Dissolution Of Soil Native Iron By Chelating Agents

When the Fe-chelate releases Fe to the plant, the chelating agent may dissolve native Fe (Figure 5-1), which can be then transported to the plant (Chen and Barak, 1982; Lindsay, 1995; Lucena, 2003). However, the importance of this dissolution process in plant Fe nutrition is not well known. Schwertmann (1991) reviewed the knowledge on the processes of dissolution of Fe oxides, indicating that most of the studies concerning chelating agents have been done for the estimation of the plant-available Fe. The chelating agent may dissolve native Fe present in the solid phases. This is a kinetically controlled process that depends on the chelating agent (Stone, 1977), the nature of the solid phase (Schwertmann 1991; Perez-Sanz and Lucena 1995; Nowack and Sigg, 1997) and the soil conditions. The rate of dissolution may be the limiting factor for the whole process. It has been hypothesized that kinetics, rather than equilibrium, probably controls the speciation of low stability complexes (such as phytosiderophores; Stone, 1977). Perez-Sanz and Lucena (1995) studied the kinetic parameters for the Fe dissolution of different Fe oxides in presence of o,o-EDDHA. Results showed that amorphous Fe(OH)3 had a higher dissolution rate than goethite, maghemite and magnetite. Dissolution rate from amorphous Fe(OH)3 and goethite follows a first order kinetics, while from maghemite and magnetite the order of the reaction was 2. This implies that for the last two the dissolution rate increases with the amount of free o,o-EDDHA in the media, conversely to what happens for amorphous Fe(OH)3 and goethite. In the same study, the dissolution rate was compared with the Fe nutrition of sunflower grown in hydroponics in presence of both o,o- EDDHA/Fe3+ and different Fe oxides separated from the nutrient solution by a dialysis bag. Authors concluded that Fe uptake was higher when using the Fe oxides with higher dissolution rates. Recently, the dissolution rate of Fe from different soils and Fe oxides by EDTA, o,o-EDDHA and o,p-EDDHA has been compared (Garcia-Marco et al., 2005). The amount of Fe solubilized with time fitted well to the equation:

Fe(pmol / g) = —(1) t1/2 + t where Fe (^mol/g) is the amount of Fe solubilized by the chelating agent for a time t, Femax is the maximum amount of Fe that can be dissolved for that chelating agent and t1/2 (half time) is the time needed to dissolve half of the Femax. This last parameter is an index of the rate of dissolution, and it is presented for the materials studied in Table 5-1. The lower the value of t1/2, the faster the dissolution process. It can be observed that the dissolution of o,p-EDDHA is faster than that of o,o-EDDHA, and also that EDTA dissolves Fe from all the materials used. However, the maximum amount of Fe that can be dissolved was largest for o,o-EDDHA and very low for EDTA.

Table 5-1. Kinetic parameters for the dissolution process of Fe oxides and soils in 100 ^M chelating agents.

Maghemite

Goethite

Fe(OH)3amp

Soil-1

Soil-2

SCS

Half time (t1/2,

hours)

EDTA

10.7

472.1*

3.5

1.91

1.38

1.62

o,o-EDDHA

2.3

91.5

3.1

0.56

1.80

3.14

o,_p-EDDHA

1.3

6.1

0.5

0.22

0.22

0.13

Femax (% of the chelating capacity)

EDTA

12.3

4.4

1.1

4.1

8.4

2.8

o,o-EDDHA

79.5

101.5

29.4

18.2

47.9

40.0

o,_p-EDDHA

59.6

10.6

2.4

5.2

22.4

7.9

Soil-1 and Soil-2: calcareous agricultural soils. SCS: standard of calcareous soils. *Predicted from the experimental data.

Soil-1 and Soil-2: calcareous agricultural soils. SCS: standard of calcareous soils. *Predicted from the experimental data.

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