Solute rejection measurements

In order to characterise the real (intrinsic) properties of the membrane, these boundary layer phenomena must be taken into account. Modified cut-off values are obtained in this way and in some cases this seems to be a good approach. The method can be improved further by taking a test molecule as dextran which has both a broad molecular weight distribution and a relatively low adsorption tendency. Using gel permeation 

The concentration at the membrane surface [C (membrane)] cannot be measured directly and must be calculated from equations incorporating boundary layer phenomena (see chapter VII). Another approach is to employ experimental conditions such that c j (membrane) = c j (feed). This implies that the experiments should be carried out at low driving forces (low pressures) and very low feed concentrations. 

An even more simple approach on solute rejection based on hydrodynamics was already developed by Ferry in 1936 [22] assuming a simple sieving mechanism. The rejection of a non-adsorbing spherical molecule can be related to the ratio of the solute radius and the pore radius, X  

Although the Ferry equation is based on a rather sinqile concept since it does not take into account surface effects (adsorption), flow induced deformation, hindered diffusion and other interactive and hydrodynamic effects, it may be helpful as a first estimate of the rejection of a solute in relation to the pore size of the membrane. Several investigators have shown that this simple concept can be well applied when the pore size is larger than the solute size, i.e., when X. 1. A number of similar equations have been derived [23.24] showing about the same t) e of retention curves as obtained from eq. FV - 16. 

A polymer chain in solution can be considered as a random coil.The size of the macromolecular solute can be expressed as the radius of gyration or as the hydrodynamic radius or Stokes-Einstein radius r, . If there is a possibility of rotation aibout covalent bonds in the polymer backbone there is a continuous motion and in fact there is no well defined shape. In solution the chain is rather coiled than stretched and therefore it is better to define coil dimensions. 

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In summary, solute rejection measurements provide a very simple technique for indicating the performance of a given membrane. For this reason they are very frequently used for... 

Membrane Rejection. Both cellulose acetate and FT-30 composite membranes were evaluated for rejection of solutes. Sodium chloride rejections were confirmed and listed in Table III. Typical organic rejections of model compounds are listed in Tables IV and V for cellulose acetate and FT-30 composite membranes, respectively. Rejections were measured during screening and concentration tests solute levels were in the parts-per-billion range. Measurement of feed and permeate stream solute concentrations provided the necessary information to calculate solute rejection. Eq 1 was used to calculate rejection values. 

Rejections of solute compounds measured during the concentrations generally corresponded well with those measured in the screening test. Rejection values determined in the concentration test are presented in Tables IV and V along with values measured in the screening test. Rejection measurements made at 8- and 40-fold volume reductions are reported with the value determined at the lower concentration factor listed first (e.g., 87/83, see Tables IV and V). 

Rejection rate measures the ability of the membrane to retain a certain molecule. The observed solute rejection rate R, for a given specie i is given by ... 

J/p) and solute rejections (r) over a broad range of pH. Experiments were carried out at temperatures between 30 and 70 C, pressures up to 6.9 MPa (1,000 psig), and cross flow velocities of 1 to 2 m/s. Electrolyte rejections were determined by measuring the conductivity of the feed and permeate solutions. The concentrations of the sugar solutions were measured with a refractometer. 

A third possible complicating factor in rejection measurements, especially with protein solution, is physical Interaction of the solute with the membrane surface. Solute adsorption, for example, could alter the parameters L, a or P and cause anamo-lous rejection. An earlier search O for adsorption effects by a cellulosic membrane was negative. Although other investigators have reported such effects with non-cellulosic membranes, the effects with Cuprophan fibers were not observed, and adsorption parameters were not included in this transport model. 

Test solutes were dissolved in 0.85% NaCl stabilized with 200 ppm NaN. The rejection of four test solutes was measured ... 

The effect of serum on rejection of solutes, as compared with rejection measured out of saline, can be addressed in light of data for cellulosic membranes. Figures 10 and 11 demonstrate an effect consistently observed for the solutes myoglobin and cytochrome C. The observed and corrected rejection values fell off at approximately J >0.6 x 10 cm/sec from a plateau when saline was the solvent, whereas in serum the R versus J graphs were as predicted from the Spiegler-Kedem equation. The relative effects of serum or BSA on R, , and on resultant values for a... 

Solute rejection for four solutes and ultrafiltration rates for two protein solutions have been measured for high-flux cellu-losic hollow-fiber bundles of three lengths. 

Myoglobin, cytochrome-C, inulin, and vitamin B-12 were the solutes studied in saline, calf serum, and BSA systems at 37 C and pH 7.4. Observed solute rejections were corrected to intrinsic values by using uniform-wall-flux boundary layer theory for the developing and fully-developed asymptotic regions. The Splegler-Kedem equation ( ) for rejection versus volume flow was used to calculate reflection coefficients and diffusive permeabilities for each solute. There was no significant difference between rejection parameters measured in saline and protein solutions. 

