The phytoremediation research that I’m doing here at Cal Poly requires a bit of creativity. To give some background before I delve into the interesting tests we are running, phytoremediation is a form of environmental remediation that uses plants to clean up chemical contaminants in the soil. I am working with three native California plants–coyote brush, mulefat, and purple needlegrass–to determine their ability to take up chemicals in the contaminated soil.
Here is a summary of the organic contaminants that the soil contains:
- Polychlorinated biphenyls (PCBs), which are a man-made chemical found in transformer fluid and banned for use by U.S. Congress in 1979
- Chlorinated dioxins, which are produced from smelting and also naturally from forest fires and volcanoes
- Polycyclic aromatic hydrocarbons (PAHs) and other petroleum hydrocarbons, which are found in crude oil
There’s also a plethora of metals in the soil, some of which are dangerous…like mercury. What I am going to explain in this post is how we plan to help the plants extract mercury from the soil they’re planted in.
The mercury in the soil is in Hg(II) form. Basically, Hg(II) is taken into the plant by its roots and travels through the stems to its foliage. During this process, Hg(II) is reduced to Hg(0) by mechanisms in the plant that are still being researched. Once the Hg(0) reaches the foliage, it is released into the air as a gas via the stomata. The stomata are essentially holes in the plant’s leaves that open and close depending on the pressure differential from the moisture in the air.
The beauty of mother nature’s setup is this: Hg(0) is less likely to be harmful to people and in the environment than Hg(II). Because Hg(II) can more easily form organic mercury compounds, which are the most dangerous, it is better that mercury be in the Hg(0) form and out of the soil, which is teeming with organic molecules that have Hg(II) binding potential.
So, to recap, we are researching to determine which of the three plants, if any, have this potential to uptake mercury from the soil into their roots and volatilize it as a way less-harmful gas into the air.
Next is the fun part. We decided to add a chelating agent, EDTA, to the soil to increase the plant uptake. Ethylenediaminetetraacetic acid (EDTA) is an organic molecule that forms multiple bonds with an ion, encapsulating it. It’s like putting a wrapper around a sticky piece of candy so that it won’t get stuck in your pocket. The wrapper is the chelating agent, the sticky piece of candy is the ion, and the surroundings are the soil. Because contaminants adsorb, or “stick”, to the soil, they need help detaching from it. Once the chemicals are not adsorbed to the soil, they are more easily guided through the plant root cells. Below is a picture of the chelating agent (clear) that we added to the soil:
So far, the plants that received a dose have started to get brown leaves and die. This could be because the EDTA did what we wanted it to and increased the sequestration of Hg in the roots, stems, and foliage. However, it’s likely that EDTA prevented the plant from receiving other vital metals that it naturally uptakes from the soil.
In my next post I will explain the process of analyzing what chemicals the plants volatilize.
On another note, this weekend the other REU students and I went to Montana De Oro, home to giant sand dunes and starfish!
I have never seen a real-life starfish, so this was exciting! We spent the afternoon walking on the rocks and finding sea anemones. I noticed a black, sticky tar that covered most of the rocks and think this may be from oil pollution in the ocean. It was nice to see that the rest of the beach and park was clean though!