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Drinking tea has been linked to various health benefits, including a lower risk for heart disease and degree of inflammation in the body. Now, a new study by researchers from Northwestern University suggests another surprising perk of the beverage: The brewing process may help purify the water.
Brewing tea may naturally adsorb heavy metals such as lead and cadmium, according to the study published Monday in the journal ACS Food Science & Technology. “Adsorb” refers to the ability of a solid material to hold molecules — of a gas, liquid or dissolved substance — as a thin film on its outside surface or within internal surfaces such as crevices.
“This is one of very few ‘systematic’ studies that provide rational control, meaningful statistics and ‘nanotechnology’-level of sophistication brought to a seemingly mundane topic of tea brewing,” said senior study author Dr. Vinayak Dravid, the Abraham Harris Professor of Materials Science and Engineering at Northwestern University in Illinois, via email. “So little is studied about its capture potential, over and beyond ‘release.’”
The authors investigated how the adsorption of heavy metals depended on different types of tea — black, green, oolong, white, chamomile and rooibos — brewing methods, and whether it was looseleaf or in bags made of cotton, nylon or cellulose (a natural biodegradable material made from wood pulp).
For the adsorption experiment, the team created solutions from water and concentrations of lead and other metals such as cadmium, chromium, copper and zinc, then heated them to just below the boiling point. The authors then added tea leaves or bags and steeped them for various time periods, from seconds to 24 hours, before measuring how much of the metals remained in the water.
Cellulose bags performed the best, while cotton and nylon bags barely adsorbed any of the metals.
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“Nylon tea bags are already problematic because they release microplastics, but the majority of tea bags used today are made from natural materials, such as cellulose,” said Dr. Benjamin Shindel, first study author and contract engineer with the US Department of Energy’s National Energy Technology Lab, in a news release. Shindel worked on the research study while a doctoral student at Northwestern.
The type and grind of tea also mattered to a minor degree. Finely ground leaves, especially those of black tea, adsorbed more contaminants than whole leaves.
“When tea leaves are processed into black tea, they wrinkle and their pores open,” Shindel said. Those wrinkles and pores, as well as a finer grind, also increase surface area, increasing the capacity for contaminant binding.
Brewing tea can generally filter out about 15% of lead from drinking water, the authors found. This estimate is based on one mug of water and one tea bag brewed for three to five minutes. However, the most significant factor among all types of experiments was the length of time the researchers steeped the tea — the longer they let it steep, the better the metal adsorption was.
These findings were true for lead concentrations as high as 10 parts per million, or ppm, though concentrations of that level aren’t likely in highly developed areas.
“Given the testing conditions of an initial concentration of 1 ppm (1000 ppb), it’s difficult to say how that would translate to real-world scenarios,” Dr. Tasha Stoiber, a senior scientist at the Environmental Working Group who wasn’t involved in the study, said via email. “Brewed tea (in distilled water) contains about 1-4 ppb of lead. Drinking water in the U.S. from public water systems ranges from about 1 to 10 ppb for most systems, and levels in the range of 30-100 ppb would be highly contaminated. So the concentrations tested in the study represent an extreme amount of lead in water.”
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The authors contend that tea brewing may adsorb heavy metals via heavy metal ions sticking to the surfaces of tea leaves and staying trapped until the bag is disposed. In the case of cellulose bags, those may have greater surface area, and thus more binding sites, than the synthetic materials.
“I’m not sure that there’s anything uniquely remarkable about tea leaves as a material,” Shindel said. “They have a high active surface area, which is a useful property for an adsorbent material and what makes tea leaves good at releasing flavor chemicals rapidly into your water.”
What is special, however, is that besides water, “tea happens to be the most consumed beverage in the world,” Shindel added. “You could crush up all kinds of materials to get a similar metal-remediating effect, but that wouldn’t necessarily be practical. With tea, people don’t need to do anything extra.”
The study promotes or reinforces the idea that one can clean or filter one glass at a time in situations in which cleaning the surroundings may not be possible, Dravid said. But it doesn’t suggest that tea brewing is a substitute for cleaning water with a filter, since unfiltered water may have other contaminants that tea leaves or bags may not adsorb.
Those impurities can include nitrate (used for fertilizers), disinfection byproducts, arsenic, or per- and polyfluorinated alkyl substances, also called PFAS, Stoiber said.
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Lowering lead levels in drinking water is “easy to do with a simple carbon filter,” which is also the best method, Stoiber said. “The carbon acts in a similar way to the tea leaves by (adsorbing) the lead, but does so much more efficiently.
“And, as the paper demonstrates at higher concentrations, there are limits to the (adsorption), which is why it’s important to remember to change your filter cartridge on time as recommended,” Stoiber added. “Overall, the study is a good reminder to be aware of what’s in our drinking water.”
This is especially important for children, for whom there is no safe level of lead exposure, Stoiber said.
For more information on water quality, contaminants and finding out about the health of your own drinking water, Stoiber suggested viewing the Environmental Working Group’s tap water database.
