sorry: it reads:  "phytoremediation"

Hallo Ewald, sorry, I am in a hurry, will write you more later. Concerning the enhanced phytoremediation of heavy metals, it is not my field, but on this topic I saw earlier some papers by Domen Lestan from Slovenia, and his review in the book Phytoremediation and Rhizoremediation, Theoretical Background, published by Springer, Mackova M., Dowling D., Macek T., editors, also available as e-book. ISBN-10 1-4020-4952-8 or e.g. ISBN-13 978-1-4020-4999-4

There is a lot of experiments done in pots, but I am looking for true field experiments, which also investigate the fate of EDTA in the soil!

In 2002, we investigated speeding up the Transfer of various metals from a mine tailing site to lettuce by EDTA. Roughly, concentrations in the tissues increased, but yield decreased. It has been published in Poland.

Manfred Sager, Hyo Taek Chon, Soo Young Lee: Growth of lettuce (lactuca sativa) at mine tailings from Shiheung/ Korea, and the effect of added organic complexants on metal mobilities. Ecol. Chem. Eng. 17,  2010

I cannot follow the rationale: complexing heavy metals with EDTA or related chelators, whether in the Erlenmeyer or in the field, will prevent them from being taken up into plant root cells, due to the much higher molecular mass, dimension and sterical hindrance during transport if siderophore systems cannot deal with them, reduced charge, and probably some more factors..

Dear Ewald,

I can give a theoretical point of view. EDTA will increase the concentration of total metal in the soil solution but will decrease the fraction of the free ionic species, the one taken up by roots. Therefore, assuming the metal-complex is not taken up by the roots (only the free ion is absorbed) then the uptake will decrease. Increased uptake is often observed at high concentrations of EDTA which can then enter the roots with the metal complex but due to damages to the root tissues. Here is a paper we did that explains the story. Lin, Z., Schneider, A., Nguyen, C., Sterckeman, T., 2014. Can ligand addition to soil enhance Cd phytoextraction? A mechanistic model study. Environmental Science and Pollution Research 1–16. doi:10.1007/s11356-014-3218-8

To my knowledge, it remains uncertain, whether the metal complex is taken by the roots as such, or cation and EDTA enter at different sites, and recombines. If the complex is formed, transport in the xylem is facilitated. EDTA can interfere with Ca, Fe and other metabolisms also. Therefore we tried Citrate and Oxalate instead. As These organic acids can be used by soil microbes, the survival period in the soil was uncertain, and we had no means to measure this during our pot experiment. EDTA is not consumed, but may be hazardous to groundwater formation; there is a threshold about..

then if you use carboxylates for which the complex is not assumed to be taken up by roots, their addition to the soil is expected to decrease the concentration of the free ion and therefore the uptake as simulated in the paper I previously referred to unless there is a  pH decrease resulting in a change in the metal speciation. The complexes are generally not  enough labile to increase the uptake by roots compared to the buffer capacity of the soil. The complex may increase the uptake in soisl with a very low buffer capacity : Lin, Z., Schneider, A., Sterckeman, T., Nguyen, C., 2015. Ranking of mechanisms governing the phytoavailability of cadmium in agricultural soils using a mechanistic model. Plant and Soil 1–19. doi:10.1007/s11104-015-2663-6

very valuable information, dear colleagues!

Please remember that soil is a Substrate full of micro-organism, and never in Equilibrium. They consume the organic ligands at various extents, depending on Nitrogen and Oxygen supply, and humidity. Life is never in equilibrium; under Equilibrium conditions under Aerobic conditions, I would turn to CO2 and Ca- Phosphate. Therefore I do not trust any calculations based on stability constants, to simulate conditions in soils.

This was looked at in the US in the late 1990 's, where a lead-contaminated field was planted with Mustard, the crop grown to maturity and watered with EDTA. Soil lead levels were measured before and after treatment as well as plant lead content and plant dry matter.  Soil lead levels decreased after treatment.  This could not be accounted for by plant uptake and the difference was taken to be the chelated fraction that was leached into the lower soil horizons and eventually to ground water.  EDTA kills plant roots, removes selective barriers to ion uptake and means that treated plants act as sponge.  Similar results were found in field trials in Poland, where expensively-modified EDTA derivatives were used in conjunction with Sunflowers in a large scale field trial with similar disappointing results.  This is a technology promoted by a now-defunct company and should be thoroughly discredited. Far better to use 'soft' remediation strategies that minimise soil-plant transfers and protect groundwater.  In conclusion, don't bother.

I agree with Nicholas. EDTA induced leaching is most likely responsible for the accelerated heavy metal removal from upper soil horizons. This method is not acceptable under current US EPA guidance. 

Good question by Dr.Schnug,well discussed by all colleagues and finally well answered by Dr.Nicholas.

Thanks for all the constructive contributions, it helped a lot!

I am providing a link to an article prepared evaluating treatment technologies for Small Arms Firing Ranges. Phytoremediation is discussed beginning page 43 and EDTA chelation on page 51. Field studies are discussed on page 55.  Studies IV and V in particular used EDTA.

https://www.researchgate.net/publication/235204246_Treatment_and_Management_of_Closed_or_Inactive_Small_Arms_Firing_Ranges

Technical Report Treatment and Management of Closed or Inactive Small Arms Fi...