Functionalized carbon nanotube immobilized membrane for low temperature ammonia removal via membrane distillation

Enhanced ammonia separation via direct contact membrane distillation (DCMD) using carbon nanotube immobilized membranes (CNIMs) is presented. The CNIMs were prepared using raw carbon nanotubes (CNTs) as well as more polar carboxylated carbon nanotubes referred to as f-CNTs. The ammonia removal by both CNIMs were markedly superior to that of the original PTFE membrane, while functionalized CNIM (CNIM-f) showed the best performance in terms of flux, mass transfer coefficients and selectivity. The flux with f-CNTs was 63% higher than the PTFE membrane and 22% higher than raw CNTs. Similar trends were observed for ammonia removal efficiency and mass transfer coefficients that were also significantly higher for the f-CNTs. The ammonia selectivity with respect to water vapor decreased with temperature for the PTFE and CNIM with raw CNTs, but in the presence of f-CNTs the selectivity did not show any decrease with temperature. The enhancement in ammonia removal with f-CNTs is attributed to the favored chemisorption of ammonia on the f-CNTs which was evident from contact angle measurements that showed a dramatic decrease for aqueous ammonia in the presence of the carboxylic groups.