Henning M. Krieg

The removal of acid sulphate pollution by nanofiltration

Abstract Mine water, specifically from gold and coal mines in South Africa, contains large quantities (up to 3 g/l=3000 ppm) of sulphate and can be very acidic (pH 2–4). A study was done to investigate the performance of commercially available nanofiltration (NF) membranes for the removal of sulphates and other ions at acidic conditions. The NF measurements were carried out in a dead-end membrane set-up at pressures between 3 and 20 bars. At neutral pH, membranes were identified with a sulphate rejection of 95–99% and a water flux of 2–7 l/m2.h.bar. At lower pH values, the performance decreases due to the presence of a higher fraction of monovalent HSO−4 ions and as well as a possible change in the membrane charge from negative to positive. However, there are still some membranes (from Filmtec) which are suitable for mine water with a pH of 4 and a salt loading of 2500 ppm. The water flux of these membranes is, however, below 4 l/m2.h.bar. Surprisingly multi-charged ions like Ca2+ and SO2−4 as well as single-charged ions like Na+ and Cl− are rejected with values larger than 90%.

Chiral resolution by β-cyclodextrin polymer-impregnated ceramic membranes

Abstract In this study, a porous tubular ceramic membrane was impregnated with a β-cyclodextrin (CD) polymer to obtain a chiral-selective membrane. The membrane was evaluated in terms of the effectiveness of impregnation as well as the ability to separate the enantiomers of the racemic pharmaceutical chlorthalidone (CT). Impregnation into the pores of the ceramic support was shown by the increase in weight and confirmed by porosimetry. In spite of polymer loss from the membrane during the permeation of CT, an average separation factor (α) of 1.24 was attained. The proposed mechanism of selectivity permits continual retrieval of both enantiomers in a process similar to the permeation swing adsorption.

Novel morpholinium-functionalized anion-exchange PBI–polymer blends

Novel morpholinium-functionalized anion-exchange blend membranes are presented in this study. The blended membranes consist of highly brominated poly(aryl ether) (PAE-Br) or poly(2,6-dimethyl-1,4-phenyleneoxide) (PPO-Br), partially fluorinated polybenzimidazole (F6-PBI) as the polymer matrix and/or a sulfonated polyethersulfone from 1,1′-biphenyl-4,4′-diol and potassium 5,5′-sulfonylbis(2-fluorobenzenesulfonate), abbreviated in this study as SAC. One of the manufactured blend membranes (IEC: 2.4 mmol g−1) had a better alkaline stability than the Tokuyama membrane A201. In addition, excellent thermal stability and moderate water uptake were observed for the blend. The enhanced thermophysical properties were ascribed to the PBI matrix, whereas the high bromination degree of the ionomer contributed to the high ion-exchange capacities and conductivities obtained. The significantly increased chemical stability noted for these blends can partially be attributed to supplementary ionic cross-links formed between acidic and basic components. These results, combined with the ability of PPO-Br and PAE-Br to form macroscopically homogeneous blends with PBIs, resulting in stable and flexible polymer films, reiterate the potential suitability of the presented membranes as polymer electrolytes for electrochemical applications such as alkaline fuel cells.

Centrifugal casting of ceramic membrane tubes and the coating with chitosan

Abstract In this paper, the centrifugal casting of ceramic membrane tubes is presented using different powder particle sizes and powder mixtures. The inner surface of the tubes has a very regular pore structure and is also very smooth. The strength of the tubes increases with increasing sintering temperature and decreasing particle size up to stress values of 1500 MPa. For the strongest materials, the pores sizes and porosities are in the order of 50 nm and 30%, respectively. The water permeability varies between 5 and 50 l/m 2 .h.bar. The inner surface of the tubes was coated with a highly porous chitosan biopolymer layer of 30–50 μm thickness. The chitosan layer is prepared by a phase-inversion method using water with low and high pH values as a solvent/non-solvent system. Silica, which dissolves at high pH values, is used as porogen and creates the pores. This biopolymer can adsorb heavy metals like copper. With this membrane system, Cu 2+ (50 ppm as CuSO 4 ) can be removed almost completely with a membrane capacity of 0.1 g Cu 2+ /g chitosan.

