How to improve water permeability and rejection performance of RGO membranes? RGO-MXene membranes give answer

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Membrane separation technology has developed into one of the most effective methods of water treatment due to its advantages of high separation efficiency, convenient operation and low energy consumption. The membranes fabricated from graphene oxide (GO) have attracted a lot of attention due to their high water permeability attributed to the fast water transport in the atomically smooth sp2 regions (non-oxidized regions) of their membrane channels.

Reduced graphene oxide (RGO) membranes are theoretically more conducive to the rapid transport of water molecules in their channels compared to graphene oxide (GO) membranes because they have fewer oxygen-containing functional groups and more non-oxidized regions.

However, experimental results from the literature showed that the RGO membranes generally have very low water permeabilities, even below 1.0 L/(m2h bar). The reduction process from GO to RGO can decrease the number of oxygen functional groups on RGO nanosheets, hence RGO membranes exhibit weaker hydrophilicity and closer interlayer spacing.

Despite the rapid transport of water molecules in the non-oxidized areas of RGO membrane channels, their weakly hydrophilic and narrow membrane channels could impede the entry of water molecules into the channels, resulting in lower water permeability. In addition, the reduction of oxygen functional groups on the RGO membrane surface would weaken the electrostatic interactions between the membrane and charged species, which could decrease the rejection rates for these charged species.

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To solve the problem that the weak hydrophilicity of the RGO membrane inhibits the water entry channel and leads to low water permeability, Prof. Xie Quan from Dalian University of Technology and his team members designed electrically conductive MXene-intercalated RGO (RGO-MXene). . Membranes with wettability-regulated channels through intercalation of MXene in RGO membranes.

This study entitled “RGO-MXene electroconductive membranes with wettability-regulated channels: improved water permeability and electroenhanced repulsion performance” is published online in Frontiers of environmental science and engineering.

The research team found that the RGO-MXene composite membrane had a high pure water permeability of 62.1 L/(m2h bar), about 16.8 times that of the RGO membrane (3.7 L/(m2h bar)). Wettability test results and molecular dynamics simulations suggest that the improved water permeability results from the improved wettability of the RGO-MXene membrane and the increased rate of water molecules entering the RGO-MXene channels.

The electro-assisted RGO MXene membrane benefits from its good conductivity and shows significantly increased rejection rates for negatively charged dyes (from 56.0% at 0V to 91.4% at 2.0V for Orange G) without increasing the permeate flux reduce what could be attributed to increased electrostatic repulsion under electro-assist.

In this work, electrically conductive RGO MXene membranes are fabricated by intercalating MXene into RGO membranes. Membrane wettability is regulated by adjusting the mass percentage of MXs in the composite membrane. Wettability properties and molecular dynamics simulations indicate that membrane wettability and the rate of water molecules entering the membrane channels are important factors in enhancing water flux.

In addition, the RGO MXene membrane demonstrates improved charged dye rejection rates under electroassist. This work is expected to provide a new perspective for future research to construct novel membrane channel structures with high water transport and molecular repulsion.

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More information:
Xiaoying Wang et al, Electroconductive RGO-MXene membranes with wettability-regulated channels: improved water permeability and electroenhanced repulsion performance, Frontiers of environmental science and engineering (2022). DOI: 10.1007/s11783-023-1601-8

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