Pine tree’s tissue for moving sap holds promise as water filter

Rohit Karnik has focused on developing new water filtration technologies for six years, often drawing on detailed physical knowledge of how fluids behave at the smallest scales.

At a conference designed to bring together researchers from different disciplines, he found himself transfixed by a scientist’s description of how sap flows through plants.

Karnik and a team that includes a high school teacher and a high school student reported the details last month in the journal PLoS ONE of a promising next-generation water filter that might be effective, cheap, and biodegradable. The technology? A branch from a pine tree stripped of its bark.

“There is a community of people who do look at sap flow and drying in plants because it’s obviously important, but that community doesn’t intersect with the water purification community,” said Karnik, an associate professor of mechanical engineering at Massachusetts Institute of Technology.

Trees have tissue inside called xylem that transports sap. They use an interconnecting structure of channels within the xylem — similar to pipes in parallel — each connected by a membrane that allows fluid through but blocks small particles or air bubbles. Karnik realized that the way trees have evolved to prevent air bubbles from blocking their circulatory system might be repurposed to filter pathogens from water.

The researchers harvested sapwood from pine trees. They stripped off the bark and fixed a cross-section of the branch — about an inch long and half an inch in diameter — in a tube. They dissolved microscopic red dye particles in water and filtered it through the wood, determining the filter could strain particles the size of bacteria.

Next, they tried the filter on E. coli bacteria that were dead, and found it was effective. The wood filter they tested would be able to cleanse about four liters of water a day.

“We did not file for a patent. I just felt one shouldn’t patent something that’s so universal,” Karnik said. “But I think that how do we process xylem or how do we make filters out of it — that’s where I think there’s a lot of potential to develop this technology.”