2. CHELATES

The word chelate was first used by Sir Gilbert T. Morgan and H.D.K. Drew. The

word was coined from the Greek word Chela, meaning great claw (Morgan and Drew

1920). Chelates are coordination or complex compounds which consist of a

centralized metal atom, bound cyclically to a large chelating agent or molecule called

a Ligand. In other words, a chelate is a complex compound made up of a metal ion

and a chelating agent.

Figure: Formation of a Chelate from the reaction between a Chelating Agent and a

Metal

A chelating agent or ligand is a molecule or ion that is attached to a central metal

atom or ion by coordinate bonds with the ligand existing as a Lewis base and the

metal ion existing as a Lewis acid (Natarajan 2014). Ligands could be monodentate,

bidentate or polydentate. Monodentate ligands possess only one donor atom while

bidentate ligands possess two donor atoms. Polydentate ligands possess more than

two donor atoms and could be tridentate, tetradentate, pentadentate or hexadentate

depending on the number of donor atoms or ligand groups (Hueey 1993).

Ligands, which are anions, have an affinity for metallic cations thus making chelating

agents useful for trapping metal ions like Pb, Cu, Mg, Fe, Co, Ca and Mn and

releasing them slowly for plant uptake (Sekhon 2003). The extent of affinity of a

chelating agent for a metal is one of its important characteristics which describes its

stability relative to other chelating agents. In essence, chelating agents have the

propensity to disturb the natural speciation and influence the bioavailability of metals.

Chelating agents can be organic or synthetic in nature. The decomposition of organic

matter gives rise to natural chelating agents like lignosulphates, phenols, poly

flavonoids, siderophores, phytosiderophores, and ligninipolycarboxylates.etc.

Synthetic chelating agents include Ethylene diamine-tetra-acetic acid (EDTA),

Diethylene-triamine Penta-acetic acid (DTPA), Nitrilo-tri-acetic acid (NTA), Citric

acid, Ethylen-diamine-di (o-hydroxyphenylacetic acid) (EDDHA).

EDTA is the most common synthetic chelating agent (Agro Services International

2014) and is prevalent at high concentrations than any other anthropogenic organic

compound found in European surface waters (Nowack and VanBriesen 2005). This is

as a result of its widespread use in domestic, industrial and chemical processes and its

slow process of degradation under environmental conditions. DTPA is mostly used

for chelates administered on alkaline soils and is more effective than EDTA but more

expensive.

Organic acids like citric acids are important for the transportation of divalent atoms

but hardly chelate monovalent atoms. They aid the effective uptake of metals by

neutralizing the metal charge and remain the natural system through which plants

translocate and solubilize metals. However, they form weaker bonds with metals and

are more expensive to produce on a commercial scale as compared to other chelating

agents (Biogro 2014).

Hence, EDTA appears to be the most effective chelating agent for phytoremediation

with the disadvantage of not being biodegradable. NTA was the first chelating agent

to be synthesized in 1862 (Heintz 1862) with the industrial production of NTA and

EDTA commencing in 1936 and 1939 respectively in Germany. EDTA and Citric

acid was utilized for the bioavailability of metals in this work.

Morgan, G., and Drew, H.D.K. (1920) J. Chem Soc., 1920, 117Hueey, J. (1993) Inorganic Chemistry 4th ed., Harper Collins

Biogro (2014) Chelating Agents [online] available from http://biogro.com/resources/CFSM/BG

%20CFS%2003%20-%20Amino%20Minerals%20and%20Foliar%20Feeding.pdf [2nd September 2014]

Nowack B.and VanBriesen, J.M. (2005) Chelating Agents in the Environment [online] available from

http://www.empa.ch/plugin/template/empa/*/82536/---/l=1 [2nd September, 2014]

Sekhon, B.S. (2003) Chelates for Micronutrient Nutrition among Crops [online] available from

http://www.ias.ac.in/resonance/Volumes/08/07/0046-0053.pdf [2nd September, 20114]

Agro Services International (2014) Chelated Micronutrients [online] available from

http://www.agroservicesinternational.com/Articles/Chelates.pdf [2nd September, 2014]

Heintz, W. (1862) Ann. Chem. Pharm., I22, 257-294

Natarajan, S. (2014) Coordination Chemistry and Ligand Exchange Reactions in the Synthesis of New

Functional Inorganic Compounds [online] available from

http://fen.nsu.ru/posob/htt/xtt_natarajan_mof.pdf [2nd September, 2014]