Sieving Salt from Seawater-Next generation nanofiltration membranes for water desalination

Haiqing Lin Membrane Research Lab

PhD candidate Xiaoyi Chen is developing next-generation nanofiltration (NF) membranes for water desalination from graphene oxide (GO).

Overview

graphene oxide (GO) molecular structure.

Figure 1. Structure of graphene oxide (GO)

The world is made up of ~70% water. However, only 2.5% of the 70% is useable fresh water, whereas the rest is saline and ocean-based water. Seawater desalination is a critical way to increase the quantity of accessible fresh water. There are various desalination techniques such as reverse osmosis (RO), nanofiltration (NF), electrodialysis (ED), electrodialysis reversal (EDR), forward osmosis (FO), and membrane distillation (MD). The Lin research group focuses on the nanofiltration, as it is one of the leading pressure-driven membrane processes that can separate turbidity, hardness, fluoride, and nitrate, and dissolved salts from water. Also, the desalination process can be operated at low pressures, which in return saves energy.

The objective of this project is to develop next-generation nanofiltration (NF) membranes from graphene oxide (GO) with enhanced water permeance and high salt rejection properties. GO has recently been investigated for NF applications due to its unique 2D molecular structure with multiple functional groups, high hydrophilicity, molecular sieving effect, and tunable interlayer spacing, as shown in Figure 1. However, Go-based membranes have low ion rejection property. One approach to enhance the salt rejection is to reduce GO, which results in a significant decrease in water flux. However, membranes with high water permeance are needed to lower the cost. The trade-off between the water permeance and salt rejection is a primary challenge for the GO-based membranes.

Figure 2. Schematic of preparation of r-HGO membrane

The Lin research group tackles the challenge mentioned above by developing nanoporous GO nanosheets with high water permeance while retaining high salt rejection. These holey GO (HGO) sheets are deposited on ultrafiltration membranes and subjected to a reduction process. Figure 2 illustrates the schematic of the preparation of this NF membrane . The reduced holey graphene (r-HGO) is reported to obtain high rejection of ions and water flux properties. An example membrane with 20-nm thick r-HGO shows pure water permeance of 12 LMH/bar with the Na₂SO₄ rejection of 91.4%, which is comparable to the leading commercial NF membranes.