Supplementary material
Experimental set-up
Feed tank
Cartridge
Filter
V
Feeding
D
pump
V
VR
D
HP
RO
NF
CP
CC
TH
: Valve
: Reject valve
: Pressure regulator
: High pressure pump
: RO module
: NF module
: Pressure vessel
: Conductivity vessel
: Temperature vessel
D
Permeate
Retentat
Titration experiments
To directly determine the amount of negatively charged groups on a membrane, a titration is carried out. The
membrane is immersed for 15 min into 50 mL of a 0.1M CsCl solution. It is assumed that because of the high
concentration of the CsCl solution, the original counter-ions are exchanged for cesium and all charged groups on
the membrane have a cesium ion as counter-ion. A longer immersing time or a higher CsCl concentration did not
have any additional effect. After rinsing the membrane thoroughly in demineralized water, the membrane is
immersed for 15 min into 50 mL of a 0.01M MgCl2 solution. The magnesium ions occupy the place of the
cesium ions as counter-ions and the cesium ions set free in the solution are measured with ICP-MS. The number
of cesium ions measured is set equal to the number of negatively charged groups and is expressed in units of
equivalents per meter square of total membrane surface.
The same procedure was followed to determine the amount of positively charged groups on the membrane.
For this purpose NaF was used instead of CsCl and Na2SO4 was used to set free the fluoride ions. The number of
fluoride ions is measured in the solution with a selective fluoride electrode and is set equal to the number of
positively charged membrane groups. The solutions had a pH of 5.5–6.
Remark that initially acid–base titrations were carried out. The limitation of this method is situated at the
determination of the equivalence point, resulting in a less sensitive method compared to the determination of the
amount of cesium or fluoride ions in the samples. For the membranes where the ion exchange capacity could be
determined with this method, the results were comparable.