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5.3 Investigate

How Can Pollutants be Physically Separated Out of Air?

You now know how to use chemical reactions to remove some of the gaseous pollutants from air. But how can the particulate matter and some of the other gases in polluted air be removed? Earlier, you probably suggested several ways to do this. You will now have a chance to try out your ideas.

You will be given a mixture that contains water, salt, sand, iron filings, and foam packing material. You will use the mixture to model air. The gases in air form a homogeneous mixture. The gaseous part of air has the same composition throughout. The particulate matter in air mixes with the gases to form a heterogeneous mixture. There can be more soot or dust in some parts of a sample of air than in other parts.

The mixture you will use is a lot like air. The salt water is a homogeneous mixture, like the gases in air. The particles of sand, iron filings, and foam packing material in the salt water make the whole mixture a heterogeneous mixture.

Separate a Mixture Into Its PartsYou will be given some tools, and you will develop a procedure for separating out the sand, iron filings, foam packing material, and salt from the water. You know some things about the properties of each of these substances that will help you develop your procedure.

The contents of this stream are a heterogeneous mixture.

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PlanyourProcedureOne person in your group should collect samples of salt, sand, iron filings, and foam packing material.

1. Examine the materials. For each, what properties does it have that you could use to separate it from the rest of a mixture if all of the materials were mixed into water? On your Separating a Mixture page, list each of the materials and your observations of the properties of each. Remember that the salt will be dissolved in the water.

2. Examine the equipment that is available to you, and devise a procedure for separating out each of the four materials in your mixture to obtain a pure sample of each. Record the procedure on your Separating a Mixture page.

3. Have your teacher approve your procedure, and then send one person in your group to get a sample of the mixture you need to separate.

Mixtures can be separated by using many different procedures.

Materials•sand•ironfilings•foampackingmaterial•salt•water•handlens•magnet•coffeefilters•funnel•stirringrod•plasticcups•hotplate•aluminumpan•tweezers•plasticspoon•safetyglasses•Separating a Mixture page

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Run Your ProcedureWith the approval of your teacher, carry out your procedure. Be sure to follow the steps carefully. Record your results.

CommunicateShareYourResultsShare with the class what your group did to separate out each of the materials in your mixture. As you share, notice the similarities and differences in the ways each group separated out each of the materials.

As a class, make a list of separation methods the class used, how to make each method work, and when you think each of those methods is useful.

Reflect1. Do you think each of your separated components is pure? How can you tell?

2. If you could rewrite the procedure, what would you change?

3. Which steps of your procedure do you think would be most useful for removing particulate matter from air? Why?

Using Electricity to Remove Particulate Matter (Soot) From AirYou used magnetism, filters, mechanical separation, and heat to separate out the materials in your heterogeneous mixture. Each of these procedures is a physical procedure. Each uses some physical process to remove a material from a mixture. All of the procedures you used, plus others, are used to physically separate particulate matter from air.

One common way to physically remove particulate matter from the air is by using staticelectricity. You are familiar with static electricity. You may have used static electricity to stick a balloon to the wall. When you rub the balloon, it gives the balloon an electrical charge. The charge on the balloon and the charge on the wall are opposite, so they attract each other, and the balloon sticks to the wall. You explored with static electricity using transparent tape in Learning Set 2.

static electricity: an electrical charge on an object.

A balloon sticks to a wall because the negative charges on the surface of the balloon are attracted to the positive charges on the surface of the wall.

Project-Based Inquiry Science

Learning Set 5 • How Can Air Quality Be Improved?

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electrostatic precipitator: a device that uses a static electrical charge to remove solid particles from the air.

This same idea is used in electrostaticprecipitators to remove particulate matter from smokestacks before it enters the air. Static electricity is used to give the solid particles a charge. Then the particles collect on a surface that has an opposite charge.

DemonstrationTo help you understand how static electricity can be used to keep particulate matter from getting into the air, you will observe the operation of an electrostatic precipitator. As you watch the video, look for what happens to the particulate matter as the electrostatic precipitator works.

Reflect1. Sketch the path of the solid particles

on an Electrostatic Precipitator page.

