Conjugates of magnetic nanoparticle-actinide specific chelator for radioactive waste separation

Conjugates of Magnetic NanoparticleActinide Specific Chelator forRadioactive Waste SeparationManinder Kaurdagger Huijin ZhangDagger Leigh Martinsect Terry Toddsect and You QiangdaggerDagger

daggerDepartment of Physics University of Idaho Moscow Idaho 83844 United StatesDaggerEnvironmental Science University of Idaho Moscow Idaho 83844 United StatessectIdaho National Laboratory Idaho Falls Idaho 83415 United States

ABSTRACT A novel nanotechnology for the separation of radioactive waste thatuses magnetic nanoparticles (MNPs) conjugated with actinide specific chelators(MNP-Che) is reviewed with a focus on design and process development TheMNP-Che separation process is an effective way of separating heat generating minoractinides (Np Am Cm) from spent nuclear fuel solution to reduce the radiologicalhazard It utilizes coated MNPs to selectively adsorb the contaminants onto theirsurfaces after which the loaded particles are collected using a magnetic field TheMNP-Che conjugates can be recycled by stripping contaminates into a separatesmaller volume of solution and then become the final waste form for disposal afterreusing number of times Due to the highly selective chelators this remediationmethod could be both simple and versatile while allowing the valuable actinides to berecovered and recycled Key issues standing in the way of large-scale application arestability of the conjugates and their dispersion in solution to maintain their uniqueproperties especially large surface area of MNPs With substantial research progress made on MNPs and their surfacefunctionalization as well as development of environmentally benign chelators this method could become very flexible and cost-effective for recycling used fuel Finally the development of this nanotechnology is summarized and its future direction isdiscussed

I INTRODUCTION

One of the major requirements for sustaining human progressis an adequate source of energy Currently the largest sourcesof energy are coal oil and natural gas They will last for a whilelonger but each will probably become scarce andor harmful intens to hundreds of years1 To meet future energy needswithout injecting more carbon dioxide into the environmentnuclear energy is one of the best and cleanest sources ofenergy23 A few pounds of ldquonuclear fuelrdquo replaces thousands oftons of diesel or coal and provides electrical power without theair pollution associated with burning coal petroleum andvegetation There is however one major concern and that ishow disposal of the spent nuclear fuel is carried outldquoSpentrdquo or ldquousedrdquo nuclear fuel is fuel that has been irradiated

in a nuclear reactor (usually at a nuclear power generationplant) It is no longer useful for sustaining the nuclear reactionin an ordinary thermal reactor Although their masscontribution in spent fuel is relatively small transuranicelements such as plutonium and neptunium and minoractinides (americium and curium) are the primary contrib-utors to long-term radiotoxicitiy and heat generation in spentfuel4

The shortage of storage capacity and management of nuclearwastes is a worldwide problem5 Currently France Britain andRussia reprocess their spent nuclear fuel both for their ownfacilities and for other nations paying to have it recycled forthem No one has large scale storage plants in service while

spent fuel destined for those facilities is currently stored on-site6 One option to resolve the buildup is direct disposal into adeep geologic repository to isolate spent fuel for the hundredsof thousands of years that it may remain hazardous Anotheroption is to reprocess it and separate out the uranium andplutonium for use as new fuelAny waste processing whether for disposal or remediation

must have a minimal impact on the environment The ongoingproblem at Japanrsquos Fukushima Daiichi power plantcaused bythe March 11 2012 earthquake and tsunamihave beensignificantly exacerbated by the presence of spent fuel housed inthe reactor buildings They demonstrate the urgency of findinga way to deal with such waste especially as the amount of spentnuclear fuel housed at existing nuclear plants continue to growOne important action that might help counter the erosion ofpublic support for a renewal of nuclear power a portion ofwhich stems from the Fukushima crisis is to implement clearspent fuel policies nowCountries around the world that are looking to nuclear

power for their energy needs must consider how spent fuel willbe handled as they construct new reactors and examine existingones As more nuclear power plants become operative and

Received May 16 2013Revised August 17 2013Accepted September 26 2013

Critical Review

pubsacsorgest

copy XXXX American Chemical Society A dxdoiorg101021es402205q | Environ Sci Technol XXXX XXX XXXminusXXX

Table

ICom

parisonof

DifferentRadioactive

Waste

TreatmentMetho

ds

method

extractio

nmechanism

advantage

draw

backs

10solventextractio

nliquidminus

liquidionex-

change

1practicaltechniquein

large

2medium

scaleoperations

whensolute

concentrations

arehigh

1largeam

ount

oforganicwaste

2notforlowconcentration

11ionexchange

ionexchange

resin

1very

lowrunningcostsandvery

little

energy

isrequired

2theregenerate

chem

icalsarecheapandifwellm

aintainedresinbeds

canlastformanyyears

before

replacem

entisneeded

1foulingof

resin

2presence

oforganicmatter

3commerciallyavailableresins

arenonselective4Itisnotacontinuous

process

17precipitatio

nmetalsurfaceadsorptio

n1

good

atalkaline

2costeffective

3instrumentsareeasilyavailable

4high

degree

ofaccuracy

1notselective

2lowmetalrecovery

3tim

econsum

ingifdone

manually

18magnetic

separatio

nmagnetic

susceptibility

difference

1canseparate

almostallactin

ides

2thereareno

mediato

replaceor

disposeof

1will

notremovenonm

agnetic

materialsor

grinding

swarf

2itrequireslowflow

ratew

hich

increasesfloorarea

required

19volatility

processes

fluorid

evolatility

process

1nonaqueoustechnique

2used

toremovethecladding

onspentfuelelem

ents

1useof

halides

ampβ-diketones

2theradiolyticdecompositio

n20moltenfuelandcoolantsalt

processing

reprocessing

with

reagents

1inherent

resistance

totheradiationdamage

2thedecayheat

isuseful

formaintaining

thepyrochem

icalfluidat

processtemperature

1high

temperature

process

2lowoutput

3difficultprocessing

ofslag

3335121 M

ACS

chelator

extractio

n1

magnetic

separatio

n2

lowconcentration

3consum

ptionof

organicsolventsalmosteliminated

thechelatingagentswindup

inthewater

andenvironm

entalregulatio

nsprohibittheirdischarge

134 M

NP-Che

chelator

extractio

n1

better

kineticsfortheadsorptio

n2

efficientseparatio

nof

particles

3simpleversatileand

compact

4minimizesecondarywaste

themethodisunderdevelopment

Environmental Science amp Technology Critical Review

dxdoiorg101021es402205q | Environ Sci Technol XXXX XXX XXXminusXXXB

ldquospent fuel poolsrdquo which shield and cool the used fuel on siteapproach their capacity by 20157 the treatment of nuclearwaste materials is recognized as a matter of great urgencyAccording to a report in April 2013 from the GovernmentAccountability Office nearly 70000 metric tons of spentnuclear fuel resides at the countryrsquos 75 commercial nuclearpower plants waiting to be disposed of by the government Thisspent fuel inventory is expected to more than double by 20558

With long-term storage of used nuclear fuel there is potentialfor soil and groundwater contamination due to the performanceof interim and long-term geologic storage containers9

The next generation of nuclear power requires minimalgeneration of waste and maximum proliferation resistance (theadoption of reactor and fuel cycle technologies that are difficultto weaponize) Finding cost-effective and environmentallybenign technologies for spent fuel reprocessing with the goalof treating the radioactive liquid tank waste into a safe stableform for ultimate disposal or recycling is critical in our effortsto meet growing energy requirements protect the environmentand human health and allow our economy to flourishNuclear waste streams have traditionally been treated by

solvent extraction10 and ion exchange11 The PUREX(Plutonium Uranium Extraction) process invented byAnderson and Asprey in 1940s has become a standard nuclearreprocessing method to recover plutonium and uranium forreuse in mixed oxide (MOX) fuel1213 Besides these twoprocesses many other methods have been developed includingelectrophotolysis14minus16 precipitation17 magnetic separation18

volatilization19 molten fuel and coolant salt processing20 Someof these methods are in practical use but research intoalternative approaches that potentially simplify the remediationprocess is warranted We will first compare these majorradionuclide waste treatment methods and then describe indetail one appealing new method that uses chelating agentsgrafted onto magnetic nanoparticles (MNPs) conjugates ofMNP-Chelator (MNP-Che) in a solidminusliquid extractionprocess Table I summarizes the extraction mechanismsadvantages and drawbacks of each of these major nuclearwaste treatment methods(1) Solvent extraction processes such as PUREX are used

for extracting ion species from a liquid phase Uranium andplutonium are removed from spent fuel by dissolution in acidafter which liquidminusliquid extraction between aqueous andorganic liquid phase takes place They have been successfullyapplied in practice but there are limitations PUREX can onlyseparate U and Pu Moreover it usually does not provide theselectivity necessary to create valuable product streams suitablefor recycling or reusability(2) Ion exchange has been widely studied for the recovery of

metal ions from diluted streams Commercially available ion-exchange resins perform well but they generally exhibit poorselectivity between different metal ions A high selectivity canbe observed in some cases but the kinetics are slow due to thehydrophobic character of the polymeric backbone21

(3) Electrolysis uses electric field to separate charged speciesfrom one another in electrolyte solution due to differences intheir mobility22minus25 The driving potential of this process isprovided electrically and can be controlled very precisely toproduce a cathode product that is extremely pure Howeverthis technique has difficulties when scaled up since heatdissipation is required to prevent convection currents fromperturbing the migration pattern It is also not cost-effective fortreating low concentration waste

(4) Recently biological methods have been used toinvestigate the removal of actinides and heavy metals due totheir cost effectiveness at moderate metal concentrationsPrecipitation can be achieved by sorption onto hydrous oxidesurfaces Many strains have been isolated that precipitate outmetals on the outer membrane of the cell26minus29 This processmay not be applicable in more complex waste streams orenvironments due to the relatively low binding affinities ofcellular components for metals compared to chelators such ashumic or organic acids(5) Magnetically assisted chemical separation (MACS)30

processes utilize magnetic carrier microparticles (rare earth orferromagnetic materials embedded in a polymer material)coated with selective chemical extractants (Carbamoylmethyl-phosphine oxides (CMPO) tributyl phosphate (TBP) aminesetc) that have an affinity for the target elements Though thisprocess shows promise as an efficient and compact separationtechnology dissolution of the bare magnetic particles in acidcould be a limitation of the recyclable batch process31 Thepossibility of designing particles with large surface area shouldbe investigated for employment in large scale separations(6) Volatilization isolates uranium from bulk impurities or

fission products This process has some limitations Thevolatilization of molten fuel and coolant salt requires hightemperature In addition some fluorides of other transuranicelements are not volatile andor are unstable while a numberof elements that can be volatilized such as molybdenumtechnetium and iodine separate from the spent fuel along withthe uranium These present formidable engineering problemsto be solved before the process can be applicable32

The concept of using the MNP-Che complex to separateradionuclides while in the contaminated liquid stream is similarto the so-called MACS process first demonstrated in ArgonneNational Laboratory3133minus35 The applicable nuclear wastestream for this treatment method is aqueous and it may eithercome from the strong acidic high level waste produced by thePUREX process low level waste from other processes orunderground contaminations The MACS process combinesthe selective and efficient separation afforded by chemicalsorption with magnetic recovery of the radionuclide The keyadvantages of this MNP-Che radioactive waste remediation arethe high mobility related with nanomaterials36minus39 and therecent development of environmentally benign chelators4041

