Rightsizing Nanochannels in Reduced Graphene Oxide Membranes by Solvating for Dye Desalination

Membranes with high water permeance, near-zero rejection to inorganic salts (such as NaCl and Na₂SO₄), and almost 100% rejection to organic dyes are of great interest for the dye desalination (the separation of dyes and salts) of textile wastewater. Herein, we prepared reduced graphene oxide membranes in a solvation state (S-rGO) with nanochannel sizes rightly between the salt ions and dye molecules. The S-rGO membrane rejects >99.0% of Direct Red 80 (DR 80) and has almost zero rejection for Na₂SO₄. By contrast, conventional GO or rGO membranes often have channel sizes smaller than divalent ions (such as SO₄^2-) and thus high rejection for Na₂SO₄. More interestingly, high salinity in typical dye solutions decreases the channel size in the S-rGO membranes and thus increases the dye rejection, while the Na₂SO₄ rejection decreases because of the negatively charged surface on GO and the salt screening effect. The membranes also show pure water permeance as high as 80 L m^-2 h^-1 bar^-1, which is about 8 times that of commercial NF 90 membrane and 2 times that of a commercial ultrafiltration membrane (with a molecular weight cutoff of 2000 Da), rendering their promise for practical dye desalination.