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Journal of Hazardous Materials 163 (2009) 475510

Contents lists available atScienceDirect

Journal of Hazardous Materials

j o u r n a l h o m e p a g e : w w w . e l s e v i e r . c o m / l o c a t e / j h a z m a t

Review

Characterization and remediation of soils contaminated with uranium

Maria Gavrilescu , Lucian Vasile Pavel, Igor CretescuTechnical University Iasi, Faculty of Chemical Engineering and Environmental Protection, Department of Environmental Engineering and Management,

71 Mangeron Boulevard, 700050 Iasi, Romania

a r t i c l e i n f o

Article history:

Received 1 December 2007Received in revised form 23 July 2008Accepted 23 July 2008Available online 31 July 2008

Keywords:

BioremediationBiosorptionBioreductionChemical extractionElectrokineticsEnvironmentPolluted soilRemediationUranium

a b s t r a c t

Environmental contamination caused by radionuclides, in particular by uranium and its decay products

is a serious problem worldwide. Thedevelopment of nuclear science and technology has ledto increasingnuclear waste containing uranium being released and disposed in the environment.The objective of this paper is to develop a better understanding of the techniques for the remediation

of soils polluted with radionuclides (uranium in particular), considering: the chemical forms of uranium,including depleted uranium (DU) in soil and other environmental media, their characteristics and con-centrations, and some of the effects on environmental and human health; research issues concerning theremediation process, the benefits and results; a better understanding of the range of uses and situationsfor which each is most appropriate.

The paper addresses the main features of the following techniques for uranium remediation: naturalattenuation,physicalmethods,chemicalprocesses(chemicalextraction methods fromcontaminated soilsassisted by various suitable chelators (sodium bicarbonate, citricacid, two-stageacid leaching procedure),extraction using supercritical fluids such as solvents, permeable reactive barriers), biological processes(biomineralization and microbial reduction, phytoremediation, biosorption), and electrokinetic meth-ods. In addition, factors affecting uranium removal from soils are furthermore reviewed including soilcharacteristics, pH and reagent concentration, retention time.

2008 Elsevier B.V. All rights reserved.

Contents

1. Radionuclides in soil . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4762. Characteristics and concentrations of uranium in the environment . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 477

2.1. Uranium properties . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4772.2. Depleted uranium . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4792.3. Chemical forms of uranium . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 479

2.3.1. Uranium oxides . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4822.3.2. Uranium hexafluoride . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4822.3.3. Uranium tetrafluoride . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4822.3.4. Uranium metal . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 482

2.4. Uranium compounds toxicity . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4823. Methods and techniques for uranium removal . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 483

3.1. General description . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4833.2. Natural attenuation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4903.3. Physical processes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4913.4. Chemical methods . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 491

3.4.1. Chemical extraction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4913.4.2. Permeable reactive barriers (PRBs) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 496

3.5. Biological methods . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4973.5.1. Background . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4973.5.2. Biomineralization, formation of insoluble metal sulphides and phosphates.. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 498

Corresponding author. Tel.: +40 232 278680x2137; fax: +40 232 271311.E-mail addresses:[email protected],mgav [email protected](M. Gavrilescu).

0304-3894/$ see front matter 2008 Elsevier B.V. All rights reserved.

doi:10.1016/j.jhazmat.2008.07.103
http://www.sciencedirect.com/science/journal/03043894http://www.elsevier.com/locate/jhazmatmailto:[email protected]:[email protected]://localhost/var/www/apps/conversion/tmp/scratch_1/dx.doi.org/10.1016/j.jhazmat.2008.07.103http://localhost/var/www/apps/conversion/tmp/scratch_1/dx.doi.org/10.1016/j.jhazmat.2008.07.103mailto:[email protected]:[email protected]://www.elsevier.com/locate/jhazmathttp://www.sciencedirect.com/science/journal/03043894


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3.5.3. Phytoremediation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4993.5.4. Biosorption . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 500

3.6. Electrokinetic methods . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5014. Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 503

Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 504References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 504

1. Radionuclides in soil

Radionuclides are found in the soil as a consequence of somepathways[1]:

as a part of Earths layer (primordial radionuclides); generated and deposited by cosmic ray interactions; as a result of anthropogenic activities.

