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

Abstract Selenium (Se) radioactive wastes can be disposed through stabilization/solidification (S/S) based on the cementitious matrix on hydration products, where hydrocalumite (Ca2Al-LDH) is expected to play an important role in the retention of SeO42−. 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 Ca2Al-LDH after immobilized SeO42− under alkaline conditions. As the loading amount of amino acids on Ca2Al-LDH increasing, release of SeO42− was enhanced in HGly, H2Asp, and H2Cys series, while no enhancement was observed in HPhe and HTrp series. Density functional theory (DFT) calculation predicted ion-exchange of amino acids and CO32− with SeO42− in a unit cell of LDH model. The intercalation of Asp2− and CO32− caused 003 peaks in XRD sharper and d003 decreased from 8.15 A to 7.70 A which is assigned to Ca2Al-LDH(Asp, CO3). In H2Cys series, the 003 peaks were kept broad and SeO42− was still relatively maintained in LDH which was caused by the lower amounts of intercalated CO32− in the presence of H2Cys. Amino acids in the interlayer of Ca2Al-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.