Salt Rejection in the Absence of Organics

The salt rejection and flux ratio at the end of filtration at pH 4.5, 8, and 10 are shown in Table 7.8. The three values represent the rejection after the collection of 40, 80, and 120 mL of permeate, respectively. Jwo is the pure water flux before the experiment. The flux does not change measurably with pH, which is in accordance with the results of Hagmeyer (1999). This means that no change in pore size as reported by other workers (Braghetta (1995)) is to be expected. 

A high rejection for both calcium and sodium is observed for the TFC-ULP membrane, indicating RO behaviour. Rejection is highest at pH 10, the flux decline is high, which can be explained by concentration polarisation due to high rejection. 

Membrane pH Jwii Ca2+ Rejection [%] Na+ Rejection [%] Cl- Rejection [%] J/Jwo [-] Membrane Charge [mV] 

The TFC-S membrane shows a similar calcium rejection to the TFC-ULP membrane, but a significantly lower sodium rejection, which is more typical of NF. The TFC-S membrane shows a relatively stable performance over the pH range. However, neither membrane shows a pH effect, indicating that the ions are mostly retained due to their size. Sodium and calcium ions occur mostly in their dissociated form up to a pH of 10. 

While the TFC-ULP membrane is the membrane with the higher charge, but more open pores (see Dextran experiments), it can be concluded that the sodium rejection occurs due to charge, not ion size. This is also confirmed by the pH effect. A higher rejection of cations at pH 10 may be due to solution speciation (carbonate dominates at pH 10.3 as divalent COj- (over HCO3) which will act as a co-ion 

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