Slimy wood for clean water

Nature does a lot of filtering as part of its day-to-day activities, through various films and membranes, and this is making scientists wonder whether we can take advantage of nature's filtering prowess for our own purposes. In recent work, one group of US scientists investigated the filtering abilities of wood, while another group investigated the filtering abilities of bacterial biofilms.

Rohit Karnik and his colleagues at Massachusetts Institute of Technology are studying the filtering abilities of softwood from conifers such as pine. As with all wood, softwood contains tissue known as xylem for transporting water up from the roots. Xylem comprises long cellular tubes known as tracheids, which are connected together at various points by thin sections of the cell wall known as pit membranes. 

These pit membranes act as natural filters and so Karnik and his colleagues decided to see whether wood could be used for filtering water. They chose softwood because its tracheids are slightly shorter and more numerous than the tracheids in hardwood, allowing for thinner filters. But although initial studies showed the potential of softwood, they also highlighted a couple of problems. One is that drying softwood to make it long-lasting tends to damage the pit membranes, while another is that the pit membranes tend to become blocked with hemicellulose fibers released from the wood by the flow of water.

In a paper in Nature Communications, Karnik and his colleagues now report a solution to both these problems, which involves immersing thin cross-sections of softwood into hot water for an hour and then soaking them in ethanol. The hot water treatment dissolves and removes the hemicellulose fibers from the cross-sections, preventing them from blocking the pit membranes during use, while the ethanol treatment protects the pit membranes from damage when the cross-sections are dried.

In tests, these treated wooden filters were able to filter out disease-causing microbes such as Escherichia coli and rotavirus from spring, tap and groundwater. Karnik and his colleagues also showed how these filters could be used to produce various devices for cleaning water, both temporary and permanent, by inserting them into a 1m-long tube that connects two containers, one above the other. Contaminated water poured into the top container is then filtered as it flows through the tube, under the force of gravity, into the bottom container.

Karnik and his team deployed these devices in regions of India lacking clean water supplies and showed that they were readily adopted by the locals, who were taught how to make the wooden filters from local trees. This gave them a ready source of cheap filters, which simply had to be replaced on a regular basis to ensure a steady supply of uncontaminated water. The scientists are now investigating ways to enhance the filtration devices so they can capture chemicals as well as microbes.

One possibility is to use bacterial biofilms, as shown by Lucian Lucia and his colleagues at North Carolina State University. In a paper in Langmuir, they report that the biofilm produced by the bacterium Gluconacetobacter hansenii can separate oil from water.

Many bacteria produce biofilms as a form of protection, with different bacteria producing different types of biofilms made up of different constituents. G. hansenii produces a particularly robust type of biofilm made from cellulose.

"It's one of the purest, if not the purest, forms of cellulose out there," Lucia said. "It's very well structured. It's very water loving, and it's got a very high crystallinity, so it packs very beautifully. Once you strip out the bacteria, you have this amazingly tough material that has a real robustness, or toughness."

Lucia and his colleagues showed that a biofilm stripped of bacteria and its non-cellulose components naturally repelled oil, allowing it to be used for separating oil from water. Normally, the last thing you'd want near your drinking water is a slimy, biofilm-covered piece of wood, but it could be just what's needed to ensure the water is clean and safe to drink.

The views represented here are solely those of the author and do not necessarily represent those of John Wiley and Sons, Ltd. or of the SCI.