Degradation of Metal−Nitrilotriacetate Complexes by Chelatobacter heintzii

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-.

Subsurface interactions of actinide species and microorganisms: Implications for the bioremediation of actinide-organic mixtures

“…146,156 showed that citrate was degraded by P. fluorescens to the point where the ligand-to-metal ratio reached 1:1 in Cu-citrate and 2:3 in U-citrate systems, while degradation of citrate in the presence of other metals proceeded to completion, although at a lower rate than in systems with no significant metal concentrations. Similar results were seen by BOLTON et al 163 for NTA degradation in the presence of metals. In actinide systems, BANASZAK et al 116 showed that NTA degradation in the presence of Np(V) went to completion in the absence of metal toxicity effects, while REED et al 165 found that the degradation rate of NTA in the presence of Pu(IV), which has a NTA complex formation constant six to seven orders of magnitude greater than that of Np(V)-NTA, was significantly decreased, but not stopped, as the ligand-to-metal ratio approached 1 : 1.…”

“…225 When other, non-preferred metals form the most abundant substrate complex, substrate utilization usually is slowed. 115,145,146,163,226,227 Although several mechanisms have been proposed to account for the reduction in substrate availability and utilization in the presence of various metals, the most general effect comes about when significant metal concentrations reduce substrate availability. This reduction in availability through chemical speciation effects alone occurs when the organic substrate is a strong ligand.…”

“…256 On the other hand, substrate utilization by immobile bacteria often causes significant concentration gradients along the flow path within the aquifer; thus, the expressions describing substrate utilization and biomass synthesis must be coupled to equations describing the flow of water past the stationary organisms. 234,257 Speciation and complex-specific behavior: Recent evidence suggests that the degradation rate of many organic compounds, including organic chelates, is affected by the presence of specific metal cations, 146,226,227,258 indicating that the degradation of many organic compounds is limited to a single chemical species 163,259 or is dependent on the presence of certain metals as co-substrates for key enzymes. 166,225,260 We discussed how the bioavailability of organic compounds is affected by complexation with metals and actinides.…”

“…Other speciation-specific phenomena have been reported for various metal-ligand systems. BOLTON et al 163 suggested that the degradation of NTA by C. heintzii is dependent on the concentration of the HNTA 2-species, but VANBRIESEN and RITTMANN 228,229 reanalyzed their data using a comprehensive model that links equilibrium speciation with the kinetics of biodegradation. They concluded that the biodegradable form of NTA is NTA 3-for C. heintzii.…”

Kinetics and Mechanism of Interaction between Chromium(III) and Ethylenediaminetetra-3-Propinate in Aqueous Acidic Media

“…These include detergent, food, pharmaceutical, cosmetic, metal finishing, photographic, textile and paper industries [1][2][3]. It is also used as a component in decontamination formulation of nuclear reactors and in nuclear waste processing [4,5].…”