Ethylenediaminetetraacetate (EDTA), an industrially important chelating agent, forms very stable complexes with di- and trivalent metal ions, and in both wastewater and natural waters it is normally present in the metal-associated form. Therefore, the influence of EDTA speciation on its utilization by the EDTA-degrading bacterial strain OSM 9103 was investigated. EDTA-grown cells harvested from the exponential phase of a batch culture were incubated with 1 mM of various EDTA species and the EDTA concentration in the assay was monitored as a function of time. Uncomplexed EDTA as well as complexes with low stability constants (MgEDTA{sup}(2-), CaEDTA{sup}(2-), and MnEDTA{sup}(2-), stability constant < 10{sup}16) were found to be readily degraded to completion at a constant rate. For more stable EDTA chelates (i.e. chelates of Co{sup}(2+), Cu{sup}(2+), Zn{sup}(2+), and Pb{sup}(2+)) the data suggest that these complexes were not used directly by the cells but that they had to dissociate priorto degradation. The rate of this dissociation step possibly determined the microbial degradation of these complexes. CdEDTA{sup}(2-) and Fe(lll)EDTA{sup}- were not degraded within 48 h. In case of CdEDTA{sup}(2-) the toxicity of freed Cd{sup}(2+) ions most likely prevented a significant degradation of the complex, whereas in case of Fe(lll)EDTA{sup}- a combination of metal or complex toxicity and the very slow dissociation of the complex might explain the absence of degradation.
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