Degradation of Metal−Nitrilotriacetate Complexes by <i>Chelatobacter heintzii</i>

Abstract

Nitrilotriacetic acid (NTA) is a synthetic chelating agent that can form strong water-soluble complexes with a wide range of radionuclide and metal ions and has been used to decontaminate nuclear reactors and in the processing of nuclear materials. The co-disposal of NTA or other synthetic chelating agents with radionuclides may result in increased dispersal of radionuclides in soil and subsurface environments. Understanding the influence of aqueous geochemistry on NTA degradation is essential to predict the mobility and fate of inorganic contaminant−NTA complexes in the subsurface. Chelatobacter heintzii (ATCC 29600) was shown to degrade 14C-labeled NTA to 14CO2 with first-order kinetics at concentrations ranging from 0.05 to 5.23 μM (0.01−1 μg of NTA mL-1). The degradation of various metal−NTA complexes was investigated under conditions in which the NTA was predominantly present as the metal−NTA complex. The order for the rates of degradation was HNTA2- > CoNTA- = FeOHNTA- = ZnNTA- > AlOHNTA- > CuNTA- > NiNTA-, which is not related to the order of metal−NTA stability constants. The metal concentration used was not inhibitory to glucose mineralization, suggesting that toxicity of the chelated metal was not responsible for the differences in the rates of NTA degradation. After degradation of CoNTA- and NiNTA-, <3% of the Co or Ni was associated with C. heintzii cells. This indicates that, after degradation of the metal−NTA complex, metal ions will be predominantly present in the aqueous phase. The degradability of the various metal−NTA complexes was not related to their thermodynamic stability constants, but was related to the lability of the various metal−NTA complexes or the relative rates of formation of HNTA2-.