Both chelating agent can be performed complex with heavy metal such as Ar, pb, Hg, Cd +2 ions, but the in photoremidation concern EDTA is beneficial with strong legating of 6 coordination sites. Moreover both of them hexa valent ions, the freely solubility of molybdenum can be occur only at strong acidic condition, but in disodium EDTA is less constrain than former. Even acid treatment is prevalent in heavy metal in soil, the amount of Molybdenum restore is drawback. Hence my opinion is better choice of EDTA. Quit long ago I have remembered, my past studies of EDTA also made a complex with Mg 2+ ion present in plant of chlorophyll which responsible for slow degradation oxygen transportation, but I don’t know about the facts. please verified it.

EDTA is very effective remobilising metals, I don't know about ammonium molibdate. However, it is argued that using chelators for phytoremediation might not be a good strategy on the long run. These increase the availability of the metal greatly in a short period of time, which increases toxic effects for plants. This ultimately reduces their growth and their capacity to extract metals. Eventually plant death might occurr, as the tolerance of plants to high concentrations of metals is limited. Moreover, the amount of metal remobilised might be too much for the plants to take up, which puts you in danger of increasing metal inputs to both surface and ground water if it rains

Maybe soluble humic material instead?

Thank you all for your reply. During literature collection I found that EDTA is toxic and presents a low level of biodegradability (Dirilgen 1998; Finzgar et al. 2006). Ammonium molybdate is a fertilizer to plant. Nitrogen and Molybdenum present in ammonium molybdate are micro nutrients to plants which can promote the growth of plants and this will helps plants to uptake more toxic metals. EDTA only chelate and form soluble fractions with metals. But ammonium molybdate has the potential ability to chelate and form more soluble fraction with metals like Cd, Ni and Cu and increases bioavalibility of these metals. It also precipitate with metals like Cr,Pb and Zn and this decreases the biotoxicity of the metals. What is your opinion about this? Do you think that ammonium molybdate can be used to control biotoxicity and bioavalibility of metals to plants?

Humic material and other organic matter sources increase the capacity of the soil to retain and immobilise metals. They are good to prevent the metal from reaching water supply and trophic chain, as they are removed from the soil solution. It is a good strategy to limit the environmental impact of high metal concentration and to protect the local population. However, if the objective is to extract the metal from the soil, it might make the process much longer

Yah may be oxygen supply (transportation) is most concern in photoremidiation, and it is toxicant but i dosent know lower than molybdate. Iam not concentrating on the photosynthetic effects of molybdated (ppm level) in plant growth, but in chemistry part of coordination strength as i suggest may considering and might be acceptable by my mistake.

Take care when choosing a chelant agent. It must be sustainable. And EDTA it's not sustainable because its half live in soil and water. Try EDDS

Advances in Agriculture, Sciences and Engineering Research, Vol 2, No 9 (2012)

go trough this article, you will ultimately get the answer.

Potentials and drawbacks of chelate-enhanced phytoremediation of soils

Environmental Pollution, Volume 116, Issue 1, January 2002, Pages 109-121

THIS ALSO READ IT

Thank you Rohit .

Exactly, Cr, Pb, Zn and Ar these are primary toxicant concern as you mentioned than the others, but I won’t believe this Mo has to reduce the bio toxicity in phytoremediation effect. However the chelating effect on both complex has variance to one another and it almost problem with foreign ion interference ( ex Mo, Ar phosphorus and sulphidation). This chelating not based on the coordination sites, the relative size of the ions which involved maximum sequestration of some elements (Pb>Zn>Ar> possible my order is wrong check in reference), and you also known phytoremidation lack of complete sequestration. The stability is too chelating effect of different valence ion maintain throughout the reaction is problematic. Any way I am not come into direct answer of your question check in chronicle toxicity of MO. But I strongly suggested that Molybdenium has significant role in photosynthesis effect of chlorophyll.

I have a doubt, why should we use any chelators for Phytoremediation? The success of a phytoremediation is on how much the singular plant is able to remove the toxin from soil, isn't it? Algae and mosses and some higher plants have their natural mechanism to absorb and then detoxify heavy metals mainly using phytochelatins. If we use any chelators, then they will bind to other nutrient ions, won't they? So the plant will have nutrient deficiency. This will result in the lowering of phytoremediation efficiency.

