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Environmental fate & pathways

Adsorption / desorption

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Description of key information

Experimental results, based on chelating properties (OECD 106): HBED showed a low soil reactivity and high stability.
KOCWIN v2.00: Koc: 67.13 L/kg (MCI method, without metal component)

Key value for chemical safety assessment

Koc at 20 °C:

Additional information

Lucena, López-Rayo and Hernández (2009) conducted with different focuses on chelating properties the test substance on the basis of OECD Guideline 106 / EU Method C.18.

In the first study report, beside HBED/Fe3+ also HJB/Fe3+, o,o-EDDHA/Fe3+ and o,p-EDDHA/Fe3+ were used in an interaction experiment with two different soils, lime, clay, Fe oxide and peat. Ca-Montmorillonite, commercial black peat, standard calcareous soil (SCS) and two soils from fruit trees orchards (Sudanell and Picassent) were used as matrixes. Each of them was allowed to interact with 12.5 mL of ferric chelate solutions in polyethylene flasks. HEPES was used as pH buffer and 0.01 M CaCl2 was added in order to adjust ionic strength and to induce a Ca concentration similar to that present in calcareous soils. Chelate blanks and matrix blanks were prepared as well. The flasks were covered with aluminium foil to avoid photodecomposition of the chelates. Shaking was performed for 1 h at 25 °C at 56 cycles/min. After 3 d in a water bath at 25 °C, the supernatant was filtered through 0.45-µm Millipore membranes. Total Fe and Cu were determined by HPLC measurement. The organic matter (peat) seems to affect the test substance, HBED chelate, much more than any other matrix used. However, in SCS, containing an appreciable amount of Fe, but possesses a higher pH value, the reactivity with HBED/Fe 3+ is scarce. There are better chelating agents available for Cu than HBED. With exception of the Sudanell soil, only a low Cu displacement was observed. As conclusion, HBED/Fe3+ possesses a low soil reactivity since only some interaction with peat has been observed. Displacement by other cations seems to be of low importance.

In the second part of the available study report, the interaction of HBED/Fe3+ with calcareous soil as a function of time was the main topic. In this case, beside HBED, also o,o-EDDHA and HJB were used. A calcareous soil (Picassent) was used as incubation substrate, possessing a sandy loam texture, low organic matter and normal Cu availability. For the isotopic exchange assay, another soil (Ricla) was employed. 5 mL of each chelate solution and 5 mL of water type I were shaken together with 2 g soil at 56 cycles/min in darkness. Samples were taken in duplicate at days 1, 3, 7, 14, 28 and 56 and filtered through a 0.45-µm Millipore membrane. Chelate blanks and soil blanks were also prepared. Total soluble Fe is designated as those Fe remaining in solution after the interaction with soil and was above 95 % after 8 weeks of interaction. Chelated Fe analysis was done in a chromographic system and a significant capacity was detected to maintain chelated Fe in solution over time. The chelates used were prepared from agents of high purity, therefore, it is assumed that the total Fe added at the beginning of the interaction was in chelated form and this should be remain over the whole sampling period. Fe chelated HBED represents 95 % of total Fe at Day 56. It may be possible that degradation products have been formed and are able to complex Fe but with different retention time in the HPLC. HBED/Fe3+ presents low reactivity with soils, soil components and a high stability. Furthermore, it is able to maintain a significant amount of Fe in solution when interacting with soil for a longer time.

The soil adsorption coefficient of the test substance was determined by the computer program KOCWIN v2.00 (EPIWIN software) by US-EPA (Chemservice S.A., 2012).This tool estimates the organic carbon-normalized sorption coefficient for soil (and also for sediment), which is designated as Koc. Two different models are used for this estimation: the Sabljic molecular connectivity (MCI) method as well as with the traditional method which is based on logPow. For the substance sodium [2,2'-(ethane-1,2-diylbis{[2-(hydroxy-kO)benzyl]imino-kN})diacetato-kO(4-)]ferrat(1-) (FeHBED) the traditional method gives a Koc of 0.02829 L/kg, whereby the MCI method reveals a value of 67.13 L/kg as result. For this prediction, the metal component (Na) has been removed to perform the estimation. However, the MCI method is taken more seriously into account due to the fact, that it includes improved correction factors. It is remarked, that the estimated Koc represents a best-fit to the majority of experimental values. Furthermore, the Koc value of this structure may be sensitive to the surrounding pH.

The substance is used in high pH soils only. In alkaline soils, most nutrients solve inferior. This leads to a precipitation of i.e. iron compounds, which results in a decreased adsorption of the iron (among other nutrients) and ultimately leads to deficiency symptoms in plants (e.g. chlorosis). For counteraction, iron can be absorbed by the plants via FeHBED and the chelating agent HBED remains around the soil solution around the Rhizosphere in order to chelate another native iron from soil. If HBED would be able to adsorb strongly to soil particles, the acting as iron donor would not be possible.

Further consideration:

In accordance with REACH, Annex IX, Section 9.3.3., column 2, no additional experiment on adsorption/desorption is intended, since the physico-chemical properties of the substance leads to the expectation that it possesses only a low potential for adsorption, which is confirmed by the already available results.