Contribution of Cd-EDTA complexes to cadmium uptake by maize: a modelling approach

Chelant-enhanced phytoextraction has given variable and often unexplained experimental results. This work was carried out to better understand the mechanisms of Cd plant uptake in the presence of EDTA and to evaluate the contributions of Cd-EDTA complexes to the uptake. A 1-D mechanistic model was implemented, which described the free Cd2+ root absorption, the dissociation and the direct absorption of the Cd-EDTA complexes. It was used to explain Cd uptake by maize in hydroponics and in soil. In hydroponics, the addition of EDTA caused a decrease in Cd uptake by maize, particularly when the ratio of total EDTA ([EDTA] (T) ) to total Cd ([Cd] (T) ) was greater than 1. At [Cd] (T) = 1 mu M, when [EDTA] (T) /[Cd] (T) < 1, the model indicated that Cd uptake was predominantly due to the absorption of free Cd2+, whose pool was replenished by the dissociation of Cd-EDTA. When [EDTA] (T) /[Cd] (T) > 1, the low Cd uptake was mostly due to Cd-EDTA absorption. In soil spiked with 5 mg Cd kg(-1), Cd uptake was not affected by the various EDTA additions, because of the buffering capacity of the soil solid phase. Addition of EDTA to soil increases Cd solubility but dissociation of Cd-EDTA limits the availability of the free Cd2+ at the root surface, which finally reduces the plant uptake of the metal.