Biodegradability of ethylenediamine-based complexing agents and related compounds.

The biological degradability (Zahn-Wellens test) of ethylenediamine derivatives with carboxymethyl and 2-hydroxyethyl groups was investigated. Mixed bacterial culture (activated sludge) was used as inoculum (non-adapted sludge and sludge adapted at different mean biomass retention time, the so-called sludge age). Biodegradability of ethylene(propylene)di(tri)amine-based complexing agents depends on the character and number of substituents and nitrogen atoms in the molecule. Tetra(penta)substituted derivatives with two or more tertiary nitrogen atoms and carboxymethyl or 2-hydroxyethyl groups in the molecule (EDTA, DTPA, PDTA, HEDTA) are very stable from an environmental point of view. On the contrary, disubstituted derivatives with two secondary nitrogen atoms in the molecule (e.g., EDDA) are potentially degradable.

Biodegradability of ethylenediamine-based complexing agents.

Biological degradability of ethylenediamine derivatives depends on the type and number of substituents. The susceptibility to biodegradation decreases in the sequence of substituents -COCH3, -CH3, -C2H5, -CH2CH2OH, -CH2COOH and with polysubstitution. The biodegradability depends also on the kind and number of nitrogen atoms. Complexing agents with a single-nitrogen atom in the molecule (e.g. NTA) succumb relatively readily to biodegradation whereas, compounds with two or more tertiary amino groups are biologically highly stable and do not undergo biodegradation even in experiments with activated sludge adapted at an age of up to 30 days (EDTA, DTPA, PDTA, HEDTA). A lowering of the degree of substitution brings about an increased susceptibility to biodegradation. This holds, e.g., for replacement of tertiary amino groups with secondary ones; thus the symmetrically disubstituted ethylenediamine-N,N'-diacetic acid (EDDA) possesses still sufficient complexing ability while belonging already to the group of potentially degradable substances.

Amount concentrations in aquatic chemistry.

Usually, two main terms for concentration are used: amount concentration and mass concentration. Mass concentrations can significantly distort the actual presentation of inorganic and organic compounds in the mixture. They cannot be used when comparing quantity and properties of different compounds, when explaining reaction kinetics, reaction mechanisms and when investigating relationships between concentration and chemical or biological activity. In these cases it is necessary to prefer amount concentrations. "Equivalent concentration" is based on the charge balance of ions in the solution and not on their actual number. The dimension of the equivalent concentration is the same as that of amount concentration (e.g., mmol l-1) which concerns also ANC or BNC (alkalinity or acidity).