Free Hydroxide Concentration Research Articles

Evidence for dawsonite in Hanford high-level nuclear waste tanks

Gibbsite [Al(OH)3] and boehmite (AlOOH) have long been assumed to be the most prevalent aluminum-bearing minerals in Hanford high-level nuclear waste sludge. The present study shows that dawsonite [NaAl(OH)2CO3] is also a common aluminum-bearing phase in tanks containing high total inorganic carbon (TIC) concentrations and (relatively) low dissolved free hydroxide concentrations. Tank samples were probed for dawsonite by X-ray Diffraction (XRD), Scanning Electron Microscopy with Energy Dispersive Spectrometry (SEM-EDS) and Polarized Light Optical Microscopy. Dawsonite was conclusively identified in four of six tanks studied. In a fifth tank (AN-102), the dawsonite identification was less conclusive because it was only observed as a Na–Al bearing phase with SEM-EDS. Four of the five tank samples with dawsonite also had solid phase Na2CO3·H2O. The one tank without observable dawsonite (Tank C-103) had the lowest TIC content of any of the six tanks. The amount of TIC in Tank C-103 was insufficient to convert most of the aluminum to dawsonite (Al:TIC mol ratio of 20:1). The rest of the tank samples had much lower Al:TIC ratios (between 2:1 and 0.5:1) than Tank C-103. One tank (AZ-102) initially had dawsonite, but dawsonite was not observed in samples taken 15 months after NaOH was added to the tank surface. When NaOH was added to a laboratory sample of waste from Tank AZ-102, the ratio of aluminum to TIC in solution was consistent with the dissolution of dawsonite. The presence of dawsonite in these tanks is of significance because of the large amount of OH− consumed by dawsonite dissolution, an effect confirmed with AZ-102 samples.

Oxidation kinetics of manganese (II) in seawater at nanomolar concentrations

Manganese oxidation rates were determined at low (∼ 20 nM) concentrations in seawater by measuring dissolved manganese (Mn(II)) using flow injection analysis with chemiluminescence detection. Mn(II) was measured in samples that had been filtered (0.2 μm) and kept in the dark under controlled temperature and pH conditions for time periods up to 6 months. Eight 9 L carboys with mean pH values ranging from 8 to 8.7 were held at 25°C, another carboy (pH = 9.32) was kept at 5°C. Oxidation followed the Morgan (1967) homogeneous rate equation ( d[Mn(II)]/ dt = k 1 [O 2][OH −] 2[Mn(II)]). The mean rate constant k 1 = 1.7 ± 0.7 × 10 12 M −3 d −1 (95% CI), determined using hydroxide ion activities determined with pH measurements on the NBS scale, was in agreement with work by Morgan (1967; k 1 = 4 × 10 12 M −3 d −1) and Davies and Morgan (1989; k 1 = 1.1 × 10 12 M −3 d −1) in dilute solutions. The rate constant at 5°C was 1.3 ± 0.3 × 10 12 M −3 d −1. If free hydroxide concentrations (based on the free proton pH scale) are used, then the rate constant at 25°C was k 1 ∗ = 0.34 ± 0.14 × 10 12 M −3d −1. Autocatalytic increases in Mn(II) oxidation rates, as predicted by a heterogeneous reaction mechanism (Morgan, 1967) ( d[Mn(II)]/ dt= k 2′[Mn(II)][MnO 2]) were not observed, indicating that the homogeneous reaction dominates Mn(II) oxidation at low nM concentrations in seawater. Bacteria were enumerated by 4′,6-diamidino-2-phenylindole (DAPI) staining during the experiments. No significant correlation between bacterial concentrations and Mn(II) oxidation rates was found.