MNP-Che separation has many advantages over MACS processsuch as (1) MNPsrsquo extremely small size and high surface area tovolume ratio provide better kinetics for the adsorption of metalions from aqueous solutions (2) the high magneticsusceptibility of MNPs aids in efficient separation of particlesfrom waste solution (3) it is a simple versatile and compactprocess which is cost-effective in terms of the materials andequipment used (4) the production of secondary waste is lowwhich minimizes disposal costs and storage areaIn order to separate the radioactive waste from the aqueous

stream the MNP-Che conjugations are first added to the wastestream which can either be in a tank or in situ To maintain theconjugatesrsquo suspension the treatment stream can be mixed bymechanical stirring or other methods The actinides are thenextracted onto the MNP-Che conjugates usually taking aboutan hour to reach saturation42 The particles now loaded withactinides are magnetically collected and separated by amagnetic field gradient The waste solution is decontaminatedand can be released The actinide-loaded particles can then bestripped with a small amount of liquid (relative to the original


dxdoiorg101021es402205q | Environ Sci Technol XXXX XXX XXXminusXXXC

waste stream) and magnetically separated from the resultingsolution allowing their reuse in the next waste treatmentFinally the concentrated actinide contaminants are ready to betreated for permanent waste disposal or for a fuel recycleprocess An illustration of this process is shown in Figure 1AThe steps to make the MNP-Che for this waste treatment are

shown in the flowchart in Figure 1B First the MNPs withspecific properties suitable for the separation are synthesizedThe MNPs are coated in order to prevent their leaching underacidic nuclear waste streams Finally the most criticalconjugation process takes place between the actinide specificchelators and MNPs which produces MNPs-Che conjugatesthat are ready for the remediation process With highly selectivechelators this method can be developed into a versatile andsimple separation technology that facilitates the recovery andrecycling of heat generating minor actinides contained withinthese streams No significant reduction in radiological hazard ofthe waste can be obtained without recycling the minor actinides(Np Am Cm)4 A fully closed fuel cycle where all actinides arerecycled through fast reactors or accelerator driven systemscould potentially reduce repository storage space radiotoxicityof the waste and heat loading in the final waste form Toachieve this goal however the difficult separation of the minoractinides Am and Cm from the trivalent lanthanides has to beperformed43

Almost two decades have passed since the original researchusing chelator-loaded magnetic particles to treat the radioactivewaste stream was performed333544 Much progress has sincebeen made in radioactive waste remediation using MNP-Che

conjugates This review focuses on the design and processdevelopment of the MNP-Chersquos radioactive waste treatmentmethod First applicable MNPs suitable for this process arereviewed comparing different MNPs and their synthesisprocesses Second the chelators that are used to extractradioactive wastes are compared and their extractionmechanisms are briefly summarized Subsequently theconjugation process between MNP and chelator is describedThe experimental and pilot test results of separation usingMNP-Che are reviewed with respect to their correspondingchemical reaction parameters and waste stream conditionsFurthermore the magnetic separation and recycling of particlesare described Finally research progress is summarized and thefuture direction of radioactive waste remediation technologyusing MNP-Che conjugates is outlined

II MAGNETIC NANOPARTICLESThe first crucial step in successful waste treatment using MNP-Che conjugates is to select a MNP to be conjugated with thechelator In this section we briefly review the synthesis andcharacteristics of different MNPs that are currently used andcan be used in the future for this separation technology TableII reviews some of the particles and related chelators used toseparate actinides along with the corresponding experimentalconditions and removal efficiencies The high mobility ofMNPs with large specific surface areas and their controllableseparation by magnetic field makes it possible to use MNP-Checonjugates to extract radioactive waste This method becomesmore attractive with higher competency compared to other

Figure 1 (A) Illustration of MACS process for the radioactive waste streams treatment using MNP-Che conjugates (B) Flowchart of nuclear wasteseparation using complex conjugates of MNPs-chelator


dxdoiorg101021es402205q | Environ Sci Technol XXXX XXX XXXminusXXXD

separation technologies of minor actinides due to thecontinually decreasing production cost of MNPs45

Generally the nuclear waste stream occurs in harsh acidic oralkaline conditions46 Therefore the first important criterion for

MNP candidates is that they must be chemically stable duringsynthesis in storage and while in use Magnetic oxidesincluding iron oxides and other ferrites are usually chemicallystable with natural oxidization resistance However oxides

Table II Magnetically Assisted Chemical Separation processes


dxdoiorg101021es402205q | Environ Sci Technol XXXX XXX XXXminusXXXE

usually have low magnetic moments compared to metallicMNPs where a high magnetic moment is required for effectivemagnetic separation Metallic MNPs would therefore serve wellbut since they are extremely reactive they will oxidizeimmediately when exposed to air or dissolved in acidicconditions To solve this difficulty magnetic coreminusshellnanostructured particles with a metallic core and a protectiveoxide shell are used They are chemically stable in addition tohaving high magnetic moments Some common oxide particlessuitable for the nuclear waste separation are first reviewed withregard to their magnetic properties and chemical stabilityThen typical coreminusshell structures are reviewed with regard totheir applications for radioactive waste separation At the end ofthis section the main synthesis methods to prepare theseparticles are compared with respect to the nuclear wasteremediation processA Oxides Superparamagnetic γ-Fe2O3 (maghemite) and

Fe3O4 (magnetite) nanoparticles are commonly used formagnetic separation due to their well-established synthesisand surface functionalization processes3647The activation ofoxide MNPs is driven by two factors (1) the small particles(ltl00 nm) have a larger surface area for transuranic (TRU)waste adsorption and (2) oxide particle adsorbs hydroxideions through which the metal ions bond to the particlesurface31

B Coreminusshell Nanostructured Particles The coreminusshellstructure consists of a dark Fe metal core region surrounded bya light gray oxide shell as shown in the inset transmissionelectron microscopy (TEM) image of Figure 2 Compared to

oxides coreminusshell MNPs have a higher magnetization in thesame size of MNPs that facilitates the magnetic separation ofthe conjugates after the radionuclide adsorptionIron release is also a problem for magnetite particles which

dissolve within seconds if released into concentrated HCl48 Topreserve the high magnetic moment of MNPs in the nuclearwaste stream different types of surface coatings are used forstabilization Since the particles are more susceptible to damagefrom acid degradation than gamma radiolysis and the coatinghelps to shield the metallic part of the particle when in acidicalkaline solution the magnetic moment is preserved The typesof coating discussed here are polymer silica carbon and gold

(a) Polymer Coating Properties of polymer coatingsincluding steric repulsion prevent agglomeration of theMNPs allow the polymer coated particles to be stabilized insuspension Polymers containing different functional groupssuch as carboxylic acids sulfates and phosphates can beimmobilized or physically adsorbed on the particle surface49

The polymer layer can form a single or double layerdepending5051 on the solution conditions such as pH Thislayer creates repulsive force or steric repulsion to balance theattractive forces such as magnetic and van der Waals forcesmaking the particles more stable in solution The polymercoated MNPs can be synthesized by a single inversemicroemulsion52 or by using polyaniline53 and polystyrene54

In highly acidic conditions the polymer layers are not stableand are easily leached into the solution55 which results inprotonation of surface hydroxyl groups on the particle anddegradation of the magnetic moment Though surfacemodified polymer coated MNPs are intensively studied fordrug delivery and as contrast agents for magnetic resonanceimaging5657 they are not strong enough barriers to preventoxidation of particles in the very acidic medium of spentnuclear fuel Also the polymer coating of MNPs is verysensitive to temperature and loses its stability at hightemperature In that environment a polymer coating is stilleffective to achieve a higher chemical affinity between thechelator and the MNPs31 but only after a different protectivecoating has been applied

(b) Silica Coating The benefit of silica as a coating materialmainly lies in its high stability especially in aqueous media butother reasons include easy regulation of the coating processchemical inertness controlled porosity ease of processing andoptical transparency The main factors favoring the remarkablestability of silica sols are (i) van der Waals interactions aremuch lower than those involving other nanoparticles and (ii)cations and positively charged molecules can be tightly attachedto the characteristic polymeric silicate layer at silicaminuswaterinterfaces under basic conditions58 Thus this silicate layer canconfer both steric and electrostatic protection on different coresand act as a dispersing agent for many electrostatic colloidsThese advantages render silica an ideal low-cost material totailor surface properties Moreover this coating endows thecores with several beneficial properties such as the possibility ofsubsequent surface functionalization which can be obtained bymodifying the hydroxyls on the silica surface with aminesthiols carboxyls and methacrylate and colloidal stability over amuch wider range of solution conditions such as ionic strengthtemperature solvent polarity and pH59minus62

Two approaches have traditionally been used to coatparticles with silica The first technique described by Iler6364

involves deposition of a thin layer of silica onto oxide particlesby polymerization of silicic acid from supersaturated aqueoussilicate solutions A second technique solminusgel coating is basedupon the StOber method65 for forming uniform silica gelmicrospheres66 Other less common techniques includesodium silicate water glass methodology the two stepprecipitation method and water in oil microemulsions

(c) Carbon Coating Carbon-coated metal nanoparticleshave received considerable attention for advantages overpolymer and silica in high chemical and thermal stability67minus69

Several preparation methods such as arc-plasma70 thermaldecomposition of organic complexes71 arc discharge72 orchemical vapor deposition (CVD) have been employedThough the arc discharge method is the most popular high

Figure 2 Specific magnetization of sputtered coreminusshell MNPs withvarious sizes The inset figure is the TEM images of the coreminusshellMNPs The MNPs is totally oxidized when the size decreased to lt2nm89


dxdoiorg101021es402205q | Environ Sci Technol XXXX XXX XXXminusXXXF

yield and low costs coupled with simple procedure make thecombustion CVD method7374 a good candidate for futureindustrial fabrication of carbon-encapsulated magnetic nano-particles73 The well-developed graphitic carbon layers providean effective barrier against oxidation and acid erosion Thesefacts indicate that it is possible to synthesize carbon-coatedMNPs which are thermally stable and have high stabilityagainst oxidation and acid leaching Though carbon-coatedMNPs have many advantages such particles are often obtainedas agglomerated clusters owing to the lack of effective syntheticprocedures and a low degree of understanding of the formationmechanism The synthesis of dispersible carbon-coatednanoparticles in isolated form is one of the challenges facingthis field36

(d) Gold Coating Precious metals can be deposited on thesurface of MNPs to protect the cores against oxidation Goldseems to be an ideal coating owing to its low reactivity and theability of its surface to be further functionalized especially withthiol groups75 This treatment allows the linkage of functionalligands which may make the materials suitable for catalytic andoptical applications Gold coated particles are stable only underneutral and acidic aqueous condition76 they dissolve in highlyacidic medium such as aqua regia Gold coated particles andthiol groups would not survive the strongly acidic environmentof nuclear waste In order to apply these particles in spentnuclear fuel separation additional coatings that can resist77

highly acidic environments are required to prevent the particlesfrom dissolving in the medium The cost of gold and extra workassociated with additional coatings make this process veryexpensive and complex Hence the gold coated particles are notrecommended for nuclear waste treatmentC Synthesis of Magnetic Nanoparticles MNPs have

been synthesized with a number of different compositions andphases including iron oxides such as Fe3O4 and γ-Fe2O3

7879

pure metals such as Fe and Co8081 spinel-type ferromagnetssuch as MgFe2O4 MnFe2O4 and CoFe2O4

