Capable Carbon Filters

Introduction

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.

Background

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!

Materials

Preparation

Procedure

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.

Cleanup

More to Explore

• 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

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