Artificial radionuclides are also introduced into the environ-ment following nuclear power plant accidents or nuclear weaponstests, nuclear energy activities, scientific and other uses[25].Inaddition, soil may interact with low-level radioactive waste mate-rials that have been buried for disposal [69].Radionuclides cantravel around the world on air streams. Their weight and weatherdetermine their deposition to the ground. Also, the heavy rainscan bring the radioactive particles to the ground[10].Soils pos-sess sorbent and complexing capacities which contribute to theimmobilization of radionuclides from water in the underlyinglayers, after they were displaced from complexes or adsorptionsites.

Radionuclides existing in soil canbe dissolvedin solution,or ionexchanged in reaction, complexed with soil organics or precipitateas pure or mixed solids. They can move into the water, air and thefood supply.

The immobility of these radioactive elements in uppermost soillayers represents a problem for environment and human health,

since they can be easily integrated in the food chain[2,11,12].Ascheme for radionuclides movement in soil was proposed by Igweet al. [7] (Fig. 1). Consequently, the major part of radionuclidesreleased into the environment will finally accumulate in either theupper layer of soils or interstitial system of sediments in aquaticsystems. As a consequence, a risk forecosystems, agro-systems andhealth could be induced.

In particular, uranium mining and milling have caused enor-mous damage to the environment by means of abandoned wasteaccumulation and improper disposal of the radioactive material,waste dump after uranium prospections, other workings, espe-cially in the last 60 years since the end of the Second World War.Large amounts of uranium-containing (both high- and low-level)waste are generated from activities such as fuel fabrication, fuel

reprocessing, research and development (R&D). All these negativeimpacts influenced the quality of the environment and affectedmainly surface and ground waters, soils and simultaneously pol-luted great areas of land and endangered the catchments ofdrinking water. Also, uranium generates an important issue againstpublic perception on the risk that the environmental contamina-tion poses to the environmental and human health [14].Therefore,it is strongly evident that the contamination caused by uraniumhas severe negative biological effects on important groups of thesoil food web[3].

Thepotentialriskofuraniumsoilcontaminationisaglobalprob-lemas abouteverycountry canbe affected byone or more activitiesmentioned above. Depleted, enriched and natural uranium con-tamination in soil and water has been identified at many sites

worldwide, so that measures for preventing their assimilation by

plants should be considered a preliminary step towards the reme-diation of contaminated areas[2,1317].For a long period of timeuranium was leached commercially in a large number of depositsusingdifferent in situ technologies [1820], either alkaline leachingusing solutions containing carbonate and hydrocarbonate ions, oracid leaching[21,22].

The solubility of uranium in soil is dependent on several factorssuch as: pH, redox potential, temperature, soil texture, organic andinorganic compounds,moistureand microbialactivity[23]. Solubleforms can migrate with soil water, be uptaken by plants or aquaticorganisms or volatilized[7].

Several years ago, all commercial-scale operations for uraniumleaching were stopped due to a complex of different political, eco-

nomical and environmental reasons [24].However, regardless ofsomepreventive andremedialactionsduring theuraniumrecovery,many natural ecosystems were heavily polluted with radioactiveelements, mainly through the seepage of acid drainage waters[2527].Such waters are still a persistent environmental problemat many abandoned mine sites, while soils around the water flowpath are polluted with radioactive elements becoming unsuitablefor agricultural use, so that soil remediation has to be considered[11,28].

Another problem is the contamination of soil and water withdepleted uranium, which has increased public health concerns dueto the chemical toxicity of DU at elevated dosages[2933].For thisreason, there is great interest in developing methods for U removalfrom contaminated sources.

Fig. 1. Radionuclide transformation processes in soil (kireaction rate)[7].


