The fine-structure characteristics and isopropanol/water dehydration through pervaporation composite membranes improved with graphene quantum dots

Abstract

The facile synthesis and functionalization of graphene quantum dots (GQDs) have shown benefits in membrane separation processes since it could augment their performance. Here, the fine-structure characteristics and alcohol dehydration performance of a polymeric pervaporation composite membrane integrated with either nitrogen-doped graphene quantum dots (NGQDs) or oxygen-passivated graphene quantum dots (OGQDs) were evaluated. Calcium ion cross-linked sodium alginate (Alg) was used as the polymer matrix since the multiple hydroxyl and carboxyl groups could form hydrogen bonds with GQDs. The interfacial interaction between the two types of GQDs and Alg was evaluated through the pervaporation dehydration of isopropanol/water (i-PrOH/water) mixture and further analyzed using positron annihilation lifetime spectroscopy (PALS). The PALS results revealed that different functionalized GQDs affect the fine-structure characteristics of Alg matrix and resulted to different alcohol dehydration performance. In dry state, both NGQD and OGQD integrated membranes have smaller free volume spaces than the pristine Alg membrane indicating improved interfacial interaction between the GQD and Alg. However, in wet state, larger free volume spaces appeared and it was shown that OGQD integrated membranes have the largest. These data explained the higher permeation flux and separation factor of Alg-OGQD membranes than Alg-NGQD and Alg. The total flux reached to 5580 g m−2h−1 with a water concentration in permeate of ~99.95% in dehydrating i-PrOH/water (70/30 wt%) at 70 °C for Alg-OGQD membrane with 100 ppm of OGQD (Alg-OGQD100). Furthermore, the membrane remained stable for 720 h of operation at 70 °C with a water concentration in permeate >99% showing potential for practical applications.