Continuous population growth and urbanization as well as rapid industrialization, causing huge contamination of potable water or underground water, has been a serious concern all over the world. Due to incompetency of conventional water purification technologies to deliver complete pollutants free water at an economical price, a high performance, cost-effective and environmentally acceptable separation system is an urgent need which should not only remove macro-, micro- and nano-pollutants but also desalinate water to a significant extent. In this milieu, nanotechnology based carbon nanotube (CNT) membranes have shown impressive breakthroughs towards water purification as compared to existing energy intensive water purification systems and thus, this technology has immense potential for large scale commercial water purification in a cost effective manner.

1. Nano Porous Membranes for
Water Purification
Shrinath Ghadge
27-705 Nanostructured Materials

2. Contents
2
 Motivation for water purification
 Issues of conventional and existing water purification technologies
 Carbon nanotube (CNT)-based water purification/desalination systems
 Various problems of CNT based desalination
 Promising solutions
 Conclusion

3. Why Water Purification?
3
1. Energy
2. Water
3. Food
4. Environment
5. Poverty
6. Terrorism
7. Disease
8. Education
9. Democracy
10. Population
Humanity’s Top Ten Problems for next 50 years
U.S. Energy Information Administration - EIA - Independent Statistics and Analysis https://www.eia.gov/
Abramovitz, Janet., Imperiled waters, impoverished future: The decline of freshwater
Ecosystems, Washington, DC: World watch Institute (1996, March)

4. Water distribution-Worldwide
http://www.acquainnovations.com/water-facts/ 4

5. Existing water purification methods
Distillation Ion-exchange
http://www.freedrinkingwater.com/water-education2/46-ion-exchange-principle.htmhttp://www.desalinatedwater.info/home.php 5
Energy input : 6.5–11 kWh/m3 Limited capacity

6. Waste-Water Treatment Plant
Preliminary Treatment
http://commalinn.blogspot.com/2015/04/process-of-wastewater-treatment-plant.html 6

7. Contd…
Reverse Osmosis
http://www.dynamicscience.com.au/tester/solutions1/chemistry/nanotechnology/nanotubes.html 7
Operated at very high pressure
P ~ 40 to 82 bar for Seawater desalination

8. Issues/Problems of conventions methods
 Very high energy requirement e.g. RO needs 3 - 5.5 kWh/m3 electricity
for sea water desalination
 Highly expensive processes
 Fouling of RO membranes
 Water quality (turbidity, pH) gets affected
 Can’t operate at high pressure
 Requires a large area of land
Das, R., et al., Desalination, 2014. 336: p. 97-109.
8

9. CNT membranes for water purification
 Surface area to volume ratio (~109 m-1)
 Excellent antimicrobial activity
 Higher mechanical stability
 No fouling issues ~ longer life
 Operation at atmospheric pressure
 Higher water permeability ~ very high water flux
 Tunable physical, chemical, electrical and structural properties
Das, R., et al., Desalination, 2014. 336: p. 97-109.
9
http://www.dynamicscience.com.au/tester/solutions1/chemistry/nanotechnology/nanotubes.htm
Pure water molecule
Impurities

10. Classification of CNT membranes
Single wall CNT
http://jnm.snmjournals.org/content/48/7/1039/F1.expansion
Multi wall CNT
10

11. Key issues for CNT based desalination
 Structural issues
 Irregularities /improper alignment in CNT’s structure
 Non-uniform pore diameter of CNT
 Properties based issues
 Moderate adsorption capacity of CNT membranes
 Difficulty in functionalization of CNT’s with desired functional
groups
 Induced toxicity in permeate water streams
 Other issues/challenges
 Very high cost of SWCNT’s
 Enhancing anti-microbial activity 11

12. Solutions for the structural issues
 Growth from template approach : Omachi et al.
 Structure of template organic molecules is reproduced in the
final structures of the CNT’s
 n-Cycloparaphenylenes (CPP): Template and Et-OH: C source
n  no. of benzene rings (12)
Omachi, H., et al., Nat. Chem., 2013. 5(7): p. 572-6.
12

13. Experimental details
 C-plane sapphire substrate plate (5 mm× 5 mm)
 Reaction temperature = 500oC
 Reaction time = 15 min
 Pressure ~ 1 torr (i.e. under vacuum)
Contd…
13
Omachi, H., et al., Nat. Chem., 2013. 5(7): p. 572-6.

