Characteristics of Carbon Nanotube Membrane for Water Treatment and Its Biofouling Properties

Abstract

Carbon nanotube (CNT) membrane has been highlighted for water treatment as a future membrane to solve water shortage problem. Due to the fast transport of water molecules through CNTs and antimicrobial property of CNT, CNT membranes are expected to make a high performance membrane and biofouling-resistant membrane. Here we evaluated three types of CNT membranes such as open-ended vertically aligned (VA) CNT membrane, dense-array outer-wall CNT membrane and mixed (matrix) CNT membranes and compared to the commercial membranes in terms of their permeability, rejection property and biofouling tendency. Firstly, the feasibility of an open-ended VA CNT membrane for water treatment was investigated. The open-ended VA CNT membrane appeared to have approximately three times higher (about 1,100 LMH/bar) water permeability than the commercial UF membrane. This higher permeability was peculiarly observed in water, the most hydrophilic solvent, while other solvents showed that the permeability decreased with higher viscosity. The rejection property of the open-ended VA CNT membrane was similar to the commercial UF membrane, which examined by the MWCO measurement. Additionally, the open-ended VA CNT membrane showed better biofouling resistance with approximately 15% less permeate flux reduction and 2 log less bacterial attachment than the UF membrane. Secondly, dense-array outer-wall CNT membrane for water treatment was investigated as a novel UF membrane. It was exploited the characteristics and corresponding attributes in devising an UF membrane millimeter thick that can deliver a water permeability of close to 30,000 liters per square meter per hour (LMH) at 1 bar, which compares with the best water permeability of CNT membranes reported at 2,400 LMH/bar. The membrane consists only of vertically aligned CNT walls that provide 6 nm wide inner pores of CNTs, and also 7 nm wide outer pores that form between CNT walls when CNTs are densified. Experimental results obtained in the process of fabricating the dense-array outer-wall CNT membrane reveal that the water permeability increases as the CNTs are more densified or the pore size is decreased. It is also discovered that the dense-array outer-wall CNT membrane impedes bacteria adhesion and resist biofilm formation. Finally, two types of mixed CNT RO membranes were investigated. PA-CNT-PVA membrane was fabricated by interfacial polymerization followed by the deposition of oxidized CNTs and the coating of PVA on the surface. It showed much improved mechanical properties and durability compared with the polyamide membrane without CNTs (PA membrane). The PA?CNT?PVA membrane also exhibited better antifouling properties than the PA membrane and the commercial RO membrane. CNT embedded RO membrane, previously known to have ~30% higher flux than polyamide RO membrane, was fabricated by interfacial polymerization with CNTs in MPD solution and evaluated in terms of energy efficiency and fouling tendency compared to the polyamide RO membrane. CNT embedded RO membrane showed better energy efficiency than polyamide RO membrane because of the favorable surface properties and mechanical properties. It also showed less fouling potential for 138 h of artificial sea salt test and better flux recovery for the cleaning process. The results of three CNT membranes performance indicate that CNT membranes can be applicable for high performance membrane and fouling-resistant membrane.