In the paper, published in Carbon [DOI: 10.1016/j.carbon.2021.05.005], the researchers fabricated of a series of GO membranes by spraying a layer of graphene oxide, approximately 105 nm thick, onto polysulfone (PSU) and polytetrafluoroethylene (PTFE) substrates. The resulting membranes were characterised in terms of roughness, contact angle and surface charge before being placed into a filtration system. There, distilled water, a protein solution, and milk purchased from a local supermarket were cycled through the membranes, to evaluate their performance. Commercially available nano- and ultrafiltration membranes were put through the same tests.
The researchers were particularly interested in three related processes – fouling, milk filtration, and washing. Fouling is the build-up of organic matter on surfaces, and in membranes, it can result in clogged pores that drastically reduce the flowrate of milk through the membrane. Some of this fouling layer can be removed by washing with water, but some remains on the membrane regardless, which has an impact on its long-term effectiveness. The PSU-GO and PTFE-GO membranes exhibited less of this irreversible fouling than the commercial membranes, as well as thinner and less dense removable fouling layers. Other measurements showed that the GO membranes were also highly selective – efficiently extracting lactose from milk, but not the fat molecules that contribute to its taste.
To investigate this further, the team carried out a molecular dynamics simulation and found a weak interaction between lactose and GO, enabled by the spacing between the GO sheets and their surface charge. This allows the unwanted sugar molecule to diffuse out through the membrane, while keeping the milk’s fats and proteins intact. They also found that water naturally present in milk acts as a “driving force” for this diffusion process, pushing the lactose through nano-sized pores in the membrane.
These results suggest that GO membrane technology could be well-suited for use in the dairy industry as a means of lactose removal, exhibiting “better performance than that of typical polymeric NF [nanofiltration] membranes.”
This story was written for Materials Today: https://www.materialstoday.com/carbon/news/a-better-route-to-lactosefree-milk/
Research paper: A. Morelos-Gomez, S. Terashima, A. Yamanaka, R. Cruz-Silva, J. Ortiz-Medina, R. Sánchez-Salas, J.L. Fajardo-Díaz, E. Muñoz-Sandoval, F. López-Urías, K. Takeuchi, S. Tejima, M. Terrones, M. Endo. “Graphene Oxide Membranes for Lactose-Free Milk”, Carbon, In Press (6 May 2021). DOI: 10.1016/j.carbon.2021.05.005