EDDS

Structure and properties

EDDS has two chiral centers, and as such three stereoisomers. These are the enantiomeric (R,R) and (S,S) isomers and the achiral meso (R,S) isomer. As a biodegradable replacement for EDTA, only the (S,S) stereoisomer is of interest. The (R,S) and (R,R) stereoisomers are less biodegradable, whereas the (S,S) stereoisomer has been shown to be very effectively biodegraded even in highly polluted soils.

Synthesis

EDDS was first synthesized from maleic acid and ethylenediamine. Some microorganisms have been manipulated for industrial-scale synthesis of (S,S)-EDDS from ethylenediamine and fumaric acid or maleic acid, which proceeds as follows:

From aspartic acid

(S,S)-EDDS is produced stereospecifically by the alkylation of an ethylenedibromide with L-aspartic acid. Racemic EDDS is produced by the reaction of ethylenediamine with fumaric acid or maleic acid.

Coordination chemistry

In comparing the effectiveness of (S,S)-EDDS versus EDTA as chelating agents for iron(III):

Because of the lower stability for [Fe(S,S)-EDDS]⁻, the useful range being roughly 3<pH_(S,S)-EDDS<9 and 2<pH_EDTA<11. However, this range is sufficient for most applications.

Another comparison that can be made between (S,S)-EDDS and EDTA is the structure of the chelated complex. EDTA’s six donor sites form five five-membered chelate rings around the metal ion, four NC₂OFe rings and one C₂N₂Fe ring. The C₂N₂Fe ring and two of NC₂OFe rings define a plane, and two NC₂OFe rings are perpendicular to the plane that contains the C₂-symmetry axis. The five-membered rings are slightly strained. EDDS’s six donor sites form both five- and six-membered chelate rings around the metal ion: two NC₂OFe rings, two NC₃OFe rings, and one C₂N₂Fe ring. Studies of the crystal structure of the Fe[(S,S)-EDDS]⁻ complex show that the two five-membered NC₃OFe rings project out of the plane of the complex, reducing the equatorial ring strain that exists in the Fe[EDTA]⁻ complex. The complex also has C₂ symmetry.

Uses

(S,S)-EDDS is a biodegradable chelating agent that offers an alternative to EDTA, of which 80 million kilograms are produced annually. Under natural conditions, EDTA has been found to convert to ethylenediaminetriacetic acid and then cyclize to the diketopiperazine, which accumulates in the environment as a persistent organic pollutant. When EDDS is applied in chemical-enhanced soil remediation in excessive case (e.g., when applied for ex-situ soil washing), higher extraction efficiency for heavy metals can be achieved and the amount of extraction is less independent with the EDDS dosage; On the other hand, during soil remediation which involves continuous flushing, metal extraction is often limited by the amount of EDDS. Under EDDS deficiency, initial unselective extraction of heavy metals was observed, followed by heavy metal exchange and re-adsorption of heavy metals that have lower stability constant with EDDS.