Uranium Separation – Challenges and Opportunities: Recovery of Uranium from Seawater with Solid Sorbents

Spiro D. AlexandratosHunter College of the City University of New York

Department of Energy Nuclear Fuels Resources Workshop

October 2010

U(VI) in seawater: tricarbonatouranate [UO2(CO3)3]4-

High stability constant limits choice of ligands

Problems in recovery due to very low concentrationexcess of competing ions

Approaches

Organic polymers with amidoxime ligand polymer support

physical formimmobilization method

(other) Organic polymers Inorganic oxides

Organic polymers with amidoxime ligand

high affinity for U(VI) from seawater

Challenge is to make recovery economical:high capacityhigh sorption rateplatform from which sorbent contacts seawater

Immobilization method – Polymerization of acrylonitrile

Effect of particle size

Beads prepared by suspension polymerization AN + 5% DVB [chloroform as porogen]

sieve to particle diameters:0.35-0.42 // 0.42-0.50 // 0.50-0.59 // 0.59-0.71 //

0.71-0.84 // 0.84-1.19 mm

Convert each to amidoxime

Effect of particle size

No effect on uranyl uptake from nitrate solutions

But from seawater: Small particles preferable to large for rapid uptake

Further studies show:

High porosity preferable to low porosity for rapid uptake

More pronounced effect of porosity + swelling in uptake of U(VI) from seawater than from nitrate solutions

May be due to U(VI) in seawater being bulky [UO2( CO3)3]4-

Aim: Maximize surface area --- composite fibers

Beads prepared by suspension polymerization

AN + tetraethylene glycol dimethacrylate (4EGDM)

Mixture agitated in a vibromixer at high frequency; particles crushed to 14µm

Convert to amidoxime (AO)

Prepare composite fiber

Amidoxime particles (6 g) + silica (6 g) + polyethylene (6 g) + surfactant (0.2 g)

Add to hexane in autoclave having 1.5-mm nozzle

Stir suspension at 150 oC eject through nozzle to flash-evaporate solvent, produce

fine fibrils trapping particles in fiber structure

Fibrils of polyethylene contain adsorbent and silica

Silica gives hydrophilic channels; access by hydrophilic U(VI)

adsorption rate: 0.20 mg U / g Ads per day

Advantages of using fiber composite adsorbent:

Microparticles used as adsorbent (needed for high rate)

Network of fibers best for rapid flow of seawater

Aim: maximize sorption kinetics --- hydrogels

Prepare hydrogels of AO + comonomer (60:40)acrylic acid (AA)

methacrylic acid (MAA) ethylene glycol methacrylate phosphate (EGMP) 2-acrylamido-2-methyl-1-propane sulfonate (AMPS)3-(acrylamido propyl)trimethylammonium chloride (APTAC)

Crosslink: N,N -methylenebis(acrylamide)′UV initiator: α,α -dimethoxy-α-phenylacetophenone′

AO hydrogels: AO AO+AA AO+MAA AO+EGMP AO+AMPS AO+APTAC

%U(VI)-uptake {equilibrate hydrogels with seawater spiked with 1-5 ppm of 233U}

AO

AO + AA

AO+MAA

AO+EGMP

AO+AMPS

AO+APTACAMPS

EGMP0

102030405060708090

100

AO at equilibrium in 25 min

AA and MAA did not significantly affect timeAMPS increased time slightlyEGMP and APTAC increased time significantly

EGMP alone reached equilibrium in 7 min

Advantages of EGMP hydrogels

(i) one step synthesis using one monomer

(ii) EGMP is non-volatile

(iii) EGMP is readily polymerizable

(iv) EGMP has faster kinetics than AO or AO + comonomer

(v) EGMP can be used in seawater and acidic solutions

Immobilization method - Grafting

Use when optimum bulk + surface properties not possible by a single polymer

Construct material whose bulk is made of one polymer and surface made of different polymer

