As, Se, and Re Sorption by Mg-Al Layered Double Hydroxides
- PDF / 261,173 Bytes
- 6 Pages / 612 x 792 pts (letter) Page_size
- 94 Downloads / 252 Views
II8.5.1
As, Se, and Re Sorption by Mg-Al Layered Double Hydroxides Charles R. Bryan, Yifeng Wang, Huifang Xu1, Paul S. Braterman2, and Huizhen Gao Sandia National Laboratories, Carlsbad, NM 88220, U.S.A. 1 Department of Earth and Planetary Sciences, University of New Mexico, Albuquerque, NM 87131, U.S.A. 2 Department of Chemistry, University of North Texas, Denton, TX 76203, U.S.A. ABSTRACT Layered double hydroxides (LDHs) have high anion exchange capacities and are easily synthesized in the laboratory, giving them considerable potential as adsorbents for anionic contaminants in the environment. In this study, sorption of arsenate, selenate, and perrhenate by uncalcined Mg-Al LDH (Mg:Al ratio 2.4:1) was evaluated. To investigate sorption mechanisms, the substrate was added to the solution both as a pre-wetted slurry (allowing 24 hours for rehydration), and as a dry powder. Sorption results varied for different anionic complexes. For selenate and perrhenate, an incubation time was required for the dry material to achieve the same Kd value as the pre-wetted material. For arsenate, however, both wetted and dry materials initially had high Kds, which decreased with time. These behaviors are attributed to the structural reconstruction of the calcined LDH material in solution. Se and Re are not highly sorbed by the phases present in the calcined material, but are incorporated as interlayer anions in the LDH structure upon hydration and reconstruction, while As may be sorbing onto the dehydration products, and being partially released during rehydration. INTRODUCTION Contaminants that occur in anionic forms are commonly highly mobile in surface and subsurface environments, because they are negatively charged and are poorly sorbed by most soil and sediment components under neutral or basic conditions (pH > 7) [1-5]. Anionic contaminants include 99Tc, 79Se, and 129I, long-lived fission products (half-lives ranging from 2.13·105 to 1.57·107 years) released during reprocessing of nuclear fuel for weapons production. Under oxidizing conditions, these radionuclides are highly soluble and exist as TcO −4 , SeO 24 − , and I − . Currently, the Department of Energy (DOE) has 90 million gallons of high-level and low-level liquid waste generated by weapons production activities. This material is stored in underground tanks, some of which have known or suspected leaks. The high mobility of 99Tc, 79 Se, and 129I anionic species poses a technical challenge for tank closure. A nonradioactive anionic contaminant of concern is arsenate, AsO 34− , which exceeds the new EPA drinking water standard of 10 ppb in the municipal water supplies for many cities in the western U.S. Layered double hydroxide (LDH) materials have shown great promise as anion getters. LHDs consist of positively charged, close-packed, metal hydroxide layers, whose surplus positive charge, arising from the substitution of a trivalent cation M(III) for a divalent cation
II8.5.2
M(II), is neutralized by an interlayer anion or anionic complex. LDH materials have the gener
Data Loading...