14. Results and Conclusion
TEM of synthesized CNT’s
Conclusion:
 Crucial factors for efficient CNT’S
growth
• Reaction temperature
• Nature of reaction plate
• Carbon source
CNT’s diameter
D = 248cm-1/WRBM
~ 1. 3 to 1.7 nm
Contd…
14
Omachi, H., et al., Nat. Chem., 2013. 5(7): p. 572-6.

15. Solutions for the properties based issues
 Enhancing adsorption capacity of CNT membranes
 Synthesis of acid and plasma treated CNT membranes
CVD Fabrication and modification
Yang, H.Y., et al., Nat .Commun., 2013. 4: p. 2220.
15

16.  Modification of UCNT-based membranes
(A) Acid treatment
• Membranes were immersed into 5N HNO3, refluxed for 2 h and
then washed with DI water
(B) Plasma treatment
• Remote inductively coupled
plasma configuration
16
Yang, H.Y., et al., Nat .Commun., 2013. 4: p. 2220.

17. Assembly and ObservationsContd…
Micro-channel device Cross-sectional SEM image of the pristine
UCNT–MCE membrane
SEM image of membrane after salt adsorption
Crystal lattice of NaCl NP’s embedded in the
CNT pores 17
Yang, H.Y., et al., Nat .Commun., 2013. 4: p. 2220.

18. Results and Conclusion
 Adsorption capacity of the plasma-modified CNT’s is ~ 400% w/w (i.e. 4
gg-1 or 4,000 mg g-1)
 Activated carbons ~ 2–20 mg g-1
 Driving force for desalination of CNT-based membranes is the free
energy of adsorption
 plasma treatment : Numerous defective sites
 Partially damage of graphitic structure of CNT’s  dangling bonds
 React with the ambient gases to form various functional/charged groups,
such as –COOH and –OH
18

19. Solutions for the properties based issues
 Removal of selective pollutants
Chromium (Cr)
 Application of magnetic NP’s to solve environmental problems
Synthesis of CNT/nano-iron oxide composites
 MWCNT + FeCl3.6H2O
 5 M NH4OH added drop wise to precipitate iron oxide
 Product separation by magnet
 Washing with DI
Gupta, V.K., S. Agarwal, and T.A. Saleh, Water research, 2011. 45(6): p. 2207-2212.
19

20. ResultsContd…
XRD of MWCNTs/nano-iron oxide
Mn: Magnetite (Fe3O4)
Mh: Maghemite (Fe2O3)
• ~ 90% adsorption after 60 min
• Additional adsorbing sites
provided by oxygen atoms of iron
oxide NP’s
20
Gupta, V.K., S. Agarwal, and T.A. Saleh, Water research, 2011. 45(6): p. 2207-2212.

21. Solutions for the properties based issues
(2) Removal of selective pollutants
Zinc (Zn+2)
 Application of NaClO treated CNT’s for Zn removal
The properties of CNTs such as purity, structure and nature of the surface were greatly
improved after purification
Lu, C. and H. Chiu, Chemical Engineering Science, 2006. 61(4): p. 1138-1145. 21

22. Adsorbent Avg. Pore diameter (nm)
SWCNT 7.6
Purified SWCNT 4.12 
MWCNT 8.35
Purified MWCNT 5.17 
SWCNT MWCNT
Contd…
22
Lu, C. and H. Chiu, Chemical Engineering Science, 2006. 61(4): p. 1138-1145.

