Sorption Of Ironchelates On Soil Surfaces

Since adsorption-desorption reactions tend to be faster than precipitation-dissolution processes, adsorption at solid interfaces can be a dominant factor in regulating micronutrient concentrations in solution (Harter, 1991; Stone, 1977). Adsorption or "fixation" of chelating agents in soils was recognized in early investigations of the agricultural uses of synthetic chelating agents (Wallace and Lunt, 1956; Hill-Cottingham and Lloyd-Jones 1957; Aboulroos et al., 1983; Norvell, 1991). The effectiveness of Fe-chelates as Fe sources and carriers in soil can be severely limited by the adsorption of Fe-chelates or chelating agents in the solid phase (Sanchez-Andreu et al., 1991). The factors affecting adsorption include the type of chelating agent, the metal ion, time, pH, salt concentrations and soil texture (Aboulroos et al., 1983). Wallace et al. (1955) added various chelates, including Fe(III)-EDDHA, to soils, and found that after 4 days chelating agent losses in a calcareous loam soil ranged from 7 to 30%, whereas losses in a moderately acidic clay soil ranged from 0 to 51%. Adsorption, rather than degradation, was identified as the cause of the losses. Wallace and Lunt (1956) proposed the clay size fraction of soils as the major adsorbent of chelating agents. As it would be expected, the negatively charged surfaces of clay minerals are not effective in adsorbing the predominately anionic chelate species (Hemwall, 1958), and positively charged sites on Fe oxides and other colloids could be more important. Adsorption of chelating agents by peat was also observed, and losses of o,o-EDDHA/Fe3+ can be relatively large at low pH values (Hemwall, 1958). Negatively charged chelates adsorption by surfaces with pH-dependent charge, such as oxides or peat, decreases with rising pH (Norvell and Lindsay, 1972).

Adsorption of chelates by soil oxides may be similar to what occurs with specific Fe oxides (Norvell, 1991). Adsorption of several anionic ligands by hematite (Fe oxide) showed a degree of specific chemisorption in the binding of these anionic ligands in opposition to the net negative surface charge (Chang et al., 1983).

Figure 5-6. Sorption isotherms (T = 25°C) for the retention of o,o- EDDHA/Fe3+ and EDDHMA/Fe3+ in ferrihydrite at pH around 6.8 (after Hernández-Apaolaza and Lucena, 2001). Cs: Concentration retained; Ce: Concentration in the solution. Lines are the Langmuir fits.

Figure 5-6. Sorption isotherms (T = 25°C) for the retention of o,o- EDDHA/Fe3+ and EDDHMA/Fe3+ in ferrihydrite at pH around 6.8 (after Hernández-Apaolaza and Lucena, 2001). Cs: Concentration retained; Ce: Concentration in the solution. Lines are the Langmuir fits.

In a recent study (Hernández-Apaolaza and Lucena, 2001), we investigated the sorption of the different diastereoisomers of o,o- EDDHA/Fe3+ and EDDHMA/Fe3+ on a clay, an Fe oxide and an organic material. We concluded that the sorption on Ca-montmorillonite was probably through a Ca bridge, by the formation of an outer sphere complex montmorillonite-Ca-Fe-chelate, since Fe-chelates are negatively charged. This hypothesis was supported by the fact that both meso and racemic isomers behaved similarly, the proposed mechanism sorption depending on the net charge more than on the internal arrangement of the molecules. o,o- EDDHA/Fe3+ showed a higher sorption than EDDHMA/Fe3+ by the Fe oxide ferrihydrite (see isotherms in Figure 5-6). The meso-o,o-EDDHA/Fe3+ isomer was highly adsorbed on this oxide, but the racemic isomer was not significantly retained by ferrihydrite. In the case of EDDHMA/Fe3+ isomers, the racemic one was more retained by the oxide, but also a small degree of sorption was observed for the meso isomer. We suggested that in the sorption on ferrihydrite the o,o- EDDHA/Fe3+ and EDDHMA/Fe3+ lose some of their links with the Fe3+ ion, probably the ones with the carboxyl groups which are less stable, and then formed an electrostatic binding with the ferrihydrite surface.

In this model, the stability of the chelates is an important factor to understand the sorption process. The EDDHMA/Fe3+ chelate shows a higher stability than the o,o-EDDHA/Fe3+ (see Figure 5-4). This probably occurs because it is more difficult for EDDHMA/Fe3+ than for o,o-EDDHA/Fe3+to open its spatial arrangement, the former chelate being therefore less adsorbed at the surface. For the o,o-EDDHA/Fe3+ isomers, the meso is more retained than the racemic one, probably due to its lower stability. For the EDDHMA/Fe3+ isomers, it was also the less stable one (racemic) the more retained. This model would also explain the pH dependence of the sorption on ferrihydrite. Between pH 4 and 8 (below the isoelectric point), both chelates were largely retained by Fe oxide, but over pH 8 the sorption decreased drastically, because the oxide became negatively charged. When EDDHSA/Fe3+ was left to react with a synthetic ferrihydrite, the amount of chelate remaining in solution was 98%, whereas for o,o-EDDHA/Fe3+ it was only 66%. EDDHSA/Fe3+ has three negative charges, indicating that the sorption is not merely electrostatic. This chelate presents a high stability (see Figure 5-4), supporting that the rupture of the link between Fe and the carboxyl groups is a step previous to the sorption.