8283 and alloys suchas CoPt3 and FePt8485 Several popular methods includingcoprecipitation thermal decomposition andor reductionmicelle synthesis hydrothermal synthesis and laser pyrolysistechniques can all be directed at the synthesis of high-qualityMNPs Instead of compiling all this literature the maintechniques for MNPs synthesis (vapor condensation solid-stateprocesses chemical synthesis and coprecipitation) arediscussedi With vapor condensation evaporation of a solid metal

followed by rapid condensation forms nanosized clusters thatsettle in the form of a powder Various approaches to vaporizethe metal can be used and variation of the medium into whichthe vapor is released affects the nature and size of the particlesDifferent cluster sizes ranging from 2 to 100 nm can be

prepared using a cluster deposition system86 Nanoparticle sizeis controlled by changing the ratio of argon to helium gaspower aggregation length and temperature The ionized Arions sputter atoms from the surface of a target The sputteredatoms form nanoparticles in the aggregation chamber whichthen travel to the deposition chamber due to pressuredifference There they react with a small amount of oxygen(sim2 to 5 sccm) to form a protective oxide shell on the particlesbefore finally landing softly onto a room-temperaturesubstrate8788 Figure 2 shows the magnetization of coreminusshellFe MNPs as a function of particle size89 A major advantage ofthis method is that the particles have much smaller size

dispersion than grains obtained in any typical vapor depositionsystemii In solid-state processes grinding or milling can be used to

create nanoparticles90 For example high energy ball milling hasbeen utilized to break particles into nanosized subparticlesthrough high energy bombardment The milling materialmilling time and atmosphere during milling affect theproperties of resultant nanoparticles This approach can beused to produce nanoparticles that are hard to synthesize fromthe chemical and gas aggregation methods Contaminationfrom the milling material can be an issue in this methodThough mechanical milling is a versatile technique to producemetallic micropowders the particle size below 100 nm is notachievable that limit to get the high surface area nanoparticles91

iii Chemical methods92 have been widely used to producenanostructured materials due to their straightforward natureand their potential to produce large quantities of the finalproduct Particle sizes ranging from nanometers to micrometerscan be achieved by controlling the competition betweennucleation and growth during synthesis It is well-known 93 thata short burst of nucleation followed by slow controlled growthis critical to produce monodisperse particles Chemicalsynthesis methods are generally low-cost and high-volumebut contamination from the precursor chemicals can be aproblem90

iv Coprecipitation3694 is a facile and convenient way tosynthesize iron oxides (either Fe3O4 or γ-Fe2O3) from aqueousFe2+Fe3+ salt solutions by the addition of a base under inertatmosphere either at room temperature or elevated temper-ature95 The size shape and composition of the MNPs dependon the type of salts used (eg chlorides sulfates nitrates)Fe2+Fe3+ ratio reaction temperature pH value and ionicstrength of the media Magnetic saturation values of magnetitenanoparticles are experimentally determined to be in the rangeof 30minus50 emug which is lower than the bulk value 90 emugParticles prepared by coprecipitation unfortunately tend to berather polydisperse Also this method is limited to the synthesisof colloidal solution of iron-oxide particles

III CHELATORS FOR SORPTION OF RADIOACTIVEELEMENTS

Chelators or ligands are defined by American Society forTesting and Materials (ASTM-A-380) as ldquochemicals that canform soluble complex molecules with certain metal ionsinactivating the ions so that they cannot normally react withother elements or ions to produce precipitates or scalerdquo Anyreagent used in the separation process must be chosen to fulfillseveral challenging criteria96 The chelator must show a goodlevel of selectivity for the actinides so that the separationprocess can be carried out in a relatively small number ofextraction stages The organic phase solubility of both thereagent and its extracted complexes should be high to minimizethe possibility of third-phase formation or precipitation Theligand must show a sufficiently high level of or acceptableresistance toward acidic hydrolysis and radiolysis and anydegradation products that form must not interfere with theseparation process to any significant degree Finally thechelator should be able to extract from nitric acid (HNO3)solutions of low pH (le4 M HNO3) that are produced in thePUREX process If possible it should only be composed of theelements C H O and N (CHON principle) so that any spentsolid residues solvent streams or ligands may be degradedwithout producing corrosive products at the end of their


dxdoiorg101021es402205q | Environ Sci Technol XXXX XXX XXXminusXXXG

valuable life reducing or avoiding altogether the generation ofsecondary waste In order to make possible large-scaleproduction of these ligands the synthesis of the chelatorshould be as simple cost-effective and practical as possibleSeveral European Union funded research projects studied a

number of substituted diamide ligands such as

i N Nprime-dimethyl-N Nprime-dibutyltetradecylmalonamide(DMDBTDMA)97

ii NNprime-dimethyl-NNprime- dioctlyhexoxyethylmalonamide(DMDOHEMA)98

iii NNNprimeNprime-tetraoctly diglycolamide (TODGA)99 for theDIAMEX (diamide extraction)100 process

Trivalent actinide (III) ligands can be categorized into threemajor groups O-donating ligands S-donating ligands and N-donating ligands40The general rules of coordination chemistrystate that soft donor atoms favor binding to soft metal ionswhile hard donor atoms favor hard ions The hard or softcharacter of a metal ion depends on oxidation state forexample Fe(III) is harder than Fe(II) and Cu(II) is harder thanCu(I) Based on these general rules oxygen is a hard donoratom that binds well to hard cations such as Ca(II) and Mn(II)sulfur is a soft donor atom that binds well to Cd(II) and Cu(I)and nitrogen is of intermediate hardness between oxygen andsulfur101

1 O-Donating Ligands This group of ligands covers abroad range of O-bearing functionalities (eg phosphonatesamides ketones and phenols) CMPO (octyl (phenyl)-NN-di(iso-butyl)) and carbamoyl-methylphosphonates (CMPs) arewell-known O-donating ligands A derivative of CMPOs ispresently used in the transuranium extraction (TRUEX)process102103 Upon complexation with the metal cation itforms a six membered chelate ring CMPO chelator and itschelation complex with americium (Am) are shown in theFigure 3A

The O-donor ligands are generally strong extractants owingto the hard nature of the oxygen atom This type of ligandsusually lack discrimination between the same oxidation state ofAm(III) and Eu(III) which results in a relatively low separationfactor104one distribution ratio divided by the other It is ameasure of the ability of the system to separate two solutes Allsuccessful aqueous processing options for group separation offission product lanthanides from trivalent actinides have reliedon the applications of ligand donor atoms ldquosofterrdquo than oxygen(N S Cl)105106 The appearance of hard donor oxygen atoms(resulting from degradation of the soft-donor extractantmolecule) has been shown to significantly degrade theeffectiveness of many soft donor separation systems107

2 S-Donating Ligands Cyanex 301 is a dialkyldithio-phosphinic acid extractant containing C H P and S Thissulfur-containing compound is a good example of an Am(III)chelator having a very high separation factor due to thepreferable covalent binding of Am(III) to the relatively softersulfur donor atom The oxidation behavior of Cyanex 301reduces its selectivity when it oxidizes to Cyanex 302 while itincreases again when further oxidized to Cyanex 272 Thischaracteristic minimizes its applicability in industrial processesWade et al have discussed the application of soft donor ligandsto the separation of trivalent actinides and actinides108 The S-donor bidentate dithiophosphinic acids were the first reagentsthat show very high selectivity for actinides over lantha-nides109minus112

3 N-Donating Ligands N-donor ligands are classified asan intermediate between the O-donor and S-donor ligands withrespect to extraction efficiency and Am(III) selectivity Due tothe chelate effect extraction efficiencies are also generally betterfor ligands having a higher valency The nitrogen atom isintegrated into an aromatic ring in most of the N-donor ligandsSuch ligands are able to replace all coordinated watermolecules which is most likely one reason for its high

Figure 3 (A) Traditional CMPO chelator and the its chelation complex with Am40 (B) Environmentally friendly chelator DMOGA and itschelation complex with UO2

41 (C) MNP coupled by DTPA chelator wraps around Am (III)


dxdoiorg101021es402205q | Environ Sci Technol XXXX XXX XXXminusXXXH

extraction efficiency The first N-donor ligand found toselectively extract Am(III) from Eu(III) was tridentate aromaticnitrogen donor ligand 22prime62Prime-terpyridine (TERPY)3

Tripyridyltriazine (TPTZ) is a terdendate N-donor ligandwhich can selectively extract actinide An(III) from actinideand lanthanide solution when applied in synergistic combina-tion with an organic cation exchanger113114

More information about recently developed ligands can befound in ref 3The choice of the ligand donor atom depends on the

chemical nature of the metal Actinides are considered to behard Lewis acids115 and will be preferentially bonded witheither oxygen or nitrogen donors over sulfur donors116

Traditional ligand such as CMPO have phosphorus whileenvironmentally friendly ligands contain only of C H O andN alkyl-substituted amides which have attracted moreattention recently due to their more efficient separationprocess41 Also stripping actinides from the organic amide-containing solvents is relatively easy In contrast to the largeamounts of liquid andor solid radioactive waste generated intraditional organophosphorus-based separation processes theamount of solid radioactive waste generated in the amide-basedprocesses is significantly reduced An environmentally friendlychelator NN-dimethyl-3-oxa-glutaramic acid (DMOGA)117

and its complex with UO2 is shown in Figure 3C

IV CONJUGATION OF CHELATORS AND MNPSAn important consideration for applications of the MNP-Checonjugates is the absorption or binding strengths exhibitedbetween the consequent coating and actinide specific chelatorsLoss of interfacial integrity in either case under high level wastetank conditions is undesirable To combine the chemicalseparation capability of chelators and the convenient magneticseparation of MNPs strong conjugation is a key step torealizing nuclear waste remediation using MNP-Che con-jugatesThe strong acidic medium of nuclear waste streams makes it

necessary to protect the surface of the particles In many casesthe protective coatings not only stabilize the MNPs but canalso be used for further surface functionalization with forinstance various ligands Physical (or electrostatic) adsorptionand chemisorption are the principal types of interactionbetween an organic ion (chelatorligand) and the metal surfaceof a coated particle118

A Physical Adsorptive Coatings Physical adsorp-tion119120 is the result of electrostatic attractive forces betweenorganic ions or dipoles and the electrically charged surface ofthe metal (particle) This type of interaction is known ashydrophobic or ionic In the early report of the MACS processthe extractant was usually mixed with the magnetic particles andevaporated to obtain coated particles for waste treatment(plutonium and americium separation) The particles wereprepared by coating iron or other ferromagnetic material witheither an organic polymer or ion-exchange resin The ion-exchange resin was attached to the particle by an adhesive or bydirect bonding The report found that organic solvents could beadsorbed onto the polymeric surface by contacting the particleswith the solvent in volatile diluents that were subsequentlyremoved by evaporation Both coatings selectively separatecontaminates onto the particles due to their chemical natureOnce loaded the particles can be recovered from the tank usinga magnet The most reported magnetic particles are theCharcoal poly bis-acrylamide coated magnetite particles3334121

Usually particles coated by this method are only partiallycovered Magnetite is soluble in nitric acid thus the dissolutionof magnetite prevents recovery of the particles and hinders theseparation of radioactive waste

B Covalent Bonded Coatings Another type ofinteraction between metal particles and organic ions knownas chemisorptions forms a covalently bonded coating Thisprocess involves charge sharing or charge transfer from theorganic molecules to the metal surface in order to form acoordinate bond Once the protective coating is on theMNPrsquosurface the particles can be covalently conjugated withactinide specific chelators Such chemically modified particlesshould show increased long-term stability since the ligandscannot be desorbed or leachedThe availability of specific functional groups on the

polymeric coated particles enables them to chemicallyconjugate the chelators such as CMPO and TBP Theselection criteria of polymers include structural similarity toand chemical affinity for the specific chelator solubility andmelting point A slightly porous polymer with differentchemical affinity than phosphine oxides allows for greateracceptance of CMPOTBP31 The vaporization method forimpregnation of the solvent extractant CMPO yielded productthat had a high sorption capability for radionuclide europiumas compared to product of the wet impregnation method122

