Tea is already one of the healthiest drinks you can reach for, but researchers have discovered another perk to the popular drink: Steeping tea leaves may make for cleaner drinking water.
“We found that brewing tea has the potential to remove a meaningful fraction of metal content in drinking water. The longer the tea is steeped for, the more removal you can observe,” says first author Benjamin Shindel, a PhD candidate in materials science and engineering at Northwestern University in Evanston, Illinois.
Then there’s the concern that tea leaves themselves can have metals that are released to the water they’re brewed in, says Ana Navas-Acien, MD, PhD, chair of environmental health sciences at Columbia University Mailman School of Public Health in New York City. “To actually think of the tea leaves themselves as a way to adsorb the metals from the water was a very interesting finding that I don’t think is very commonly known or thought about,” says Dr. Navas-Acien, who was not involved in the research.
How Tea Brewing Time and Methods Affect Toxic Metal Adsorption
Heavy metal ions adsorb onto the surface of the tea leaves and bags during brewing, where they stay trapped after the tea is removed. Researchers designed the new study to explore how different types of tea, tea bags, and brewing methods affect heavy metal adsorption.
Tested varieties included black, green, oolong, white, chamomile, and rooibos teas. They also examined the differences between loose-leaf and commercially bagged tea.
The researchers created water solutions with known amounts of lead and other metals such as chromium, copper, zinc, and cadmium, then heated the solutions to just below boiling temperature. Next, they added the tea leaves and steeped them for various amounts of time, from just a few seconds to 24 hours.
Then investigators measured how much of the metal content remained in the water. By comparing metal levels before and after adding the tea leaves, they were able to calculate how much was removed by the brewing process.
Key findings from the experiment include:
The type of tea bag matters. After testing different types of bags without tea inside, the researchers found cotton and nylon bags only adsorbed trivial amounts of contaminants, but cellulose bags worked very well.
The key to a successful sorbent (or molecule-collecting) material is high surface area, researchers reported. They theorize that cellulose, a biodegradable natural material made from wood pulp, has higher surface area and therefore more binding sites compared to the other materials.
Tea type and grind have minimal impact on metal adsorption. Tea type and grind played minor roles in adsorbing contaminants. But finely ground tea leaves, particularly black tea leaves, adsorbed slightly more metal ions than whole leaves, which the researchers attributed to surface area that the metals could bind to.
It’s likely that decaffeinated tea would also have adsorbing properties, says Shindel.
Longer steeping times filter out more metals. Out of all the experiments, steeping time made the most difference in tea leaves’ ability to adsorb metal ions. The longer the steeping time, the more contaminants were adsorbed.
“Some people brew their tea for a matter of seconds, and they are not going to get a lot of remediation. But brewing tea for longer periods or even overnight — like iced tea — will recover most of the metal or maybe even close to all of the metal in the water,” says Shindel.
Why Tea Leaves Might Adsorb Heavy Metals
Tea leaves have a high active surface area, which is a useful property for an adsorbent material, and is what makes tea leaves good at releasing flavor chemicals rapidly into your water, he explains.
It’s likely that other kinds of materials could have the same effect, but it wouldn’t be as practical or convenient, says Shindel.
“Tea happens to be the most consumed beverage in the world. People don’t need to do anything extra. Just put the leaves in your water and steep them, and they naturally remove metals,” he says.
Health Risks Posed by Heavy Metals
What Does This Mean for the Average Tea Drinker?
From their experiments, the researchers estimate that a typical cup of tea (one mug of water, one bag, brewed for three to five minutes) can get rid of around 15 percent of lead from drinking water.
If tea accounts for about one-fifth of a person’s beverage consumption, that might reduce a person’s lead intake by about 3 percent, wrote the authors.
Is this enough to make a meaningful difference in health? “We’re not recommending that tea drinkers change their habits or try to brew bitter tea to remove more metals. But we think it’s an interesting finding that on the margins, tea consumption might be lowering metal intake to a small degree for people who frequently drink tea,” says Shindel.