Chromate adsorption on amorphous iron oxyhydroxide was investigated in dilute iron suspensions as a single solute and in solutions of increasing complexity containing C02(g), SOZ-(aq), H4Si04(aq), and cations [K+, Mg2+, Ca"(aq)]. In paired-solute systems (e.g., Cr0:-H2C03*), anionic cosolutes markedly reduce Cr042-adsorption through a combination of competitive and electrostatic effects, but cations exert no appreciable influence. Additionally, H4Si04 exhibits a strong time-dependent effect: Cr0:-adsorption is greatly decreased with increasing H4Si04 contact time. In multiple-ion mixtures, each anion added to the mixture decreases Cr0:-adsorption further. Adsorption constants for the individual reactive solutes were used in the triple-layer model. The model calculations are in good agreement with the Cr042-adsorption data for paired-and multiple-solute systems. However, the model calculations underestimate Cr042-adsorption when surface site saturation is appr6ached. Questions remain regarding the surface interactions of both C02(aq) and H4Si04. The results have major implications for the adsorption behavior of Cr0:-and other oxyanions in subsurface waters.

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-.

No abstract

No abstract