CMPO groups can be attached to the surface of MNPs byreaction of p-nitrophenyl(diphenylphosphoryl) acetate with theterminal amino groups of the coated MNPs For example thecovalent attachment can take advantage of the preorganizationof the chelating sites on various macrocyclic platforms likecalix[4]arenes The length and the number of linkers betweenthe calixarene and the particle surface also affects the extractioncapacity of chelator-functionalized MNPs Linkers are used toprovide a ldquochemicalrdquo spacer between the solid surface (particle)and the receptor that is anchored to the surface by anappropriate functional group They spatially extend thereceptor from the surface increasing its accessibility by thesolution ligand and removing any nonspecific adsorption Ingeneral increasing the chain length between the ligand and thecalix[4]arene seems to reduce the selectivity while enhancingthe distribution coefficients However for calix[6]arenesubstituted on the narrow rim increasing the chain lengthbetween the ligand and the calixarene tends to reduce both theselectivity and the distribution coefficients Structural repre-sentation of calix[4]arenes is shown in Figure 3B

V SORPTION OF THE NUCLEAR WASTE

After the selection of suitable MNPs and chelators andcompletion of the conjugation process the MNP-chelatorcomplex is ready to be applied in the treatment of aqueousnuclear waste streamsThe distribution coefficient (Kd)

34 a measure of extraction isusually used to assess the extractant in adsorption of nuclearwaste If a solute is introduced into any two-phase systemwhether the system is gassolid gasliquid liquidliquid orliquidsolid it will become distributed between the two phasesAfter equilibrium is reached the solute distribution is definedby the distribution coefficienttab

=minus

KC C

CVm

( )fd

i

f


dxdoiorg101021es402205q | Environ Sci Technol XXXX XXX XXXminusXXXI

where ci and cf are the metal ion concentrations before and afterextraction V is the volume and m is the dry weight of thecoated (or functionalized) MNPsA higher Kd value implies greater effectiveness of the chelator

in capturing and holding the metal ions The values of 103 mLg are considered good and those above 104 mLg areoutstanding123

The sorption performance of chemical extractants dependson the pH and ionic strength of the waste stream which isexplained later in the text These factors are closely related tothe surface charge of the particles Extractant concentration inturn affects the sorption through reaction kinetics Due to thechemical coordination between the nuclear waste and theextractant the effects of temperature and sorption time shouldalso be consideredThe sorption process is generally studied by plotting a

compoundrsquos concentration in the sorbent as a function of eitherits gas phase or solution concentration all of which aremeasured at equilibrium and constant temperature Aclassification of isotherms has been reported Some of themsuch as the Langmuir BrunauerminusEmmettminusTeller (BET) andGibbs models often fail to adequately describe sorption data inthe liquid phase This leaves only Freundlich and linear modelsas usable fits to the sorption data124

Some research results using MNP-Che processes arecompared in Table I The main process includes adding theMNP-Che complexes into the waste stream which can be inthe reaction chamber or in situ as some cases in tanks Tomaintain particle suspension the tank contents can be mixed bymechanical stirring ultrasonication or other methods After therequired reaction time physical separation of the particles fromthe solution can be achieved by using a magnetic field to collectthe actinide adsorbed MNP-Che complexes The left overuncontaminated stream can be removed for safe disposal Asmall amount of strip feed is added to the MNP-Che-Actinidecomplex to separate the actinide from the MNP allowing thelatter to be recycled Usually a volume reduction (processingwaste materials to decrease the space they occupy by biological(composting) mechanical (compacting shredding) or thermal(incineration vitrification) means) factor higher than 100 canbe achieved With similar magnetic separation the MNPs orMNP-che conjugates can be removed magnetically leaving acondensed form of actinide waste that is ready for final disposalor recyclingThe key element of the technology is the particlesrsquo selective

adsorption These composite particles consist of a magneticcore a polymer coating for durability and either a ldquofunction-alizedrdquo resin coating or selective seed materials embedded inthe polymer coatingIn this next section the effects of important parameters on

sorption are discussed in detailA Effect of MNPs and Chelators The chemistry between

the chelator and the actinides plays a significant role in thesorption of radionuclides High temperature concentratedreactant and catalysts all increase the reaction speed As thechelatorrsquos contact area increases more sorption sites becomeavailable to bind the actinides consequently increasing the netefficiency of sorption The sorption capacity of a given chelatormay depend on a series of properties such as particle-sizedistribution cation exchange capacity solution pH and ionicstrength125 Examining these properties will both help topredict the partitioning of the various radionuclides in a realsystem and provide a means to optimize the waste stream

separation using the open gradient magnetic separation(OGMS) system126

OGMS is a type of high gradient magnetic separation systemthat consists of multiple magnetic field sourcesThe sorption efficiency can be also optimized during the

chelator conjugation process As reported by Buchholz et al121

the best Kd values for Am were obtained for extractantsprepared with concentrations in the range of 10minus12 M CMPOin TBP 121 Particles prepared with lower concentrationsproduced less homogeneous coatings due to insufficientCMPO whereas particles prepared with 15 M CMPO inTBP remained sticky after heating and did not disperse wellTherefore coatings prepared with 10 and 12 M CMPO inTBP yielded high and consistent Kd values

B Temperature and Extraction Time 1 TemperatureGenerally sorption coefficients decrease with increasingtemperature127 However some examples of increasingequilibrium sorption with increasing temperature128 and noeffect of temperature on sorption equilibrium were also foundChiou et al 129 observed that an inverse relationship130

between sorption coefficients and solubilities exists for organiccompounds Lower Kd values are found at higher temperaturesfor most organic compounds for which solubility increases withtemperature while increased sorption at higher temperaturescan be expected for compounds for which solubility decreaseswith temperature Therefore due to the dependence of bothsorption coefficients and solubility on temperature themeasured effect of temperature on sorption isotherms is thecombined result of sorption and solubility trendsTo make the MACS process applicable to wastes that have

been stored in tanks for decades the reaction temperatureshould be in the range of 20minus50 degC The extraction ofamericium by 1 M CMPOTBP coating as a function of nitricacid concentration was analyzed at three different temperatures(10deg 25deg and 50 degC)31131 The extraction result of Am inFigure 4A shows that all Kd curves have the same general shapeirrespective of temperature On the other hand the Kd value ofmany solvent extraction systems decreases with increasingtemperature between 0 and 60 degC132

2 Extraction Time The amount of time required tocomplete extractions differed between solvent types133 Theseparation relies on the chemical reaction between theextractant and the extractor The kinetics usually reachequilibrium within 1 h Some extractors show slower kineticsfor a certain extractant which may be related to the viscosityand chemical reaction conditions Due to these saturationphenomena the Kd values are usually not constant and thusonly values obtained under identical conditions (concentrationin the liquid phase amount of solid phase) can be compared Itcan be seen from Figure 4B that the Cyanex 923-coated42

magnetic particles exhibit slightly slower uptake kinetics forU(VI) (30 min) Am(III) (30 min) and Eu(III) (30 min) thanfor Th(IV) (15minus20 min) The sorption efficiency of actinidesremained the same after 30 min irrespective of contact time orextraction timeThe uptake for various actinides - Am(III) Np(V) Pu(IV)

and U(VI) at pH 1 and Am(III) U(VI) and Np(V) at pH 3by diethylene triamine pentaacetic acid (DTPA) chelatorconjugated MNPs (DTPA-MNPs) shows maximum sorptionefficiency for Am at pH 3 and Pu at pH 1 in Figure 5 Thesorption reaches saturation in less than 7 min and remainsstable until total extraction time reaches 4 h134


dxdoiorg101021es402205q | Environ Sci Technol XXXX XXX XXXminusXXXJ

C Waste Stream Condition The separation depends onthe pH and ionic strength of the waste stream1 Effect of pH Solution pH is the single most important and

dominant variable that influences the behavior of metals in the

environment uptake and percent removal of metal ions fromthe waste solution and adsorption rates The pH determinesthe surface charge of the adsorbent (organic ion or chelator)the degree of ionization and the specification of the adsorbate(metal ion) This is partly due to the fact that the hydrogen ionitself is a strong competing adsorbate135136

The hydroxyl groups present on the MNPrsquos surface give it acationic nature As the pH of the solution decreases thehydronium ion concentration increases resulting in proto-nation of surface sites and a net increase in positive charge onthe surface On the other hand as pH increases the samesurface sites deprotonate and cause the surface to be negativelycharged As a result free metal cations exhibit near zeroadsorption at low pH and free metal anions exhibit 100adsorption Hydroxyl carbonate chloro- etc complexeschange the size and charge (ie charge density) of the cationand that affects the net adsorption efficiency137

For pure mineral phases and natural sediments U(VI)adsorption tends to increase with increasing pH from pH 35 toabout 8 At pH gt 9 the adsorption declines due to theformation of anionic carbonatehydroxyl complexes Theincrease in adsorption as the pH rises from acidic to neutralis attributed to the opening of sorption sites vacated by thecorrespondingly decreased proton concentration in clays andmineral surfaces138

The chemistry of Pu is remarkably complex due to its manyoxidation states (III IV V VI) in solution the tendency forPu(IV) to disproportionate and the slow rate of reaction of Pu-oxygen species (eg PuO and PuO+)139 The Kd value of Pustudied at four different pH values (8 10 12 and 14) showsthe maximum at pH 12 Pu removal is decreases if the pH isbelow 115 or above 135140141

The influence of pH on the extraction of actinides likesAm(III) at pH 1 and 3 by DTPA-MNPs shows lower removalof Am(III) in highly acidic medium as given in Figure 6134 Itcan be explained either by the degree of protonation of thecarboxylate groups of DTPA which is higher at lower pH(strong acid) or by the stability of the chelator conjugates ofMNPs (DTPA-MNP) which might be lesser in high acidicmedium

Figure 4 (A) Extraction of Am as a function of temperature for 1 MCMPOTBP coating131 (B) Contact time effect on the separationefficiency of the actinides with (A) Cynax923 and (B) CMPOTBP4279

Figure 5 Distribution coefficient (Kd) as a function of variousactinides such as Am(III) Pu(IV) U(VI) and Np(V) by MNPscoupled through DTPA chelator134

Figure 6 Removal percentage of Am(III) by DTPA-MNPs as afunction of sorption time at pH 1 and pH 3 The inset figure shows theMNP coupled by DTPA cheltor wraps around Am (III)134


dxdoiorg101021es402205q | Environ Sci Technol XXXX XXX XXXminusXXXK

Dendrimer-coated magnetic silica particles with picolinamidederivative show strong dependence of Am and Eu extraction onthe pH value The Kd values for Am and Eu are extremely highat pH 3 compare to those at pH 1 and 2142

Generally sorption increases as pH increases In order toremove the sorbed species the pH would need to be adjustedto the acidic range provided that aging effects are minimal thefate of sorbed species on oxide surfaces is not well understoodthough it is believed that as time passes a slow reactionbecomes important2 Ionic Strength Effect In general surface complexation is

influenced by pH values whereas ion exchange is influenced byionic strength143 The ionic strength dependency of themagnetic separation process is important when applying thisprocess to waste streams that carry a wide range of acidities144

