Effectiveness of a Phytoremediation Plant in Controlling Water Pollution

Aim

To investigate the effectiveness of a phytoremediation plant, such as water hyacinth (Eichhornia crassipes) or Pistia (water lettuce), in reducing the level of pollutants in polluted water over time, compared with polluted water left without the plant.

Variables

• Manipulated variable: the presence or absence of the phytoremediation plant in the polluted water sample.
• Responding variable: the level of a chosen pollutant indicator, such as turbidity (measured with a turbidity tube or by visual comparison) or a nitrate/nutrient reading from a test strip, measured at set time intervals.
• Controlled variables: the volume and initial concentration of the polluted water sample, the size and type of container, the light intensity and temperature, and the total duration of the investigation.

Materials and apparatus

• A phytoremediation plant, such as water hyacinth or Pistia
• A sample of polluted water (or water with a known amount of fertiliser or dye added to simulate pollution)
• Two identical containers
• A turbidity tube or a nitrate/nutrient test kit
• A ruler
• A calendar or stopwatch to keep track of the measuring intervals

Procedure

1. Prepare two identical containers, each filled with an equal volume of the same polluted water sample.
2. Place the phytoremediation plant into one container, which becomes the test set-up, and leave the second container without any plant, which becomes the control.
3. Place both containers under the same light intensity and temperature conditions.
4. Measure and record the pollutant indicator reading, such as turbidity or the nitrate test-kit reading, in both containers at the start of the investigation (Day 0).
5. Measure and record the same reading in both containers at regular intervals, such as every 2 to 3 days, over a period of 1 to 2 weeks.
6. Compare the readings from the test container and the control container over the whole period.

Expected results

The pollutant indicator reading is expected to fall steadily in the container with the phytoremediation plant, while the control container without the plant shows little or no change, as illustrated in Table 1.

Conclusion

The phytoremediation plant reduces the level of pollutants in the water, for example by absorbing excess nutrients or other dissolved substances through its roots, so the aim of the investigation is achieved. The clear difference between the test and control containers shows that the plant is effective in helping to control water pollution compared with water left untreated.

Paper 3-style questions

Context. A student placed water hyacinth in one container of polluted water (the test set-up) and left an identical container without the plant (the control). The turbidity of the test container fell from high to very low over 9 days, while the control stayed slightly high.

Question 1 (hypothesis). State a suitable hypothesis for this investigation.

Model answer. Polluted water containing a phytoremediation plant shows a greater fall in the pollutant level (turbidity) over time than identical polluted water without the plant.

Question 2 (variables). State the manipulated variable, the responding variable, and two variables that must be kept constant.

Model answer. The manipulated variable is the presence or absence of the phytoremediation plant. The responding variable is the pollutant level, measured as the turbidity reading at set intervals.

Variables kept constant include the volume and starting concentration of the water sample and the light intensity and temperature around both containers.

Question 3 (tabulation and graph). Describe how the results should be recorded and displayed.

Model answer. Record the day and the turbidity reading for both containers in one table with clear headings. Plot a line graph with the day on the horizontal axis and the turbidity reading on the vertical axis, drawing one line for the test container and one for the control on the same axes so the two trends can be compared.

Question 4 (inference). Using the readings, state the conclusion and explain the role of the control.

Model answer. Turbidity fell far more in the test container than in the control, so the plant is effective in lowering the pollutant level, for example by absorbing dissolved nutrients through its roots. The control shows how little the pollutant level changes without the plant, so the extra fall in the test container can be attributed to the plant rather than to natural settling.

Safety

• Wear gloves when handling polluted water and wash your hands thoroughly afterwards; never taste or swallow the water samples.
• Handle any fertiliser or dye used to simulate pollution with care, following the label, and keep it away from the mouth and eyes.
• Water hyacinth and Pistia spread very quickly, so do not release them into ponds, drains or rivers; dispose of the plants as instructed by your teacher.
• Label both containers clearly and place them where they will not be knocked over or mistaken for drinking water.

Common mistakes

Source:SRC-DSKP-EN

Frequently asked questions