The Influence of β-Cyclodextrin on the Solubility of Chlorthalidone and Its Enantiomers

The solubility of chlorthalidone in 16 solvent systems was determined in the absence and presence of different amounts of beta-cyclodextrin (beta-CD). Chlorthalidone (CT) was shown to be more soluble in hydrophilic organic solvents, with the highest solubility in ethylacetate (EtOAc) saturated with water. The solubility of CT in water, butanol, octanol, and dichloromethane (DCM) was enhanced by the addition of beta-cyclodextrin. The enantioselective partitioning of CT between water and EtOAc, DCM, butan-1-ol, butan-2-ol, and octan-1-ol was determined in the presence of beta-CD at pH 5, 7, and 9. According to the results, both the solubility and partitioning properties of CT are affected by beta-CD in aqueous solution. It was also shown that the solubility of the individual enantiomers differs in the presence of beta-CD.

Poly(vinylbenzylchloride) based anion-exchange blend membranes (AEBMs): influence of PEG additive on conductivity and stability

In view of the many possible applications such as fuel cells and electrolysers, recent interest in novel anion exchange membranes (AEMs) has increased significantly. However, their low conductivity and chemical stability limits their current suitability. In this study, the synthesis and characterization of several three- and four-component anion exchange blend membranes (AEBMs) is described, where the compositions have been systematically varied to study the influence of the AEBM’s composition on the anion conductivities as well as chemical and thermal stabilities under strongly alkaline conditions. It was shown that the epoxide-functionalized poly(ethylene glycol)s that were introduced into the four-component AEBMs resulted in increased conductivity as well as a marked improvement in the stability of the AEBMs in an alkaline environment. In addition, the thermal stability of the novel AEBMs was excellent showing the suitability of these membranes for several electrochemical applications.

Synthesis of a composite inorganic membrane for the separation of nitrogen, tetrafluoromethane and hexafluoropropylene

The advanced use of inorganic membranes, such as zeolites, in large-scale industrial processes is hindered by the inability to manufacture continuous and defect-free membranes. We therefore aimed to construct such a defect-free membrane. Various zeolites were synthesised on the inner surface of α-alumina support tubes by a hydrothermal process. Gas permeation properties were investigated at 298 K for single component systems of N2, CF4 and C3F6. Ideal selectivities lower than Knudsen selectivities were obtained as a result of defects from intercrystalline slits and crack formation during synthesis and template removal. A composite ceramic membrane consisting of a ceramic support structure, a mordenite framework inverted intermediate zeolite layer and a Teflon AF 2400 top layer was developed to improve separation. The Teflon layer sealed possible defects present in the separation layer forcing the gas molecules to follow the path through the zeolite pores. Ideal selectivities of 88 and 71 were obtained for N2/CF4 and N2/C3F6 respectively. Adsorption experiments performed on materials present in the membrane structure suggested that although adsorption of C3F6 onto Teflon AF 2400 compared to CF4 results in a considerable contribution to permeation for the composite ceramic membrane, the sealing effect of the zeolite layer by the Teflon layer is the reason for the large N2/CF4 and N2/C3F6 selectivities obtained. The Teflon layer effectively sealed intercrystalline areas in-between zeolite crystals, which resulted in high ideal selectivies for N2/CF4 and N2/C3F6.

