(567a) Engineering the Nanochannels in Reduced Graphene Oxide Membranes for Dye Desalination

Membranes that are highly permeable to salts (such as NaCl and Na₂SO₄) and reject organic dyes are of interest for dye desalination in textile industries. Commercial nanofiltration (NF) and ultrafiltration (UF) membranes cannot efficiently desalinate the dye solutions because NF membranes reject both divalent salts and dyes and UF membranes have a low rejection to dye molecules. Graphene oxide (GO) based membranes with tunable nanochannels formed by stacking nanosheets in parallel present a new opportunity for this application. Current research is often focused on reducing the size of nano-channels to enhance the salt rejections, which cannot be adopted for dye desalination. Herein we develop a facile approach in preparing solvated reduced GO (S-rGO) membranes with the increased nanochannel size, which lies between the size of the salt ions and dye molecules (such as Direct Red 80). Specifically, these membranes were prepared by filtration of rGO solutions through microporous Nylon support. The membranes were always kept in the wet and swollen state to control the channel sizes for the dye desalination. The S-rGO membranes can reject about 99% of Direct Red 80 with almost no rejection for NaCl and Na₂SO₄. More importantly, the nanochannels in S-rGO membranes can also be controlled by adjusting the salt concentration in the dye solutions. For example, as the Na₂SO₄ content increases to 60 g/L, the rejection of Direct Red 80 in the S-rGO membranes increases to 99.9%, while the rejection of Na₂SO₄ remains negligible. The reduction in the channel size is also validated by the decrease in the molecular weight cutoff (MWCO) of the S-rGO membranes in the salt solution. These membranes exhibit water permeance as high as 80 L/m²·h·bar, which is about eight times that of NF membranes and two times that of UF membranes (MWCO: 2000), suggesting their promise for practical applications of dye desalination.