Chelates: Meaning, Stability and Implications

After reading this article you will learn about:- 1. Meaning of Chelates 2. Reaction of Organic Matter and Metal Ions 3. Stability of Chelates 4. Practical Implications in Chelation Reactions in Soils.

Meaning of Chelates:

The term “Chelate” is derived from a Greek word which means as a type of chemical compound in which a metallic ion is firmly combined with a molecule by means of multiple chemical bonds. The chelates have a marked tendency to hold tightly certain cations attracted to them.

The chelates may be formed either naturally between organic matter and metal ions (natural chelates) or artificially between different chelating agents [Diethylene thiamine penta acetic acid (DTPA), EDTA (Ethylene diamine tetra acetic acid) etc.] and metal ions (Synthetic chelates).

A chelate is an organic compound which combines with and protects certain metallic cations such as Fe, Mn, Cu and Zn etc. The cation-chelate combinations are complex ring structures and the metals so bound essentially lose their usual ionic characteristics (Fig. 20.19). Chelates are sometimes called siderophores.

The chelated metals remain in soil solution at much higher soil pH than do the inorganic ionic forms. The protected cations are not generally subject to precipitation as insoluble hydroxides. Specific example for the protective action of chelating compounds is shown below:

If an inorganic iron salts are applied to calcareous soils, the iron precipitates and becomes unavailable for the growth of the plant.

If ferric sulphate [Fe₂(SO₄)₃] is applied, the reaction occurs as follows (assuming the hydroxyl ions are sufficiently present):

If instead of ferric sulphate, an iron chelate is used, the ferric ion (Fe³⁺) would be protected from this precipitation. Although the chelated metals are protected against various soil reactions, the nutrients remain in these combinations are considered available to plants.

The rate of absorption of chelating agents associated with metallic cations is relatively higher than that of the chelating agents without the association of cations. A simple mechanism showing the mobility of metal ions by chelates from soil to plant root surfaces is depicted diagrammatically (Fig. 20.20).

The metal-chelate complex diffuses to the root cell where metal ions are absorbed leaving the chelating agent to re-diffuse and mobilize a further metal ions from the solid phases of soil.

Reaction of Organic Matter and Metal Ions:

The chemistry of heavy metals in soil is related to the formation of complexes with organic matter whereas monovalent cations (Na⁺, K⁺, etc.) are held mainly by simple cation exchange through the formation of salts with —COOH groups (RCOONa, RCOOK), multivalent cations (Cu²⁺, Zn²⁺, Mn²⁺ and others) have the potential for forming co-ordinate linkages with organic molecules.

With regard to complex formation and plant nutrition, the metals can be placed into the following three groups:

(i) Those which are essential to plants but which are not bound in co-ordinate, compounds like all monovalent cations, and divalent cations Ca²⁺ and Mg^:+.

(ii) Those metal which are essential to plants and which form co-ordinate linkages with organic ligands. They include nearly all of the metals in the first transition series, including Cu²⁺, Zn²⁺, Mn²⁺ and Co²⁺ as well as Mo of the second transition series.

(iii) Those without known function in plants but which accumulate in the environment e.g. Cd²⁺, Pb²⁺, Hg²⁺, chromium, gold, vanadium and uranium.

Reaction mechanism is shown below:

Stability of Chelates:

Chelates may be applied to the soil or as a spray to the foliage.

When Zn-chelate is applied to soils containing high amounts of Fe²⁺, the following reaction occurs:

It is obvious that the applied metal-chelate combination must be stable within the soil if it is to have any persistency. Therefore, Fe-chelate is not so effective in plant nutrition because of its relatively higher stability, whereas Zn, Mn and Cu chelates are effective in plant nutrition.

However, the stability of a metal-chelate complex is determined by a variety of factors like number of atoms forming a bond with the metal ion, number of rings that are formed, the nature and concentration of metal ions and pH.

Because of the possibility of reaction of Cu²⁺, Zn²⁺ and Mn²⁺ with Fe²⁺ in the soil, Cu, Zn and Mn-chelates can be applied as spray and localised (or banded) soil placement. The stability sequence for some selective divalent cations is as follows:

Cu²⁺> Ni²⁺> Co²⁺> Zn²⁺> Fe²⁺> Mn²⁺

Practical Implications in Chelation Reactions in Soils:

The formation of metal-organic complexes would have the following implications in soils:

1. Metal ions that would ordinarily convert to insoluble precipitates at the pH values found in most agricultural soils would be maintained in solution. It is known that many biochemicals synthesized by micro-organisms such as amino acids, simple aliphatic acids etc. form soluble complexes with metal ions.

2. Organic complexing agents may influence the availability of many trace elements like Fe, Mn, Cu, and Zn etc. to higher plants as well as to soil micro-and macro-faunal organisms.

3. Under certain conditions the concentration of a metal ion may be reduced to a level of non-toxic through such complexation. This would be particularly true to the metal organic complexes of low solubility e.g. complexes between metal ions and humic acid, and other high molecular weight components of the organic matter.

4. Chelation plays a key role in the chemical weathering of rocks and minerals. Lichens as well as bacteria and fungi bring about the disintegration of rock surfaces to which they are attacked through production of organic chelating agents.

5. Natural complexing agents are of tremendous importance in the transportation and concentration of metals in a variety of commercially important biogenic deposits such as peat and coal.

It has been found that the toxicity of Cu arises in the peat to a series of events such as, uptake from the surrounding mineral soil by plant roots, translocation to leaves, incorporation into humus of the forest floor following leaf-fall and migration to the swamp in seepage waters as soluble organic complexes.

6. Complexing agents or chelating compounds of various kinds acts as carriers of heavy metals in soils.

7. The interaction of Al³⁺ with organic matter may be of considerable importance in controlling soil solution levels of Al³⁺ in acid soils.

Some important soluble metal-chelates are shown below:

Common chemical names of some chelating agents: