1Managed by UT-Battelle for the U.S. Department of Energy Summary of Uranium Adsorbents Chris Janke Oak Ridge National Lab

2Managed by UT-Battelle for the U.S. Department of Energy 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

3Managed by UT-Battelle for the U.S. Department of Energy Favorable Attributes of U Adsorbents High U adsorption capacity in seawater – Japan U adsorbents 1 st generation Nonwoven stack system g U/kg Ads. 2 nd generation Braid system 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.

4Managed by UT-Battelle for the U.S. Department of Energy 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

5Managed by UT-Battelle for the U.S. Department of Energy 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

6Managed by UT-Battelle for the U.S. Department of Energy Irradiation Parameters (Volumetric Process) Pre-irradiation (indirect) method is preferred – Adsorbent is 1 st 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.

7Managed by UT-Battelle for the U.S. Department of Energy 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

8Managed by UT-Battelle for the U.S. Department of Energy 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 uranyl tricarbonate to uranyl ions (improves ads. rate of U in sea water) – Other monomers studied include acrylic acid (AA), glycidyl methacrylate (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 (N 2 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 H 2 SO 4 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)

9Managed by UT-Battelle for the U.S. Department of Energy 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 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 (-CH 2 -.CH-CH 2 -) – major product – allyl radicals (-CH 2 -.CH-CH=CH-) – polyenyl (-CH 2 -.CH-(CH=CH) n -CH 2 -)

10Managed by UT-Battelle for the U.S. Department of Energy 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.)

11Managed by UT-Battelle for the U.S. Department of Energy 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

12Managed by UT-Battelle for the U.S. Department of Energy Comparision of Uranium Elution Methods HCl vs. tartaric acid vs. Na 2 CO 3 –HCl method ( M) 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 –Na 2 CO 3 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

13Managed by UT-Battelle for the U.S. Department of Energy 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

14Managed by UT-Battelle for the U.S. Department of Energy 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 UO 2 2+ complexation? Amount of UO 2 2+ complexation on adsorbent surface vs. interior of adsorbent? – Can UO 2 2+ 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?

15Managed by UT-Battelle for the U.S. Department of Energy 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