Mechanical properties determined by matrix polymer, independent of adsorbent

Free radicals produced with chemical initiators or irradiation by γ-rays or electron beams

(Efficient grafting methods needed to reduce cost of radiation-grafted polymers)

Graft Polymerization

Prepare amidoxime grafted onto polyethylene(AN:MAA ratio --- 70:30)

U(VI) uptake dec’s with # of cycles: no uptake at 5 cycles

Elute with NaOH after HCl - 73% uptake at 5 cycles

SEM shows: surface layer comes off at 3rd HCl elution

Replace HCl with 1M tartaric acid: elute 100% U(VI) 90% uptake at 5 cyclesreduced damage on fiber surface

Amidoxime membranes

Graft polym’n of AN onto polyethylene (convert to AO)

Membrane 10-4 m thick with 70% porosityAmidoxime uniformly distributed; 1.8 mmol /g membrane

Adsorbed 0.85 mg U(VI) /g of Ads in 50 days

Adsorbent stable to repeated loading-elution cycles

Effect of crosslinker on amidoxime sorption (seawater)

U(VI) adsorption rate on polyAO crosslinked with 4EGDM is much higher than with DVB

Polymer with 4EGDM is hydrophilic: with more water uptake, U(VI) diffuses readily into polymer interior

Adsorption of U(VI): 0.20 mg/g of Ads in 10 days

Mechanism:Is the –NH2 in amidoxime active in binding U(VI)?

Mechanism:Is the –NH2 in amidoxime active in binding U(VI)?

Prepare acrylamide analogue

Mechanism:Is the –NH2 in amidoxime active in binding U(VI)?

Prepare acrylamide analogue

No affinity for U(VI)

Braid adsorbents

Beads need container for effective contact with seawater

National Institute of Advanced Science & Technology (Japan) developed AO fibers from AN

Fibrous adsorbents use ocean current when moored to seabed

But mechanical strength was insufficient for mooring

Intrinsic mechanical strength lost after amidoximation

To increase strength, graft polym’n applied to prepare fibrous amidoxime

Graft AN onto polyethylene non-woven fabric

Graft AN onto polyethylene non-woven fabric

Graft copolym’n of AN with hydrophilic methacrylic acid improved adsorption rate ; mechanically strong

Collection system for braid adsorbent: Anchor to seabed

Modified ligand: Bis(amidoxime)

Seawater circulated upward through column containing bis(amidoxime) particles; flow rate 6 mL/min

Adsorption capacity1 h 13.08 mg U / g adsorbent1 day 28.1

Time to equilibrium: 3 h

Modified ligand: Polyhydroxamic acid

Vary acrylamide-crosslinker mole ratios

0.95/0.05, 0.85/0.15, 0.75/0.25

0.95/0.05 gave highest uptake capacities

Place 1 g in glass column - 50 L seawater at 3mL/min

Recovery of metals from seawater

seawater(µg/L) uptake(µg/g)

Ti 1 0.18U 3.3 18.2V 1.9 6.0 Co 0.4 5.6Mo 10 1.1

Other polymers: polyallylamine

Uranium removal from seawater , 24 h contact 23% 35% 78%

Other polymers: Uranyl ion-imprinted polymers (IIP)

Two-step synthesis of IIP resins:(i) complex formation(ii) copolymerize complex with monomers

UO22+ + 5,7-dichloroquinoline-8-ol + 4-vinylpyridine

Copolymerize with [2-hydroxyethyl methacrylate] and [ethylene glycol dimethacrylate]

After polymer formation

Remove UO22+ with 5 M HCl

Contact seawater; remove 83% of the uranyl present

Stability constant of U–DCQ (1.29×1021) is greater than U-carbonate (1.67×1016)

Other polymers: immobilized tannin

Adsorption capacity: 2.35 mg U / g adsorbent (22 h)

Application of microbial biomass

R. arrhizus and P. chrysogenum are effective U(VI) sequestering agents

From seawater: Amount of U(VI) sorbed by R. arrhizus is much less than that sorbed from U(VI) solutions

