Science Buddies: "Ask an Expert"

Hi,
In my experiment, I investigated the aquatic macrophyte, Azolla, and its phytoremediation capabilities to remove metallic ions from water.
The inspiration for my project can be found here: http://www.scienceinschool.org/2011/issue21/azolla
I prepared two test samples for my experiment: a bin with 500 mL of tap water and 10 grams of Azolla, and my control, which was just a bin with 500 mL of tap water.
I hypothesized that the conductivity of the water in the bin with Azolla would decrease, since the Azolla would remove any available metallic ions in the water, whereas the conductivity in the control bin would remain the same.
I measured and recorded the conductivity of each bin every day for 14 days.
To my shock, the conductivity of the water in the bin with Azolla actually increased over the first three days of my experiment, rather than decreasing like I hypothesized. However, after the first three days, the conductivity began decreasing at a steady rate until the end of my experiment.
In my control, the conductivity of the water actually steadily increased over the course of my experiment, rather than being stagnant.

Why did the conductivity of the water in the bin with Azolla increase over the first three days, and then decrease for the remainder of the experiment?
Why did the conductivity of the water in my control steadily increase, rather than staying the same?

Thank you.

Hi,

There are several possible reasons for your results. Did you measure the temperature of the solutions? Conductivity increases with increasing T and is usually standardized to 25 Celsius.

You said you used tap water for the experiment. Did you compare its conductivity to that of distilled water as a control? CO2 from the air will dissolve in water to form bicarbonate ions that conduct electric current. If your tap water had no dissolved CO2 to begin with then its conductivity might have increased as the gas dissolved and formed ions. What was the conductivity of the water without Azolla at the beginning of your experiment? At the end.? How did it compare to water with Azolla?

Did you have the containers exposed to sunlight or artificial light? For how many hours? Azolla would photosynthesize in the light, but during the dark it would do respiration and produce CO2 that would dissolve in the water to form bicarbonate ions that conduct electricity. You should check the pH of the tap water as it comes out of the faucet with pH paper or a meter then again after a few days to see if the pH goes down which would indicate that CO2 is dissolving.
I read the project notes in the link you sent and it said that Azolla has a symbiotic relationship with a cyanobacterium that is able to fix atmospheric nitrogen to produce nitrate ions. These could temporarily increase conductivity until the Azolla used up the nitrate.
That’s all I can think of right now to explain your results. Maybe some of the other experts can come up with other possibilities. Let us know further what you find out. I am curious as to why your conductivity measurements increased with tap water alone and I would like to know the answer. You could call a water-testing company and ask them what they think the reason might be.
Good luck and keep us posted.
Sybee

Hi SciB,

No I did not measure the temperature of the water, but the trends in my results are still constant, as the conductivity of the water in the control steadily increased, and the conductivity of the water in the bin with Azolla steadily increased and then steadily decreased.
I calibrated my conductivity meter using distilled water, as it gave a reading of 0 μs when immersed in distilled water.
The conductivity of the water in the control was 313 μs at the start of the experiment, and 511 μs at the end of the experiment, which was 14 days later.
The conductivity of the water with Azolla started off higher at 359 μs, but it finished at a lower conductivity at 286 μs. On day three of the experiment, the highest conductivity of the water with Azolla was recorded; it was 443 μs.
My bins were on a windowsill, so they had exposure to natural sunlight for about 8 to 10 hours a day. After the sun had set, the bins would experience about 1 hour total of artificial light daily. Other than that, they were in darkness.
In terms of pH, the pH of the water with Azolla decreased from 7.2 to 6.8 within the first two days of the experiment, whereas the the pH of the water in the control remained the same at 7.2 for the whole experiment.
I also measured the nitrite and nitrate levels in the water. In the bin with Azolla, the nitrites decreased from 0.5 ppm to 0 ppm within the first two days of the experiment, and the nitrates decreased from 20 ppm to 0 ppm also within the first two days of the experiment. In the control, the nitrite and nitrate levels remained the same (at 0.5 ppm and 20 ppm, respectively) throughout the experiment.

Does this additional information help you come to any other conclusions about my results?
Even though the pH in the control was constant at 7.2 during the entire experiment, could your explanation of CO2 from the air dissolving into the water to form bicarbonate ions that conduct electric current and increase conductivity still be valid?
Could evaporation of the water have any effect on its conductivity?

Thank you.

It looks like you have covered all the possibilities I thought of already!

I'm afraid I am stumped as to how to explain the increase in conductivity of the plain water. Do you still have that water? I would continue to measure the conductivity and the pH for as many days as you can just to see if the conductivity continues to rise. And yes, evaporation of the water would concentrate the ions and increase the conductivity. The conductivity of the water increased by almost 2/3 so I would think that the volume would have to decrease by about 1/3 to increase the reading that much. That seems like a lot of evaporation in just 14 days. How hot was it on your window sill?

Did you only do one control and one experimental test? You really should do at least three of each so you can do some statistical analysis of your data. Do you have time to repeat the experiment? If so, then do that and see if you confirm the initial result. Measure the volume of the water at the beginning so you can know how much evaporation occurs. Also, take the temp of the water when you do the conductivity reading just so you can report it if anyone asks, since temp does affect conductivity.

Maybe someone who knows more about conductivity than I do will read this and have a logical explanation of the results. In the meantime I would try to contact someone who does water tests professionally and see what they say about your readings on tap water increasing with time.

Good luck and do keep us posted.

Sybee

I have a link to an experiment that tested whether azolla's potential use in phyremediation. The link is shown in below. You would want to make sure that the volume of water that you used in the experiment remained constant for all treatments, because water volume would affect conductivity measurements. This experiment recommended using tap water or contaminated water (ideal). Since the latter may not be safe to use, you may want to add metals to tap water instead. Conductivity was measured using a conductivity meter, while commercial strip tests were used to measure ion concentrations. I do not know exactly what type of equipment you used, but you might want to check the quality of your equipment, because that might interfere with your results. In addition, control treatments included utilizing plants that don't absorb heavy metals, using spectrophotometers to measure bacterial content and turbidity, or counting colony forming units via the dilution method. Bacteria can interfere with your results by taking up nutrients and metals. In sum, you might want to investigate the quality of your equipment, include more control treatments, and measure your bacteria level. I hope that this helps.

http://www.scienceinschool.org/print/2565