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M. Gavrilescu et al. / Journal of Hazardous Materials 163 (2009) 475510 477

Various methods for remediation of soils contaminated withradioactive elements are known but only few of them have beenapplied under field conditions. The excavation and transportationoftheheavilypollutedsoilstospecificdepositoriesisstillacommonpractice in most countries[34].

The aim of this article is to provide an overview on the mainaspects concerning the presence of uranium in the environment,in particular in soil, as well as on techniques for remediation ofpolluted soil together with recent advances in the application ofthese processes to the detoxification of uranium contamination.

2. Characteristics and concentrations of uranium in the

environment

2.1. Uranium properties

Uranium can be characterized as a heavy, ductile and slightlyparamagnetic metal, silver-white in color and pyrophoric whenfinely divided and in this form it can react with cold water[23,35,36]. It is the first element, with the atomic number 92,which was found by Henry Becquerel (in 1896) to possess the prop-

erty to be radioactive [37]. As a naturally occurring element, itcontributes to low levels of natural background radiation in theenvironment. Uranium is found in all rock types in varying, butusually small concentrations[12,38].Uranium is widely dispersedin the earths crust, rocks and soils at a level of about 24ppmby weight[12,37,39].The natural concentration of uranium in theearth crust is about 106 g/g. Also, the uranium concentration inocean water, plants, and animal organisms is around 107 g/g, as aresult of the solubility of U(VI) compounds in water[40].

Volcanic eruption is another natural phenomenon that mayincrease theconcentration of natural uranium in thesoil.The redis-tribution of uranium and uranium progeny to both soil and wateroccurs often naturally in environmental circuit.

Uranium is in fact more abundant than gold, silver, mercury,antimony or cadmium, and more or less as common as tin, cobalt,lead,molybdenumandarsenic [4,41]. Itisanextremelydensemetal(at 19 g/cm3, about twice as heavy per unit volume as lead), beingthe heaviest chemical element to be found in the nature. Naturaluranium exists in three different forms (isotopes), all of which areradioactive[41,42].The two most abundant isotopes, uranium 235(0.72%) and uranium-238 (99.27%), have radioactive half-lives ofabout 7 108 and 4.4 109 years, respectively[12,43].An impor-tant characteristic is its toxicity[4,37].

Uranium is found in virtually all rock and soil (being derivedfrom erosion of the rocks), it is essentially everywhere in ground-

Table 2

Radioactive characteristics of natural uranium[51]

Characteristic Uranium series Actinium series

238 234 235

Nuclide half-life (years) 4.46 109 245.5 103 704 106

Natural abundance (%) 99.2742 0.0054 0.7204Type of emission Alpha Alpha AlphaDecay energy (MeV) 4.270 4.859 4.679% of-activity 0.0123 0.0127 0.0006

Total % of-activity 0.0256

water [4345]. Lower concentrations of uranium are found in basicrocks, while acidic rocks contain higher uranium concentrations[46,47].AsTable 1shows, the average radioactivity in soils is simi-lar to that in the rocks, usually bedrock, from which it derives[46].The average radioactivity in soil of234U from Table 1 is 0.61pCi/g.Since the activity of234 U accounts for approximately one-half ofthe total activity in natural uranium, the value inTable 1may bemultiplied by two to obtain the total uranium radioactivity in soils

(approximately 1.2pCi/g)[46,47].However, there are wide varia-tions from the values presented in the table, particularly in areaswhere uranium minerals are more concentrated[4,37,5155].

The mineralogy of uranium-containing minerals has beendescribed by Frondel [48]. In essentially all geological environ-ments, +4 and +6 are the most important oxidation states ofuranium[4951]. The characteristics of naturally occurring ura-nium are shown inTable 2.

Primordial uranium consists of three isotopes, 238U and 234Ufrom the uranium series and 235U from the actinium series. Innature, an imbalance between 234U and 238U may exist due toalpha particle withdraw from the decay of238U that increases theavailability of234U for transport through geological processes.