The ionic strengths of 0001 001 and 01 M NaNO3 werechosen to investigate their effect on Th(IV) sorption onto Na-rectorite The sorption of Th(IV) decreased with increasingionic strength (Figure 7)145 This phenomenon could be

attributed to two causes (1) Th(IV) ions form electricaldouble layer complexes with Na-rectorite which favor sorptionwhen the concentration of the competing salt is decreased Thismay indicate that the sorption interaction between thefunctional groups of Na-rectorite and Th(IV) ions is mainlyionic in nature which is in agreement with an ion exchangemechanism (2) ionic strength of the solution influences theactivity coefficient of Th(IV) ions which limits their transfer toNa-rectorite surfaces The sorption of Th(IV) onto γ-Al2O3 andSiO2 in the absence or presence of humic acid (HA)fulvic acid(FA) was weakly dependent on ionic strength Jakobsson 146

investigated the sorption of Th(IV) onto bare TiO2 and foundthat the sorption was independent of ionic strength Reiller etal147 studied the sorption of Th(IV) onto hematite (α-Fe2O3)and goethite (α-FeOOH)148 and also found that the sorptionof Th(IV) was not strongly influenced by ionic strength in thepresence of HA

VI SEPARATION AND RECYCLING OF MNPSThere are two common and simple ways used to separate thecomplex conjugates of MNPs with actinides from the wastesolution centrifuge149 and magnetic attraction39150151 Com-pared to the centrifuge magnetic attraction that relies onpermanent magnets or electromagnets is more energy efficient

for large quantity applications After actinide extraction theparticles can be magnetically separated by one of the threemethods34 (1) placing a magnetic field around the treatmenttank (2) pumping the colloidal solution through a magneticfilter (eg commercially available magnetic units) or (3)introducing an electromagnetic device into the tank

A High Gradient Magnetic Separation (HGMS) AHGMS30152153 system generally consists of a column throughwhich a particle suspension flows This column is packed with abed of wires and placed inside an electromagnet The wireshave a high magnetic susceptibility (diameter lt50 μm) sowhen a magnetic field is applied across the column itdehomogenizes The result is strong field gradients radiatingfrom each wire thatattract magnetic particles trapping them tothe wire surfaces The effectiveness of this system depends ongenerating strong magnetic field gradients as well as on theparticlesrsquo size154155 and their magnetic propertiesThe magnetic properties are highly dependent on the particle

size86 Larger particles have higher saturation magnetizationsand strong interparticle interaction that can lead toagglomeration Until now micrometer scale particles havebeen used for magnetic separation Yamaura et al156 used silanecoated magnetite particles for the sorption of Eu ions Reddy etal42157 used iron oxide particles (MagaCharc-AA) of size 1minus60μm and Gruttner142 Verboom158 Matthews159 and Wang160

et al used magnetic silica particles whereas Nunez andKaminski34121 used micrometer size cross-linked polyacryla-mide entrapped charcoal and magnetite particles for theextraction and separation of lanthanides and actinides fromnuclear waste streams Compared to micrometer size particlesnano size particles have unique properties due to greater surfacearea relative to volume which enhances the loading capacity ofchelators thereby extracting many more of ions from nuclearwaste The best particle size for magnetic separation dependson the type of magnetic nanoparticles Coating and surfacefunctionalization reduces the particlesrsquo saturation magnet-ization hence the selection of particle size should balance thenonmagnetic coating on particlesrsquo surface and their magnet-ization Superparamagnetic particles can experience reductionof their net magnetic moment if the thickness of the deadcoating (protective coatingpolymerchelator) on the particlesrsquosurface exceeds its limit because the magnetization is measuredwith respect to the total mass of the particle The super-paramagnetic size limit depends on the type of particles It iscategorized as single domain thermally unstable particles Thesize of single domain of particles is given as 161 Fe = 14 nm Co= 55 nm Ni = 70 nm Fe3O4 = 128 nm Therefore a particlelarger than its superparamagnetic limit is a suitable candidatefor magnetic separation Monodispersity of MNPs also countsfor magnetic separationBoth saturation magnetization and interactions between the

particles increase with the size of the ferromagnetic particlesLarger particles interact more than smaller ones Super-paramagnetic particles are not a suitable candidate for magneticseparation because the particlesrsquo magnetization drops signifi-cantly after surface functionalization and silica coating Coreminusshell particles exhibit a higher saturation magnetization andweak interaction between the particles Even after surfacefunctionalization coreminusshell particles show high enoughmagnetization to magnetically separate out of the solutionAlso the oxide shell carries hydroxide groups that give a strongbase to the silica coating

Figure 7 Isotherms of Th (IV) sorption at different ionic strength ofNaNO3

145


dxdoiorg101021es402205q | Environ Sci Technol XXXX XXX XXXminusXXXL

For HGMS to succeed the magnetic force attracting theparticles toward the wires must dominate all other forcespresent including forces due to fluid drag gravitation inertiaand diffusion Typically HGMS has been used to separatemicrometer-scale or larger particles or aggregates In somecases MNPs have been used as separation agents howeverthese nanoparticles have usually been present as micrometer-scale aggregates or encapsulated in larger polymer beads Usinga larger volume of these particles makes magnetic collection byHGMS (or other means) relatively straightforwardB Stripping of the Actinide The physical separation

(desorption) of actinides from the complex conjugate of surfacefunctionalized MNPs after sorption is known as stripping It is akey process to enable the reuse of MNP-Che for futureseparation of actinides from nuclear waste streams Successfulcompletion of the stripping process without adversely affectingthe MNP-chersquos sorption efficiency is one of the most importantcriteria to make this separation method economical and cost-effective Numerous studies have reported that the recycle ofmagnetic sorbents (particles) is possible by using appropriatestripping agents such as strong acid solution42 or deionizedwater142 to back-extract radionuclides from the particle surfaceThe stability and reliability of magnetic nanosorbents can bestudied by performing sorptionstripping cyclesIn a series of experiments using deionized (DI) water Eu was

back-extracted from the particle surface by shaking the particlesin 10 mL water three times where each time was for 10 min innew water142 Generally more than 90 of Eu activity wasrecovered by stripping the particles twice with DI water and nosignificant decrease of the Eu extraction capacity was observedduring 10 extractionstripping cycles The activity of extractedand back-extracted Eu at each step is represented in Figure 8142

These back-extraction studies demonstrate the feasibility ofrecycling the particles

The reusability of the MNP-Che is checked using 50Cyanex 923 coated MNPs42 and 50 NNprime-dimethyl-NNprime-dibutyl tetradecyl malonamide (DMDBTDMA)157 coatedMNPs it is found that after repeated extraction Kd dropsfrom 132 (I cycle) to 130 (II cycle) for Cyanex and 2794 (Icycle) to 2590 (II cycle) for DMDBTDMA These resultsdemonstrate the stability and recycling capacity of the MNP-Che

VII CURRENT STATUS OF SPENT NUCLEAR FUEL

Reprocessing spent nuclear fuel is a key component of nuclearenergy because recovered fissile and fertile materials providesfresh fuel for existing and future nuclear power plants RussiaJapan and several European countries have policies in place toreprocess nuclear waste by extracting residual plutonium anduranium and making it into usable fuel The governments inmany other countries have not yet addressed the variousaspects of reprocessing or even whether or not to begin it162

Currently the recycling process is much more expensive thanthe production of new fuel but it significantly cuts down on theamount of radioactive waste that must be disposed ofAdditionally recycling allows elements with particularly longhalf-lives present in the waste to be extracted and reusedultimately making the wastersquos storage period much shorterResearch is being conducted in several countries to improve theefficiency and efficacy of reprocessing technologies163

Removing uranium and plutonium from waste andconverting the fuel cycle to shorter-lived fission productswould eliminate most of the volume of radioactive material thatcurrently requires disposal in a deep geologic repository anddrastically diminish the long-term radioactivity in nuclear wasterespectively However the waste resulting from reprocessingwould have nearly the same short-term radioactivity and heat asthe original spent fuel because recently reprocessed wasteconsists primarily of the same fission products that areresponsible for the near term radioactivity and heat in spentfuel Since heat is the main limiting factor on repositorycapacity conventional reprocessing would not provide majordisposal benefits in the near futureTo address that problem various proposals have been made

to further extract the primary heat-generating fission products(Cs Sr) from high level waste for separate storage and decayCurrent US policy favors direct disposal of high level waste

instead of recycling or reprocessing it because current recyclingmethods are cost-effective when compared to producing newfuel164 The Department of Energy strategy with respect toused nuclear fuel and high-level radioactive waste has two majorcomponents a firm commitment to a once-through fuel cycleand a plan for legislative and administrative action to facilitatesafe permanent disposal Important elements include institu-tional and public cooperation to arrange for waste trans-portation routes and storage facilities development of apermanent geological disposal site and a new agency tooversee it all to fulfill the federal governmentrsquos 1985 contractualobligations165

Now that the Obama administration has canceled plans tobuild a permanent deep disposal site at Yucca Mountain inNevada spent fuel at the nationrsquos 104 nuclear reactors will islikely to remain onsite for years as it continues to accumulateThe Administration has however kept the door open to saferrecycling methods as used in France

VIII SUMMARY

Nuclear energy can be a safe and cost-effective energy source ifthe radioactive waste is handled properly The extraction ofprimary elements (U Pu) from waste for reuse can be cost-saving but extraction of heat generating elements includingAm Cm and Np is an essential first step to any disposal actionWithout recycling these minor actinides no significantreduction in the radiological hazard of the waste can beobtained and unlike current processes MNP-Chelator nano-

Figure 8 Activity of extracted and back-extracted 152Eu during 10extractionstripping cycles with particles MC1142


dxdoiorg101021es402205q | Environ Sci Technol XXXX XXX XXXminusXXXM

technology promises to do that It is an efficient separationtechnology that uses chelator extractants covalently bonded tothe surface of MNPs to remove the actinides from nuclearwaste solution The process is intended to (1) reduce thecomplexity of equipment required for reprocessing (2) facilitatescaling because of its simplicity (3) target specific minoractinides with highly selective chelators (4) ease separationsince MNP-chelator conjugates require only a small amount ofmagnetic field and (5) recycle the MNPs Magnetic separationnanotechnology has been summarized with respect to differenttypes of MNPs their synthesis processes surface functionaliza-tion sorption conditions and stripping method

AUTHOR INFORMATIONCorresponding Author(YQ) E-mail youqianguidahoeduNotesThe authors declare no competing financial interest

ACKNOWLEDGMENTSThis work was supported by the LDRD Program of IdahoNational Laboratory administered by the Center for AdvancedEnergy Studies under the Department of Energy IdahoOperations Office Contract DE-AC07-05ID14517 We wouldlike to thank Christopher Bert (University of Massachusetts atAmherst) a summer 2013 REU student at University of Idahofor helping us to read the manuscript

