[Paper Review] Membrane fouling: microscopic insights into the effects of surface chemistry and roughness

Mao Wang, John Wang+1 more|arXiv (Cornell University)|Aug 25, 2021

Membrane Separation Technologies43 references39 citations

TL;DR

This molecular simulation study investigates how surface chemistry and roughness affect membrane fouling, revealing that increased roughness reduces fouling on hydrophilic alumina membranes due to weaker foulant-surface interactions and enhanced water structuring in ridge regions, while on hydrophobic graphene, roughness increases fouling due to stronger foulant-surface adhesion. The findings demonstrate that surface hydrophilicity and morphology jointly govern fouling behavior, offering design principles for anti-fouling membranes.

ABSTRACT

Fouling is a major obstacle and challenge in membrane-based separation processes. Caused by the sophisticated interactions between foulant and membrane surface, fouling strongly depends on membrane surface chemistry and morphology. Current studies in the field have been largely focused on polymer membranes. Herein, we report a molecular simulation study for fouling on alumina and graphene membrane surfaces during water treatment. For two foulants (sucralose and bisphenol A), the fouling on alumina surfaces is reduced with increasing surface roughness; however, the fouling on graphene surfaces is enhanced by roughness. It is unravelled that the foulant-surface interaction becomes weaker in the ridge region of a rough alumina surface, thus allowing foulant to leave the surface and reducing fouling. Such behavior is not observed on a rough graphene surface because of the strong foulant-graphene interaction. Moreover, with increasing roughness, the hydrogen bonds formed between water and alumina surfaces are found to increase in number as well as stability. By scaling the atomic charges of alumina, fouling behavior on alumina surfaces is shifted to the one on graphene surfaces. This simulation study reveals that surface chemistry and roughness play a crucial role in membrane fouling, and the microscopic insights are useful for the design of new membranes towards high-performance water treatment.

Motivation & Objective

• To understand the microscopic mechanisms of membrane fouling influenced by surface chemistry and roughness.
• To investigate how surface roughness affects foulant mobility and adhesion on alumina and graphene membranes.
• To explore the role of water structuring and hydrogen bonding in fouling mitigation on hydrophilic surfaces.
• To determine how surface hydrophilicity can be tuned to shift fouling behavior from alumina-like to graphene-like responses.
• To provide design guidelines for next-generation anti-fouling membranes based on surface chemistry and morphology.

Proposed method

• Molecular dynamics simulations were performed on flat and sinusoidally patterned alumina and graphene surfaces with varying roughness (amplitude A = 0, 3, 5 Å).
• Two foulants—sucralose and bisphenol A—were simulated on these surfaces to assess mobility and interaction energies.
• Harmonic potential was applied to drive foulant motion along the x-axis to quantify mobility via mean square displacement (MSD).
• Interaction energies and contact areas between foulants and surfaces were calculated to evaluate adhesion strength.
• Hydrogen bond formation and stability between water and alumina surfaces were analyzed using number counts and autocorrelation functions.
• Surface hydrophilicity was modulated by scaling atomic charges of alumina to emulate hydrophobicity levels and compare fouling behavior with graphene.

Experimental results

Research questions

• RQ1 How does surface roughness affect foulant mobility and adhesion on hydrophilic alumina versus hydrophobic graphene membranes?
• RQ2 Why does roughness reduce fouling on alumina but enhance it on graphene, despite similar surface topologies?
• RQ3 What role do water-surface hydrogen bonds play in enhancing surface hydrophilicity and anti-fouling performance on rough alumina?
• RQ4 How does the hydrophilicity of alumina influence the transition from reduced fouling (on smooth surfaces) to enhanced fouling (on rough surfaces)?
• RQ5 Can the fouling behavior on alumina be tuned to mimic that of graphene by adjusting surface charge and hydrophilicity?

Key findings

• On rough alumina surfaces, foulants exhibit increased mobility and reduced adhesion in ridge regions due to weaker foulant-surface interactions, leading to decreased fouling.
• On rough graphene surfaces, foulants remain strongly adsorbed in both ridge and valley regions due to strong hydrophobic interactions, resulting in enhanced fouling with increasing roughness.
• Roughness increases the number and stability of water-alumina hydrogen bonds, with longer lifetimes on rougher surfaces, enhancing surface hydrophilicity.
• The contact area between foulants and alumina surfaces decreases to zero in ridge regions on A5 roughness, indicating foulant desorption and reduced interaction.
• Scaling alumina’s atomic charges to reduce hydrophilicity shifts fouling behavior from alumina-like (reduced fouling with roughness) to graphene-like (increased fouling with roughness), confirming the role of surface energy.
• The simulation results align with experimental observations of 3D-printed alumina membranes showing reduced fouling with patterned roughness, validating the model's predictive power.