Superhydrophobicity of single-walled carbon nanotubes for oil separation

Industrial oily wastewater has become a worldwide challenge and petrochemical and marine industries deal with a number of oil spillage incidents[1]. These issues have caused severe environmental destruction. Emulsified oils from industries and daily life will further aggravate the crisis of potable freshwater[1].

There is a need for a superabsorbent material capable of separating oil-water mixture, especially with high absorption capacity and mechanical strength. Traditional absorbent materials for oil-water separation are microporous absorbers such as wool, zeolites, and activated carbon, which have drawbacks of poor absorption capacity, non-selective water/oil absorption and pose challenges in reusability[2]. These demand development of a superhydrophobic surface with greater water contact angle[2], [3].

Single-walled carbon nanotubes(SWCNT) as an absorbent material show excellent selective absorption, fast absorption kinetics, and good working capacity due to good wettability and porosity[3]. SWCNT offer the diversity of controlling the degree of cross-linking, the nanostructure, porous features, and specific surface area, to attain the efficient adsorption capacities[4].

Maurizio Prato et.al developed a novel methodology – a HiPCO® SWCNT based-adsorbent for purification of water from oil spilling by a high degree cross-linking of SWCNT with benzidine functionalization. High cross-linking degree (HCD) of SWCNT enables the formation of more bridges between the CNTs resulting in the formation of smaller cavities and pores (pore diameter 9.73 nm) in the adsorbent material. Also, the increase in cross-linking degree led to an increase in the specific surface area (SSA-427 m2 /g) which exhibits a large number of active sites. The presence of smaller and new pores due to bridges of SWCNT increases the SSA. The adsorption capacity is directly proportional to the SSA as the number of active sites available for adsorption is higher[4].

The wettability of HCD-SWCNT was investigated by placing a water droplet (~3 μL) onto the HCD-SWCNT adsorbent surface. The water remained there without spreading (contact angle equal to 105°) showcasing the superhydrophobic surface of HCD-SWCNT. In contrast, oil droplets spread quickly (nearly zero contact angle) showing super-oleophilic properties. This behavior is due to the increased surface roughness, the number of linkers among the nanotube strands that lower the surface free energy after functionalization[4].

Single-walled carbon nanotube/polydopamine/polyethyleneimine (SWCNT/PD/PEI) ultrathin film composite with superoleophobic property is successfully prepared and used for ultrafast separation of oil-in-water nanoemulsions. The ultrathin SWCNT/PD/PEI composite with pore size of ~10 nm and thickness of ∼160 nm separates oil from oil-in-water nanoemulsions with an ultrahigh permeation flux up to 6000 L m-2h -1bar-1 and with a high separation efficiency. Permeation flux (volume flowing through the membrane per unit area per unit time) of the SWCNT/PD composite film depends on the film thickness and effective pore size. The micro-porous structure of small diameter SWCNT entraps the oil droplets of tens of nanometers[5].

Single-walled carbon nanotubes are excellent adsorbent material due to their superhydrophobicity and good wettability properties with tunable porous structure and high SSA being added advantages. Hence, the unique properties of SWCNT encourage water purification from oil spilling.

References :

[1]       M. O. Adebajo, R. L. Frost, J. T. Kloprogge, O. Carmody, and S. Kokot, “Porous Materials for Oil Spill Cleanup: A Review of Synthesis and Absorbing Properties,” p. 2.

[2]       “A novel carbon nanotubes reinforced superhydrophobic and superoleophilic polyurethane sponge for selective oil–water separation through a chemical fabrication – Journal of Materials Chemistry A (RSC Publishing).” [Online]. Available: https://pubs.rsc.org/en/content/articlelanding/2015/ta/c4ta03945a#!divAbstract. [Accessed: 04-Nov-2019].

[3]       “(1) Hydrophobic carbon nanotubes for removal of oils and organics from water | Request PDF.” [Online]. Available: https://www.researchgate.net/publication/264243093_Hydrophobic_carbon_nanotubes_for_removal_of_oils_and_organics_from_water. [Accessed: 04-Nov-2019].

[4]       “Cross-Linked Carbon Nanotube Adsorbents for Water Treatment: Tuning the Sorption Capacity through Chemical Functionalization | ACS Applied Materials & Interfaces.” [Online]. Available: https://pubs.acs.org/doi/abs/10.1021/acsami.8b20557. [Accessed: 04-Nov-2019].

[5]       “Superwetting polymer-decorated SWCNT composite ultrathin films for ultrafast separation of oil-in-water nanoemulsions – Journal of Materials Chemistry A (RSC Publishing).” [Online]. Available: https://pubs.rsc.org/en/content/articlelanding/2015/ta/c4ta05624h/unauth#!divAbstract. [Accessed: 04-Nov-2019].