2. How do you think the electrostatic precipitator works? How does it remove the solid particles?

3. What kind of air pollutants did the electrostatic precipitator remove from the air? How are these particles similar to and different from the particles in the smokestack of a power plant?

4. If you were to install an electrostatic precipitator on a coal-burning furnace, at what step in the process would you install the precipitator to clean the pollutants out of the air? Look back at your tour of the coal-burning power plant to support your answer.

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5.3 Investigate

Using Mechanical Filters to Remove Pollutants From the AirPerhaps you have played with sand by letting it flow through an object, such as a sieve. Or maybe you have let flour flow through a sifter. Sieves and sifters have small holes in them. Only particles small enough to fit through the holes can pass through. Larger particles or chunks of material cannot pass through and remain behind.

Mechanical filters are another way to remove particulate matter from polluted air. They work much like sieves and sifters. They are usually made of cloth with very small holes. The cloth is usually charged, as in the electrostatic precipitator. The cloth attracts particles just as an electrostatic precipitator does. Large particles are caught by the cloth and cannot go through the small holes. Small particles that could get through the holes of the cloth are caught and held by the electrostatic charge on the cloth. Fabric filters are used in factories and also in homes. For example, vacuum cleaners have fabric filters. So do some furnaces.

A sieve catches large particles and allows only small particles to pass through.

Fabric filters can be used in vacuum cleaners or in furnaces.

Project-Based Inquiry Science

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Filters are also used in vehicles. For example, diesel oil is a type of fuel that is used in many trucks and buses. Diesel engines are among the most polluting sources of power because of their high emissions of particulate matter and harmful gases. Often, you can even see the large pieces of particulate matter from the exhaust of a diesel engine being released into the air. One advance in clean technology for diesel engines is the diesel particulate filter. This device uses a catalyst to remove gas pollutants, such as VOC’s and CO. In addition, a filter bag fits over the exhaust stream and removes particulate matter.

Using Water to Remove Pollutants From the AirYou already read about how water can be used to chemically separate out gases from the air. Liquids can also be used to remove small particulate matter from the air.

You already know that a wetscrubber is one of the most common pollution-control devices used by industry. It operates on a very simple principle: polluted air is brought into contact with a liquid, such as water.

Pollutants react with the liquid and are removed from the air. Very small particulate matter is also captured by liquid droplets and can be removed with the liquid.

Usually, a wet scrubber works in three or more phases. In the first phase, the polluted air is passed through a liquid that can

wet scrubber: a device that cleans gases from the smokestacks of power plants and factories.

Vehicles with diesel engines are one of the most polluting sources of power.

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5.3 Investigate

dissolve most of the gaseous pollutants and neutralize them. In the next phase, the air is passed through another liquid that can remove the particles and dissolve the rest of the gases. In the third phase, the liquids are treated to remove the pollutants and particulate matter and pumped back into the wet scrubber to be used again.

You might be surprised that a wet scrubber does not actually do any scrubbing. The liquid does its work just by being there. What is important is that the liquid and the polluted air must come into contact with one another. The liquid in a wet scrubber can be sprayed into the air, or the polluted air might be forced into the liquid.

Stop and Think1. What happens to pollutant gases in the wet scrubber? Why does

this happen?

2. What do you think happens to particulate matter in a wet scrubber? Why does this happen?

3. If you were to install a wet scrubber on a coal-burning furnace, at what step in the process would you install the scrubber to clean the pollutants out of the air?

ReflectYou have read about many different types of pollution control—chemical methods, electrostatic methods, mechanical methods, and wet scrubbing methods.

1. Which kinds of pollutants does each method reduce?

2. Why do you think so many different kinds of pollution-control methods are needed?

3. If you wanted to get the air as clean as possible, how would you do it?

4. Why do you think car manufacturers and power plant operators might not always want to clean the air as much as they are required to by government regulations?

5. Why do you think it might take so long to pass laws that require industries to remove pollutants before they enter the air?

Project-Based Inquiry Science

Learning Set 5 • How Can Air Quality Be Improved?

