PHAL on the rate and selectivity of the reaction has been elucidated in detail.     

Surfactant assisted preparation and characterization of carboxymethyl cellulose Sn(IV) phosphate composite nano-rod like cation exchanger

Carboxymethyl cellulose Sn(IV) phosphate composite nano-rod like cation exchanger with diameter in the range of 20–40 nm, length in the range of 100–150 μm and particle size in the range of 21–38 nm have been successfully prepared by surfactant assisted sol–gel method. Scanning electron microscopy, transmission electron microscopy, X-ray powder diffraction, fourier transform infra red spectroscopy and thermogravimetric analysis-differential thermal analysis studies were carried out to study the structure and morphology of this composite nano-rod like cation exchanger. Freundlich adsorption isotherm is well fitted for the adsorption of pyridine on the surface of this composite nano-rod like cation exchanger. The thermodynamic parameters such as Freundlich constant, thermodynamic equilibrium constant (K ₀), standard free energy changes (ΔG ⁰), standard enthalpy changes (ΔH ⁰) and standard entropy changes (ΔS ⁰) have been evaluated. These parameters indicated that the adsorption of pyridine on the surface of composite nano-rod like cation exchanger was feasible, spontaneous and exothermic in nature which suggests for the potential application of pyridine removal from water.     

Biosorption studies of uranium (VI) on cross-linked chitosan: isotherm,kinetic and thermodynamic aspects

The cross-linked chitosan (CS) gels synthesized by using glutaraldehyde (GLA), epichlorohydrin (EC), and ethylene glycol diglycidyl ether (EGDE) as cross-linkers respectively were used to investigate the adsorption of U(VI) ions in an aqueous solution. The pure chitosan (PCS) and the cross-linked chitosan gels were characterized by FTIR and SEM analysis. The kinetic, thermodynamic adsorption and adsorption isotherms of U(VI) ions onto unmodified and modified cross-linked chitosan were studied in a batch adsorption experiments. The effect of pH, contact time and temperature on the adsorption capacity were also carried out. At the optimum pH, the maximum adsorbed amount of PCS, GLACS, ECCS and EGDECS were 483.05, 147.05, 344.83 and 67.56 mg/g, respectively. The uranium (VI) adsorption process of PCS and ECCS followed better with pseudo-second-order kinetic model, while GLACS and EGDECS followed pseudo-first-order kinetic model well. The results obtained from the equilibrium isotherms adsorption studied of U(VI) ions were analyzed in two adsorption models, namely, Langmuir and Freundlich isothms models, the results showed that the Langmuir isotherm had better conformity to the equilibrium data. The thermodynamic parameters such as enthalpy (ΔHo), entropy (ΔSo), and Gibbs free energy (ΔGo) showed that the adsorption process was both spontaneous and endothermic.     

In situ kinetic studies of the trifluoromethylation of caffeine with Zn(SO2CF3)2

In situ monitoring of the trifluoromethylation of caffeine using Zn(SO₂CF₃)₂ provides mechanistic insight into this important reaction. The concentration dependences of each of the reagents are probed, along with the effect of various additives. The kinetic profile is characterized by a rapid initial regime followed by a period of slower rate. Increasing the concentration of the Zn reagent extends the initial rapid period to higher conversions. Neither the caffeine concentration nor the concentration of the oxidant has a strong effect on the reaction rate. A multiple aliquot method is introduced to help deconvolute factors associated with each rate regime.     

Digestion treatments and risks of Cr(III)–Cr(VI) interconversions during Cr(VI) determination in soils and sediments—a review

Threshold values for Cr(VI) in various types of solid matrices have been set up to protect human health and biota. To ascertain the compliance of solids with these limits different types of extractants and different conditions of pH and temperature have been proposed in the literature. These extraction procedures are reviewed and their potentialities in quantitatively extracting Cr(VI) from solids without inducing undesired Cr(VI)–Cr(III) interconversions during the extraction are carefully evaluated. This evaluation takes into account the knowledge of the kinetics of most important redox reactions of chromium gathered in recent years. Among possible Cr(VI) reductants made available during the digestion, a number of species including Fe(II), sulphide, sulfite and humic matter were considered, while oxidants included hydrogen peroxide, dissolved oxygen, manganese oxides. Theoretical calculations suggest that pH higher than 10, high temperature and high concentrations of carbonate and magnesium ions minimize Cr(III)–Cr(VI) interconversions. The EPA Method 3060A meets these basic requirements. However, the applicability of this method to the analysis of Cr(VI) in soil and sediment samples, whose extracts may suffer from the interference by humic matter, is questionable.