Effect of etching and reduction time on the structure, performance, and stability of GO-based membrane for water purification

Holey graphene oxide (HGO) and reduced HGO membranes (rHGO) can overcome high tortuosity and poor stability of GO-based membranes, respectively. In this study, HGO and rHGO membranes were fabricated to try to quantitatively elucidate the etching and reduction time on structure, stability and rejection performance for slat and pharmaceuticals. Water flux of HGO membrane linearly increased (Jv=2.07t + 1.02, R2=0.9931) and Na2SO4 rejection linearly decreased (RNa2SO4=- 4.37t + 88.26, R2=0.9511), while the membranes stability substantially increased with etching time from 1 to 5 h (water flux variation from 22% to 5% and rejection ratio variation from 17% to 7%) under long-term vibration . Subsequently, water flux of rHGO membranes exponentially decreased (Jv=exp(2.89-0.12t + 0.0039t2), R2= 0.9996) with reduction time, while slightly improved salt rejection (RNa2SO4=exp(2.89-0.12t + 0.0039t2), R2= 0.9971) were observed. Effective length of actual pathways for GO, HGO4 and rHGO4-5 membranes calculated by the mass transfer-based model was 7.37 & PLUSMN; 0.47, 4.48 & PLUSMN; 0.68 and 7.23 & PLUSMN; 1.29 & mu;m, respectively, which were much higher than that geometric thickness of the GO membranes. These results indicated that d-spacing, tortuosity and porosity should be comprehensively considered for membrane design and fabrication. With a series of saccharides as probes, average pore size slightly decreased for HGO4 (rp=0.36 nm, Sp=0.34) and rHGO4-5 (rp=0.33 nm, Sp=0.28) membrane compared with GO membranes (rp=0.38 nm, Sp=0.3), demonstrating that etching and following reduction marginally affected the pore size distribution. H2O2 etching increased both water permeability (A) and its selectivity (A/B) for three pharmaceuticals, while the thermal reduction substantially decreased the water flux. Moderate and low rejection ratios for the three pharmaceuticals also demonstrated the importance of trade-off between etching and reduction time and the urgent need for narrowing pore-size distribution for GO-based membranes for water purification.