Lesson Six: Removing Iron from Drinking Water

Grades: 9-12 (Science, Chemistry and Biology)
Time: 60 minutes
Space Requirement: Classroom with sink and ample counter space
Methodology: Hands-on and Cooperative Learning, Group Discussion

Materials not included in the kit:

For each group: Two 250mL beakers, a marker, scissors, stir stick, two coffee filters

For the teacher: Beaker (at least 100mL), a marker

Materials included in the kit that will be used:

For each group: Three small plastic beakers, plastic cup, raw well water sample, bio-oxidized sample, four aluminum packets of iron testing reagent

For the teacher: Six small plastic beakers, bio-oxidized sample

Objectives: The students will learn about the problems associated with iron in drinking water. Students will learn about the chemical states of iron. Students will learn about removing iron from water using biological treatment processes. Students will gain an appreciation for the use of natural processes to perform tasks that would otherwise require chemicals.

Directions/Procedure:

  1. Distribute printed copies of the Lesson Six Handout to the class and read through it with them. In the second paragraph the students are instructed to take a look at the raw well water samples provided in the kit. Pass one or two of these samples around the room for the students to see. Instruct them not to shake them up until everyone has seen the iron settled at the bottom. They can then be shaken up and passed around again.
     
  2. Read through the rest of the Lesson Six Handout until you reach the activity.
     
  3. The class will split into their groups and each student should carefully read over the procedure.
     
  4. Separate the materials listed for this activity from the rest of the kit and have someone from each group retrieve what they need. Remind the students that they need to be careful with the bio-oxidized sample and not to shake it or break the floc up too much.
     
  5. Have each group work through the procedure of filtering the iron solutions and preparing samples for comparison.
     
  6. While the students are working label each of the six remaining small plastic beakers “unfiltered bio-oxidized”.
     
  7. Once each group has their three samples ready (Step 10) and you have their attention you can prepare the unfiltered bio-oxidized sample that will be used for comparative purposes. It is best that the students watch you do this so that they know what this sample is; there should be no mysteries here.
    a. Put 100mL of tap water in a beaker.
    b. There will be at least one tube of bio-oxidized sample remaining after each group has taken one. Point out that the tube you are using is just like the ones they used.
    c. Shake up this tube and break apart the floc inside then pour this sample into the beaker and stir it up. Point out that the tube you are using is just like the ones the students used and that it is being mixed into the same amount of water, the only difference is that this one was shaken up. This sample will have the same iron concentration as the one the students prepared before they filtered it.
    d. Pour 10mL of this solution into each of the six small plastic beakers and give one to each group so they can continue with the activity
     
  8. When everyone is finished the activity and have recorded their observations you can begin a class discussion with the following questions. Following many of the questions are brief explanations and descriptions of the desired outcomes.
    a. Before you added the colour changing reagent did you think the two unfiltered samples had the same amount of iron?
    b. Did you change your mind about this after adding the reagent?
    The bio-oxidized sample should appear to have more iron because even though it has been shaken up the iron will still be sticking together a little bit. These clumps will make it look like there is more iron than in the raw well water sample because the iron in that sample is hardly visible. These two samples should actually have the same iron concentration. The iron test should indicate this fact.
    c. Why do you think it is important for this experiment that these two samples have the same iron concentration? The two samples being filtered need to start with the same iron concentration or else it would not be fair to compare the samples to each other after filtering. You want to be able to judge which sample lost more iron going through the filter.
    d. Before you added the colour changing reagent how different did the filtered and unfiltered raw water samples look?
    e. How different did the filtered and unfiltered bio-oxidized samples look?
    There will probably be very little difference between the filtered and unfiltered raw water samples but the filtered and unfiltered bio-oxidized samples should look quite different.
    f. Which of the two filtered samples appeared to have the most iron in it?
    g. Was this confirmed by the iron test?
    h. Which sample filtered better?
    The test should show that there is much less iron in the filtered bio-oxidized sample than in the filtered raw water sample.
    i. Why do you think one sample filtered better than the other?
    The key to this is the floc. The large clumps of iron get caught in the filter much easier than small pieces.