REFERENCES(1) Kharecha P A Hansen J E Prevented mortality andgreenhouse gas emissions from historical and projected nuclearpower Environ Sci Technol 2013 47 4889minus4895(2) Markandya A Wilkinson P Electricity generation and healthThe Lancet 15 370 979minus990(3) Lewis F Hudson M Harwood L Development of highlyselective ligands for separations of actinides from lanthanides in thenuclear fuel cycle Synlett 2011 2011 2609minus2632(4) Agency I A E Status of Minor Actinide Fuel Development (IAEANuclear Energy Series) International Atomic Energy Agency 2010(5) OrsquoBrien J N International auspices for the storage of spentnuclear fuel as a nonproliferation measure Nat Resources J 1981 21857(6) Andrews A Holt M Managing the Nuclear Fuel Cycle PolicyImplications of Expanding Global Access to Nuclear Power 2010(7) NRC Nuclear Fuel Pool Capacity Energy Resources Internationaland DOERW-0431 minus Revision 1 1995(8) Commercial Spent Nuclear Fuel Observations on the KeyAttributes and Challenges of Storage and Disposal Options httpwwwgaogovproductsGAO-13-532T (accessed Jul 23 2013)(9) Enos D Bryan C Technical Work Plan EnvironmentalDegradation of Materials Relevant to Interim Storage and PermanentDisposal of Used Nuclear Fuel 2012(10) Choppin G R Sci Teacher Vol 48(11) Nash K L Actinide phosphonate complexes in aqueoussolutions J Alloys Compounds 1993 213-214 300minus304(12) Anderson H H Asprey L B Solvent Extraction Process forPlutonium 1960(13) Birkett J E Carrott M J Fox O D Jones C J Maher CJ Roube C V Taylor R J Woodhead D A Recent developmentsin the Purex process for nuclear fuel reprocessing Complexant basedstripping for uraniumplutonium separation CHIMIA Int J Chem2005 59 898minus904(14) Kinoshita K Inoue T Fusselman S Grimmett D KruegerC Storvick T Electrodeposition of uranium and transuranic elementsonto solid cathode in LiCl-KClCd system for pyrometallurgicalpartitioning J Nucl Sci Technol 2003 40 524minus530

(15) Koyama T Iizuka M Shoji Y Fujita R Tanaka HKobayashi T Tokiwai M An experimental study of molten saltelectrorefining of uranium using solid iron cathode and liquidcadmium cathode for development of pyrometallurgical reprocessingJ Nucl Sci Technol 1997 34 384minus393(16) Uozumi K Kinoshita K Inoue T Fusselman S GrimmettD Pyrometallurgical partitioning of uranium and teansuranic elementsfrom rare earth elements by electrorefining and reductive Extraction JNucl Sci Technol 2001 38 36minus44(17) Choppin G R Morgenstern A Radionuclide separations inradioactive waste disposal J Radioanal Nucl Chem 2000 243 45minus51(18) Avens L R Gallegos U F McFarlan J T Magneticseparation as a plutonium residue enrichment process Sep Sci Tech1990 25 1967(19) Rosenthal M W Haubenreich P N Briggs R B TheDevelopment Status of Moltan Salt Bleeder Reactors ORNL-4782 1972(20) Choppin G R Liljenzin J-O Rydberg J Chapter 21 - TheNuclear Fuel Cycle In Radiochemistry and Nuclear Chemistry 3rd ed Butterworth-Heinemann Woburn 2002 pp 583minus641(21) Barbette F Rascalou F Chollet H Babouhot J Denat FGuilard R Extraction of uranyl ions from aqueous solutions usingsilica-gel-bound macrocycles for alpha contaminated waste watertreatment Anal Chim Acta 2004 502 179minus187(22) Ebert W E Testing to Evaluate the Suitability of Waste FormsDeveloped for Electrometallurgically Treated Spent Sodium-BondedNuclear Fuel for Disposal in the Yucca Mountain Reporsitory ANL2006(23) Gourishankar K V Redey L Williamson M A Electro-chemical Reduction of Metal Oxides in Molten Salts Argonne NationalLaboratory Argonne IL 2001(24) Willit J L Miller W E Battles J E Electrorefining ofuranium and plutoniumA literature review J Nucl Mater 1992195 229minus249(25) Li S X Johnson T A Westphal B R Goff K M BenedictR W Experience for pyrochemical processing of spent EBR-II driverfuel Global 2005 2005(26) Boswell C D Dick R E Eccles H Macaskie L EPhosphate uptake and release by Acinetobacter johnsonii incontinuous culture and coupling of phosphate release to heavy metalaccumulation J Ind Microbiol Biotechnol 2001 26 333minus340(27) Macaskie L E Jeong B C Tolley M R Enzymicallyaccelerated biomineralization of heavy metals Application to theremoval of americium and plutonium from aqueous flows FEMSMicrobiol Rev 1994 14 351minus367(28) Podda F Zuddas P Minacci A Pepi M Baldi F Heavymetal coprecipitation with hydrozincite [Zn5 (CO3)2 (OH)6] frommine waters caused by photosynthetic microorganisms Appl EnvironMicrobiol 2000 66 5092minus5098(29) Renninger N McMahon K D Knopp R Nitsche H ClarkD S Keasling J D Uranyl precipitation by biomass from anenhanced biological phosphorus removal reactor Biodegradation 200112 401minus410(30) Nunez L Kaminski M D Magnetically assisted chemicalseparation process Filtr Sep 1998 35 349minus352(31) Nunez L Bradley C Buchholz B A Landsberger S Aase SB Tuazon H E Vandergrift G F Kaminski M MagneticallyAssisted Chemical Separation (MACS) Process Preparation andOptimization of Particles for Removal of Transuranic Elements ArgonneNational Laboratory Argonne IL 1995(32) Committee on Separations Technology and TransmutationSystems Board on Radioactive Waste Management Commission onGeosciences Environment and Resources National Research CouncilNuclear wastes technologies for separations and transmutation NationalAcademies Press 1996(33) Nunez L Buchholz B A Vandegrift G F Waste remediationusing in situ magnetically assisted chemical separation Sep SciTechnol 1995 30 1455minus1471


dxdoiorg101021es402205q | Environ Sci Technol XXXX XXX XXXminusXXXN

(34) Nunez L Kaminski M D Transuranic separation usingorganophosphorus extractants adsorbed onto superparamagneticcarriers J Magn Magn Mater 1999 194 102minus107(35) Nunez L Buchholz B A Kaminski M Aase S B Brown NR Vandegrift G F Actinide separation of high-level waste usingsolvent extractants on magnetic microparticles Sep Sci Technol 199631 1393minus1407(36) Lu A-H Salabas E L Schuth F Magnetic nanoparticlesSynthesis protection functionalization and application Angew ChemInt Ed 2007 46 1222minus44(37) Laurent S Forge D Port M Roch A Robic C VanderElst L Muller R N Magnetic iron oxide nanoparticles Synthesisstabilization vectorization physicochemical characterizations andbiological applications Chem Rev 2008 108 2064minus2110(38) De M Ghosh P S Rotello V M Applications ofnanoparticles in biology Adv Mater 2008 20 4225minus4241(39) Pankhurst Q A Connolly J Jones S K Dobson JApplications of magnetic nanoparticles in biomedicine J Phys DAppl Phys 2003 36 R167minusR181(40) Dam H H Reinhoudt D N Verboom W Multicoordinateligands for actinidelanthanide separations Chem Soc Rev 2007 36367minus377(41) Tian G Rao L Teat S J Liu G Quest for environmentallybenign ligands for actinide separations thermodynamic spectroscopicand structural characterization of UVI complexes with oxa-diamide andrelated ligands ChemndashEur J 2009 15 4172minus4181(42) Shaibu B S Reddy M L P Bhattacharyya A Manchanda VK Evaluation of Cyanex 923-coated magnetic particles for theextraction and separation of lanthanides and actinides from nuclearwaste streams J Magn Magn Mater 2006 301 312minus318(43) Wigeland R A Bauer T H Fanning T H Morris E ESeparations and transmutation criteria to improve utilization of ageologic repository Nucl Technol 2006 154 95minus106(44) Kaminski M Landsberger S Nunez L Vandegrift G FSorption capacity of ferromagnetic microparticles coated with CMPOSep Sci Technol 1997 32 115minus126(45) De Berti I P Cagnoli M V Pecchi G Alessandrini J LStewart S J Bengoa J F Marchetti S G Alternative low-costapproach to the synthesis of magnetic iron oxide nanoparticles bythermal decomposition of organic precursors Nanotechnology 201324 175601(46) NEA Nuclear Science Committee OECD Nuclear EnergyAgency Actinide Separation Chemistry in Nuclear Waste Streams andMaterials Nuclear Energy Agency Organisation for Economic Co-Operation and Development [Sl] 1997(47) Front Matter In Oxide Ultrathin Films Pacchioni G Valeri SEds Wiley-VCH Verlag GmbH amp Co KGaA 2011 pp iminusxvi(48) Yantasee W Warner C L Sangvanich T Addleman R SCarter T G Wiacek R J Fryxell G E Timchalk C Warner MG Removal of heavy metals from aqueous systems with thiolfunctionalized superparamagnetic nanoparticles Environ Sci Technol2007 41 5114minus5119(49) Cornell R M Schwertmann U The Iron Oxides StructureProperties Reactions Occurrences and Uses Wiley com 2003(50) Shen L Laibinis P E Hatton T A Bilayer surfactantstabilized magnetic fluids synthesis and interactions at interfacesLangmuir 1999 15 447minus453(51) Sousa M H Tourinho F A Depeyrot J da Silva G J LaraM C F New electric double-layered magnetic fluids based on coppernickel and zinc ferrite nanostructures J Phys Chem B 2001 1051168minus1175(52) Dresco P A Zaitsev V S Gambino R J Chu B Preparationand properties of magnetite and polymer magnetite nanoparticlesLangmuir 1999 15 1945minus1951(53) Deng J Ding X Zhang W Peng Y Wang J Long X LiP Chan A S Magnetic and conducting Fe3O4minuscross-linkedpolyaniline nanoparticles with coreminusshell structure Polymer 200243 2179minus2184

(54) Xu X Friedman G Humfeld K D Majetich S A Asher SA Superparamagnetic photonic crystals Adv Mater 2001 13 1681(55) Farrell D Majetich S A Wilcoxon J P Preparation andcharacterization of monodisperse Fe nanoparticles J Phys Chem B2003 107 11022minus11030(56) Harris L A Goff J D Carmichael A Y Riffle J S HarburnJ J St Pierre T G Saunders M Magnetite nanoparticle dispersionsstabilized with triblock copolymers Chem Mater 2003 15 1367minus1377(57) Thunemann A F Schutt D Kaufner L Pison U MohwaldH Maghemite nanoparticles protectively coated with poly (ethyleneimine) and poly (ethylene oxide)-b lock-poly (glutamic acid)Langmuir 2006 22 2351minus2357(58) Vigil G Xu Z Steinberg S Israelachvili J Interactions ofsilica surfaces J Colloid Interface Sci 1994 165 367minus385(59) Gerion D Pinaud F Williams S C Parak W J ZanchetD Weiss S Alivisatos A P Synthesis and properties ofbiocompatible water-soluble silica-coated CdSeZnS semiconductorquantum dots J Phys Chem B 2001 105 8861minus8871(60) Tan W Wang K He X Zhao X J Drake T Wang LBagwe R P Bionanotechnology based on silica nanoparticles MedRes Rev 2004 24 621minus638(61) Selvan S T Tan T T Ying J Y Robust non-cytotoxic silica-coated cdse quantum dots with efficient photoluminescence AdvMater 2005 17 1620minus1625(62) Guerrero-Martiacutenez A Perez-Juste J Liz-Marzan L M Recentprogress on silica coating of nanoparticles and related nanomaterialsAdv Mater 2010 22 1182minus1195(63) Iler R K Product comprising a skin of dense hydratedamorphous silica bound upon a core of another solid material andprocess of making same 2885366 1959(64) Wiegmann J The chemistry of silica Solubility polymerizationcolloid and surface properties and biochemistry Von RALPH KILER New YorkChichesterBrisbaneToronto John Wiley amp Sons1979 XXIV 866 S Lwd pound 3950 Acta Polym 1980 31 406minus406(65) Stober W Fink A Bohn E Controlled growth ofmonodisperse silica spheres in the micron size range J ColloidInterface Sci 1968 26 62minus69(66) Johnson A K Kaczor J Han H Kaur M Tian G Rao LQiang Y Paszczynski A J Highly hydrated poly(allylamine)silicamagnetic resin J Nanopart Res 2011 13 4881minus4895(67) Sutter E Sutter P Calarco R Stoica T Meijers RAssembly of ordered carbon shells on GaN nanowires Appl Phys Lett2007 90 093118minus093118minus3(68) Sutter E Sutter P Au-induced encapsulation of Ge nanowiresin protective C shells Adv Mater 2006 18 2583minus2588(69) Geng J Jefferson D A Johnson B F Direct conversion ofiron stearate into magnetic Fe and Fe3C nanocrystals encapsulated inpolyhedral graphite cages Chem Commun 2004 2442minus2443(70) Bystrzejewski M Huczko A Lange H Arc plasma route tocarbon-encapsulated magnetic nanoparticles for biomedical applica-tions Sens Actuators B 2005 109 81minus85(71) Li Y Liu J Wang Y Wang Z L Preparation ofmonodispersed Fe-Mo nanoparticles as the catalyst for CVD synthesisof carbon nanotubes Chem Mater 2001 13 1008minus1014(72) Ruoff R S Lorents D C Chan B Malhotra RSubramoney S Single Crystal Metals Encapsulated in CarbonNanoparticles American Association for the Advanement of ScienceNew York 1993 Vol 259 pp 346minus346(73) Liu Z-J Yuan Z-Y Zhou W Xu Z Peng L-M Controlledsynthesis of carbon-encapsulated Co nanoparticles by CVD ChemVapor Deposition 2001 7 248minus251(74) Flahaut E Agnoli F Sloan J OrsquoConnor C Green M L HCCVD synthesis and characterization of cobalt-encapsulated nano-particles Chem Mater 2002 14 2553minus2558(75) Love J C Estroff L A Kriebel J K Nuzzo R GWhitesides G M Self-assembled monolayers of thiolates on metals asa form of nanotechnology Chem Rev 2005 105 1103minus1170


