Synergistic Effects of Microwave Radiation and Nanocarbon Immobilized Membranes in the Generation of Bacteria-Free Water via Membrane Distillation

In this study, we introduce microwave-induced membrane distillation (MIMD) where microwave radiation is applied not only to heat water but also to enhance the biocidal effects of nanocarbon immobilized membranes. The three types of membranes used in this study were carbon nanotube immobilized membranes (CNIM), one functionalized with carboxylated nanotubes (CNIM-COOH), and graphene oxide immobilized membrane (GOIM). The membrane performances were evaluated based on the production of water vapor flux and the percentage cell growth inhibition due to the combined effect of microwaves and nanocarbon membranes. These combinations were most effective at a temperature of 80 and 60 °C for the removal of thermophilic and mesophilic cells, respectively. Under microwave heating, the CNIM exhibited the maximum biocidal effect (99.6% for thermophilic and 95.5% for mesophilic cells) followed by CNIM-COOH (92.3% for thermophilic and 65.8% for mesophilic cells) and GOIM (90.1% for thermophilic and 59.4% for mesophilic cells). They were all higher than a plain poly(tetrafluoroethylene) (PTFE) (82.3% for thermophilic and 41.6% for mesophilic cells) membrane without nanocarbons. In MIMD, the biocidal performance as well as the flux were improved due to thermal and nonthermal factors of microwave irradiation. The latter caused higher cell destruction due to the interaction of the microwave with the cellular matter, an improved water vapor flux (30–40%) due to localized superheating, and enhanced hydrogen bonding breakdown of water molecules. Furthermore, MIMD required much lesser (20–25%) energy than conventional MD to carry out the experiments under the same conditions.