Graphene oxide composite membranes cross-linked with urea for enhanced desalting properties

Cross-linking reaction is an important method to prepared stable graphene oxide (GO) membrane, however, the cross-linking reaction with common diamine always induces the water flux decline and ion rejection decrease because GO is partly reduced to result in some oxygen functional groups lose and the cross-linking diamine was inserted to enlarge the free spacing of GO layers. Herein, GO was cross-linked with urea (UR) to show high water flux and good ion rejection. XPS results demonstrated that UR was chemically linked with GO sheets and cellulose acetate (CA) membrane support. In comparison with GO/CA membrane, the water contact angle increase of UR modified membrane (GO-UR/CA) is 10 degrees, but the water contact angle increase of ethane diamine (EDA) modified GO membrane (GO-EDA/CA) achieved 40 degrees, which resulted in GO-UR/CA membrane almost achieving the similar water permeation flux with the pristine GO/CA and the much lower water flux of GO-EDA/CA at same GO specific mass deposition. The d spacing of GO-UR/CA in wet state is about 9.7 nm, and GO-EDA/CA reaches a 0.95 nm d spacing, however, GO-UR/CA membrane yielded a best separation performance with the ion rejections reaching 21.6%, 26.8%, 63.2%, 73.5%, 81.9% for KCl, NaCl, CaCl2, MgSO4, and CuSO4, respectively. The reasons for that was the higher O content, the introduced the polar ketone groups and smaller free empty space in GO-UR/CA membrane. This finding may provide a new viewpoint to comprehend the mechanism of molecule and ions permeation in GO membrane.