In a new study, published in the journal ACS Food Science & Technology, Northwestern University researchers have demonstrated that tea brewing naturally adsorbs heavy metals like lead and cadmium, effectively filtering dangerous contaminants out of drinks.
Heavy metal ions stick to, or adsorb to, the surface of the tea leaves, where they stay trapped until the used tea bag is disposed of.
“We’re not suggesting that everyone starts using tea leaves as a water filter,” said Northwestern’s Vinayak Dravid, the study’s senior author. “For this study, our goal was to measure tea’s ability to adsorb heavy metals. By quantifying this effect, our work highlights the unrecognized potential for tea consumption to passively contribute to reduced heavy metal exposure in populations worldwide.”
Benjamin Shindel, the study’s first author, remarked that, “I’m not sure that there’s anything uniquely remarkable about tea leaves as a material. 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. But what is special is that tea happens to be the most consumed beverage in the world. 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. Just put the leaves in your water and steep them, and they naturally remove metals.”
Experiment
To conduct the study, the Northwestern team explored how different types of tea, tea bags and brewing methods affect heavy metal adsorption. The various varieties tested included “true” teas such as black, green, oolong and white, as well as 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 (chromium, copper, zinc and cadmium), and then heated the solutions to just below boiling temperature. Next, they added the tea leaves, which steeped for various time intervals, from mere seconds to 24 hours.
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After steeping, the team 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 effectively removed.
After multiple experiments, Dravid, Shindel and their team identified several trends. Perhaps somewhat unsurprising: The bag matters. After testing different types of bags without tea inside, the researchers found cotton and nylon bags only adsorbed trivial amounts of the contaminants. The cellulose bags, however, worked incredibly well.
“The cotton and nylon bags remove practically no heavy metals from water,” Shindel said. “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. These may release micro-particles of cellulose, but that’s just fiber which our body can handle.”
“When tea leaves are processed into black tea, they wrinkle and their pores open,” Shindel explained. “Those wrinkles and pores add more surface area. Grinding up the leaves also increases surface area, providing even more capacity for binding.”
Out of all the experiments, one factor stood out most. Steeping time played the most significant role in tea leaves’ ability to adsorb metal ions. The longer the steeping time, the more contaminants were adsorbed.
From the experiments, the researchers estimate that tea preparation can remediate about 15 percent of lead from drinking water, even up to lead concentrations as high as 10 parts per million. That estimate applies only to a “typical” cup of tea, which includes one mug of water and one bag of tea, brewed for three to five minutes. Changing the parameters remediates different levels of lead. Steeping for longer than five minutes, for example, adsorbs more lead compared to the average steeping time.
