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Environmental fate & pathways

Adsorption / desorption

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Based on the results of the available studies investigating reactivity of EDDHA chelates with soils and soil components, low absorption potential can be concluded for o,o-Fe(Na)EDDHA. Commercial iron EDDHA chelates remained mostly in solution over time and the pH range 3-12. The average value of the chelates recovered after the interaction with the soils and soil materials was 91% for o,o-EDDHA/Fe3+, and 62% for the Fe chelated by the polysubstituted byproducts, respectively. The main constituent o,o-EDDHA/Fe3+ retained more by ferrihydrite and organic matter, whereby its meso isomer retained more than racemic isomers.

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Additional information

Adsorption of Fe(III)-EDDHA chelates to soil was determined in various studies dealing with investigation of efficiency of the chelates to supply iron to plants (Hernández-Apaolaza and Lucena, 2001). Differences in the concentration of soluble Fe and Fe(III)-chelate in soil solutions and the extent of Fe(III)-chelate adsorption onto soil surfaces after Fe-fertilisation, were found between synthetic Fe(III)-chelates (even among isomers) depending on soil type (Orera et al., 2009).

Among Fe(III)-o,o-EDDHA isomers, adsorption onto highly reactive soil materials was higher for the meso isomer than for the racemic isomer on ferrihydrite and organic matter; whereas, for adsorption onto Ca-montmorillonite, no differences were observed between isomers. The retention of both isomers of Fe-EDDHA on Ca-montmorillonite increased with the concentration of the chelate.

Adsorption behaviour of the chelates depends on pH: at low pH the retention of the chelates is low, while between pH 4 and pH 8, the chelate was largely retained by the iron oxide, but over the isoelectric point that corresponded to pH 8 for a synthetic ferrihydrite, the sorption drastically decreased because the oxide became negatively charged. The meso isomer is more strongly retained by the surface along the pH range tested. Data obtained for the sorption on ferrihydrite fitted adequately to the Langmuir equation. On peat, the most retained isomer of Fe-EDDHA on the acid peat surface is the meso isomer, possibly due to its lower stability constant in respect to the racemic one (Hernández-Apaolaza and Lucena, 2001).

In a study by López-Rayo et al. (2009), the racemic o,o-EDDHA/Fe3+ was less adsorbed than meso o,o-EDDHA/Fe3 +. The amount of chelated Fe was higher than 90% for o,o-EDDHA/Fe3 + for all of the soils and soil materials, only slightly affected by acid peat and ferrihydrite. As expected, peat and ferrihydrite react more extensively with meso-o,o-EDDHA/Fe3+. By the Fe dissolution study, a slight chelate decrease was observed with the time. The chelating agent EDDHA solubilized 51.7, 65.4 and 92.3 % Fe from ferrihydrite, maghemite and goethite, respectively. Besides, EDDHA is able to maintain the soluble Fe in solution during the entire experiment.

The authors concluded that sorption of the chelate was proportional to the concentration of the Fe-EDDHA solution added. The higher concentration of the chelates is in the soil, the little retention occurred. The adsorption of the chelates depends also on their stability constants and purity. More retention is expected for less pure commercial chelates. More stable chelating structures retain less than their weaker analogues (Hernández-Apaolaza et al., 2001; Nowak, 2002).“At high concentrations, chelating agents decrease metal adsorption by forming dissolved complexes but at low concentrations they increase metal adsorption onto mineral surfaces, because they themselves are adsorbed onto surfaces”(Nowack, 2002) Further factors that affect adsorption include time, pH, salt concentration and soil texture (Norwell, 1991, cited in Álvarez-Fernández et al., 2002).

Similar results have been obtained byÁlvarez-Fernández et al. (2002):“Among soil materials, ferrihydrite and acid peat retain both chelated and non-chelated Fe to the greatest extent. The type of chelating agent is a factor that affects chelated Fe availability in soil. FeEDDHA and FeEDDHMA were retained more by soil surfaces than FeEDDHSA”. The results obtained by Álavrez-Fernández et al. (2002) showed that EDDHA and its other structural analogues EDDHMA, and EDDHSA in solution remain fully associated with Fe from pH 4 to 9 despite competition from Ca. The reaction of Fe chelates in soils or in soil surfaces was studied by assessing the Fe that remains in solution. The losses of chelated Fe from the solution after interaction showed that among the solid phases tested only ferrihydrite and acid peat significantly affect the presence of chelated Fe in solution. With regard to total chelated Fe added, losses of total soluble iron by ferrihydrite were 6.9 % and 3.1 % for commercial samples of EDDHA and EDDHMA, respectively. Acid peat retained 7.2 % of commercial EDDHA and EDDHMA. Losses of non-chelated Fe by interaction with ferrihydrite were 70.3 % and 74.2 %, for EDDHA and EDDHMA, respectively. Losses of non-chelated Fe by interaction with acid peat were 63.6 % and 70.6 %, for EDDHA and EDDHMA, respectively. The values for EDDHSA were significantly lower. The retention of soluble non-chelated Fe of the studied chelates by soils was in the range of 15-56 %. A higher retention of the non-chelated soluble Fe is due to the bonds of Fe with -OH and -CH3 positional isomers which form also stable hexaccordinated complexes (Álavrez-Fernández et al., 2002)

Hernández-Apaolaza et al. (2006) studied the ability of polycondensation EDDHA-like products to solubilize iron after their interaction with different iron oxides and soils. Compared to o,o-EDDHA, o,p-EDDHA and EDTA polycondensation products solubilised negligible amounts of iron from all the solid phases. Regarding their retention onto soil materials, they were only slightly more retained than the o,o- and o,p-EDDHA complexes, except for ferrihydrite, in which a higher retention was obtained (Hernandez-Apaolaza et al., 2006). The average value of the chelates recovered after the interaction with the soils and soil materials was 91% for o,o-EDDHA/Fe3+, and 62% for the Fe chelated by the polysubstituted byproducts. The retention by calcareous soil presented a 93, and 26% recovery of chelated Fe for o,o-EDDHA/Fe3+, and Fe chelated by the polysubstituted byproducts, respectively. The most reactive material was the ferrihydrite that allowed a low recovery of the chelates (76, vs 0% for o,o-EDDHA/ Fe3+ and the Fe chelated by the polysubstituted byproducts, respectively). Regarding desorption ability, compared to o,p-EDDHA/Fe3+, o,o-EDDHA/Fe3+, condensation products solubilized almost negligible amounts of iron from all the solid phases. In conclusion, the condensation products do not have the long-lasting effect presented by o,o-EDDHA/ Fe3+ and present a less efficient fast-action effect than the o,p-EDDHA/Fe3+.


In conclusion, EDDHA/Fe3+ had low reactivity with soils and soil components.