No abstract

Constant pH adsorption isotherms for nonradioactive Cs + , Sr 2+ , and Co 2+ on pure magnetite and a 80% (w/w) magnetite-silica composite were measured at 25 °C over a wide range of metal ion concentrations. The adsorption studies were carried out at four different pH's: 6, 7, 8, and 9 for Cs + and Sr 2+ and 5, 6, 7, and 8 for Co 2+ . All of the constant pH isotherms exhibited type I behavior with a saturation capacity that was pH-dependent and increased with increasing pH. The corresponding distribution coefficients increased with increasing pH but decreased with increasing metal ion concentration; they were also 10-1000 times lower than those reported in the literature for more selective but more expensive adsorbents. These two magnetite-based adsorbents also exhibited moderate regeneration conditions, with nearly 90-100% regeneration achieved in most cases at pH values between 1 and 3. A Langmuir model with pH-dependent parameters was also fitted successfully to all of the constant pH adsorption isotherms. This experimental data and the corresponding pH-dependent Langmuir correlation should find considerable use in the design and development of inexpensive fixed-bed adsorption processes for the removal of the radioactive isotopes of Cs + , Sr 2+ , and Co 2+ from aqueous solutions that are produced in nuclear facilities. Magnetite, when encased in silica and placed in a packed column, can also be used as the charging element in high gradient magnetic separation, thereby removing not only metal ions via surface complexation (adsorption) but also nanoparticles of a paramagnetic nature.

No abstract

No abstract

No abstract

No abstract

No abstract

Storage tanks at the Savannah River Site (SRS) contain highly enriched uranium (U) and dibutyl phosphoric acid (HDBP), formed from the hydrolysis and radiolysis of tributylphosphate (TBP) in the PUREX process. Highly enriched dibutyl phosphoric acid, a relatively strong acid with a pKa (acid dissociation constant) of about one, forms insoluble compounds with U and other actinides in acid solutions. Accumulation of solids in these storage tanks presents a criticality hazard and must be avoided. In this work, U and HDBP compounds were formed from batch reactions in 0.2-6.0 M nitric acid (HNO 3 ) and two HDBP concentrations, forming either a homogenous or heterogeneous solution based on solubility. A 4 : 1 HDBP : U ratio was used in all experiments. The chemical composition of the compounds was characterized using liquid scintillation, ion chromatography, and stannous chloride reduction. The physical characteristics of the U and HDBP compounds were characterized using a combination of infrared spectrometry (IR), electronic spectrophotometry (UV=Vis), and Phosphorous-31 Nuclear Magnetic Resonance ( 31 P NMR). The physical appearance of the solids ranged from a pale yellow powder at 0.2 M HNO 3 loading to a bright yellow gel at 6.0 M HNO 3 , with constant modifications as the acid concentration increased. The U and HDBP precipitate from 0.2 M HNO 3 was characterized as UO 2 (DBP) 2 (a polymer with ten units) in homogenous U and HDBP solutions and UO 2 (DBP) 2 (HDBP) x (where x ¼ 1 or 2) in heterogeneous U and HDBP solutions. Indications of nitrate complexation appeared in precipitates from 0.8 M HNO 3 for both HDBP loadings, however a 1 : 1 uranium : nitrate ratio was not reached at that acid concentration. UO 2 (NO 3 )(H(DBP) 2 )(HDBP) 2 precipitated from ! 3.0 M HNO 3 with high HDBP concentrations. This compound was not formed until 6.0 M HNO 3 was used in lower HDBP concentrations. It is assumed that the change in physical characteristics and addition of nitrate coincides with the breakdown of the UO 2 (DBP) 2 polymer. Supporting data for these findings are presented and discussed.