Yushan Zhang

Partially reduced graphene oxide and chitosan nanohybrid membranes for selective retention of divalent cations

A tremendous quantity of brackish water with a high proportion of divalent cations is in great need of water softening. Layer-stacked graphene oxide membranes show potential in membrane processing due to their molecular sieving properties, but show poor selective retention of cations due to unstable interlayer spacing and electrostatic interaction. In this study, a partially reduced graphene oxide (prGO) and chitosan (CS) nanohybrid membrane (prGO–CS) was fabricated to achieve the selective retention of divalent cations by adjusting the configuration and controlling the surface charge. The prGO–CS membrane, which included a CS skin and embedded prGO sheets, showed a performance boost of 98.0% rejection of Mg2+ and 95.5% rejection of Ca2+ when compared with a CS membrane. The membrane showed good water softening performance for brackish water under low operation pressure with a high Na+/Mg2+ selectivity of 33.8. The excellent performance was attributed to the dense structure and positive charge of prGO–CS.

Enhanced separation and antifouling performance of reverse osmosis membrane incorporated with carbon nanotubes functionalized by atom transfer radical polymerization

In this paper, an approach to obtain mixed-matrix reverse osmosis (RO) membranes with enhanced separation and antifouling performance is described. Atom transfer radical polymerization (ATRP) was first introduced for functionalization of multiwalled carbon nanotubes (MWCNTs) with polyacrylamide (PAAm) structure. Subsequently, MWCNTs were incorporated into polyamide (PA) thin film composite (TFC) membranes by interfacial polymerization. The dispersion of MWCNTs in aqueous solution and compatibility between MWCNTs and polymeric matrix were effectively improved with PAAm. Membrane surface morphology, hydrophilicity and charge properties were characterized by SEM, AFM, static water contact angles and zeta potential measurement. Moreover, the effect of above properties on separation and antifouling performance of membranes was investigated. Results showed that water flux of the optimal membrane incorporated with MWCNTs grafted at 6 h (MM-6h) was 34% higher than that of virgin PA membrane. More importantly, NaCl rejection of MM-6h membrane was 98.9%, which is higher than that of virgin PA membrane and the membrane incorporated with oxidized MWCNTs. In addition, surface modified membranes showed excellent fouling resistance to BSA. The flux recovery of MM-6h membrane was about 97.6% after cleaning, while that of virgin PA membrane was only 79.2%. This work proves that MWCNTs functionalized by ATRP have potential application in simultaneously improving separation and antifouling performance of TFC RO membranes.

Establishment of transport channels with carriers for water in reverse osmosis membrane by incorporating hydrotalcite into the polyamide layer

Thin film nanocomposite (TFN) reverse osmosis (RO) membranes were prepared by incorporating hydrotalcite (HT) in polyamide layer during interfacial polymerization process using two methods: (1) dispersing HT in aqueous solution directly; (2) preparing layered double oxide (LDO) via calcination of HT and then dispersing the obtained LDO in aqueous solution to reconstruct HT. The results demonstrated that TFN RO membranes exhibited higher water flux compared with the pristine RO membrane, which could be mainly attributed to water channels constructed by HT as well as the enhancement of hydrophilicity and the increase of relative surface area. In addition, TFN RO membranes prepared using LDO showed higher water flux than those prepared using HT, which is contributed to the uniform distribution of water channels in polyamide layer resulting from the process of calcination and reconstruction reaction. Compared with pristine RO membrane, water flux of TFN RO membranes prepared with 0.075 wt% HT and those prepared with 0.050 wt% LDO was enhanced by 19.1% and 22.8% without sacrificing the salt rejection.