No dear, you r nt rite, the plants are able to accumulate the metals , and bind also , in this situtation plants also accumulate the toxic metal ,however the plants will be damaged, so using the chaliting agent and bind the toxic metal ,as you chemist you know very well when the bind the any metal to any inorganic compound then the total chemical property will be change. And then this compound will be dumped in vacule or using in positive way by the plants ok

I've just seen some articles that shows that certain plants are tolerant of toxic metals like penny-cress, arabdopsis and certain species of Asteraceae,Brassicaceae,

Caryophyllaceae, Cyperaceae, Cunouniaceae, Fabaceae, Flacourtiaceae,

Lamiaceae, Poaceae, Violaceae, and Euphobiaceae.. they do not require any addition of chelators for removal of metals. By the way while phytoremediation, after a point any way the plant will die. The plant can be just removed away to a safer containment, isn't it? When we add some chelator to enhance phytoremediation, isn't it increasing the cost? Also the chelators may bind to essential nutrients, won't they?

in my view, phytoremediation is best used to stabilise the ground and avoid dust spreading of contaminants: that is what remediation means, it does not mean purification.

Further, molybdate is not a ligand or chelator, but a polyoxometallate (like vanadate, permanganate, even aluminate or silicate, etc.)

EDTA is extremely toxic (e.g. Indian mustard - Brassica juncea) and can result in crop death. It also transfers easily to groundwaters and may remain in the soil for following crops. I would also suggest biodegradable chelating agents, although you need to supply them regularly. Anyway, phytoextraction of Pb and CrIII seems a useless pathway for phytoextraction. It really works when the bioavailable fraction is at a minimum for plant uptake.

Thank you all

Mr. Rohit Mishra in fact the question is about the usage of chelating agents. I must suggest you to ask your seniors Dr. Nandita Singh, Dr. R. D. Tripathi, Dr. Sarita Sinha about the same. In my opinion chelators enhances the availability of metal cations in soil, Plants can easily uptake such available metal. However; I agree with some of the researchers commented here about the amount of EDTA and amm. Molybdate.

And One more thing Mr. Hari Haran commented that Mo has something to do with photosynthesis but I have a little different view, Mo is required for nitrogen metabolism in plants.

Exactly EDTA can chelate heavy metals from plants and soils which were hardly or loosely bounded.

Hello. You can use EDTA, but EDDS is preferable as less toxic compound. Regards to all.

I would also like to join this useful discussion on the role of chelant and heavy metals. chealtor can enhance the avaiablity of heavy metal in soil in same time it is also taken by the plant. Number of Aminocarobxylic acid (AMC) are using like EDTA, NTA, CA etc theses all are toxic for the plant and have non degradable in nature of very slow. However, EDDS are having good capacity to bind with metal and also non toxic to plant and biodegrable in nature.

Much of the phytoremediation community now recognizes that the collateral effects of chelate addition (solubility and leaching of metals, impact on soil microbial community, etc), EDTA in particular, outweigh the potential benefit to induced phytoextraction.

So, folks, if you want phytoremediaiton, get ready to wait about 50 to 60 years for pollution removal (what you do with the collected plants is your problem). Now, if you want to accelerate with chelators, you start doing chemistry, but adding chemicals to the soil and disturbing the ecosystem, so you might as well simply add a phosphate source that with react with all multivalent metals to form insoluble, inert metal phospahtes or incorporate them in an apatite structure (solid solution of calcium and other metal phosphate), or just add fish bones or other animal phosphates, and try and see if the plants manage to extract the immobilized metals. As you know dinosaur bones are still around, and for a long time!

Of course, HM immobilisation by phosphates is an alternative solution. But I would not depreciate phytoremediation - it was already commercially used in USA, Canada, Italy, Poland... The method depends on contamination scale and the kind of pollutant.

Fish bones were used in real case funded by USA EPA in a polluted site (reported by Wright and Conca, sorry the exact ref is not in my mind), so it would be interesting to compare the phosphate treatment with any commercial use of phytoremediaiton (where, when what happened and how much it cost?). of course, nature will always have the last word (and cover the earth with plants as it does over archeological sites), but I am not convinced that toxic pollution is removed faster by nature than by industrial chemistry which usually caused the problem in the first place by some rapid spread of wastes.....it is all a question of timing.

That is true, phytoextraction is more cost-consuming, but phosphate treatment caanot be applied in any place, as it changes pH and soil sorption. Both have pros and cons, and the point is to find a proper way, especially for the scientist ;).

OK, the original question was about molybdate and or EDTA......but I like phosphate because it is a more natural product, let me explain:

we added phosphoric acid to sediments, and found that in small amounts (1% is sufficient to stabilize metals), et reacts with calcium carbonate (always present in most soils) to make hydroxyapatite, a neutral compound as is fish bones), and no impact on final pH (loss of carbonic acid explains the neutralization). I have not seen any cons on this type of treatment yet, contrary to phytoremediation which does not remediate but at best stabilizes the soils against erosion, and slowly too!

The only pro thing would be public acceptance of "biological" treatments.

The treatment depends on our requirements. If we want just to locate a house, HM immobilization by phosphate is enough. But if we want to grow plant, e.g. ornamental (I cannot imagine growing crops on spiked soils), such procedure is not enough, as mechanical agitation leads to soil oxygenation and alters microbiological processes which may ruin the previous efforts.

