Environmental impact of amino acids on selenate-bearing hydrocalumite: Experimental and DFT studies.

Selenium (Se) radioactive wastes can be disposed through stabilization/solidification (S/S) based on the cementitious matrix on hydration products, where hydrocalumite (Ca ₂ Al-LDH) is expected to play an important role in the retention of SeO ₄ ^2- . Natural organic matters (NOMs) are known to be a risk to affect the transportation and mobility of undesirable chemical species in the pedosphere which receives the low level radioactive wastes (LLW). In the present work, five amino acids were selected as the simplified models of NOMs in the pedosphere to explore their effects on the stability of Ca ₂ Al-LDH after immobilized SeO ₄ ^2- under alkaline conditions. As the loading amount of amino acids on Ca ₂ Al-LDH increasing, release of SeO ₄ ^2- was enhanced in HGly, H ₂ Asp, and H ₂ Cys series, while no enhancement was observed in HPhe and HTrp series. Density functional theory (DFT) calculation predicted ion-exchange of amino acids and CO ₃ ^2- with SeO ₄ ^2- in a unit cell of LDH model. The intercalation of Asp ^2- and CO ₃ ^2- caused 003 peaks in XRD sharper and d ₀₀₃ decreased from 8.15 Å to 7.70 Å which is assigned to Ca ₂ Al-LDH(Asp, CO ₃ ). In H ₂ Cys series, the 003 peaks were kept broad and SeO ₄ ^2- was still relatively maintained in LDH which was caused by the lower amounts of intercalated CO ₃ ^2- in the presence of H ₂ Cys. Amino acids in the interlayer of Ca ₂ Al-LDH have several possible configurations, where the most stable one is prone to be in a horizontal direction through hydrogen bonds and Ca-O chemical bonds. This provides an insight on the stability of selenate immobilized in hydrocalumite, which can be produced in cement disposing in the pedosphere for a long term of burying. Not only carbonate but also small molecular organic matters like amino acids possibly give environmental impact on the mobility of low level anionic radionuclides in LDH.
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