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

Published experimental data on ethylenediaminetetraacetic acid (EDTA) biodegradation in the presence of ferric iron (Fe(III)) showed that rapid biodegradation of EDTA suddenly stopped, leaving a residual of unbiodegraded EDTA that was equal to the concentration of dissolved Fe(III). We hypothesize that slow kinetics for the dissociation of two iron-EDTA complexes--FeEDTA(-) and FeOHEDTA(2-)--sequestered the EDTA in a form that is biologically unavailable. To evaluate this hypothesis, we added to the biogeochemical model CCBATCH a new submodel for kinetically controlled complexation. CCBATCH simulations with kinetically controlled complexation for FeEDTA(-) and FeOHEDTA(2-) and the observed concentration of total dissolved Fe(III) accurately predicted the sudden cessation of EDTA biodegradation at the exact time shown experimentally. Our simulations also correctly predicted the observed residual EDTA concentration and the amounts of biomass and NH4+. Alternate explanations for the experimental results--strong equilibrium complexation of ferric iron and EDTA and precipitation of calcium and magnesium solids--could not capture the observed trends. This analysis using CCBATCH's new sub-model for kinetically controlled complexation shows that EDTA, once it becomes complexed with Fe(III), becomes biologically unavailable.