Membrane distillation (MD) is a separation technique where the driving force is a vapor pressure gradient between a hot feed and a cold permeate. While many microorganisms get killed at higher temperatures, the challenge arises in cases where these microorganisms are stable at high temperatures namely thermophilic bacteria. In this study, carbon nanotube, functionalized carbon nanotube, and graphene oxide immobilized membranes were used in membrane distillation to generate microorganism free water. The nanocarbons degraded the bacterial cell integrity and caused cell death. The pure CNTs showed the highest biocidal activity (96.2%) followed by functionalized CNTs and graphene oxide. The biocidal activity was significantly higher than the plain polymeric membranes.
In general, the biocidal action of nanocarbons typically involves a combination of physical and chemical mechanisms. First, the nanostructured CNTs acted as a mesh to trap the bacterial cells. The presence of van der waals forces facilitated the adhesion of bacterial cells to the surface of CNTs. The sharp structures of CNTs are also known to cause significant structural damage to the cell wall and the membrane of the microorganism. On the other hand, GO exists as layers of nanosheets, and we believe that initially the bacterial cells get trapped within the nanosheets. Bacterial cell death in the case of GO is believed to be caused via oxidative stress. The presence of GO induces the production of reactive oxygen species (ROS) within the bacterial cells, which causes a decrease in membrane fluidity by altering membrane properties, ultimately leading to cell death. The CNT membranes also showed less fouling. While biofilm/bacterial sludge accumulated on the CNT-COOH and GO immobilized membrane surfaces, no biofilm was observed on the CNT membrane. Among the membranes studied, the CNTs appeared to have the highest biocidal effect and lowest biofouling.