Charged Polymer Nanofiltration Membranes for Selective Retention

1
Charged Polymer Nanofiltration Membranes for
Selective Retention
Nathan G. Lovell1, Ayse Asatekin2, Ikuo
Taniguchi1, and Anne M. Mayes1. (1) Department
of Materials Science and Engineering (2)
Department of Chemical Engineering, Massachusetts
Institute of Technology, Cambridge, MA

• Present technology commercial polymer membranes
  for water purification are limited in application
  and practical life by their susceptibility to
  foul with concurrent reduction in membrane
  permeability. Recent developments in graft
  copolymers have produced fouling-resistant
  nanofiltration membranes. Specifically,
  polyacrylonitrile-graft-poly(ethylene glycol)
  (PAN-g-PEG) coated membranes with size
  selectivity have been reported.1 We report on
  similar membranes with the addition of charged
  groups to impart charge selectivity to the
  filtration properties of the membrane and enhance
  the exclusion of salts. A membrane coated with a
  negatively-charged, sulfopropyl-modified
  PAN-g-PEG copolymer retains much of the fouling
  resistance while better retaining simple salts.
  The specificity of the coated membrane and the
  effect of environment on its properties are
  reported.

Asatekin et al., Journal of Membrane Science
(2006) 85, 81
2
Membrane Limitations

• Problem fouling (macromolecule adsorption onto
  membranes)
• ? flux
• ? membrane life
• ? energy use

Humic acid fouled membrane, 3.5x104x SEM
(Professional Water Technologies, Inc.)
80 irreversible permeability loss
(Commercial PAN UF membrane)
3
Amphiphilic Graft Copolymer Thin Film Composite
NF Membranes

• Amphiphilic graft copolymers
• Simple coating for thin selective layer
• Microphase separate into bicontinuous network
• Form hydrophilic nanochannels
• Nanochannels allow passage of water and small
  molecules1

Hydrophilic PEG nanochannel
Hydrophobic PAN glassy matrix
TEM of PAN-g-PEG section by Elsa Olivetti
1 Akthakul et al., Macromolecules (2004) 37, 7663
4
Amphiphilic Graft Copolymer TFC vs. Commercial
Membranes

• Amphiphilic graft copolymer TFC NF membranes
  have
• 300 the permeability of commercial membranes
• No permanent protein/ polysaccharide fouling
  (commercial PAN UF membrane fouled 80)

• Size cut-off 0.9 nm (by dye exclusion)
• No charge?low salt retention
• Glassy, hydrophobic matrix, often
  polyacrylonitrile (PAN)
• Hydrophobic, protein resistant side chain
  poly(ethylene glycol) (PEG)

SEM of a TFC membrane cross section with
PAN-g-PEG selective layer on a PAN UF base
PAN-g-PEG
Selective layer (lt2 µm)
Asatekin et al., Patent application. MIT Case
No.12171, filed 04/11/06
5
Charged Amphiphilic Graft Copolymer Synthesis
Characterization

• Free radical synthesis of P(AN-r-SPA)-g-PEG
• acrylonitrile
• poly(ethylene glycol) monomethyl ether acrylate
• 3-sulfopropyl acrylate potassium salt (SAPS)

• Typical properties of P(AN-r-SPA)-g-PEG
• Mw 5M, Mw/Mn gt 10
• (DMF GPC, PS standards)
• Composition
• 0.5-1.5 wt SAPS
• 30-40 wt PEG (by NMR)

DMSO NMR
6
TFC Membrane Coating Testing

• Coating
• 7 DMF solution P(AN-r-SPA)-g-PEG
• PAN base membrane (SEPRO PAN400)
• 40 µm doctor blade height (Testing Machines,
  Inc.)
• Coagulation
• transfer to isopropanol bath
• Permeability and fouling testing1
• dead-end mode
• 50 psi
• 1 g/L BSA in PBS for fouling

coat base membrane
Diagram Thin Film Composite membrane
1Asatekin et al., Journal of Membrane Science
(2006) 85, 81
7
Permeability and Retention vs. Salt Concentration
8
PEG/Water Miscibility

• Change in permeability/retention may be due to
  change in morphology
• PEG has LCST in water1
• Miscibility attributed to H-bonding or
  electrostatic interactions2
• Potential disruptions
• ionic strength
• salt size and valence
• (eg. effect of KCl gt effect of NaCl3)
• pressure
• temperature
• Salted out PEG collapses, enlarging nanochannels
  ? flux ? retention ?
• Osmotic pressure in low salt/ high retention
  solutions ? flux

0.3 M NaCl
0.0 M NaCl
Phase diagram of PEG/water showing LCST behavior
and NaCl influence1
1Saeki et al., Polymer 1977, 18, 1027 2Ashbaugh
et al., Ind. Eng. Chem. Res. 2006, 45,
5531 3Taboada et al., J. Chem. Eng. Data 2005,
50, 264
9
Permeability and Retention vs. Species
Under equal ionic strength conditions

• Divalent calcium and magnesium
• ? chloride retention
• ? flux
• ? retention of larger, divalent sulfate ion

Solutions w/ equivalent ionic strength to 200 PPM
NaCl (0.0034)
10
Fouling (Resistance)

• 86 permeability recovered with water rinse
• By comparison, PAN UF base membrane fouled 80
  irreversibly1

1Asatekin et al., Journal of Membrane Science
(2006) 85, 81
11
Conclusions

• Charged amphiphilic graft copolymer
  P(AN-r-SPA)-g-PEG synthesized via free radical
  polymerization
• TFC NF membranes produced by coating PAN UF base
  membrane with charged polymer
• ? salt ? NF membrane salt retention ?
  permeability ?
• Salt impact on membrane is species
  concentration dependent may be due to collapse
  of PEG within nanochannels
• Divalent Ca2 ? membrane permeability, ?
  retention vs. Na
• Larger, divalent SO42- is better retained than
  Cl-
• Amphiphilic graft copolymer TFC membranes
  charged group ?
• improved salt retention
• good fouling resistance
• higher charge content should improve retention

12
Acknowledgements

• Funding

• WaterCAMPWS NSF Agreement CTS-0120978

• CMSE Shared Experimental Facilities
• MRSEC Program of NSF Award DMR-02-13282

• DCIF NMR facility
• NIH grant 1S10RR13886-01

• Mayes group
• Elsa Olivetti (TEM)
• Professor Michael Rubner