Angewandte
Communications
Chemie
two fractions: the organic layer containing PP2b, and the
aqueous one containing Nafion, as well as the dye adsorbed
by it (the solution is partially colored by the dye, and Nafion
can be detected by F NMR spectroscopy, Supporting infor-
mation Figure S12).
The organic phase can be purified, reassembled, and re-
used in membrane fabrication. Nafion layer could not be
recycled, and we are currently working on a procedure that
will allow full recycling. Both BCG and Rhodamine 110
filtration using the recycled PP2b layer on which Nafion was
deposited was quantitative (Figure S13). Control dye-filtra-
tion experiments were performed using PES filters, on which
Nafion was directly deposited (it is only partially retained on
the support). We observed that the dyes are not retained
low flow rates. Cleaning the conventional covalent mem-
branes is usually a difficult and expensive process, which is
[
8,30,31]
infeasible in some cases.
In the case of our self-
assembled membrane it can be deposited from solution on
the standard filtration module (e.g. having large pore PES as
a support membrane), disassembled upon fouling, cleaned,
and reassembled again on the same module, emphasizing the
advantage of the self-assembled nature of the Nafion/PP2b
membrane.
We have also performed high-pressure experiments since
commercial processes involving membranes use pressure as
the driving force for permeation. The permeance of our
À1
À2
À1
system (3 Lh
for nanofiltration applications, which are typically driven by 3
m bar ) measured at low pressure is typical
[32,33]
(
Figure S14).
to 20 bar operating pressures.
Our systemꢀs performance
To further evaluate membrane performance regarding
in partial desalination is actually rivaling reverse osmosis
retention of pharmaceuticals (an important issue in water
membranes, which use higher trans-membrane pressures
[
23–25]
[33,34]
purification of urban sewage)
we performed filtration of
(between 10 and 100 bar).
pressure can influence membrane performance.
We note that operating
[
35,36]
Amoxicillin, a commonly used antibiotic. UV/Vis spectros-
copy indicated quantitative retention (Figure 4 f), which is
similar or superior to currently employed polymeric mem-
Hence,
the performance of hybrid membranes containing 20 mg or
50 mg Nafion deposited on top of PP2b layer was explored
under elevated pressures. With 20 mg Nafion, we observe
[
27,28]
branes.
À1
À2
À1
Targeting filtration of small water-soluble organic mole-
cules, we compared two naphthalene derivatives with similar
size but different charge: 2,3-diaminonaphtalene which is
a permeance of approximately 29 Lh m bar (2.8 bar).
Under these conditions, Pb retention is over 98.3% (from
600 ppb to less than 10 ppb). Permeance further increases
À4
À1
À2
À1
positively charged (10 m, dissolved with few drops of 1m
under higher pressure of 8.7 bar (46 Lh m bar ). Increas-
HCl) and neutral 2,3- dihydroxynaphtalene. Both molecules
are retained, with 2,3-diaminonaphtalene showing better
retention according to UV/Vis spectroscopy (Figure 4c,d).
An even smaller molecule, such as trimethylphenyl ammoni-
um chloride (Figure 4e), was also retained, while phenol and
benzoic acid were not retained. These results suggest that
both charge and molecular size contribute to the retention.
Thus, the Nafion layer acts as a membrane having ion-
exchange properties, while the densification of PP2b appears
to play a role in the size-selective rejection.
ing the pressure above 9 bar leads to a pressure drop as
evidenced by flux/permeation-pressure plots (Figure 5a,b),
probably because of membrane damage. However, when
50 mg of Nafion is deposited, permeance starts at around
À1
À2
À1
2.7 Lh
m bar at the lowest pressures, and then decreases
stepwise upon pressure increase, indicating a stepwise mem-
brane compression (Figure 5c,d). This experiment was con-
ducted over three consecutive days, providing us with the
To further probe the role of the densified supramolecular
layer in the filtration, we performed filtration of a mixture of
gold nanoparticles (1–13 nm in size, average 5.1 Æ 2.1 nm)
covered with neutral NEG (nona-ethylene glycol-based)
capping layer. EDS measurements show that the nanoparti-
cles pass the Nafion layer and then are retained by the PP2b
layer, indicating that a cut-off size of the PP2b layer is less
[
9]
than 1 nm (nondensified PP2b exhibits 5-nm cut-off),
confirming the size-selective rejection role of the densified
supramolecular layer Figures S15–S17). Thus, less-polar sol-
utes or the ones having lower affinity to the Nafion sulfonates
can pass the Nafion layer but be retained by the PP2b layer
according to their size, reflecting the synergetic function of
the membrane materials.
We have also found that decreasing the deposited Nafion
quantity from 50 mg to 20 mg maintains the good retention
efficiency for both heavy metals (Pb and Cd retention
Figure 5. Hybrid membrane flux and permeance under high pressure
over several days. The membranes were operated for the entire day,
increasing the pressure every hour, then pressure was reduced to zero
overnight and gradually ramped up again the next day. a) 20 mg
Nafion membrane, flux. b) 20 mg Nafion membrane, permeance.
c) 50 mg Nafion membrane, flux. d) 50 mg Nafion membrane, perme-
ance on three consecutive days: After the first operation up to 10 bar,
the permeance remained at this plateau over three days of variable
pressures, before the pressure was increased up to 40 bar, inducing
membrane collapse without recovery.
>
99.5%, Table S6) and organic molecules (Amoxicillin,
Figure S18). In this case we obtain a thinner layer of Nafion
and a somewhat thicker layer of PP2b (Figure S19), without
significant change in the rejection properties.
A challenging issue regarding the currently employed
[
29]
polymeric membranes is irreversible fouling that leads to
4
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Angew. Chem. Int. Ed. 2017, 56, 1 – 6
These are not the final page numbers!
Cohen, Erez
