Towards Enhanced Performance Thin-film Composite Membranes via Surface Plasma Modification

Abstract

Advancing the design of thin-film composite membrane surfaces is one of the most promising pathways to deal with treating varying water qualities and increase their long-term stability and permeability. Although plasma technologies have been explored for surface modification of bulk micro and ultrafiltration membrane materials, the modification of thin film composite membranes is yet to be systematically investigated. Here, the performance of commercial thin-film composite desalination membranes has been significantly enhanced by rapid and facile, low pressure, argon plasma activation. Pressure driven water desalination tests showed that at low power density, flux was improved by 22% without compromising salt rejection. Various plasma durations and excitation powers have been systematically evaluated to assess the impact of plasma glow reactions on the physico-chemical properties of these materials associated with permeability. With increasing power density, plasma treatment enhanced the hydrophilicity of the surfaces, where water contact angles decreasing by 70% were strongly correlated with increased negative charge and smooth uniform surface morphology. These results highlight a versatile chemical modification technique for post-treatment of commercial membrane products that provides uniform morphology and chemically altered surface properties.

Figure 1. Flux and salt rejection for control and plasma-modified membranes.

(A) Plasma treated at 10 W, (B) 50 W and (C) 80 W. Cross-flow desalination test conditions: 15 bar inlet pressure and 2,000 ppm NaCl solution at 25 °C. The data from flux and salt rejection are represented as means of four replicates and error bars corresponding to their associated standard error of the mean.

Figure 2

Morphology analysis: (A) Average roughness (Ra) calculated from AFM surface roughness maps and (B) height distribution histogram of the inset were calculated from 5 × 5 μm AFM maps, indicated by values of Z and ρ represents the density or histogram-frequency of respective height values. Each value from roughness represents the mean of three measurements in the sample associated with their estimated standard error.

Figure 3

Analysis of plasma functionalization mechanisms with XPS C1s high resolution of atomic ratio (A) peak 285.9 eV reduction: C-O; C-N/C-C from peaks 285.9/284.4 eV and (B) peak 287.6 eV increase: C=O/C-C from peaks 287.6/284.4 eV.

Figure 4

Degree of etching by FTIR analysis of the band 1045 cm⁻¹ with increasing excitation power: (A) analysis between control membranes: washed membrane with DI water (for 1 h) and a membrane with preservative materials, (B) modified membranes at 10 W, (C) at 50 W and (D) at 80 W. Normalization was performed at 1240 cm⁻¹.

Figure 5

Correlation between sulphur S_2p at% and salt rejection with increasing power and duration (A) plasma treatment at 10 W, (B) 50 W and (C) 80 W. The displayed data represents the mean of five replicates associated with their estimated standard deviations.

Figure 6

Streaming potential with increasing excitation power and plasma durations: (A) 10 W, (B) 50 W and (C) 80 W.

Figure 7. Investigation of hydrophilicity alterations by reduced water contact angles for the series of plasma powers and durations.

The displayed data represents the mean of three replicates associated with their estimated standard deviations.

Figure 8. Plasma surface reactions and resultant surface modification.