23. Langmuir adsorption model
CNT
Langmuir Freundlich
a b R2 Kf n R2
SWCNT 43.66 0.19 0.999 13.24 0.292 0.945
MWCNT 32.68 0.22 0.999 11.84 0.244 0.945
Freundlich isotherm
Adsorption isotherms for Zn2+ with purified CNT’s
Contd…
23
Lu, C. and H. Chiu, Chemical Engineering Science, 2006. 61(4): p. 1138-1145.

24. Solutions for the properties based issues
 Lowering the toxicity
 Functionalization of CNT’s
Sayes, C.M., et al., Toxicol Lett, 2006. 161(2): p. 135-42.

Cytotoxicity study on human dermal fibroblast cell cultures
24

25. 25
• As the degree of functionalization
on the surface of the CNT increases,
the cytotoxicity decreases
significantly.
Sayes, C.M., et al., Toxicol Lett, 2006. 161(2): p. 135-42.
• COO-, SO3
- groups attracts positive ions
• +ve ions attracts –ve ions
• Formation of Donnan Potential

26. Approaches for enhancing CNT’s performance
 Improvement in anti-microbial activity
Silver (Ag) NP’s-doped CNT membrane
Ihsanullah, et al., Desalination, 2015. 376: p. 82-93. 26
Porosity increases up-to 10% of Ag content

27. Results and Conclusion
• At high Ag content, the membrane
surface becomes more hydrophilic
• Flux increases
• Complete removal of all E. coli
bacteria at 60 min
• The combined antitoxic properties of
Ag and CNT’s led to enhanced
antibacterial properties
27
Ihsanullah, et al., Desalination, 2015. 376: p. 82-93.

28. Solutions to lower the CNT’s cost
 CNT doping to other polymers or matrices for cost reduction
Das, R., et al., Desalination, 2014. 336: p. 97-109.
Filler Advantages
Polysulfonate • Increased water flux (160%)
Poly (vinylidene
fluoride)
• Eliminated E. coli cells (~2 μm) through size exclusion.
• Inactivated 80% of the bacteria within 20 min contact time.
Polyamide–
polysulfone
• Increased water permeability.
• Enhanced bacterial cytotoxicity (60%/h).
Polyether sulfone
• Increased water refluxing capacity.
• Greater antifouling activity against whey proteins
Poly (methyl
methacrylate)
• Increased water flux (62%) with improved selectivity and
sensitivity and Retained Na2SO4 (99%).
28

29. Conclusion
 CNT based membranes could potentially lead to more effective
means of filtration:
 Remove more impurities/ pollutants
 Faster
 Extended service life
 More economical
 Most promising method for future seawater desalination
 Exhaustive research still needed to assess any harmful effects of
CNT’s on environment and living beings.
29

30. References
• R. Das, M. E. Ali, S. B. A. Hamid, S. Ramakrishna, and Z. Z. Chowdhury, "Carbon nanotube
membranes for water purification: A bright future in water desalination," Desalination, vol. 336, pp.
97-109, 2014.
• H. Omachi, T. Nakayama, E. Takahashi, Y. Segawa, and K. Itami, "Initiation of carbon nanotube
growth by well-defined carbon nanorings," Nat Chem, vol. 5, pp. 572-6, 2013.
• C. Bower, W. Zhu, S. Jin, and O. Zhou, "Plasma-induced alignment of carbon nanotubes," Applied
Physics Letters, vol. 77, p. 830, 2000.
• L. Ding, A. Tselev, J. Wang, D. Yuan, H. Chu, T. P. McNicholas, et al., "Selective Growth of Well-
Aligned Semiconducting Single-Walled Carbon Nanotubes," Nano Letters, vol. 9, pp. 800-805, 200
• H. Y. Yang, Z. J. Han, S. F. Yu, K. L. Pey, K. Ostrikov, and R. Karnik, "Carbon nanotube membranes
with ultrahigh specific adsorption capacity for water desalination and purification," Nat Commun, vol.
4, p. 2220, 2013.
30