A dual-channel gas chromatography method for the quantitation of low and high concentrations of NF3 and CF4 to study membrane separation of the two compounds

A dual-channel gas chromatographic method is described in this paper that can be conveniently used for quantitation of NF3/CF4 mixtures with a thermal conductivity detector (TCD) on one channel for the quantitation of high-concentrations, and a pulsed discharge helium ionization detector (PDHID) on a second channel for the quantitation of low concentrations. It is shown that adequate separation is achieved on both channels with this dual single-column setup in which column switching as used for NF3/CF4 analysis in industrial chromatographic methods are not required, thus yielding an effective analysis method for laboratory-scale investigations. In addition, the use of packed columns with purified divinylbenzene-styrene co-polymers as the sole stationary phase yields satisfactory resolution between NF3 and CF4 at isothermal conditions of 30°C, with elution times of less than 8min on the TCD channel and less than 4min on the PDHID channel. Consequently, this method allows for reliable, straight-forward quantitation of NF3/CF4 mixtures, which is necessary when studying the commercially important problem of NF3 and CF4 separation by different methods. Therefore, the applicability of the method to studying membrane separation of NF3 and CF4 is briefly discussed and illustrated, for which the dual-channel setup is especially beneficial.

Mechanistic study of hafnium and zirconium extraction with organophosphorus extractants

ABSTRACT Research on the purification of Zr has increased in the last few decades and although effective processes have been developed, little is known about the mechanism of Hf and Zr extraction; particularly, at low acid concentrations. This study’s aim was to determine the influence of alterations in alkyl substituents on the extraction mechanism of Hf and Zr from sulfate media. Seven organophosphorus extractants (Dio-PA, TPPA, DPPA, TPPO, TPPS, D2EHPA and Ionquest 801) were screened with four of them (Dio-PA, DPPA, D2EHPA and Ionquest 801) resulting in promising extraction results. This was attributed to the activity of the acidic hydroxy phosphinyl (=P(O)OH) group which is responsible for the dimerization and cation exchange properties of the extractant. The extractant and hydrogen ion stoichiometries involved in the extraction mechanism were determined for these four extractants by slope analysis from extractant- and acid-variation experiments. It was found that Dio-PA, D2EHPA, and Ionquest 801 extract via a cation-exchange mechanism, according to [M(OH)2]2+ + 2 HA ⇌ M(OH)2A2 + 2 H+. In contrast, the extraction with DPPA was best described by a solvated cation exchange (SCE) mechanism, according to [M(OH)]3+ + 6 HA ⇌ M(OH)A3.3HA + 3 H+. It was concluded that the presence of electron-withdrawing substituents produced softer Lewis base extractants, which favored the extraction of Hf over Zr from acidic sulfate media.

Application of Novel Anion-Exchange Blend Membranes (AEBMs) to Vanadium Redox Flow Batteries

Both cation-exchange membranes and anion-exchange membranes are used as ion conducting membranes in vanadium redox flow batteries (VRFBs). Anion-exchange membranes (AEMs) are applied in vanadium redox flow batteries due to the high blocking property of vanadium ions via the Donnan exclusion effect. In this study, novel anion-exchange blend membranes (AEBMs) were prepared, characterized, and applied in VRFBs. Bromomethylated poly(2,6-dimethyl-1,4-phenylene oxide), poly[(1-(4,4′-diphenylether)-5-oxybenzimidazole)-benzimidazole] (PBI-OO) and sulfonated polyether sulfone polymer were combined to prepare 3-component AEBMs with 1,2,4,5-tetramethylimidazole (TMIm) for quaternization. 3-component AEBMs showed significantly enhanced chemical and mechanical properties compared with those of 2-component AEBMs, resulting in an improved performance in VRFBs. The compositions of the anion-exchange polymers in 3-component AEBMs were systematically varied to optimize the AEBMs for the redox-flow battery application. While the 3-component AEBMs showed comparable efficiencies with Nafion® 212 membranes, they displayed improved vanadium ions cross-over as was confirmed by open circuit voltage tests and capacity fade tests conducted in VRFBs. In addition, one of the synthesized 3-component AEBM had a superior coulombic efficiency and capacity retention in a charging–discharging test over 300 cycles at a current density of 40 mA/cm2. It can thus be concluded that 3-component AEBMs are promising candidates for long-term operation in VRFBs.