dxdoiorg101021es402205q | Environ Sci Technol XXXX XXX XXXminusXXXO

(76) Ban Z Barnakov Y A Li F Golub V O OrsquoConnor C JThe synthesis of coreminusshell iron gold nanoparticles and theircharacterization J Mater Chem 2005 15 4660minus4662(77) Liu S H Han M Y Synthesis functionalization andbioconjugation of monodisperse silica-coated gold nanoparticlesRobust bioprobes Adv Funct Mater 2005 15 961minus967(78) Illes E Tombacz E The effect of humic acid adsorption onpH-dependent surface charging and aggregation of magnetitenanoparticles J Colloid Interface Sci 2006 295 115minus123(79) Buchholz B A Nunez L Vandegrift G F Argonne N LRadiolysis and hydrolysis of magnetically assisted chemical separationparticles Sep Sci Technol 1996 31 1933minus1952(80) Park S-J Kim S Lee S Khim Z G Char K Hyeon TSynthesis and magnetic studies of uniform iron nanorods andnanospheres J Am Chem Soc 2000 122 8581minus8582(81) Puntes V F Krishnan K M Alivisatos A P Colloidalnanocrystal shape and size control the case of cobalt Science 2001291 2115minus2117(82) Chen Q Rondinone A J C Chakoumakos B John ZhangZ Synthesis of superparamagnetic MgFe2O4 nanoparticles bycoprecipitation J Magn Magn Mater 1999 194 1minus7(83) Park J An K Hwang Y Park J-G Noh H-J Kim J-YPark J-H Hwang N-M Hyeon T Ultra-large-scale syntheses ofmonodisperse nanocrystals Nat Mater 2004 3 891minus895(84) Sun S Murray C B Weller D Folks L Moser AMonodisperse FePt nanoparticles and ferromagnetic FePt nanocrystalsuperlattices Science 2000 287 1989minus1992(85) Shevchenko E V Talapin D V Rogach A L Kornowski AHaase M Weller H Colloidal synthesis and self-assembly of CoPt3nanocrystals J Am Chem Soc 2002 124 11480minus11485(86) Kaur M McCloy J S Jiang W Yao Q Qiang Y SizeDependence of inter- and intracluster interactions in coreminusshell ironminusiron oxide nanoclusters J Phys Chem C 2012 116 12875minus12885(87) Antony J Qiang Y Baer D R Wang C Synthesis andcharacterization of stable iron-iron oxide core-shell nanoclusters forenvironmental applications J Nanosci Nanotechnol 2006 6 568minus572(88) Kaur M Dai Q Bowden M Engelhard M H Wu Y TangJ Qiang Y Watermelon-like iron nanoparticles Cr doping effect onmagnetism and magnetization interaction reversal Nanoscale 2013 57872minus7881(89) Qiang Y Antony J Sharma A Nutting J Sikes D MeyerD Ironiron oxide core-shell nanoclusters for biomedical applicationsJ Nanopart Res 2005 8 489minus496(90) Holister P Weener J-W Vas C R Harper T NanoparticlesTechnology White Papers 2003 3 1minus11(91) Gogotsi Y Nanomaterials handbook CRC press Boca RatonFL 2006(92) Wang C Sun S Chemical synthesis of monodisperse magneticnanoparticles In Handbook of Magnetism and Advanced MagneticMaterials John Wiley amp Sons Ltd 2007(93) Hyeon T Chemical synthesis of magnetic nanoparticles ChemCommun 2003 927minus934(94) Maaz K Karim S Mumtaz A Hasanain S K Liu J DuanJ L Synthesis and magnetic characterization of nickel ferritenanoparticles prepared by co-precipitation route J Magn MagnMater 2009 321 1838minus1842(95) Gupta A K Gupta M Synthesis and surface engineering ofiron oxide nanoparticles for biomedical applications Biomaterials2005 26 3995minus4021(96) Hudson M J Some new strategies for the chemical separationof actinides and lanthanides Czech J Phys 2003 53 A305minusA311(97) Chan G Y Drew M G Hudson M J Iveson P BLiljenzin J-O Skalberg M Spjuth L Madic C Solvent extractionof metal ions from nitric acid solution using NNprime-substitutedmalonamides Experimental and crystallographic evidence for twomechanisms of extraction metal complexation and ion-pair formationJ Chem Soc Dalton Trans 1997 649minus660

(98) Serrano-Purroy D Baron P Christiansen B Malmbeck RSorel C Glatz J-P Recovery of minor actinides from HLLW usingthe DIAMEX process Radiochim Acta 2005 93 351minus355(99) Modolo G Asp H Schreinemachers C Vijgen HDevelopment of a TODGA based process for partitioning of actinidesfrom a PUREX raffinate Part I Batch extraction optimization studiesand stability tests Solvent Extr Ion Exch 2007 25 703minus721(100) Cuillerdier C Musikas C Hoel P Nigond L Vitart XMalonamides as new extractants for nuclear waste solutions Sep SciTechnol 1991 26 1229minus1244(101) Bertini I Biological Inorganic Chemistry Structure andReactivity University Science Books 2007(102) Horwitz E P Chiarizia R Dietz M L Diamond HNelson D M Separation and preconcentration of actinides fromacidic media by extraction chromatography Anal Chim Acta 1993281 361minus372(103) Cecille L Casarci M Pietrelli L New Separation ChemistryTechniques for Radioactive Waste and Other Specific ApplicationsElsevier Applied Science New York 1991(104) Helfferich F G Ion Exchange Courier Dover Publications1962(105) Nash K L A review of the basic chemistry and recentdevelopments in trivalent f-elements separations Solvent Extr IonExch 1993 11 729minus768(106) Diamond R M Street K Jr Seaborg G T An ion-exchangestudy of possible hybridized 5f bonding in the actinides 1 J Am ChemSoc 1954 76 1461minus1469(107) Modolo G Odoj R Influence of the purity and irradiationstability of Cyanex 301 on the separation of trivalent actinides fromlanthanides by solvent extraction J Radioanal Nucl Chem 1998 22883minus89(108) WadeKSchroederNJarvinenGSmithBBarransRGibsonRSoftdonor ligands for separation of trivalent actinides and lanthanides InEighteenth Annual Actinide Separations Conference 1994(109) Xu Q Wu J Chang Y Zhang L Yang Y Extraction of Am(III) and lanthanides (III) with organo dithiophosphinic acidsRadiochim Acta 2008 96 771minus779(110) Coupez B Boehme C Wipff G Importance of interfacialphenomena and synergistic effects in lanthanide cation extraction bydithiophosphinic ligands A molecular dynamics study J Phys Chem B2003 107 9484minus9490(111) Ionova G Ionov S Rabbe C Hill C Madic CGuillaumont R Krupa J C Mechanism of trivalent actinidelanthanide separation using bis (2 4 4-trimethylpentyl) dithiophos-phinic acid (Cyanex 301) and neutral O-bearing co-extractantsynergistic mixtures Solvent Extr Ion Exch 2001 19 391minus414(112) Bhattacharyya A Mohapatra P K Manchanda V K Solventextraction and extraction chromatographic separation of Am3+ andEu3+ from nitrate medium using Cyanexreg 301 Solvent Extr Ion Exch2007 25 27minus39(113) VitorgeP Lanthanides and Trivalent Actinides Complexation byTripyridyl Triazine Applications to Liquid-Liquid Extraction FranceAtomic Energy Comm Rep CEA 1984(114) YS Chan G GB Drew M J Hudson M S Isaacs N ByersP Madic C Complexation of 246-tri-tert-butylpyridine-135-triazineligand (L) with the cerium(IV) nitrate anion encapsulation ofprotonated [LHn]n+ with the hexafluorophosphate nitrate andhydroxide anions formation and crystal structures of Ce(NO3)4L2[LH3]3+[Ce(NO3)5(OH2)]-[Ce(NO3)5(EtO)]2- 2[NO3]-[OH]-2[LH2]2+[PF6]-15[BF4]-05[PO4]3- and [LH4]4+[3NO3]-[PF6]-Polyhedron 1996 15 3385minus3398(115) Ionova G Ionov S Rabbe C Hill C Madic CGuillaumont R Modolo G Krupa J C Mechanism of trivalentactinidelanthanide separation using synergistic mixtures of di(chlorophenyl) dithiophosphinic acid and neutral O-bearing co-extractants New J Chem 2001 25 491minus501(116) Grenthe I Puigdomenech I Agency O N E Modelling inAquatic Chemistry OECD Publishing 1997


