Effect of adsorption on the biodegradation of nitrilotriacetate by Chelatobacter heintzii

Nitrilotriacetic acid (NTA) is a synthetic chelating agent that was used to decontaminate nuclear reactors and was disposed at Department of Energy waste sites. NTA may influence the mobility and fate of radionuclides in soils and sediments if it is not biodegraded. Because it was unclear how adsorption of NTA would influence its degradation, experiments were conducted in a model system containing gibbsite and Chelatobacter heintzii with (60)CoNTA at pH 7 and NTA at pH 6 and 8. The rates of NTA desorption from gibbsite were pH dependent (desorption half-lives at pH 6, 7, and 8 were 80, 16, and 1 h, respectively, while that of Co at pH 7 was 2.5 h for equal molar CoNTA). Degradation rates of NTA in solution by C. heintzii decreased as pH decreased and depended on the dominant form of NTA in solution (e.g., biodegradation rates for HNTA(2-) > AIOHNTA(-) CoNTA(-)). The degradation of NTA was significantly slower when NTA was adsorbed to gibbsite. This difference was observed at pH 6 and 8 with NTA and at pH 7 with NTA and equal molar cobalt. A coupled process model successfully simulated NTA desorption and degradation at all pHs. Both experimental data and simulations demonstrated that adsorbed NTA was unavailable for biodegradation and that the rate of desorption limited the rate of biodegradation. Only a fraction of the total Co-60 (i.e. 4%) was associated with C, heintzii cells after NTA had been degraded in the presence of gibbsite. Therefore, the biodegradation of NTA by microorganisms similar to C. heintzii should not significantly alter Co-60 sorption to aluminum oxides or Co-60 transport where NTA and Co-60 have been co-disposed.