In the same study it was found that o,o-EDDHA/Fe3+ retention in peat was also larger than that of EDDHMA/Fe3+, in good agreement with previous findings (Alvarez-Fernandez et al., 1997). The most retained isomer of o,o-EDDHA/Fe3+ on the acid peat surface was the meso isomer, possibly due to its lower stability constant with respect to the racemic one. The mechanism of sorption of o,o-EDDHA/Fe3+ on peat surfaces is not known, but the higher retention of the meso- o,o- EDDHA/Fe3+ could suggest an electrostatic process, where Fe3+ can be the binding element. Alvarez-Fernandez et al. (2002b) found that retention of EDDHSA/Fe3+ and EDDCHA/Fe3+ was negligible in organic materials.

Despite the advances in the knowledge of the sorption processes in soil materials, the adsorption of metal chelates by soils still remains poorly understood. Full understanding of these reactions will always be difficult, because of the complexity of soil surfaces, the continuing changes in chelate speciation, and the concomitant degradation of the chelating agent. Most of the knowledge has been obtained by batch interaction experiments, where global processes are considered. In general, the most stable Fe-chelates are able to maintain 90-100% of the Fe-chelate in solution after short periods of interaction (Alvarez-Fernandez et al., 2002b). In fact, EDDHSA/Fe3+ and

EDDCHA/Fe were generally as efficient as o,o- EDDHA/Fe3+ and EDDHMA/Fe3+ (the most common and effective Fe fertilisers) to maintain Fe in soil solution for short periods. However, the type of chelating agent is a factor that affects the chelated Fe availability in soil solution, since EDDHMA/Fe3+ and o,o-EDDHA/Fe react with oxides and organic matter more extensively than EDDCHA/Fe3+ and EDDHSA/Fe3+. Alvarez-Fernandez et al. (2002b) also found that the non-chelated Fe present in commercial Fe chelates was stable in solution at the typical pH of calcareous soils, but still exhibited an elevated reactivity in soils, mainly with some components such as oxides and organic matter. Recently, we have studied the interaction of o,p-EDDHA/Fe3+ with calcareous soils (Garcia-Marco et al., 2005) and we observed (Figure 5-7) that higher amounts of this chelate were maintained in the soil solution as compared with EDTA/Fe3+, but the amounts were much lower than those of o,o- EDDHA/Fe3+. These results are similar to the predicted by theoretical calculations (see Figure 5-4) but in this case we observed an even larger decrease of o,p- EDDHA/Fe3+, possibly due to the sorption of the neutral species Fe-Ho,pEDDHA0 of this chelate.

S 20

Soil 1

Soil 2

Soil 3

Figure 5-7. Percentage of Fe-chelate remaining in solution after three days of interaction with three calcareous soils.

Recently, García-Mina et al. (2003) observed that in a calcareous soil treated with different commercial Fe chelates containing o,o-EDDHA, approximately 25-30% of the soluble Fe found at 50 days was not attributable to o,o-EDDHA/Fe3+. They did not determine o,p-EDDHA/Fe3+, but suggested the existence of other isomers of EDDHA besides the o,o isomer, which could also be available for plant uptake. Similarly, for EDDHSA/Fe3+ commercial chelates the amount of soluble Fe not attributable to the monomer EDDHSA/Fe3+ was 36-40% of the total Fe, which is in good agreement with the presence of soluble, stable Fe-chelates, possibly consisting in condensation products formed in the industrial synthesis. With respect to the presence of the Fe-chelates in the soluble fraction, they found that both EDTA/Fe3+ and DTPA/Fe3+ were easily lost from the soil solution, likely due to Ca competition (Figure 5-8). Among the stronger Fe-chelates, EDDHSA/Fe3+ and EDDCHA/Fe3+ (only the monomers were measured in that study) are those maintained in larger amounts in the soluble fraction, whereas EDDHMA/Fe3+ and o,o- EDDHA/Fe3+ slowly decreased with time.

Figure 5-8. Percentage of the Fe-chelate remaining in solution after interaction with a calcareous soil. (Data after Garcia-Mina et al, 2003).

time (days)

Figure 5-8. Percentage of the Fe-chelate remaining in solution after interaction with a calcareous soil. (Data after Garcia-Mina et al, 2003).

While Fe precipitation implies the destruction of the chelate, Fe-chelate sorption could be reversible. In fact, most of the proposed mechanisms are electrostatic. Therefore, sorbed Fe-chelates could serve as a reservoir, acting as slow release fertilizers and increasing the Fe presence in the soil solution in the long term. However, while sorption may provide a longer lasting effect of the Fe-chelates, it also may reduce the fast action normally needed by chlorotic crops.

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