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How Clean Does the Air Need To Be to be Safe?The table below contains the 2008 standards for ambient air quality in the United States. The term ambient describes anything that surrounds you, such as air. The standards are meant to be applied to the air in your community as you walk and breathe the air. They are the maximum allowable amounts for each pollutant. Of course, any concentrations less than these are better. These are not standards for emissions directly out of the tailpipe of a car or the smokestack of a power plant. Those values would be much higher. Because the composition of pollutants in air changes from hour to hour and from day to day, the values in the table are often given as averages.

These seem like very small numbers and although each of these pollutants is a poison, you might think that, because there is so little of each, their effects are harmless.

µg: micrograms, 0.000001 g, one millionth of a gram.

micron: 0.000001 m, one millionth of a meter. You could line up 10,000 of PM-10 particles in a 1 cm line. These particles are very small but very large compared to molecules. For example, about 2500 oxygen molecules would need to be lined up to make a line 1 micron long.

National Ambient Air Quality Standards (NAAQS)

Pollutant Standard Value

Particulate Matter (PM-10, PM-2.5)* 50 µg/m3 (about 30 ppb)

CarbonMonoxide,CO 9 ppm (8 hour average)

Ozone,O3 0.12 ppm (1 hour average)

NitrogenDioxide,NO2 0.053 ppm (yearly average)

SulfurDioxide,SO2 0.03 ppm (yearly average)

Mercury,Hg 1 ppm in fish (to be eaten)

*ParticulateMatterismeasuredbysize.PM-10isparticulatematterthatis10micronsinsize; PM-2.5isparticulatematterthatis2.5micronsinsize.PM-2.5issmallerandmoredangerous becauseitpenetratesmoredeeplyintothelungs.

AIR QUALITYAQ 275

Stop and Think1. Which substance in the table is the most dangerous, on a per gram basis?

How did you decide that?

2. Which substance in the table is the least dangerous, on a per gram basis? How did you decide that?

However, the amount that is harmless depends on how much air you take into your lungs. An adult human takes about 24,000 breaths each day, and each breath contains about 0.5 liters of air. The table below shows how much of each pollutant is safe for a healthy adult human to breathe in a day.

As you can see, the amount of each pollutant is still very small. However, this chart shows how very toxic each pollutant is. If all the pollutants were present in the air at these levels all day, every day, you would be a very sick person. But usually, you are not in the places all day that have the large amounts of these pollutants. For example, the concentration of carbon monoxide, nitrogen dioxide, and ozone is higher close to a busy street than it is in the classroom where you are sitting now.

5.3 Investigate

Maximum Allowed Pollutant Inhaled per Day in Grams

PM-10 (soot) 0.00060 g

Carbonmonoxide,CO 0.135 g

Ozone,O3 0.0031 g

Sulfurdioxide,SO2 0.000103 g

Nitrogendioxide,NO2 0.0013 g

Mercury,Hg 1100-gservingoffishperweek 0.00010 g

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3. These numbers show the amounts of pollutants that are safe for a healthy adult to breathe in a day. How do you think these amounts of pollutants would affect a baby, an elderly person, or a sick person?

4. If you lived in a large city, which substance would be the hardest to avoid? Which would be easiest to avoid? Why?

5. Which substances do you think would be most important to remove from the air? Why?

What’s the Point?Advances in technology can clean the pollutants from emissions or keep pollutants from being made. Some of the ways to improve the quality of the air are by using physical means, such as electrostatic filters or mechanical filters, or chemical means, such as wet scrubbers, to separate pollution from the air. If you wanted to make air really clean, you might use chemical means of cleaning it and then filter it several different ways. But cleaning air is expensive, and some ways of cleaning air are more cost-effective than other ways. Some manufacturers and power-plant and factory owners do not want to improve air quality because of the high cost of installing air-control devices.

The EPA has set standards for how much of each pollutant is allowed in the air. These standards are based on what scientists say is safe for a healthy adult to breathe. But these amounts could still be harmful for babies, small children, elderly people, and those who are sick.

Many industrial plants use electrostatic precipitators, such as the one shown here.