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Summary of Uranium Adsorbents

Summary of Uranium Adsorbents. Chris Janke Oak Ridge National Lab. Uranium Adsorbents . Favorable attributes of U adsorbents Possible causes of U adsorbent degradation Uranium adsorbent synthesis Adsorbent material Irradiation Grafting reaction

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Summary of Uranium Adsorbents

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  1. Summary of Uranium Adsorbents Chris Janke Oak Ridge National Lab

  2. Uranium Adsorbents • Favorable attributes of U adsorbents • Possible causes of U adsorbent degradation • Uranium adsorbent synthesis • Adsorbent material • Irradiation • Grafting reaction • Parameters affecting free radical concentration and decay rate • Amidoxime reaction • KOH Conditioning • Uranium elution • Characterization techniques and structural determination • Uranium adsorption capacity

  3. Favorable Attributes of U Adsorbents • High U adsorption capacity in seawater • Japan U adsorbents • 1st generation Nonwoven stack system - 0.5 g U/kg Ads. • 2nd generation Braid system - 1.5 g U/kg Ads. • Due to its lower mechanical props. NW system must be supported which limits it accessibility to seawater resulting in lower U ads. cap. and higher cost vs. Braid system • Current goal is 6 g U/kg Ads. • High selectivity for U in seawater • Rapid loading kinetics of U in seawater • Rapid elution kinetics of U from adsorbent • Long lifetime with excellent cycling performance • To be economical adsorbent must undergo repeated adsorption/desorption cycling with little or no performance knockdown • High mechanical strength & durability (Braid > NW) • Over entire life adsorbent must have high tensile strength, high % E-B, high fatigue resistance • Low cost (Braid < NW) • Includes raw material and initial adsorbent synthesis, repeated adsorption/desorption cycling, and costs associated with storage, placement and retrieval from seawater, etc.

  4. Possible Causes of U Adsorbent Degradation • Chemical • Seawater • Acids & bases & solvents (repeated cycles of U elution and conditioning) • Competing complexation with other species • Mechanical • Continuous stresses from ocean currents & wave action • Physical handling during adsorbent placement & retrieval in seawater and during repeated adsorption/desorption cycling • Thermal • Range of seawater temperatures • Exposure to cycling temperatures during storage, transport, and seawater placement/retrieval • UV exposure • Other

  5. Past/Present Uranium Adsorbent Materials • Adsorbent Forms • Microbeads (i.e. PiBMA) • Plastic films • Spunbonded nonwoven fabric (made from short, discontinuous fibers) • Continuous Fiber (i.e. Braid) • Adsorbent Trunk Polymers • Continuous HDPE fiber (Japan’s preferred matl.) • Porous PE hollow fibers • PE (sheath)/PP(core) nonwoven (i.e. 50/50 – Japan’s NW system – more costly, lower mechanical props. and lower U. ads. cap. vs. HDPE ) • PP fiber or nonwoven (lower mechanical properties and U. ads. cap. vs. HDPE) • PAN fiber (has low mechanical properties after adsorbent synthesis) • 100% HDPE continuous fiber in braided form is currently the preferred U adsorbent material due to its higher U ads. cap. and mechanical properties and its lower cost

  6. Irradiation Parameters (Volumetric Process) • Pre-irradiation (indirect) method is preferred • Adsorbent is 1st irradiated and then brought into contact with monomer solution • Homopolymerization is less of a problem vs. simultaneous (direct) method • Irradiation source • Gamma (generally has lowest dose rate but can process very thick materials) • Electron Beam (offers high dose rate and high throughput) • X-ray (generally has lower dose rate than EB but is highly penetrating and is amenable to irradiating thicker materials vs. EB) • Total dose • 200 kGy is std. dose with HDPE • Irradiation time • Minimum processing time is preferred (i.e. Japan - EB-20 min., 20 passes, 10 kGy/pass; Gamma – 10 hrs.) • Irradiation atmosphere • Irradiating under inert conditions extends free radical lifetime and is the preferred approach with HDPE • Adsorbent temperature during & after irradiation (typically RT to -78C) • Lower temperatures increase lifetime of free radicals • Heating of adsorbent during irradiation can occur due to beam passing through adsorbent , adsorbent rxns. and thermal conduction from supporting substrate • Goal is to generate and preserve free radicals in trunk polymer and minimize cross-linking and chain scission rxns.

  7. Grafting Reaction • Diffusion controlled process • Grafting essentially takes place in amorphous region due to the decreased ability of the monomer to penetrate crystallites and access free radicals • Grafting proceeds via grafting front mechanism • Initial grafting occurs on fiber surface which leads to swelling of surface layers • Further grafting proceeds into fiber interior by progressive diffusion of monomer • Eventually results in higher viscosity of system which effectively lowers rates of monomer diffusion, chain propagation and chain termination

