The feed‐to‐glass conversion, which comprises complex chemical reactions and phase transitions, occurs in the cold cap during nuclear waste vitrification. To investigate the conversion process, we analyzed heat‐treated samples of a simulated high‐level waste feed using X‐ray diffraction, electron probe microanalysis, leaching tests, and residual anion analysis. Feed dehydration, gas evolution, and borate phase formation occurred at temperatures below 700°C before the emerging glass‐forming melt was completely connected. Above 700°C, intermediate aluminosilicate phases and quartz particles gradually dissolved in the continuous borosilicate melt, which expanded with transient foam. Knowledge of the chemistry and physics of feed‐to‐glass conversion will help us control the conversion path by changing the melter feed makeup to maximize the glass production rate.
To understand feed‐to‐glass conversion for the vitrification of nuclear waste, we investigated batch reactions and phase transitions in a simulated nuclear waste glass melter feed heated at 5 K/min up to 700°C using optical microscopy, scanning electron microscopy with energy‐dispersive X‐ray spectroscopy, and X‐ray diffraction. To determine the content and composition of leachable phases, we performed leaching tests; the leachates were analyzed by inductively coupled plasma atomic emission spectroscopy. By 400°C, gibbsite and sodium borates lost water and converted to amorphous phase, whereas other metallic hydroxides dehydrated to oxides. Between 400°C and 700°C, carbonates decomposed before 500°C; amorphous aluminum oxide and calcium oxide reacted with the sodium borate and formed the more durable amorphous borate phase along with intermediate crystalline products; above 500°C, quartz began to dissolve, and hematite started to convert to trevorite.
Silicate glasses containing CdS and Ag2O were made by the melt‐quenching method. CdS quantum dots (QDs) were precipitated inside the glass matrix by heat treatment at 570–590 °C for 10 h, and the influence of Ag on photoluminescence (PL) of CdS QDs was investigated. The emission located at 478–493 nm in wavelength originated from the direct recombination of electron/hole pairs was quenched due to charger transfer between Ag and CdS QDs. Modification of PL from CdS QDs by Ag provides potentials toward developing the color changing materials for light‐emitting diodes (LEDs).
This article reports the effect of Ag+ ion‐exchange on the precipitation of PbS quantum dots (QDs) in silicate glasses. The glasses were subjected to ion‐exchange in AgNO3 salt melts at 260°C for 60 s. Ag nanoparticles (NPs) and PbS QDs precipitated after heat treatment at temperatures of 440°C–460°C for 10 h. Photoluminescence wavelength λPL of PbS QDs was longer (~1525 ≤ λPL ≤ ~1580 nm) in Ag+ ion‐exchanged regions than in Ag+‐free regions (~1030 ≤ λPL ≤ ~1200 nm). Average diameter D of PbS QDs estimated by calculation and TEM images was larger in Ag+ ion‐exchanged regions (~5.2 ≤ D ≤ ~5.5 nm) than in Ag+‐free regions (~2.4 ≤ D ≤ ~3.3 nm). Ag NPs that formed during heat treatment provided the sites for heterogeneous nucleation of PbS QDs and promoted the precipitation of PbS QDs in glasses.
Multiple Fe(III)-reducing Geobacter species including the model Geobacter sulfurreducens are thought to be incapable of carbon dioxide fixation. The discovery of the reversed oxidative tricarboxylic acid cycle (roTCA) for CO 2 reduction with citrate synthase as key enzyme raises the possibility that G. sulfurreducens harbors the metabolic potential for chemolithoautotrophic growth. We investigate this hypothesis by transferring G. sulfurreducens PCA serially with Fe(III) as electron acceptor and formate as electron donor and carbon source. The evolved strain T17-3 grew chemolithoautotrophically with a 2.7-fold population increase over 48 h and a Fe(III) reduction rate of 417.5 μM h −1 . T17-3 also grew with CO 2 as carbon source. Mutations in T17-3 and enzymatic assays point to an adaptation process where the succinyl-CoA synthetase, which is inactive in the wild-type, became active to complete the roTCA cycle. Deletion of the genes coding for the succinyl-CoA synthetase in T17-3 prevented growth with formate as substrate. Enzymatic assays also showed that the citrate synthase can perform the necessary cleavage of citrate for the functional roTCA cycle. These results demonstrate that G. sulfurreducens after adaptation reduced CO 2 via the roTCA cycle. This previously hidden metabolism can be harnessed for biotechnological applications and suggests hidden ecological functions for Geobacter.
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