Environmental impact of amino acids on the release of selenate immobilized in hydrotalcite: Integrated interpretation of experimental and density-functional theory study

The environmental impact of amino acids on the release of SeO₄^2- immobilized into hydrotalcite (Mg₂Al-LDH) which belongs to the layered double hydroxides (LDHs) family was investigated by experimental study and the observed layer structure of hydrotalcite was verified through density-functional theory (DFT) calculations. Glycine, l-cysteine, and l-aspartic acid, which have smaller molecular sizes, can release SeO₄^2- largely due to intercalation, unstabilization of Mg₂Al-LDH and simple dissolution, while l-tryptophan and l-phenylalanine caused limited SeO₄^2- release due to their larger sizes and aromaticity. XRD patterns for the solid residues after intercalation of amino acids revealed that the layer distance of Mg₂Al-LDH was partially expanded. The main peaks and shoulder features corresponding to d₀₀₃ diffraction were well explained by DFT simulations using glycine as a model: the layer spacing of the main peak is responsible for the remaining SeO₄^2- and singly stacked glycine molecule and the layer spacing of the shoulder peak was well explained by doubly stacked glycine molecules. Hydrogen bonds between amino acids and hydroxyl ions in the metallic layers of Mg₂Al-LDH were responsible for the stable configuration of the intercalated Mg₂Al-LDH. This study indicates potential limitations to the stability of low-level radioactive wastes of ⁷⁹Se in repositories which are affected by smaller molecules of amino acids released through degradation of organic matters in the pedosphere.

What's up down there?

The development of careful quality assurance criteria assuring freedom from contamination in all aspects of sample recovery has opened the window to studies of a fascinating new microbial biome in the deep subsurface. Organisms have been recovered with unusual metabolic capabilities and a chemosynthetic lifestyle independent of the recent surface photosynthetically derived energy inputs. The properties of the subsurface microbiota are critical when assessing aspects such as the utility of burying radioactive waste, the remediation of mixtures of organics, metals, and nuclides, and the search for life in extreme environments on Earth as well as on Mars and other extraterrestrial sites. In addition this pioneering work provides a foundation for examining life processes in extreme environments, such as the environment beneath the ocean floor.