Negative osmosis

The negative osmosis term, proportional to the concentration gradient is expected to be of the same order of magnitude as the conventional electro-osmotic term and thus should be included in consideration. 

Bifurcation analysis in the local Teorell model accounting for negative osmosis and concentration dependence of the electro-osmotic factor as prescribed by (6.4.44). The analysis should be essentially identical to that of 6.3 with the generalized Darcy s law (6.3.11) replaced by the expression... 

Figure 2.5 Anomalous positive and negative osmosis through cation exchange membranes (change in solvent flow in various concentrations). Concentration ratio of solutions in both sides (C2/Ci) 8 C-l membrane (o ion exchange capacity, 0.253 meq./g dry membrane water content 1.96g H20/g dry membrane thickness 0.30 mm) (A) KCl (B) NaCl (C) LiCl C-3 membrane ( ion exchange capacity 0 meq./g dry membrane, water content 1.76g H20/g dry membrane,) (D) KCl in C-3 membrane.

The electrolyte flux is naturally affected by osmosis. Namely, a strong positive osmosis carries the electrolyte from the dilute solution to the concentrated one, which is incongruous salt flux. Conversely, electrolyte diffusion is retarded when the mobility of the co-ion is faster (negative osmosis). The flux of the solvent provides the energy required to transfer the electrolyte against its chemical potential gradient. 

Electrophoresis and electro osmosis can be used to enhance conventional cake filtration. Electrodes of suitable polarity are placed on either side of the filter medium so that the incoming particles move toward the upstream electrode, away from the medium. As most particles carry negative charge, the electrode upstream of the medium is usuaHy positive. The electric field can cause the suspended particles to form a more open cake or, in the extreme, to prevent cake formation altogether by keeping aH particles away from the medium. 

Reverse osmosis models can be divided into three types irreversible thermodynamics models, such as Kedem-Katchalsky and Spiegler-Kedem models nonporous or homogeneous membrane models, such as the solution—diffusion (SD), solution—diffusion—imperfection, and extended solution—diffusion models and pore models, such as the finely porous, preferential sorption—capillary flow, and surface force—pore flow models. Charged RO membrane theories can be used to describe nanofiltration membranes, which are often negatively charged. Models such as Dorman exclusion and the... 

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. 

The phenomenon of electro-osmosis can be studied by using a U-tube [fig. (9)] in which a plug of moist clay (a negative sol) is fixed. The two limbs of the tube are filled with water to the same level. The platinum electrodes are dipped in water and potential applied. It is observed that water level rises on the cathode side, while it falls on the anode side. This motion of the medium towards the negative electrode, shows that the charge on the medium is positive. Similarly, for a positively charged sol, electro-osmosis will occur in the reverse direction. 

Electrokinetic phenomena are generally characterized by the tangential motion of liquid with respect to an adjacent charged surface. In the above example the surface was that of a negatively charged clay particle the particle moved with respect to the stationary liquid. The surface may also be that of a droplet as in emulsions. Alternatively, the particles may be stationary with the liquid moving, as for Instance in electro-osmosis. For sand this phenomenon was also discovered by Reuss I... 

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