Capable Carbon Filters
Do you filter your tap water before drinking? Maybe at home you have a water filter on your faucet, in the fridge, or use special pitchers that have a filter unit in them. You have probably heard in commercials that these filters are supposed to make your water cleaner and safer to drink. But have you ever wondered what, exactly, these filters are doing and if the water is really cleaner in the end? Find out about the cleaning power of water filters in this activity; but for results you can see, you will "clean" a colored sports drink or colored soda.
To find out how water filters work, it is probably best to have a look inside the filter. Most of the filters that are used for home water treatment are carbon filters. That means that the material inside the filter is carbon, or a special form of carbon, called activated carbon. What makes activated carbon special is that it is a very porous form of carbon—almost like a sponge—that has many tiny microscopic pores that soak up water. All these little micropores create a huge inner surface area; about 5 teaspoons (10 grams) of granular activated carbon has a surface area that is approximately the size of a football field.
When the water or liquid travels through the porous structure of the filter, impurities can be removed from the water by a process called adsorption. Adsorption occurs when compounds physically or chemically adhere to the carbon surface and a film of the adsorbate, or the chemical impurities, is created on the adsorbent (the carbon surface). This is also why the surface area matters. The more surface area, the more possible bonding sites there are for the impurities. If all the bonding sites are taken up, then the impurities have to stay in the water and it is time to replace your water filter. Besides the surface area, the time the water spends in contact with the activated carbon is also an important factor that determines the efficiency of the filtration process. The longer the contact time or the slower the flow rate of the water, the more adsorption can take place. See for yourself in this activity; time to get started with your own adsorption experiment!
- Colored sports drink or colored soda
- 5 plastic cups (2 oz)
- Activated carbon (you can get this from a pet store)
- Coffee filters (at least 4)
- Permanent marker
- Paper towels
- White sheet of paper
- Choose a working area that can tolerate some liquid splashes.
- Label four 2 oz plastic cups with 0, 1a, 1b, 2a and 2b.
- Fill cups 1a and 2a with about 1 tablespoon of activated carbon (about 2 grams). It should be the same amount for each cup.
- Prepare two double-layered coffee filters. Put one filter into another filter to generate two layers of coffee filter.
- Get your timer and colored sports drink ready.
- Take the empty 2 oz plastic cup, labeled 0, and add about 1 tablespoon (approximately 10 mL) of colored sport drink. How does the drink look inside the cup? Is the color very intense? Smell the sports drink in the cup. Does it have a specific smell?
- Set your timer for 5 minutes. Do not start it yet.
- To the two cups with activated carbon, labeled 1a and 2a, add about 1 tablespoon (circa 10 mL) of colored sports drink. Make sure you have approximately the same amount in each cup. Carefully swirl each cup to make sure that the solution mixes well with the activated carbon. What happens when you add the solution to the activated carbon? What does the mixture look like?
- Start your timer immediately after you have filled both cups.
- Take the double-layered coffee filter and hold it above cup 1b. Once the timer rings after 5 minutes, pour the contents of cup 1a (liquid and activated carbon) into the double-layered coffee filter. Collect the liquid that runs through the filter in cup 1b. Did the appearance of the sports drink change? How? Smell the liquid again; do you notice a difference compared to the original sports drink?
- Set your timer for 25 minutes and start it immediately after you finished filtering the solution from cup 1a.
- While waiting, swirl cup 2a occasionally.
- Now prepare your second double layered coffee filter and once your timer rings after 25 minutes, hold it over cup 2b. Pour the liquid and activated carbon from cup 2a in the filter and collect the liquid in cup 2b. How does this solution look compared to the original sports drink and the solution in cup 1b? Do you notice any change in smell?
- Finally, line up cup 0 with your original sports drink, cup 1b and 2b. Put them on a white sheet of paper so you can see the solutions better. Look at all three cups. How did the sports drink change over time when it was in contact with the activated carbon?
- Optional: If you used food-grade activated carbon, you can taste each of your solutions. Do you notice any differences compared to the original sports drink? How does the taste change?
Extra: Try different colors of sports drinks or sodas. Do you see the same effect for all of them or does one drink work better than the others?
Extra: What happens if you change the ratio of activated carbon and liquid? Try this experiment with different amounts of activated carbon in the same volume of sports drink. Or change the volume of your solution and keep the amount of activated carbon the same. Do different activated carbon/liquid ratios lead to different results? What do you see after 5 minutes; for example, when you add more sports drink to the same amount of activated carbon?
Extra: Try to find other materials, instead of activated carbon, that could clean your solution. Can you think of other adsorbing materials that would work the same way as activated carbon? What property does this material need to have?
Observations and Results
Sports drinks come in a variety of colors. Looking at your plain drink in the plastic cup, you probably found that the color was very intense. Depending on the color or flavor, you might have noticed a fruity or sweet smell. When you poured your drink into the cups with activated carbon, did you see it fizzing and bubbling? This is because once the activated carbon becomes wet, all the air that is inside its many pores is replaced by liquid and, therefore, is released, which leads to the bubbling and fizzing.
The mixture now looks like a black slurry, and if you filtered it into cup 1b after 5 minutes, you should have noticed that the original color of the solution got much less intense. And the fruity smell probably became very faint or was not even recognizable anymore. If you check the ingredients list of your sports drink or soda, you probably will find that there are specific ingredients listed that make the color and flavor, such as “blue 1,” “caramel color” or “artificial flavor.” These are mostly chemicals or organic compounds that will adsorb to the activated carbon surface and get trapped in its huge pore space. Thus, they will be removed from your drink. When treating your sports drink longer with the activated carbon, you can even remove all its color so you end up with an almost clear solution. Did your sports drink look clear in cup 2b once you filtered out the activated carbon after 25 minutes? The longer the liquid is in contact with the activated carbon, the more of the color and fragrance chemicals can adsorb to its surface and if you wait long enough, all the color and smell is gone.
However, if you play around with the ratio of liquid volume and amount of activated carbon, you might find that if you have a lot of liquid and only very little activated carbon, you will not be able to remove the color completely. This is because the activated carbon has a limited adsorption capacity, which means that once all the surface area is covered with color and flavor compounds, there is no more space to adsorb more. This is also the reason why you have to replace your water filters at home regularly. If you used food-grade activated carbon and tasted the colorless sports drink, it probably did not taste like the original one at all! Now think of the water filter that you use at home; it works exactly the same way. All the organic compounds that make a bad taste or give the water a murky color will be trapped in the activated carbon filter and the only thing left is pure, clean water for you to drink.
Ask an Expert
- You can pour all the solutions into the sink. Collect the activated carbon in one container and discard it with your regular trash. Wipe your work area down with wet paper towels. If you have leftovers of your colored sports drink or soda, you can drink it.
- Activated-carbon filtering pitcher significantly reduces chemicals in tap water, from Science Daily
- Adsorption (activated carbon), from Sustainable Sanitation and Water Management (SSWM)
- From contaminated to clean: How filtering can clean water, from Science Buddies
- Science Activity for All Ages!, from Science Buddies