Membranes negative rejection

Negative rejections were consistently measured for several compounds by using the cellulose acetate membrane system. Compounds of this nature must possess a strong affinity for the membrane material and have relatively high transport rates through the membrane. 

In summarizing Tables VIII-XI, several points should be highlighted (1) Recoveries were higher, as expected, for the FT-30 composite membrane than for the cellulose acetate membrane. (2) Compounds exhibiting negative rejections with the cellulose acetate membrane were not recovered. (3) Many compounds were not recovered at their predicted levels in tests with either type of membrane. 

As expected, compounds demonstrating consistent negative rejections with cellulose acetate membranes (dichlorophenol, biphenyl, furfural, chloroform) were not recovered to any extent. Compounds with the best rejections (>90%) were the better recovered substances. The FT-30 composite membrane clearly demonstrated superior performance to the cellulose acetate membrane for organic rejection, concentration, and recovery. Sodium chloride rejection was no indicator of potential organic rejection. 

Negative rejection coefficients, that is, a higher concentration of solute in the permeate than in the feed are occasionally observed, for example, for phenol and benzene with cellulose acetate membranes [48],... 

The cation rejection depends strongly on the electrolyte concentration. The calcium rejection increases with calcium concentration and in the presence of 20 mM NaCl. Sodium rejection also increases with concentration, but decreases in the presence of calcium, even reaching negative rejections when the calcium concentration increases to 2.5 mM. Macoun (1998) attributes negative rejection to a faster permeation of ions than water under certain conditions of enhanced driving force, and Hagmeyer (1999) states that the multivalent ion may pump the monovalent ion across the membrane. The final flux depends on the electrolyte concentration. Calcium is more effective in reducing flux than sodium chloride. 

The average rejection of nitrate is significandy less than that of other monovalent ions. Negative rejection is sometimes observed for phenol and benzene with CA membranes. 

Cationic NF membrane — negative charges repel anions such as S04 while attracting cations especially Ca2+. Salt rejection is Na2S04>NaCl>CaCl2. 

The ability of living organisms to differentiate between the chemically similar sodium and potassium ions must depend upon some difference between these two ions in aqueous solution. Essentially, this difference is one of size of the hydrated ions, which in turn means a difference in the force of electrostatic (coulombic) attraction between the hydrated cation and a negatively-charged site in the cell membrane thus a site may be able to accept the smaller ion Na (aq) and reject the larger K (aq). This same mechanism of selectivity operates in other ion-selection processes, notably in ion-exchange resins. 

Electroultrafiltration (EUF) combines forced-flow electrophoresis (see Electroseparations,electrophoresis) with ultrafiltration to control or eliminate the gel-polarization layer (45—47). Suspended colloidal particles have electrophoretic mobilities measured by a zeta potential (see Colloids Elotation). Most naturally occurring suspensoids (eg, clay, PVC latex, and biological systems), emulsions, and protein solutes are negatively charged. Placing an electric field across an ultrafiltration membrane faciUtates transport of retained species away from the membrane surface. Thus, the retention of partially rejected solutes can be dramatically improved (see Electrodialysis). 

Electrodialysis. Electro dialysis processes transfer ions of dissolved salts across membranes, leaving purified water behind. Ion movement is induced by direct current electrical fields. A negative electrode (cathode) attracts cations, and a positive electrode (anode) attracts anions. Systems are compartmentalized in stacks by alternating cation and anion transfer membranes. Alternating compartments carry concentrated brine and purified permeate. Typically, 40—60% of dissolved ions are removed or rejected. Further improvement in water quaUty is obtained by staging (operation of stacks in series). ED processes do not remove particulate contaminants or weakly ionized contaminants, such as siUca. 

Nanofiltration membranes are negatively charged and reject multivalent anions at a much higher level than monovalent anions, an effect described as Donnan exclusion. Nanofiltration membranes have MgS04 retention and water permeability claims. 

Nanofilters incorporate negative membrane charge for higher anion rejection. High feed salinities can passivate these charges and reduce anion retention. 

Geong and coworkers reported a new concept for the formation of zeolite/ polymer mixed-matrix reverse osmosis (RO) membranes by interfacial polymerization of mixed-matrix thin films in situ on porous polysulfone (PSF) supports [83]. The mixed-matrix films comprise NaA zeoHte nanoparticles dispersed within 50-200 nm polyamide films. It was found that the surface of the mixed-matrix films was smoother, more hydrophilic and more negatively charged than the surface of the neat polyamide RO membranes. These NaA/polyamide mixed-matrix membranes were tested for a water desalination application. It was demonstrated that the pure water permeability of the mixed-matrix membranes at the highest nanoparticle loadings was nearly doubled over that of the polyamide membranes with equivalent solute rejections. The authors also proved that the micropores of the NaA zeolites played an active role in water permeation and solute rejection. 

Reverse osmosis for concentrating trace organic contaminants in aqueous systems by using cellulose acetate and Film Tec FT-30 commercial membrane systems was evaluated for the recovery of 19 trace organics representing 10 chemical classes. Mass balance analysis required determination of solute rejection, adsorption within the system, and leachates. The rejections with the cellulose acetate membrane ranged from a negative value to 97%, whereas the FT-30 membrane exhibited 46-99% rejection. Adsorption was a major problem some model solutes showed up to 70% losses. These losses can be minimized by the mode of operation in the field. Leachables were not a major problem. 

Fig. 7.10 Electrostatic rejection of anions by negatively charged pores in the Nation membrane...

The cationic nanofiltration membrane has negative groups attached to the polymer backbone. These negative charges repel negative anions, such as SO42, while attracting positive cations, particularly divalent cations such as Ca2+. The result is an order of salt rejection... 

Mass-transport limitations are common to all processes involving mass transfer at interfaces, and membranes are not an exception. This problem can be extremely important both for situations where the transport of solvent through the membrane is faster and preferential when compared with the transport of solute(s) - which happens with membrane filtration processes such as microfiltration and ultrafiltration - as well as with processes where the flux of solute(s) is preferential, as happens in organophilic pervaporation. In the first case, the concentration of solute builds up near the membrane interface, while in the second case a depletion of solute occurs. In both situations the performance of the system is affected negatively (1) solute accumulation leads, ultimately, to a loss of selectivity for solute rejection, promotes conditions for membrane fouling and local increase of osmotic pressure difference, which impacts on solvent flux (2) solute depletion at the membrane surface diminishes the driving force for solute transport, which impacts on solute flux and, ultimately, on the overall process selectivity towards the transport of that specific solute. 

Pervaporation - photocatalysis In the described systems the membrane usually permeates water and rejects the reactants, enhancing their residence time in the photoreactor. However, it is known that some intermediate products of the photo-catalytic degradation of organic compounds can negatively affect the reaction rate, therefore, in some cases it is useful to eliminate these by-products in order to improve the thermodynamic and/or the kinetics of the reaction. 

In a pressure-driven membrane process the molecules are generally rejected by the membrane and therefore their concentrations in the permeate are lower than those in the feed solution. However, an accumulation of excess particles can occur at the membrane surface with the creation of a boundary layer. This phenomenon, called concentration polarization, causes a different membrane performance. In particular, with low molecular weight solutes the observed rejection will be lower than the real retention or, sometimes, it could be negative. 

---