James M. Dickson

A new class of polyelectrolyte-filled microfiltration membranes with environmentally controlled porosity

Abstract A new type of membrane composed of a microfiltration substrate and a pore-filling polyelectrolyte has been produced by UV-induced grafting of 4-vinylpyridine onto polyethylene and polypropylene microfiltration membranes. By imposing the rigid structure of the polyolefinic substrate on the highly charged but extremely flexible and swellable gel of polyelectrolyte, stable membranes with very high charge density (> 3.5 mequiv/g) have been developed. The membranes show an outstanding pH valve effect and the capability of rejecting small inorganic ions in the process of reverse osmosis. The pressure-driven transport and separation with these membranes differ markedly from those of conventional reverse osmosis membranes being governed by the properties of the polyelectrolyte. Salt rejection depends strongly on the feed concentration, decreasing substantially with increase in concentration. The most characteristic features of separation with these membranes are higher rejection of polyvalent co-ions and lower rejection of polyvalent counter-ions compared to monovalent ions.

Porous, polyelectrolyte-filled membranes: Effect of cross-linking on flux and separation

Abstract A new type of membrane composed of a microfiltration substrate and a pore-filling polyelectrolyte has been produced by UV-induced grafting of 4-vinylpyridine (4VP) with varying amounts of divinylbenzene (DVB) onto polypropylene microfiltration membranes. Using the same irradiation conditions, it has been found that graft yield increases substantially in the presence of DVB. The increase in graft yield was shown to be accompanied by a substantial increase in the thickness of the grafted membranes and a small but significant decrease in water content. The composite membranes have very high charge densities and good mechanical properties. The membranes with various amounts of DVB were quaternized (methylated) and examined for reverse osmosis of various salts. In addition to an expected drop in flux due to the increased thickness and decreased water content, there was significantly different salt rejection for membranes with cross-linking. While, for example, there is practically no difference in rejection of NaCl by a membrane with 0.55% DVB and one having no cross-linker, the Na 2 SO 4 rejection by the cross-linked membrane is, on average, twice as high as that by the non-cross-linked one. Large differences between the cross-linked and non-cross-linked membranes were found in the ratios of pure water to NaCl permeate fluxes of the membranes at various pressures. The results are discussed in terms of the physicochemical nature of the membranes and conformational changes of the pore-grafted poly(4-vinylpyridine).

Acid recovery using diffusion dialysis with poly(4-vinylpyridine)-filled microporous membranes

Abstract Two series of membranes have been produced by photoinitiated polymerization of 4-vinylpyridine (4VP) and divinylbenzene (DVB) within the pores of polypropylene microfiltration membranes. The first series was comprised of membranes with varying mass gain and constant DVB content. The second series of membranes had similar mass gains but varying DVB content. The membranes were tested by diffusion dialysis of acid/salt solutions (HCl/NaCl/MgCl2) in order to determine the effects of both mass gain and degree of crosslinking on dialysis coefficients and acid/salt separation. It was found for the first series of membranes that the dialysis coefficients of the acid and salts decreased and then leveled off with increasing mass gain while separation increased and then also leveled off. The second series of membranes showed a decrease in acid and salt dialysis coefficients but a dramatic increase in separation as the DVB content was increased. These results are interpreted in terms of the fixed charge concentration and the water content of the membranes. A comparison is made with a commercial diffusion dialysis membrane.

Salt separation and hydrodynamic permeability of a porous membrane filled with pH-sensitive gel

The effect of pH on permeability and the pressure-driven ionic separation of a microporous membrane incorporating a pH-sensitive cross-linked poly(4-vinylpyridine) gel in the pores was examined. Membranes were prepared with cross-linked poly(4-vinylpyridine) constrained in the pores of a poly(ethylene) microporous host membrane. The degree of ionization (protonation) of poly(4-vinylpyridine) in the membranes as a function of pH was determined by potentiometric titration. The pressure-driven flux and salt separation of these membranes were monitored as a function of pH and, consequently, the degree of ionization of the pore-filling polyelectrolyte. The pure water flux was found to decrease reversibly by an order of magnitude when the pH of water was changed from 5.5 to 2.6 by the addition of HCl. Similar reversible flux changes were found with pH-adjusted municipal tap water used as a feed. On the other hand, the rejection of cations in tap water increased sharply with addition of HCl and increase of ionization of the incorporated poly(4-vinylpyridine). The results obtained in this study are explained by microphase transitions taking place in the gel network enmeshed with the porous structure of the support membrane.

Formation of pore-filled ion-exchange membranes with in situ crosslinking: Poly(vinylbenzyl ammonium salt)-filled membranes

Robust, polyelectrolyte-filled, microporous membranes were prepared by the introduction and crosslinking of a preformed polymer within the pores of a poly(propylene) host membrane. Specifically, poly(vinylbenzyl chloride) (PVBCl) was reacted with piperazine or 1,4-diaminobicyclo[2.2.2]octane in an N,N-dimethylformamide (DMF) solution contained in the pores of the microporous base membrane. The remaining chloromethyl groups were reacted with an amine, such as trimethylamine, to form positively charged ammonium sites. This simple two-step procedure gave dimensionally stable, anion-exchange membranes in which the degree of crosslinking and the mass loading were determined by the concentration of PVBCl and crosslinker in the starting DMF solution. The incorporated polyelectrolyte gel was evenly distributed within the pores of the host membrane with no surface layers present. The membranes are fully characterized.