31. • Ihsanullah, T. Laoui, A. M. Al-Amer, A. B. Khalil, A. Abbas, M. Khraisheh, et al., "Novel anti-
microbial membrane for desalination pretreatment: A silver nanoparticle-doped carbon nanotube
membrane," Desalination, vol. 376, pp. 82-93, 2015.
• Ihsanullah, A. M. Al Amer, T. Laoui, A. Abbas, N. Al-Aqeeli, F. Patel, et al., "Fabrication and
antifouling behaviour of a carbon nanotube membrane," Materials & Design, vol. 89, pp. 549-558,
2016
• C. M. Sayes, F. Liang, J. L. Hudson, J. Mendez, W. Guo, J. M. Beach, et al., "Functionalization density
dependence of single-walled carbon nanotubes cytotoxicity in vitro," Toxicol Lett, vol. 161, pp. 135-
42, 2006
• V. K. Gupta, S. Agarwal, and T. A. Saleh, "Chromium removal by combining the magnetic properties
of iron oxide with adsorption properties of carbon nanotubes," Water research, vol. 45, pp. 2207-
2212, 2011.
• P. Yenphan, A. Chanachai, and R. Jiraratananon, "Experimental study on micellar-enhanced
ultrafiltration (MEUF) of aqueous solution and wastewater containing lead ion with mixed
surfactants," Desalination, vol. 253, pp. 30-37, 2010.
31

32. Thankyou!
http://www.newwatersupply.org/news/quotes.htm 32

33. *Supplementary slides*
33

34. MEUF for water purification
 Removal of heavy metal ions based on their charge
Yenphan, P., A. Chanachai, and R. Jiraratananon, Desalination, 2010. 253(1–3): p. 30-37.
• Removal of positive metal ions (Ni+2, Cu+2, Cd+2, Zn+2 etc.) using anionic surfactant
(e.g. SDS)
• Cationic surfactant : Trimethylhexadecyl ammonium chloride, to remove negative
metal ions (e.g. Br-)
34

35. Microwave plasma-enhanced chemical vapor
deposition (MPECVD) : Brower et al.
 The CNT’s have been grown perpendicular to the local substrate surface
regardless of the substrate tilt or shape
Bower, C., et al., Applied Physics Letters, 2000. 77(6): p. 830.
35

36. Chemical vapor deposition : Ding et al.
 Substrates : single-crystal ST-cut quartz
 Carbon source : ethanol/methanol mixture
 Catalyst : Cu nanoparticles
SEM image • Diameter ~ 1.55-1.78 nm
• 95% of nanotubes being semiconducting due to
strong interaction between SWNT’s and the quartz latticeDing, L., et al., Nano Letters, 2009. 9(2): p. 800-805
36

37. Approaches for enhancing CNT’s performance
Improvement in anti-microbial activity
Silver (Ag) NP’s-doped CNT membrane
AgNO3 dissolved in Et-OH + CNT dissolved in Et-OH

Sonicating (homogeneity )

Evaporation of Et-OH in oven

Calcination for 4 h at 350 oC
Ihsanullah, et al., Desalination, 2015. 376: p. 82-93. 37

38. Approaches for enhancing CNT’s performance
(1) Improvement in membrane performance
Fe2O3 NP’s-doped CNT membrane
Ihsanullah, et al., Materials & Design, 2016. 89: p. 549-558.
Diametral compression test curves
Mechanical strength of 11.2 MPa at 20 % Fe2O3
A maximum removal of 90 and 88% of SA was
achieved for membranes with a 10 and 1%
Fe2O3 content, respectively, after 3 h
38

39. Dissolved air floatation (DAF)
http://commalinn.blogspot.com/2015/04/process-of-wastewater-treatment-plant.html 39

40. Anaerobic Sludge Digester
http://commalinn.blogspot.com/2015/04/process-of-wastewater-treatment-plant.html 40

41. Secondary sedimentation tank
http://commalinn.blogspot.com/2015/04/process-of-wastewater-treatment-plant.html 41

42. Operation of CNT membranes for water purification
http://mw.concord.org/modeler/showcase/nano/waterrace.html
 Water molecules with different initial velocities move through a cluster of CNTs
42