Slow complexation kinetics for ferric iron and EDTA complexes make EDTA non-biodegradable

Area of Science:

• Environmental chemistry
• Biogeochemistry
• Microbial degradation

Background:

• Ethylenediaminetetraacetic acid (EDTA) is a widely used chelating agent.
• EDTA biodegradation can be inhibited by the presence of ferric iron (Fe(III)).
• Previous studies observed a sudden cessation of EDTA biodegradation correlating with Fe(III) concentration.

Purpose of the Study:

• To investigate the hypothesis that slow dissociation kinetics of iron-EDTA complexes inhibit EDTA biodegradation.
• To evaluate the role of kinetically controlled complexation in EDTA's biological unavailability.

Main Methods:

• Incorporation of a new submodel for kinetically controlled complexation into the CCBATCH biogeochemical model.
• Simulation of EDTA biodegradation using the modified CCBATCH model with experimental data for Fe(III) concentrations.
• Comparison of model predictions with experimental observations of biodegradation cessation, residual EDTA, biomass, and ammonium (NH4+) concentrations.

Main Results:

• The CCBATCH model with kinetically controlled complexation accurately predicted the timing of EDTA biodegradation cessation.
• Simulations correctly reproduced the observed residual EDTA concentration, which equaled the dissolved Fe(III) concentration.
• The model successfully predicted biomass and NH4+ levels, outperforming equilibrium complexation and precipitation models.

Conclusions:

• Slow dissociation kinetics of Fe(III)-EDTA complexes sequester EDTA, rendering it biologically unavailable.
• Kinetically controlled complexation is the primary mechanism limiting EDTA biodegradation in the presence of ferric iron.
• This finding clarifies the fate of EDTA in iron-rich environments.