DEGRADATION OF METAL-NITRILOTRIACETATE COMPLEXES BY CHELATOBACTER-HEINTZII

Nitrilotriacetic acid (NTA) is a synthetic chelating agent that can fo rm strong water-soluble complexes with a wide range of radionuclide an d metal ions and has been used to decontaminate nuclear reactors and i n the processing of nuclear materials. The codisposal of NTA or other synthetic chelating agents with radionuclides may result in increased dispersal of radionuclides in soil and subsurface environments. Unders tanding the influence of aqueous geochemistry on NTA degradation is es sential to predict the mobility and fate of inorganic contaminant-NTA complexes in the subsurface. Chelatobacter heintzii (ATCC 29600) was s hown to degrade C-14-labeled NTA to (CO2)-C-14 with first-order kineti cs at concentrations ranging from 0.05 to 5.23 mu M (0.01-1 mu g of NT A mL(-1)). The degradation of various metal-NTA complexes was investig ated under conditions in which the NTA was predominantly present as th e metal-NTA complex. The order for the rates of degradation was HNTA(2 -) > CoNTA(-) = FeOHNTA(-) = ZnNTA(-) > AIOHNTA(-) > CuNTA(-) > NiNTA( -), which is not related to the order of metal-NTA stability constants . The metal concentration used was not inhibitory to glucose mineraliz ation, suggesting that toxicity of the chelated metal was not responsi ble for the differences in the rates of NTA degradation. After degrada tion of CoNTA(-) and NiNTA(-), <3% of the Co or Ni was associated with C. heintzii cells. This indicates that, after degradation of the meta l-NTA complex, metal ions will be predominantly present in the aqueous phase. The degradability of the various metal-NTA complexes was not r elated to their thermodynamic stability constants, but was related to the lability of the various metal-NTA complexes or the relative rates of formation of HNTA(2-).