As the world’s population grows and climate change brings more wildfires and drought, clean water is becoming an increasingly precious resource. Converting seawater to fresh could solve this problem. The challenge is to do it on a large scale using low cost, sustainable methods.
Researchers from the University of Colorado Boulder and the University of Maryland believe wood could be the answer. They have made a super-thin membrane from wood to desalinate and purify water. The 0.5 mm-thick membrane, reported in Science Advances, a sustainable alternative to the petroleum-derived plastic membranes used at desalination plants.
One in nine people around the world do not have access to clean drinking water. Residents in regions such as the Middle East and California already get water from desalination plants. Most of these plants rely on reverse osmosis, a method in which seawater is forced through a porous membrane that blocks salt and impurities. But the method requires a lot of energy and expensive plastic membranes that tend to get clogged.
Another attractive technology that hasn’t yet been commercialized is membrane distillation. It uses solar power to evaporate water. The steam passes through a water-repelling membrane, condensing into clean water on the other side. Membranes for this technology are also made from plastic, and the technology has been limited by cost and energy use.
So the research team decided to make distillation membranes from American basswood. They use a chemical process to remove the fibrous lignin from the wood slices. This leaves behind a colorless, ultralight styrofoam-like material. The team has previously demonstrated the use of this material for insulation and cooling.
This time, they coated it with a chemical that makes it repel water molecules. The arrangement of the cellulose fibers in the material allow it to conduct heat along its surface, but keep the heat from passing through the material because of pores that form between the fibers.
The researchers heat one side of the membrane so that it can evaporate water flowing over it. The method uses much less energy than boiling the water. The membrane is more porous and allows steam to pass through more easily than the best commercial plastic membranes. But it still did not perform as well, filtering about 20 kilograms of water per square meter per hour, about half the rate of the commercial membranes.
This is because the membrane is thicker and has smaller pores, the team writes. Making thinner membranes from other wood with ideal pore sizes should do the job. They also plan to engineer membranes that are more durable under high temperatures and harsh chemical conditions.
Source: Dianxun Hou et al. Hydrophobic nanostructured wood membrane for thermally efficient distillation. Sci. Adv., 2019.
Photo: S. He and T. Li at the University of Maryland College Park