Synthesis of micron sized Tc-incorporated trevorite
EMSL Project ID
48273
Abstract
Abstract: Low retention of Tc in a final borosilicate glass is attributed to high volatilization rate of Tc species from the melt process. Two major Tc species, Tc4+ and Tc7+ show different volatilization rate because Tc2O7 melts at 119.5 oC, while TcO2 sublimes at 900 oC. Therefore, reduced condition is likely to increase Tc retention in a final glass. In addition, if reduced Tc is incorporated within stable mineral structure like spinel present in glass, Tc retention in a final glass will be also increased. Trevorite, one of spinel structure minerals (A2+B3+2O4) can be formed with sub-micron size at mild temperature (75-90 oC) condition to capture Tc and increase Tc retention in a glass waste form during the cold cap glass melting process. Previous studies for Tc removal using trevorite, nickel-iron spinel (NiFe2O4) was not successful, because trevorite was synthesized at high Temperature (500-800 oC) using molten salts in which Tc can be escaped during the high temperature synthetic process. Alternative synthesis was conducted with dissolved Ni and Fe ion solutions in wet-chemistry method and react with alkaline pH solution at 70-90 oC. However, wet-solution synthesis results in nano-sized trevorite particle. This nano-sized trevorite may capture Tc in mineral structure, but high solubility of nano-sized trevorite including Tc can easily be melted again to release Tc during glass melt process. The objective of this proposed work is to develop a synthesis method for trevorite containing Tc with submicron sizes using Fe(OH)2(s) powder as initial precursor. Recent tests have shown that Tc can be removed from alkaline solution by a process that reduces pertechnetate, Tc7+ to Tc4+ which then co-precipitates with the maghemite/magnetite mixture during mineral transformation from Fe(OH)2(s) at 75oC (Um et al., 2013). Removal of Tc from solution by Fe(OH)2(s) transformation process was fast and more than 95% of the initial Tc (10-5 M) was removed from slurry containing Fe(OH)2(s), even without aqueous Fe(II) addition as reductant. Since Fe(OH)2(s) can be oxidized to form magnetite (Fe3O4) or maghemite (?-Fe2O3) by reacting with H2O even in anaerobic conditions (Skiorr, 1933), the presence of Fe(OH)2(s) itself can provide reducing condition to reduce Tc(VII) to Tc(IV). In addition, because the final pH of a slurry mixed with Tc and Fe(OH)2(s) is fairly alkaline (pH = 9–11.5), Tc removal is not considered to be from surface adsorption, but incorporation into transformed mineral product, a mixture of magnetite and maghemite which both belong to spinel mineral group. The XAFS analysis also confirms Tc speciation in the final transformed product from Fe(OH)2(s) that consists of Tc4+ (96%) with at most 4% TcO4- (Figure 1). So, in this proposal, we will develop and use micron-sized Fe(OH)2(s) as an initial solid substrate, and test mineral transformation of this with addition of Ni from NiCl2 or Ni(NO3)2 to form trevorite at low temperature (<100 oC) which is similar condition to the Cold-cap region formed at outside layer of glass formation.
Project Details
Start Date
2014-03-07
End Date
2014-09-30
Status
Closed
Released Data Link
Team
Principal Investigator
Co-Investigator(s)