P. chrysogenum has negligible uptake

Carbonates in seawater inhibit biomass from sorbing U(VI)

Inorganic oxides

First pilot plant for uranium recovery from seawater with hydrous TiO2: Ministry of International Trade & Industry (Japan), 1981 – 1988

Adsorption capacity: 0.1 mg U / g adsorbentMust increase > 10 X to decrease recovery cost

Adsorbent attrition resistance is low

U(VI) from seawater using hydrous TiO2: effect of particle size

U(VI) from seawater using hydrous TiO2: effect of pH

Inorganic adsorbents: slow adsorption rates; low mechanical stability

Hydrous zirconium oxide (13 µg/g)Hydrous tin oxide (17 µg/g)Hydrous lanthanum oxide (38 µg/g)Hydrous iron(III) oxide (60 µg/g)Hydrous aluminum oxide (61 µg/g)Hydrous titanium oxide

freshly precipitated (1550 µg/g)after >60 days storage (200 µg/g)

Silica titania gel (27 µg/g)

Composites

Hydrous TiO2 on activated carbon

Zinc carbonate on activated carbon

Composite: carboxylate-functionalized graft copolymer of PMAA on TiO2-densified cellulose

Matrix (Cell-Ti)

Prepare cellulose xanthate viscose (cellulose + CS2 / NaOH)Add TiO2 to viscose (1.5 : 10 weight ratio)

Disperse in solution of chlorobenzene + oilAgitate suspension at 90 ◦C for 1 h

Filter, wash particlesDecompose xanthate in HOAc + EtOH

Graft PMAA onto Cell-Ti

TiO2 embedded in cellulose to form composite

TiO2 particles increase density of the composite

Adsorbent stable in mineral acids and alkalies

Apply to removal of U(VI) from simulated nuclear industry wastewater

2.5 g adsorbent/L sample: ≈100% U(VI) removal

Can preparation of Cell-Ti be modified to give synergism between Ti and ligands grafted onto cellulose for removal of U(VI) from seawater?

Future Directions:

Broad study of immobilized ligands:Amidoxime (long-term stability, regenerant scheme)Mechanistic studyPhosphates, phenolics, …Immobilization method:Homopolym’n, graft polym’n, blends, IPNs,…Physical form: nanobeads on membranes, fiber,…Support polymer: PPE, PE, rayon, Lucite,…

Organic / inorganic oxide composites

References

Ind. Eng. Chem. Res. 2010, 49, 6559J. Macromol. Sci., Part A: Pure Appl. Chem. 2006, 43, 735J. Phys. Chem. B 2009, 113, 6328React. Funct. Polym. 2005, 63, 143Ind. Eng. Chem. Res. 1994,33, 657Ind. Eng. Chem. Res. 1988, 27, 1461Anal. Chim. Acta 2007, 587, 263Ind. Eng. Chem. Res. 2009, 48, 6789Coll. Surf. A: Physicochem. Eng. Aspects 2010, 361, 180

Sep. Sci. Tech. 2005, 39, 3753Rad. Phys. Chem. 2000, 57, 187Erice seminar 2009Can. J.Chem. Eng. 1984, 62, 559Bull. Chem. Soc. Jpn. 1980, 53, 1BARC Newsletter, December 2003Ind. Eng. Chem. Res. 1987, 26, 1977Ind. Eng. Chem. Res. 1987, 26, 1970Macromolecules 1985, 18, 2357Adsorption 2004, 10, 309Sep. Sci. Tech. 1987, 22, 1609

Shikoku Kogyo Gijutsu Shikensho Hokoku 1988, 19, 62Haiyang Xuebao (Zhongwenban) 1985, 7, 313-16.Jpn. Kokai Tokkyo Koho 1977, JP 52114585 A 19770926J. Nucl. Sci. Tech. 1977, 14, 811