Pervaporation performance of oligosilylstyrene-polydimethylsiloxane membrane for separation of organics from water

A novel silicone rubber was prepared by crosslinking silylstyrene-oligomer containing -SiH groups with divinyl-polydimethylsiloxane using Karstedts catalyst at room temperature. Membranes cast from this silicone rubber were found capable of separating chlorinated hydrocarbons and aromatics from water containing trace amount of the organics through pervaporation (PV). Synthesis of this silylstyrene-oligomer, crosslinking it to divinyl-polydimethylsiloxane to produce the silicone rubber and preparation of membranes from this rubber are discussed. PV test results are presented.

Nanofiltration using pore-filled membranes: effect of polyelectrolyte composition on performance

Several series of membranes composed of microporous poly(propylene) substrates filled with polyelectrolyte gels of different chemical structure, polymer concentration and charge densities have been prepared in order to examine the effect of polyelelctrolyte composition on the performance of these membranes in nanofiltration. The membranes were made by two different routes involving either in situ chemical cross-linking of poly(4-vinylpyridine) or poly(vinylbenzyl chloride), or by in situ polymerization of acrylic acid with tetra(ethylene glycol) diacrylate or N,N-methylenebisacrylamide as cross-linking agents in the pores of the substrates. The resulting pore-filled membranes were characterized by the concentration (volume fraction) of the polyelectrolyte in the pores, ion exchange capacity (charge density), water content, and thickness. The different series of membranes were tested under pressure to determine their hydrodynamic permeabilities and salt separation properties (NaCl). It was found that there was a good correlation between hydrodynamic (Darcy) permeability and gel polymer concentration that holds irrespective of the gel polymer chemistry in the pore-filling gels. Membranes with different pore-filling gels, whether positively or negatively charged, followed the same relationship. The separation properties of the membranes are very good and the salt rejection was found to be practically constant over a wide range of permeabilities (gel concentrations). It was also found that the increase in the nominal charge density above approximately 0.2 mmol/cm3 of the swollen gel had a negligible effect on the separation properties of the gel-filled membranes. The results of this study provide a basis for the further design and optimization of polyelectrolyte filled membranes for nanofiltration applications.

Simulation of a pervaporation system on the industrial scale for water treatment Part I: Extended resistance-in-series model

The resistance-in-series model has been used by several authors to describe the pervaporation of dilute solutions. This paper extends the use of this model to encompass support layer (or vapour side) resistance, membrane swelling and component concentrations in the vapour mixture. The definition of an enhancement factor for the membrane top layer diffusion coefficient, similar to conventional heat and mass transfer equations, both facilitates solution of the equations and helps interpret the effect of swelling on transport. Operating conditions, properties of the membrane and the solution, and geometric parameters of the membrane and the module are represented in the derived equations. The model is then numerically integrated over the entire module. The use of the extended model is illustrated by simulations using a hollow fiber module for treatment of wastewater containing volatile organic compounds.

Nonequilibrium molecular dynamics simulation of water transport through carbon nanotube membranes at low pressurea)

Nonequilibrium molecular dynamics (NEMD) simulations are used to investigate pressure-driven water flow passing through carbon nanotube (CNT) membranes at low pressures (5.0 MPa) typical of real nanofiltration (NF) systems. The CNT membrane is modeled as a simplified NF membrane with smooth surfaces, and uniform straight pores of typical NF pore sizes. A NEMD simulation system is constructed to study the effects of the membrane structure (pores size and membrane thickness) on the pure water transport properties. All simulations are run under operating conditions (temperature and pressure difference) similar to a real NF processes. Simulation results are analyzed to obtain water flux, density, and velocity distributions along both the flow and radial directions. Results show that water flow through a CNT membrane under a pressure difference has the unique transport properties of very fast flow and a non-parabolic radial distribution of velocities which cannot be represented by the Hagen-Poiseuille or Navier-Stokes equations. Density distributions along radial and flow directions show that water molecules in the CNT form layers with an oscillatory density profile, and have a lower average density than in the bulk flow. The NEMD simulations provide direct access to dynamic aspects of water flow through a CNT membrane and give a view of the pressure-driven transport phenomena on a molecular scale.

Theoretical modification of the surface force-pore flow model for reverse osmosis transport

Abstract One relatively new and popular model used to describe and predict the performance of reverse osmosis type membranes, the surface force-pore flow (SF-PF) model, has been modified and extended. The two most serious problems with the SF-PF model are that an incorrect form of material balance is used and that the potential function in the pore is inconsistent with the cylindrical pore geometry. A modified model, the MD-SF-PF model, which is based on the same physical precepts is derived. Equations describing the concentration profile for both models have been derived and compared. It has been shown that the SF-PF model can predict physically unacceptable results. Simulation results for the MD-SF-PF model are consistent with what is expected for reverse osmosis type membranes.