  8. Grafting Reaction Parameters(Diffusion Controlled Process) • Grafting monomer type & concentration (Japan prefers 50 wt. % AN/MAA (70:30) co-monomer system) • AN readily forms amidoxime groups and has high U ads. cap. • MAA is hydrophilic and has good compatibility w/ seawater and facilitates decomplexation of uranyltricarbonate to uranyl ions (improves ads. rate of U in sea water) • Other monomers studied include acrylic acid (AA), glycidylmethacrylate (GMA), GMA modified w/ 3,3’-iminodipropionitrile (IDPN), divinylbenzene, acryloylchloride, dicyanoethylated polystyrene • Grafting solvent type & concentration (50 wt. % DMSO preferred w/ HDPE or PE/PP) • Chain transfer constant (higher values result in faster termination of growing chain and lowers DOG) • Grafting atmosphere (N2 is preferred w/ HDPE or PE/PP) • Grafting temperature (40C is typical w/ HDPE) • Grafting time (4-5 hrs. is typical w/ HDPE) • Viscosity of grafting system (higher viscosity limits accessibility of monomer to trunk polymer) • Additives • Presence of H2SO4 and/or water in grafting solution has shown increased DOG • Hansen solubility parameters • DOG increases as solubility parameters of materials become closer in value (i.e. solvent, monomer, growing polymer chains and trunk polymer)

  9. Parameters affecting free radical concentration and decay rate • Polymer type & crystallinity (Due to its higher crystallinity UHMWPE has lower DOG vs. HDPE but offers much longer free radical lifetimes) • Total Dose • Free radical concentration generally increases w/ dose but eventually reaches max. value (i.e. HDPE - 200 kGy) • Dose rate (not as critical with preirradiation method vs. simultaneous method) • Irradiation temperature (lower is better) • Irradiation time (shorter is better) • Irradiation atmosphere (inert is better with HDPE) • Storage time (shorter is better) • Grafting should be performed ASAP after irradiation • Note:ESR studies on irradiated PE has revealed the formation of 3 types of free radicals including: • alkyl radicals (-CH2-.CH-CH2-) – major product • allyl radicals (-CH2-.CH-CH=CH-) • polyenyl (-CH2-.CH-(CH=CH)n-CH2-)

  10. Amidoxime Reaction Parameters • Nitrile groups are converted to amidoxime groups • Solvent (typically 50/50, water:methanol) • Hyroxylamine hydrochloride concentration (normally 3% or 10%) • pH must be adjusted to 7 by base neutralization • Reaction temperature (typically 60C (10% HA) or 80C(3% HA)) • Reaction time (1 hr.)

  11. KOH Conditioning • Prior to U adsorption experiments the adsorbent requires KOH conditioning in order to: • De-quarternize or remove H from amidoxime groups • Neutralize MAA • Increase free volume or enlarge space between polymer chains thereby facilitating sorption of water and Uranium • Typical conditions include 2.5% KOH at 80-90C for a min. of 60 min., then washing w/ DI water

  12. Comparision of Uranium Elution Methods HCl vs. tartaric acid vs. Na2CO3 HCl method (0.5-1.0M) Requires KOH conditioning AO groups damaged after 5 adsorption/desorption cycles Tartaric acid method (1.0M) Requires KOH conditioning AO groups last longer than HCl method Offers higher U adsorption after adsorption/desorption cycling vs. HCl method Higher mechanical strength than HCl method Na2CO3 method (Pandey-2008) Doesn’t require KOH conditioning for reuse AO groups not damaged after repeated cycling U quantitatively desorbed > 90% at equilibration times of 60 min. vs. 40 min. for HCl method

  13. Characterization Techniques and Structural Determination of U Adsorbents • ESR – determination of free radical types, concentrations and decay rates under different processing conditions • XPS – quantitative determination of surface chemical composition and chemical bonding • Microprobe Raman and FTIR-ATR - distribution and depth profile of the graft • SEM and X-ray microprobe analysis – distribution of graft on micrometer scale and elemental analysis by coupling SEM to X-ray spectrometer • AFM – surface morphology and 3-D profile information of grafted structures • Confocal Raman microscopy – investigation of changes in composition • Contact angle measurements – wetting and surface energy properties • SAXS and SANS – structural investigations • DSC and TGA – Tg, Tm, degree of crystallinity and thermal stability • X-ray diffraction – crystallinity observations • Mechanical property and durability testing – provides important info. regarding the changes in the tensile strength, modulus, % E-B of the adsorbent material during its lifetime

  14. Uranium Adsorption Capacity • U ads. cap. is one of the most important properties that directly impacts the economics and any eventual implementation • What is the relationship between DOG, AO group density and U adsorption capacity? • Influence of steric factors WRT UO22+ complexation? • Amount of UO22+ complexation on adsorbent surface vs. interior of adsorbent? • Can UO22+ adsorp to and/or elute from interior of fiber? • Is higher U ads. cap. favored by small no. of long chains or large no. of short chains? Can chain transfer agents be used to form large no. of short chains? If so, which ones would be most effective and will homopolymerization be much of a problem?

  15. Degree of co-grafting (%) • 100 x (W1 – W0)/W0 where W0 and W1 are the dry wts. of ads. before grafting and the ads. after grafting • AO group density (mol/kg) • 1000 x (W2 – W1)/33xW2 where W1 and W2 are the dry wts. of the ads. After grafting and the ads. After amidoximation • Grafting efficiency • The ratio of the wt. of the grafted monomer(s) to the total wt. of monomer(s) converted in both grafting and homopolymerization

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