Improved toxic and radioactive elements immobilization in tin slag-blended lithium aluminate borate glass

Abstract

Ever-increasing production of tin slag (TS) by various tin smelting industries in Malaysia remains a major environmental concern, wherein a large amount of naturally occurring radioactive materials (NORM) as wastes are dumped annually since 1982 due to the rare earth elements extraction-related activities. Based on this fact, we proposed some tin slag-included lithium aluminate borate (LAB) glass for improved immobilization of both toxic and radioactive elements present in waste using vitrification. These glasses of composition (50–0.5x)Li2O-xAl2O3-(50–0.5x)B2O3 (0.0 ≤ x ≤ 4.0 mol%) blended with 5, 15 and 30 wt% of TS were made by the standard melt-quenching method and characterized to determine their radioactive wastes immobilization potency. The obtained glasses were immersed in deionized water (DW) at 90 °C for up to 28 days to evaluate their durability. The weight loss of the samples was assessed by applying leaching tests. Inductively coupled plasma-optical electron spectra confirmed the ion exchange-mediate interdiffusion leaching of glass, indicating the presence of Li, Al, and Th in contacting water. The pH values of the solutions were increased from 7.00 to 11.93 with the increase in immersion time which was due to the leaching of Li (alkali metal) from the glass. FTIR spectra exhibited distinct absorption bands characteristics to the vibrations of various chemical functional groups like Al–O, B–O–B, BO4, BO3 and O–H stretching, confirming the association between radioactive waste and glass constituents. The observed bands at 850-1750 cm−1 and 3200 cm−1 corresponded to the vibration of the borate network and O–H bonds, indicating the occurrence of a leaching-mediated hydrolysis process. High-purity germanium gamma-ray spectroscopy was used to determine the radioactivity level of radionuclide (Bq g−1) in all samples powder. The radioactivity levels were found to be below the exempted waste limit set by IAEA safety standard for NORM wastes which is 1 Bq g−1 for 232Th and 238U and 10 Bq g−1 for 40K. In other words, the studied LAB glass containing NORM waste is suitable for conventional waste disposal as it is considered an ecologically safe material as other waste. The 90% reduction in radioactivity levels of the LAB glass containing TS compared to the TS itself indicated a successful dilution process of the studied glass, a part of excellent hydrolytic resistance characteristics. Therefore, it is suggested that the proposed LAB glass system in the present study is practical for NORM radioactive waste immobilization.