# Oxygen Plasma-Modified Graphene Composite Membranes for Enhanced Forward Osmosis Performance: Mitigating Reverse Salt Flux and Improving Permeability

## TL;DR

This paper introduces a new method to improve forward osmosis membranes by using oxygen plasma etching and PDLLA doping to enhance water flow and reduce salt leakage.

## Contribution

A defect engineering strategy using oxygen plasma etching and PDLLA doping to optimize graphene composite membranes for forward osmosis.

## Key findings

- Short oxygen plasma etching (10 s) increased water flux by an order of magnitude while maintaining high salt rejection.
- PDLLA doping at 0.1 wt.% improved mechanical strength without compromising low reverse salt flux and high permeability.
- Prolonged etching or excessive PDLLA caused degradation in membrane performance due to hydrophobicity and structural changes.

## Abstract

Forward osmosis (FO) membranes face challenges in balancing high water permeability, low reverse salt flux (RSF), and mechanical durability. Although nanopores in graphene have great theoretical potential, the existing methods make it difficult to independently optimize the nanopores of the graphene layer and the microstructure of the substrate without damaging each other. Here, we propose a defect engineering strategy based on oxygen plasma etching to address this collaborative optimization challenge. Monolayer porous graphene (PG) was integrated with polysulfone (Psf) substrates, followed by oxygen plasma etching to introduce nanopores and oxygen-containing functional groups (e.g., carboxyl, hydroxyl). By controlling the etching time to 10 s, the resulting membrane (S-PG10) exhibited a water flux of 0.24 LMH in 0.5 M NaCl, representing an order-of-magnitude increase compared to the pristine graphene membrane (S-G). Remarkably, S-PG10 maintained a high salt rejection (>96%) and a low Js/Jw (<0.35 g·L−1). Substrate modification via short-term plasma etching (5 min) further doubled the water flux of S*5-PG10 (0.47 LMH in 0.5 M NaCl) by increasing porosity (81.8%→85.6%) and hydrophilicity. However, prolonged etching (>15 min) degraded mechanical strength and increased RSF due to pore structure disruption. To enhance robustness, Poly(D,L-lactic acid) (PDLLA)-doped substrates (S#-PG) were engineered, with 0.1 wt.% PDLLA optimizing mechanical properties while maintaining low RSF and high flux. Excessive PDLLA (10 wt.%) induced hydrophobicity and crystalline structures, reducing permeability. The study demonstrates that synergistic optimization of plasma etching duration on the graphene selective layer (5~10 s) and substrates (5 min) as well as PDLLA doping (0.1 wt.%) balances pore architecture, surface chemistry, and substrate integrity, achieving FO membranes with superior water-salt selectivity and mechanical stability. These findings provide critical insights into designing high-performance graphene-based membranes for sustainable desalination and water purification.