Commercially available polysulfone membranes are known to be hydrophobic an
d are prone to fouling due to strong interaction with variety of solutes. I
n order to overcome this. the 5 K molecular weight cut off (MWCO) and 20 K
MWCO polysulfone ultrafiltration membranes have been modified by gas phase
nitration using NOx (a mixture of NO and NO2). These membranes were subsequ
ently aminated using hydrazine hydrate and as a result of this, membranes w
ere observed to give considerably higher rejection for nonionic molecules (
polyethylene glycol) with simultaneous increase in resistance to the flow.
Experimental measurements have shown that the modified membranes have the e
xchange capacity of 4.17 meq per dry g of membrane and we show that this ex
change capacity can be obtained only when there are about two exchangeable
amine groups per repeat unit of every polysulfone molecule. A careful exami
nation of the increase in resistance suggests that one of the mechanisms of
separation of nonionic solutes is due to the reduction of average pore siz
e of the membranes, However, for sodium chloride solution we show that the
modified membranes have a 13%, rejection (as opposed to none for unmodified
one), while for a complex feed, like alkaline black liquor. the rejection
(,For all dilutions) is twice that for unmodified membranes. Due to this mo
dification, it appears that the membrane is entering into nanofiltration zo
ne. (C) 2001 Elsevier Science B.V. All rights reserved.