Rooted in more than 3000 years of history, tea has found its place in many cultures around the world. As a result, it has become the second most popular beverage in the world next to water. Aside from generational practices and pure enjoyment, a large number of people drink tea for its health benefits. Many studies suggest that the frequent consumption of tea can benefit the cardiovascular system, reduce the risk of cancer, and aid weight loss.[2]
Tea is typically derived from the leaf of the camellia sinensis plant. As the plants grow, they extract trace metals from the soil around them. Unfortunately, some of these metals are toxic to humans and are released when the tea is brewed. Therefore, it is of great interest to prevent the metals from leaving the tea leaves while the tea is brewing.
Plastic tea bags have the potential to adsorb these metals, allowing them an alternative pathway into the water and ultimately our bodies. In hot water, plastic tea bags break down into microplastics, which significantly increases the surface area to volume ratio. The microplastics are then able to adsorb metals, effectively pulling them away from the tea leaves and depositing them into the water.
The adsorption process is a three-step endothermic mechanism, meaning that the introduction of heat would increase the kinetic favorability of the isothermic reaction.[1] A proposed mechanism for the tea bag experiment is as follows:
Metal in tea leaves <-> metal(aq)inside tea bag
Metal(aq)inside + microplastic <-> metal::microplastic
Metal::microplastic <-> metal(aq)outside
In the proposed mechanism, the metal in the tea leaves (solid metal) is in equilibrium with the aqueous metal inside the tea bag. After the metal dissolves in water, some is absorbed by microplastics floating in the tea bag. Because the water is not perfectly still due to convection and external forces, the metal::microplastic couple is able to migrate from the inside to the outside of the tea bag.
It is important to note that the microplastics bind to metals in solution similarly to why activated charcoal binds to metals: high surface area to volume ratio and affinity for metals. Activated charcoal is so effective at absorbing metals that it is used industrially to do so. Therefore, it would be reasonable to suspect that microplastics would also be effective at adsorbing metals.
Both microplastic and activated charcoal adsorption are modeled using the Langmuir adsorption isotherm, which describes how an increase in surface area leads to an increase in adsorption.
Θ = KP/(1+KP)
Annotation: as surface area increases, fractional coverage, decreases. More metal must be adsorbed to restore the ratio.
MP-AES analysis was used to determine metal concentration in the tea samples because of its high accuracy, precision, and sensitivity. We will use the MP-AES to run trials for each permutation of tea bag, brewing time, and brewing temperature. It is predicted that for consistent temperature and brew time, metal concentration will be higher for trials done with the plastic tea bag than for trials done with the cotton tea bag.