Evaluation: Each group should submit their observation sheet and can be evaluated on the completeness of this log. Students can be evaluated on participation in both the experiment and the class discussion.

For the Teacher:

Related Links:

IBROM Treatment System
http://www.safedrinkingwaterteam.org/ibrom.html

Operation Water Biology

Lesson Six

Removing Iron from Drinking Water

Two Species of Iron Bacteria

Iron is an essential element for humans. People usually get all the iron they need from the food they eat so extra iron in water is not necessary for good health. The guideline for iron in the Guidelines for Canadian Drinking Water Quality is 0.3mg/L. This guideline is in place primarily for aesthetic reasons since iron discolours water and promotes bacterial growth. It is not a health-based guideline because iron, and the kinds of bacteria that grow around iron, are not usually harmful to peoples’ health. These iron bacteria do cause many other problems though. When iron is present in water, bacteria that use iron as their energy source will grow and build up anywhere that water flows. These bacteria form thick layers called biofilm in reservoirs and pipes, causing a lot of damage to distribution systems that is difficult and expensive to repair. Excessive bacterial activity also results in water with an unpleasant taste and smell which makes it bad for both cooking and washing. These bacteria can still grow in water that has less iron than the 0.3mg/L guideline. If there is any iron at all that these bacteria can use for energy then they will grow and spread. To keep iron bacteria out of drinking water all of the iron must be removed, not just enough to meet the guideline.

Take a look at the tubes of raw well water that are included in the kit. You will see that the iron in this water has settled to the bottom of the tubes, you should see many very small orange particles there. If you shake up one of the tubes and look very closely you can see the tiny orange particles floating all around in the water. These particles are visible because this iron is insoluble in water. This is not always the case though, when this water first comes out of the well there would not be any tiny orange particles visible in it. This was because of a difference between the two chemical states of iron. Iron can either be in a reduced state or an oxidized state (also known as rust). One of the biggest deciding factors of which state iron is in is exposure to oxygen. When the iron is deep in a well it is not exposed to oxygen so it will be in its reduced state. This reduced iron is soluble in water so it dissolves and you can not see it even though it is still there. When the water is brought up from the well and is exposed to oxygen it becomes oxidized. This is when the iron separates from the water because it is no longer soluble.

Making sure that all of the iron in the water is oxidized is an important step in filtering it out. This is because when the iron is still reduced it is dissolved in the water and can pass right through filters along with the water. To filter out the iron it must first be oxidized so that the particles separate from the water and then can be picked up by the filter. There are a few different methods that treatment facilities use to oxidize iron and other contaminants. The simplest is to let the water sit in a large open pool called a reservoir so that it is exposed to the oxygen in the air, most of the iron will eventually oxidize and settle to the bottom but this process is slow and often leaves some reduced iron in the water. Another option is to use oxidizing chemicals; these can be added to the water to oxidize all of the iron more quickly and efficiently than the reservoirs. These processes create water like the raw well water that you saw in the tubes with many very small iron particles floating around. Even though the iron is no longer dissolved in the water it is still very difficult to filter out because the particles are so small. This iron can pass right through most regular filters and when very fine filters that can catch particles of that size are used they clog up very quickly and need to be constantly cleaned and replaced. The usual solution to this is to add another chemical called a flocculent. The flocculent makes the little particles stick together in clumps which are called floc. This floc is easier to filter out because its larger size makes it easier to catch.

