photo credit: Bacteria (green) trapped over pores (red and blue) in the sapwood after filtration / Boutilier et al.
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To turn dirty lakewater into drinkable H2O, peel away the bark from a nearby tree branch and slowly pour water through the wood. According to new research, this neat, low-tech trick ought to trap any bacteria, leaving you with uncontaminated water.
Okay, time for a little tree physiology. To get water and minerals up a tree, wood is comprised of xylem, porous tissue arranged in tubes for conducing sap from the roots upwards through a system of vessels and pores. Xylem tissue is found in sapwood, the younger wood that lies in concentric circles between the central heartwood and the bark. Tiny pores called pit membranes are scattered throughout the walls of the vessels, allowing sap to flow from one vessel to another, feeding various structures along a tree’s length.
Turns out, the same tissue that evolved to transport sap up the length of a tree also has exactly the right-sized pores to allow water through while blocking bacteria. Additionally, the pores also trap air bubbles, which could kill a tree if spread in the xylem. “Plants have had to figure out how to filter out bubbles but allow easy flow of sap,” study author Rohit Karnik from MIT says in a news release. “It’s the same problem with water filtration where we want to filter out microbes but maintain a high flow rate. So it’s a nice coincidence that the problems are similar.”
As Karnik’s team finds, a small piece of sapwood can filter out more than 99 percent of the E. coli from water, at the rate of several liters per day.
To study sapwood’s water-filtering potential, the team collected white pine branches and stripped off their outer bark. They attached inch-long sections of sapwood to plastic tubing, then sealed it with epoxy and secured it with clamps.
They tested their improvised filter using water mixed with particles ranging in size. They found that while sapwood naturally filters out particles bigger than 70 nanometers, it wasn’t able to separate out 20-nanometer particles.
When they poured water contaminated with inactivated E. coli through the sapwood filter, they saw how bacteria had accumulated around the pores in the first few millimeters of the wood. In the false-color electron microscope image above, (green) bacteria are trapped over pit membranes (red and blue).
Existing water-purification technologies that use chlorine treatments and membranes with nano-scale pores are expensive. Even boiling water requires fuel for heat. Here, just take some wood and make a filter of it — it’s low-cost, efficient, and readily accessible for rural communities as well as dehydrated campers in the Northeast. “Ideally, a filter would be a thin slice of wood you could use for a few days, then throw it away and replace at almost no cost,” Karnik explains.
The group is looking into the filtering potential of other types of sapwood. Flowering trees, for example, tend to have smaller pores than coniferous trees and may be able to filter out even smaller particles, like viruses.
The work was published in PLOS ONE last week.