Membrane madness

by Tyler Shendruk

Published: Apr 7

The problem
ONE IMPORTANT STEP in water treatment is filtration. Nobody wants little gritty pieces of dreg or oily bits of gunk in their drinking water. River water is passed through membranes in water treatment plants which block oversized contaminants from going any farther. Making membranes with large surface areas, but with small enough pores (micro- or nanofiltration), is possible by casting polymers into a film. These films are either hydrophilic (water-loving) or hydrophobic (water-hating).
It turns out that most contaminants are oily, hydrophobic gook, and since “the enemy of my enemy is my friend” the contaminants are very likely to bury themselves in the hydrophobic membrane to hide from the water. They can’t get through at first, but eventually contamination degradates the membrane’s performance.
On the other hand, hydrophilic membranes have their own set of problems. The contaminants don’t like the membrane, don’t bury themselves in it, and so degradation is slower. But hydrophilic membranes tend to be significantly weaker than hydrophobic ones, and so will often break during water treatment.

The researcher
Takeshi Matsuura is a chemical engineer at the University of Ottawa who develops modified membranes that can improve the distillation and filtration processes. In particular, he is interested in modifying surfaces using large macromolecules that can be attached to membranes.

The project
Filtration science would really benefit from filters that are strong, and that do not rapidly degrade. Hydrophobic and hydrophilic membranes each have their drawbacks, but by combining them, Matsuura hopes that he can get the best of both worlds.

The key
While other scientists cast a strong hydrophobic membrane, and later modify it by grafting hydrophilic polymers on top to make a protective coating, Matsuura thinks this is too slow (and costly). He mixes the hydrophilic and hydrophobic polymers together in solution and then casts them. As the water evaporates, the polymers naturally separate. Matsuura is left with a single membrane with a strong bottom layer and protective coating on top. In just one step, he gets a surface modified film that has the strength of a hydrophobic filter, but degrades slowly like a hydrophilic filter. That really is the best of both worlds.