Improved dye and heavy metal ions removal in saline solutions by electric field-assisted gravity driven filtration using nanofiber membranes with asymmetric micro/nano channels

The present study presents a new strategy to remove dye and heavy metal ions from saline solution by using a membrane with asymmetric micro/nano channel structures for electric field-assisted gravity-driven filtration (GDF). The asymmetric channels were obtained by dissolving the bottom of an ethanolamine-grafted polyacrylonitrile (PAN) nanofiber membrane and then immersing the partly dissolved membrane in water for solidification. The channel structures were assessed though their dye rejection properties. Among five dyes tested, the highly charged ones, such as methyl blue (MB) and Congo red (CR), could be effectively removed under an electric field, and their rejections were further increased in saline solutions. At 10 V and 0.1 bar, the flux was 75 L m⁻²·h⁻¹, and MB rejection reached 98% with less than 5% salt rejection for the solution of 100 ppm MB and 3% NaNO₃. Similarly, increased rejections to Cu²⁺ and Ni²⁺ ions were found in their saline solutions under an electric field. The rejection of above 99% for 1000 ppm Cu²⁺ or Ni²⁺ was achieved in a solution with 5000 ppm NaCl at 10 V, and the flux was around 7.4 L m⁻² h⁻¹ at 0.01 bar. Finally, the mechanism of improved rejections of dyes and heavy metal ions in saline solutions was elucidated through the increased electrochemical effect as a result of salt adsorption on membrane surface.