Dear readers: This pationate debate is worth pursuing, and I will explain why: if nothing is done about pollution, our planet will become poisonous to many (even polar bears!), and I would like to show Groenland as an example. You may find pictures on internet, but can start by looking at this publication

National Environmental Research Institute

University of Aarhus, Denmark, NERI Research Note No. 241, 2008

Environmental Impact of the Lead-Zinc Mine at Mestersvig, East Greenland

see: http://www2.dmu.dk/pub/ar241.pdf

It shows how lead and zinc poisoned fish, plants, seeweeds, etc, even the sand on the beaches, for a long time!! Perhaps now someone will also find gold and upset the earth again? In my view, it also shows that phytoremediation does not work (in cold climate, OK.), but natural slow dilution and spread does occur.

I plead against phytoremediation because it means avoiding a real problem, that is not solved by plants, but can be solved by proper chemistry (I mean incorporating metals in hydroxyapatites for immobilization). The result is as longlasting as dinosaur bones are. Even though I am aware of the phytochelating agents released by plants.

In some of our phosphate remediation trials we heated the treated sediments to 600°C to burn off organic pollutants and cristallize the apatites to make them more inert. It is hard to find a chemical reaction simpler than that, even though mixing and heating does cost money, but it does decrease bioavailable fractions of metals. But of course one may just sit on the problem (build a parking lot) and let the metals infiltrate the water tables. regulations should require miners to return the land to a stable condition after use, and take care of the mine tailings, which will only increase in quantities as we push forward towards lower grade ores for more metal for cars or other things we consume. We used a polluted sediment treated with phosphates to construct a road base, and got drinking quality water seeping out from under. Can anyone show this using phytoremediation? I have seen nature slowly allow plants to grow on asbestos tailing in Quebec, or on industrial tailings in Belgium (near Charleroi), but this does not prevent polluting the local river sediments. However, many people still insist that phytoremediation is better! cheaper to implement (let the grass grow) perhaps, and better for the eyes, yes, but what about the invisble metals poisoining the environment?

Jon,

I do not think that we should count peas (Patrick mentioning asbestos).

All you have made your points.

Both for immobilization and phytoremediation (remediation in general), it has to be ensured that the contaminants stay in place at least, i.e. do not spread through air, soil and water. Immobilization mostly goes with a substantial reduction in soil bioactivity, i.e. release might mostly slow enough to be close to environmental background levels. Managed phytoremediation in general is more than growing grass on top soil, however, usually prevents contaminant spreading.

At the end, economics considering long term effects will determine which is the most appropriate approach to a contamination in place.

Thank you all for participating in this discussion

EDTA is the good chelating agent in soil environment

There was no answer because the question shows ignorance of what a chelating agent is: it is an organic molecule with donor groups (like amino or carboxylate functions just like in edta) that complex metal ions, that is, wrap around them and changes their properties (solubilities or color for example), whereas ammonium molybdate is only a salt of a polyoxometallate (like permanganate or vanadate anions), so cannot be considered a chelating agent.

to try to answer the question properly, I would say that the chelating strength depends on which metal is concerned, the local pH, and the type of agent. EDTA is among the strongest chelating molecules for most metals,a s measured by the formation constants of edta metal chelates (widely available in the litterature)

To come back to the initial phytoremediation story, let me point out another fact (I will no longer give pictures like dinosaur bones, because despite their familiarity so that readers may get the message, the extrapolation is a bit hazardous indeed): it is about lead. Lead ions are toxic, particularly the Pb2+ free ions, such as those released from acid batteries in cars. However, phosphate bound lead ions (pyromorphite) are amongst the least soluble lead salts known, and most inert ones too. That is, unlike sulphides, which can and do get oxidized in air and turn into more soluble sulfates, the phosphates are stable in aerobic and anaerobic coonditions.

So, I still claim phosphate immobilization is the best option for treating polluted soils with metals, and for a long time period, perhaps not exactly geologic times (who knows what will our earth be doing in another era?), but at least for as long as humans are on the surface.

On the other hand, phyto remediation techologies estimate at about 50 years the time needed to extract a significant amount of toxic metals from contaminated soils. And even then, you burn the plants, recover the ashes (containing the metals) and dispose them in landfills, and the polution continues, unless you fire the metals in glasses formed in stable melts (and I do not know of any that do not lixiviate on the long run, except phosphate ones!)

However, I agree phosphates are valuable minerals needed for crop growth and it is a pity to waste them, it would be better not to extract ores, not to make cars, not to make babies, and let the earth grow just animals and plants that survive well.

There are basic factors that determines the chelating strength as rightly stated by Patrick Sharrock. Soil pH and organic matter content plays a key role in determining the chelating strength. Hence more emphasis should not only be on the AGENT, but the soil properties.