More information on biological water treatment can be found at
http://www.safedrinkingwaterteam.org/ibrom.html

A different option for filtering iron is to use a biological process like the one for ammonia that was discussed earlier. Rather than putting chemicals in it, the water can be passed through a filter containing bacteria that do an even better job of oxidizing all of the iron. These are actually the very same iron bacteria as we mentioned at the beginning of this handout, we may want to keep them out of the water pipes but we can still put them to work for us in our filters. Iron bacteria take in reduced iron and oxygen and perform a bio-oxidization reaction on them which produces oxidized iron. These iron bacteria gain energy from performing this reaction and they are specialized to do it very effectively. There is a second benefit to using these bacteria as well, as they are oxidizing the iron they are also making a little bit of sticky gel which automatically forms a floc with the iron. This allows the bacteria to do the job of both the oxidizing and flocculating chemicals at once. During the experiment for this lesson you will see an iron floc formed by iron bacteria in the other tubes included in the kit. These bio-oxidized water samples were taken from real biological filters in a water treatment facility where iron bacteria oxidized the iron and formed the floc. Once this floc is formed it gets caught in the filter and the water passes through iron free. There is no longer an energy source in this water for any other iron bacteria to use so they can not grow and form biofilm and thus all of the problems associated with iron in water are avoided.

In this experiment you will be able to see many of the things discussed in this lesson for yourself. You will filter water samples that have been oxidized in different ways and see if they give different results.

Materials needed for this experiment are:

Three small plastic beakers
Two 250mL beakers
Plastic cup
Raw well water sample
Bio-oxidized water sample
Two coffee filters
Four aluminum packets of iron test reagent
Marker
Scissors
Stir stick

1. Label one of the small plastic beakers “raw unfiltered” then shake up the raw well water sample, open it and pour 10mL into that beaker. Set this beaker aside for now.

2. Fold your coffee filter into a cone just like in Lesson Four and hold it in place over the cup.

3. Slowly pour the rest of the raw well water sample through the filter into the cup.

4. Label one of the small plastic beakers “raw filtered” then pour 10mL of filtered raw well water from the cup into that beaker. This beaker should also be set aside for now. Any water still in the cup can be disposed of.

5. Put 100mL of tap water into one of the large beakers.

6. Very gently and slowly turn the tube of bio-oxidized sample upside down. If the floc is stuck to the bottom of the tube turn it over a few times until all of the floc is knocked loose, you want all of it to come out when you pour the water out. You must do this carefully; you do not want to break the floc apart too much.

7. Open the tube of bio-oxidized sample and very carefully pour it into the beaker with 100mL of water (not through a filter). Try to hold the tube close to the surface of the water when pouring because the floc might break apart when it hits the water if it falls too far.

8. Fold your second coffee filter into a cone and hold it in place over the second large beaker.

9. Very slowly pour the sample you just mixed in step 7 through the coffee filter. Try to hold the beaker close to the filter when pouring so that the water does not fall as far.

10. Label the third small plastic beaker “filtered bio-oxidized” and pour 10mL of the filtered bio-oxidized sample into that beaker.

Once everyone has prepared their three samples the teacher will prepare the unfiltered bio-oxidized samples and give one to your group.

11. You can now begin observing and comparing the different samples.
The pairs of samples that you should compare are:
a. Unfiltered raw water to unfiltered bio-oxidized
b. Unfiltered raw water to filtered raw water
c. Unfiltered bio-oxidized to filtered bio-oxidized
d. Filtered raw water to filtered bio-oxidized

On a piece of paper write down the names of the two samples you are comparing and then write down your observations in a sentence or two. Leave lots of space to add things later then put the names of the next pair of samples and continue.

Examine them closely, does there seem to be differences between them? Are there different sizes or number of particles visible? Does one look like it has more or less iron than the other?

The aluminum packets contain a reagent that is used to indicate the presence of iron. When it is added to a water sample it will turn pinkish red if there is iron in it. The darker the colour gets the more iron there is. This is very useful for comparing samples, it can be put into a number of samples and you will know that the one that turns the darkest shade of red has the most iron or if some turn the same colour you will know that they have the same iron concentration.

12. With scissors very carefully cut open the tops of your four packets and pour one of the packets into each of your four samples. Use a stir stick to mix the powder into the water samples. You must wait at least three minutes for the reaction to finish but once the colour changes there is no hurry, the samples will stay that colour for a long time.

13. When the three minutes are up compare the same four pairs of samples again. Which sample really had more iron? Judging by how different the colours of the samples are does the difference in iron concentration seem to be large or small? Write down your observations in a sentence or two in the spaces that you left under your first observations.