dxdoiorg101021es402205q | Environ Sci Technol XXXX XXX XXXminusXXXP

(117) Rao L Tian G Teat S J Complexation of Np (V) withNN-dimethyl-3-oxa-glutaramic acid and related ligands thermody-namics optical properties and structural aspects Dalton Trans 201039 3326minus3330(118) Mansfeld F Corrosion Mechanisms CRC Press Boca RatonFL 1987 Vol 28(119) PE P A S Fundamentals of Corrosion Mechanisms Causesand Preventative Methods CRC Press Boca Raton FL 2009(120) Masereel B Dinguizli M Bouzin C Moniotte N FeronO Gallez B Vander Borght T Michiels C Lucas S Antibodyimmobilization on gold nanoparticles coated layer-by-layer withpolyelectrolytes J Nanopart Res 2011 13 1573minus1580(121) Buchholz B A Optimizing the coating process of organicactinide extractants on magnetically assisted chemical separationparticles Sep Purif Technol 1997 11 211minus219(122) Ambashta R D Wattal P K Singh S Bahadur DMagnetic carrier for radionuclide removal from aqueous wastesParameters investigated in the development of nanoscale magnetitebased carbamoyl methyl phosphine oxide Sep Sci Technol 2006 41925minus942(123) Fryxell G E Lin Y Fiskum S Birnbaum J C Wu HKemner K Kelly S Actinide sequestration using self-assembledmonolayers on mesoporous supports Environ Sci Technol 2005 391324minus1331(124) Delle Site A Factors affecting sorption of organic compoundsin natural sorbentwater systems and sorption coefficients for selectedpollutants A review J Phys Chem Ref Data 2001 30 187(125) KleinW31 Mobility of environmental chemicals includingabiotic degradation In Ecology and Climte John Wiley amp Sons NewYork 1989(126) Fukui S Nakajima H Ozone A Hayatsu M YamaguchiM Sato T Imaizumi H Nishijima S Watanabe T Study on opengradient magnetic separation using multiple magnetic field sourcesIEEE Trans Appl Supercond 2002 12 959minus962(127) Liu C Zachara J M Qafoku O Smith S C Effect oftemperature on Cs+ sorption and desorption in subsurface sedimentsat the Hanford Site USA Environ Sci Technol 2003 37 2640minus2645(128) Qian L Hu P Jiang Z Geng Y Wu W Effect of pHfulvic acid and temperature on the sorption of uranyl on ZrP2O7 SciChina Chem 2010 53 1429minus1437(129) Chiou C T Freed V H Schmedding D W Kohnert R LPartition coefficient and bioaccumulation of selected organicchemicals Environ Sci Technol 1977 11 475minus478(130) Kleineidam S Schuth C Grathwohl P Solubility-normalized combined adsorption-partitioning sorption isotherms fororganic pollutants Environ Sci Technol 2002 36 4689minus4697(131) Nunez L Vandegrift G F Plutonium and AmericiumSeparation Using Organophosphorus Extractant Absorbed onto Ferro-magnetic Particles Argonne National Lab Argonne IL 1994(132) Rydberg J Musikas C Choppin G R Principles and Practicesof Solvent Extraction M Dekker 1992(133) Dobush G R Ankney C D Krementz D G The effect ofapparatus extraction time and solvent type on lipid extractions ofsnow geese Can J Zool 1985 63 1917minus1920(134) Kaur M Johnson A Tian G Jiang W Rao LPaszczynski A Qiang Y Separation nanotechnology of diethylene-triaminepentaacetic acid bonded magnetic nanoparticles for spentnuclear fuel Nano Energy(135) El-Ashtoukhy E-S Amin N K Abdelwahab O Removal oflead (II) and copper (II) from aqueous solution using pomegranatepeel as a new adsorbent Desalination 2008 223 162minus173(136) Caykara T Inam R Determination of the competitiveadsorption of heavy metal ions on poly (n-vinyl-2-pyrrolidoneacrylicacid) hydrogels by differential pulse polarography J Appl Polym Sci2003 89 2013minus2018(137) Serne R J Cantrell C J Lindenmeier C W Owen A TKutnyakov I V Orr R D Felmy A R Radionuclide-Chelating AgentComplexes in Low-Level Radioactive Decontamination Waste Stability

Adsorption and Transport Potential Pacific Northwest NationalLaboratory (PNNL) Richland WA 2002(138) Organic Seminar Abstracts Department of Chemistry andChemical Engineering University of Illinois at Urbana-Champaign1962(139) Alloway B J Heavy Metals in Soils Springer 1995(140) Slater S A Chamberlain D B Aase S A Babcock B DConner C Sedlet J Vandegrift G F Optimization of magnetitecarrier precipitation process for plutonium waste reduction Sep SciTechnol 1997 32 127minus147(141) Kochen R L Thomas R L Actinide Removal from AqueousSolution with Activated Magnetite Rockwell International CorpGolden CO 1987(142) Gruttner C Bohmer V Casnati A Dozol J-F ReinhoudtD N Reinoso-Garcia M M Rudershausen S Teller J Ungaro RVerboom W Wang P Dendrimer-coated magnetic particles forradionuclide separation J Magn Magn Mater 2005 293 559minus566(143) Wang L K Chen J P Hung Y T Shammas N K HeavyMetals in the Environment (Advances in Industrial and Hazardous WastesTreatment) CRC press Boca Raton FL 2009(144) Nunez L Buchholz B A Vandegrift G F Wasteremediation using in situ magnetically assisted chemical separationSep Sci Technol 1995 30 1455minus1471(145) Xu D Chen C Tan X Hu J Wang X Sorption of Th(IV)on Na-rectorite Effect of HA ionic strength foreign ions andtemperature Appl Geochem 2007 22 2892minus2906(146) Am J Measurement and modeling of Th sorption onto TiO2J Colloid Interface Sci 1999 220 367minus373(147) Reiller P Casanova F Moulin V Influence of addition orderand contact time on thorium(IV) retention by hematite in thepresence of humic acids Environ Sci Technol 2005 39 1641minus1648(148) Reiller P Moulin V Casanova F Dautel C Retentionbehaviour of humic substances onto mineral surfaces and con-sequences upon thorium (IV) mobility Case of iron oxides ApplGeochem 2002 17 1551minus1562(149) StolarskiMKellerKEichholzCFuchsBNirschlI HContinuousselective high gradient magnetic bio separation using novel rotatingmatrix centrifugation American Filter amp Separation Society AnnualConference Pennsylvania 2008(150) Schaller V Kraling U Rusu C Petersson K Wipenmyr JKrozer A Wahnstrom G Sanz-Velasco A Enoksson P JohanssonC Motion of nanometer sized magnetic particles in a magnetic fieldgradient J Appl Phys 2008 104 093918minus093918minus14(151) Furlani E P Analysis of particle transport in a magneto-phoretic microsystem J Appl Phys 2006 99 024912minus024912minus11(152) Fletcher D Fine particle high gradient magnetic entrapmentMagnetics IEEE Transactions on 1991 27 3655minus3677(153) Moeser G D Roach K A Green W H Alan Hatton TLaibinis P E High gradient magnetic separation of coated magneticnanoparticles AIChE J 2004 50 2835minus2848(154) He Y T Wan J Tokunaga T Kinetic stability of hematitenanoparticles The effect of particle sizes J Nanopart Res 2008 10321minus332(155) Yavuz C T Mayo J T William W Y Prakash A FalknerJ C Yean S Cong L Shipley H J Kan A Tomson M Low-fieldmagnetic separation of monodisperse Fe3O4 nanocrystals Science2006 314 964minus967(156) Yamaura M Camilo R Felinto M C F Synthesis andperformance of organic-coated magnetite particles J Alloys Compd2002 344 152minus156(157) Shaibu B S Reddy M L P Prabhu D R Kanekar A SManchanda V K NNprime-dimethyl-NNprime-dibutyl tetradecyl malonamideimpregnated magnetic particles for the extraction and separation ofradionuclides from nuclear waste streams Radiochim Acta 2006 94267minus273(158) Reinoso-Garciacutea M M Jan czewski D Reinhoudt D NVerboom W Malinowska E Pietrzak M Hill C Baca J GrunerB Selucky P CMP (O) tripodands synthesis potentiometric studiesand extractions New J Chem 2006 30 1480minus1492


dxdoiorg101021es402205q | Environ Sci Technol XXXX XXX XXXminusXXXQ

(159) Matthews S E Parzuchowski P Bohmer V Garcia-CarreraA Dozol J-F Gruttner C Extraction of lanthanides and actinides bya magnetically assisted chemical separation technique based onCMPO-calix[4]arenes Chem Commun 2001 417minus418(160) Bohmer V Dozol J-F Gruttner C Liger K Matthews SE Rudershausen S Saadioui M Wang P Separation of lanthanidesand actinides using magnetic silica particles bearing covalently attachedtetra-CMPO-calix [4] arenes Org Biomol Chem 2004 2 2327minus2334(161) Gubin S P Koksharov Y A Khomutov G B Yurkov G YMagnetic nanoparticles Preparation structure and properties RussChem Rev 2005 74 489(162) Association W N Processing of used nuclear fuel 2013 httpworld-nuclear orginfoinf69 html (accessed May 2 2012)(163) Lake J A Bennett R G Kotek J F Next Generation NuclearPower Scientific American 2009(164) Sikkema L Savage M Nuclear Renaissance StateLegislatures 2007 33(165) Strategy for the Management and Disposal of Used Nuclear Fueland High-Level Radioactive Waste US Department of Energy 2013(166) Kaminski M D Nunez L Separation of uranium from nitric-and hydrochloric-acid solutions with extractant-coated magneticmicroparticles Sep Sci Technol 2000 35 2003minus2018


dxdoiorg101021es402205q | Environ Sci Technol XXXX XXX XXXminusXXXR

Table

ICom

parisonof

DifferentRadioactive

Waste

TreatmentMetho

ds

method

extractio

nmechanism

advantage

draw

backs

10solventextractio

nliquidminus

liquidionex-

change

1practicaltechniquein

large

2medium

scaleoperations

whensolute

concentrations

arehigh

1largeam

ount

oforganicwaste

2notforlowconcentration

11ionexchange

ionexchange

resin

1very

lowrunningcostsandvery

little

energy

isrequired

2theregenerate

chem

icalsarecheapandifwellm

aintainedresinbeds

canlastformanyyears

before

replacem

entisneeded

1foulingof

resin

2presence

oforganicmatter

3commerciallyavailableresins

arenonselective4Itisnotacontinuous

process

17precipitatio

nmetalsurfaceadsorptio

n1

good

atalkaline

2costeffective

3instrumentsareeasilyavailable

4high

degree

ofaccuracy

1notselective

2lowmetalrecovery

3tim

econsum

ingifdone

manually

18magnetic

separatio

nmagnetic

susceptibility

difference

1canseparate

almostallactin

ides

2thereareno

mediato

replaceor

disposeof

1will

notremovenonm

agnetic

materialsor

grinding

swarf

2itrequireslowflow

ratew

hich

increasesfloorarea

required

19volatility

processes

fluorid

evolatility

process

1nonaqueoustechnique

2used

toremovethecladding

onspentfuelelem

ents

1useof

halides

ampβ-diketones

2theradiolyticdecompositio

n20moltenfuelandcoolantsalt

processing

reprocessing

with

reagents

1inherent

resistance

totheradiationdamage

2thedecayheat

isuseful

formaintaining

thepyrochem

icalfluidat

processtemperature

1high

temperature

process

2lowoutput

3difficultprocessing

ofslag

3335121 M

ACS

chelator

extractio

n1

magnetic

separatio

n2

lowconcentration

3consum

ptionof

organicsolventsalmosteliminated

thechelatingagentswindup

inthewater

andenvironm

entalregulatio

nsprohibittheirdischarge

134 M

NP-Che

chelator

extractio

n1

better

kineticsfortheadsorptio

n2

efficientseparatio

nof

particles

3simpleversatileand

compact

4minimizesecondarywaste

themethodisunderdevelopment


dxdoiorg101021es402205q | Environ Sci Technol XXXX XXX XXXminusXXXB



















































7879











































=minus

KC C

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( )fd

i

f
















































145















VIII SUMMARY













































































7879











































=minus

KC C

CVm

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i

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145















VIII SUMMARY

























































=minus

KC C

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145















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VIII SUMMARY



























Conjugates of magnetic nanoparticle-actinide specific chelator for radioactive waste separation

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Transcript of Conjugates of magnetic nanoparticle-actinide specific chelator for radioactive waste separation