Clean Technology 2008
F. Fornasiero, H.G. Park, J.K. Holt, M. Stadermann, S. Kim, J.B. In, C.P. Grigoropoulos, A. Noy, O. Bakajin
Lawrence Livermore National Laboratory, US
carbon nanotube membrane, desalination, ion selectivity, Donnan theory
Both MD simulations and experimental studies have shown that liquid and gas flow through carbon nanotubes with nanometer size diameter is exceptionally fast. For applications in separation technology, selectivity is required together with fast flow. In particular, for water desalination, coupling the enhancement of the water flux with selective ion transport could drastically reduce the cost of brackish and seawater desalting. In this work, we use pressure-driven filtration experiments to study ion exclusion in silicon nitride/CNT composite membranes as a function of solution ionic strength, pH, and ion valence. The pores of these membranes are sub-2-nm diameter CNTs whose entrance is decorated by negatively charged carboxylic groups. We show that carbon nanotube membranes exhibit significant ion exclusion at low salt concentration. Our results support a Donnan-type rejection mechanism, dominated by electrostatic interactions between fixed membrane charges and mobile ions, while steric and hydrodynamic effects appear to be less important. Comparison with commercial nanofiltration membranes for water softening reveals that our carbon nanotube membranes provides far superior water fluxes for similar ion rejection capabilities.