Codisoposal of anthropogenic chelating agents such as nitrilotriacetate (NTA) with radioactive and heavy metals can enhance environmental transport of the metals, extending subsurface contamination and threatening groundwater sources. The biodegradation of the chelating agent can lead to the immobilization of the chelated metal and radionuclide contaminants. The rate of biodegradation of the organic complexing agent may depend on the concentration of a specific, biologically available form of the chelate. In mixtures of metals and chelating agents, the relative distribution of different chemical forms of the chelate at equilibrium is controlled by the total concentrations of organic and inorganic constituents and thermodynamic stability constants for the aqueous complexes that form. In this paper, we evaluate experimental results for biodegradation of NTA by Chelatobacter heintzii in different metal/NTA systems in order to identify the chelate form controlling the rate of degradation. The CaNTA{sup}- is the only species that can control the rate of NTA degradation in our systems. Our analysis of the potentially rate-limiting reactions in the biodegradation of NTA indicates that kinetically controlled complexation in the NTA system is not affecting the biodegradation of the chelate. The rate of transport of CaNTA{sup}- into the cell appears to control the overall rate of NTA degradation. Thus, we expect enhanced rates of biological degradation of the chelate and immobilization of codisposed metals when CaNTA{sup}- is available to C. heintzii.
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