The GPC analysis of feed and permeate solution is ideally suited for rapid simultaneous rejection measurements. Simulta-eous rejection is of great Importance in ultrafiltration practice. As an example, we show here a simultaneous measurement of rejection of proteins and of lactose in whey ultrafiltration (Figure 13). The membrane used was the ABCOR HFK membrane and the feed solution had a typical composition of a partially concentrated... 

In terms of organic rejections, PEC-1000 membrane shows the highest values among all commercial reverse osmosis membranes. Table 5.6 lists rejection data for a variety of organic compounds. In most cases, these were measured at solute concentrations of 4 to 5%, which represents a severe test protocol. Organic solute rejections determined for other commercial membranes were typi-... 

Manufacturers tend to characterize membranes by means of rejection measurements with reference molecules, such as dextrans, proteins, or polyglycols. A parameter extensively used for membranes characterization is the cutoff value, which is defined as the lower limit of solute molecular weight for which the rejection is at least 90%. We must keep in mind, however, that rejection measurements always depend on the type of solute (shape and flexibility of the macromolecular solute), the membrane (its interaction with the solute), and the process parameters used (pressure, cross-flow velocity, geometry of the test cell, and concentration and type of solute). In particular, concentration or polarization, pore blocking, and fouling phenomena affect rejection measurements very significantly. 

Membrane fouling and the subsequent permeate flux decline is one of the major drawbacks. A number of theories about concentration polarization, cake formation and flux decline have been developed to predict membrane fouling. Generally those models are based on an assumed behavior and validated by experimental determinations. Nevertheless, typical experimental measurements are focused on macroscopic parameters, such as permeate flux, pressure drop and solute rejection, which provide little information about microscopic phenomena and, thus, are not able to validate mechanistic models. 

Weigh an amount of finely powdered tablet material containing about 100 mg of morphine sulphate, transfer to a 250-ml graduated flask, add about 100 ml of N sulphuric acid and shake for thirty minutes. Make up to volume with N sulphuric acid, mix and filter a portion of the solution, rejecting the first 30 ml. Dilute 25 ml of the filtrate to 100 ml in N sulphuric acid and measure the maximum extinction at about 285 m/ in a 1-cm cell using N sulphuric acid as the blank. Calculate the amount of morphine sulphate in each tablet. 

We assume now that, other conditions remaining constant, the intrinsic solute rejections of the ionized and unionized species remain independent of pH. Let the solute rejection of the unionized solute 1 be Ri and that of the solute 2 be i 2- Such values for phenol can be determined, for example, by making two measurements with a reverse osmosis membrane (say, the FT-30 membrane manufactured by Filmtec Inc., Minnetonka, MN) at a low pH, phenol is undissociated and so i i is obtained, while, at a very high pH, all phenol is dissociated and present as CeHsO (or, say, as sodium phenolate in caustic solution), yielding i 2-... 

In a stirred ultrafiltration cell using a flat UF membrane, an aqueous solution of the polymer Dextran 20 was ultrafiltered. Data were gathered at different values of the water flux, and the solute rejection was measured. Dextran 20 is a linear polymer, and, as the solvent flux was increased, the rejection observed for Dextran 20 decreased. A plot of the solvent flux against the quantity (1 —Robs)/K>bs in a semilogarithmic plot (logarithmic on the abscissa for (1 ilobs)/fiobs) yielded a straight line with a positive slope and an intercept of 0.05 on the abscissa. 

Kim et al. made a more detailed study of the effect of solvent in the SPPO coating solution on the performance of SPPO TFC membranes. In their study solute rejection of NaCl and MgS04 was used as a measure for the membrane selectivity. Four solvents including methanol, 2-methoxy ethanol, 2-ethoxy ethanol and 2-butoxy ethanol were used to make 0.5 wt. % SPPO... 

Solute separation is measured in terms of observed rejection, R, defined as... 

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). 

Retention Rejection and Reflection Retention and rejection are used almost interchangeably. A third term, reflection, includes a measure of solute-solvent coupling, and is the term used in irreversible thermodynamic descriptions of membrane separations. It is important in only a few practical cases. Rejection is the term of trade in reverse osmosis (RO) and NF, and retention is usually used in UF and MF. 

Membrane Characterization Membranes are always rated for flux and rejection. NaCl is always used as one measure of rejection, and for a veiy good RO membrane, it will be 99.7 percent or more. Nanofiltration membranes are also tested on a larger solute, commonly MgS04. Test results are veiy much a function of how the test is run, and membrane suppliers are usually specific on the test conditions. Salt concentration will be specified as some average of feed and exit concentration, but both are bulk values. Salt concentration at the membrane governs performance. Flux, pressure, membrane geome-tiy, and cross-flow velocity all influence polarization and the other variables shown in Fig. 22-63. 

Typically, a membrane that rejects 93% of Na+ or Cl- will reject 98% of Ca2+ or S042-when rejections are measured on solutions of a single salt. With mixtures of salts in solution,... 

The rejection coefficient R) was calculated according to the following equation / = In (Cr/C )/ln (VJVr). Cr or Vr represent the protein concentration in the retentate or the volume of the retentate Co is the concentration of the protein in the solution before filtration 1 is the initial volume of the feed. The pH value of each protein solution was immediately measured after dissolving the proteins in distilled water. 

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