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2022/10/21
Wastewater



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Phytoremediation: an option for persistent environmental contaminants?


WT Interview with Larry Green, MD, PhD.

Gillian Ward

WT: Thank you for doing this Dr. Green. My first question is, can the heavy metals that persist long in the environment: chromium, arsenic lead -- can these be detoxed from water and soil?

Dr. Green: Yes, they can. It's going to be an interesting discussion. I can explain to you the process. It's not rapid, but it very definitely can be done.

WT: Can you tell us the current pre-treatment processes common for industrial wastewater?

Dr. Green: I can’t give you all the specifics, depends on the particular toxin you are looking at. Generally, the process is a concentration of the material to try to isolate it or put it in a more concentrated form in a smaller volume of water.

Even when the system is followed perfectly, it doesn’t mean the water (effluent) is free of toxins; it’s in theory at a much lower, so-called tolerable level. I don’t believe any level is tolerable. these toxins do accumulate over time and that is our real fundamental problem.

Usually, it is a chemical process that causes a precipitation or a flocculation, which is where you have a material that is forming a gel or a precipitate that is trapping some of the (contaminant) material. For the metals, you would have to look specifically at each for the process. The problem with all of these metals is they are cations, positively charged so they flow with water very readily. Some are toxic at very low levels, there are so many problems we could address here.

WT: We have interviewed many scientists that have developed pre-treatment processes that remove contaminants from water. Am I understanding this correctly, that pre-treatment does not mean the toxins have been neutralized, just concentrated for disposal?

Green: That’s right. 

Now somebody might claim that if you had a process that caused a mineralization, say a solid crystallization, they might try to claim the toxic material could not be leached out. I have not seen any evidence of this being true. You might slow it down, but you will have accumulating, continuous release into the environment no matter what you do. Cations are hard to deal with because you can’t destroy them, unless you embed them in glass like they do with radioactive material, and try to bury the glass-entombed material in a place where there is no water. That’s not very practical.

WT: Can you explain what phytoremediation is, and is this something we should be looking at?

Dr. Green: I think phytoremediation is very attractive for almost any kind of toxin, metal material, whether it’s cadmium, chromium, arsenic, lead, it doesn’t really matter. What’s interesting about phytoremediation is it’s able to pick up all of these toxins through a very interesting process that I will describe in a just moment here.So, it doesn’t have to be one solution that fits that particular (toxin) and then you have to adopt a different solution. 

There are some interesting tests you can do to see how efficient this is. 

The process is really quite simple. 

“ Phyto” is referring to the plants that are involved in this process. 

It’s a kind of chromatography in reverse, where you are bringing something out of the ground and concentrating it above the ground as opposed to allowing it to stay in the ground and spread to a larger and larger area.

You are taking a very large number of plants growing in a contained area, and this is set up like a net to catch the toxic material. You are planting a rapidly growing plant. There are a lot of choices here: it could be grasses, sunflower, barley, or wheat. When you plant these, they will be drawing from the subterranean network, the micro-material flowing through the contained area, and they will bring this up into their leaves and stems. The contaminants, while not beneficial to the plant, by the sheer mechanics of fluid rising and travelling through the leaves and stems, the material will be carried in there because they flow with water. 

Then you harvest the plant and concentrate that material. The plants are going to keep growing, or you will plant again. You can see that what you are doing is just removing the toxic metal mass over time and putting it into a very concentrated form where you are going to have control over it. 

You can take an area that is very contaminated and reduce the contaminant amount per square meter or however you want to do your measurements. You would be surprised, you can take an enormous amount of material out in a reasonable time at very low cost. I would say within three to five years, in most cases, you could reduce the toxic material in your field to almost non-detectable.

The whole idea is that the plant is doing the work for you, you don’t have to disrupt the soil.

WT: The harvested plant now contains toxic metal. How important is it that the plant material is treated in a particular way and isolated, over against being used as a food source, potentially?

Dr. Green: Of course, the plants used in phytoremediation are not going to be used as a food source. 

In a way, you could think of this as a sophisticated way of mining and getting your material back. If you have cadmium, you originally acquired it through a mining process, now you are just recycling it back. You are going to be treating the plants, there are a variety of ways, under controlled conditions, let’s call it a modern metal refinery of sorts. You would take those plants and recover the metals for other uses.

There is a protocol to follow, it is very structured. You can accelerate this process by the plants selected.

WT: What are some of the barriers to adoption of phytoremediation as a waste mitigation practice?

Dr. Green:

Finance drives a lot of this. Businesses' margins are narrow, people will cut corners where they can. Even when someone is pointed out as the culprit or criminal (for illegal discharge of pollutants) and we make them pay, we still have the problem to deal with. 

If waste solutions were economically attractive and efficient, and profitable, it would happen.

Its known and well established that (phytoremediation) is a doable process. If we were talking about gold and silver, I am sure many companies would be interested in taking the drenched sunflower stems and stalks and processing to get the gold and silver. Cadmium and arsenic, maybe they would, maybe they wouldn’t.

 Dr. Green has a PhD in Physical Organic Chemistry, University of California, Irvine and MD from Medical College of Ohio at University of Toledo































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