Nanocarbon immobilized membranes for generating bacteria and endotoxin free water via membrane distillation

Abstract

The biocidal and endotoxin removal capabilities of carbon nanotube and graphene oxide when immobilized on a membrane surfaces are presented for the removal of thermophilic and mesophilic bacteria via direct contact membrane distillation (DCMD). The nanocarbons studied here namely carbon nanotubes (CNTs), carboxylated CNTs (CNT-COOH) and graphene oxide (GO) degraded the bacterial cell integrity and caused cell death. The membranes were characterized via SEM, TGA, AFM and water contact angles. The biocidal actions of CNTs and GO involved a mixture of physical and chemical processes: it is believed that CNTs caused significant structural damage to the cell membrane of the bacterial cells and GO caused ROS-independent oxidative stress leading to cell death. The pure CNTs showed the highest biocidal activity (96.2% for thermophilic and 83.9% for mesophilic cells) followed by CNT-COOH (84.3% for thermophilic and 49.8% for mesophilic cells), GO (81.7% for thermophilic and 47.8% for mesophilic cells) and PTFE (68% for thermophilic and 24.4% for mesophilic cells). While biofilm/bacterial sludge accumulated on the CNT-COOH and GO immobilized membrane surfaces, no biofilm was observed on the CNT membrane. Due to the amphiphilic nature of the endotoxins, surface adsorption via intermolecular interactions led to the removal of endotoxins. Endotoxin removal efficiency was as high as 99.9% for all the membranes. The nanocarbon immobilized membranes are clearly effective in generating ultrapure, medical grade water.