Almost all uranium as found in nature is the isotope 238U.It undergoes radioactive decay into a long series of 13 differentradionuclides before finally reaching a stable state in 206Pb. Theseradionuclides emit alpha or beta radiation and some also emitgammaradiation of widely varying energies[4,37,55].The ratio of234U to238U would be expected to be unity as long as the uraniumstays locked inside undisturbed crustal rock in secular equilibriumwith its progeny, butmeasurements show that the ratio is typicallydifferent than unity[5557].This disequilibrium occurs when therock is disturbed by chemical or physical changes involving water.These processes can change the uranium isotope ratios in air, soiland water.

Table 1

Average radioactivity of uranium in several types of rocks and soils[46]

Material 238U (pC i/g ) Series equi libriu m radi oactivity

Total alpha emission Total beta emission Total gamma emission

Ingenious rocksBasalt 0.20.3 1.62.4 1.21.8 0.60.9Mafic 0.20.3 1.62.4 1.21.8 0.60.9Salic 1.31.6 10.412.8 7.89.6 3.94.8Granite 1.0 8.0 6.0 3.0

Sedimentary rocksShale 1.0 8.0 6.0 3.0Sandstones 1.0 8.0 6.0 3.0Clean quartz 0.20.3 1.62.4 1.21.8 0.60.9Dirty quartz 1.0 8.0 6.0 3.0Arkose 0.30.7 2.45.6 1.84.2 0.92.1

Beach sands 1.0 8.0 6.0 3.0Carbonate rocks 0.7 5.6 4.2 2.7Soils 0.6 4.8 3.6 1.8


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Table 3

Normalized uranium effluent discharges from various activities involving uranium[58]

Uranium-238 Curies per GWy(e)

Atmospheric releasesMinning Milling 1.8 102

Mill tailings 1.9 105

Conversion 2.0 103Enrichment 9.9 104

Fabrication 2.0 105

Liquid releasesConversion 2.2 102

Enrichment 9.9 103

Fabrication 9.9 103

The uranium present in the rocks and soil as a natural con-stituent represents natural background levels. Natural processesof wind and water erosion, dissolution, precipitation, and volcanicaction acting on natural uranium in rock and soil redistribute farmore uranium in the total environment than the industries in thenuclear fuel cycle. However, those industries may release large

quantities of uranium in specific locations, mainly in the formof solids placed on tailings piles, followed by liquids released totailings ponds and then airborne releases, both directly from thefacilities and by wind erosion of the tailings piles [5558] (Table 3).

A majorlocalizedsource of enhanced natural uranium canresultfrom mining and milling operations (Table 3).As part of nuclearfuel cycle, uranium conversion, uranium enrichment and fuel fab-rication facilities also release small amounts of uranium to theatmosphere[5560](Table 3andFig. 2)

Contamination of the soil can occur either from deposition ofuranium originally discharged into the atmosphere, or from wasteproducts discharged directly into or on the ground (e.g., water

containing uranium from either underground or open-pit mines).Examples of industrial activities that may result in soil depositioninclude uranium mining and milling, uranium processing, phos-phate mining, heavy metal mining, coal use and inappropriatewaste disposal (Fig. 2).

The contamination could be specific for various locations,depending on the contamination source and this lead to unam-biguous and significant effort for conditioning, treatment, storageand safe disposal of nuclear waste at the repository[58].Availabledata on concentrations of uranium in different places confirmthese conclusions and reveals the fact that efforts have to bedevoted for remediation and for public health preservation. SomeAmerican contaminated sites are relevant from this perspective[1,10,44,45,56,58] (http://www.atsdr.cdc.gov/toxprofiles/tp150-c5.pdf; http://www.epa.gov/superfund/sites/npl/nar1605.htm):

concentrations of uranium in Louisiana soils ranged from 2.35 to3.98g/g (average radioactivity 1.62.7 pCi/g);

uranium concentrations in phosphate rocks in north and centralFlorida ranged from 6.8 to 124g/g (4.583.4pCi/g);

soil samples adjacent to Los Alamos, NM, taken during

19741977 contained total uranium in the range of 0.15.1g/g(0.0673.4 pCi/g), with a mean concentration of 2.4g/g(1.6 pCi/g);

the concentrations of uranium in soils adjacent to the HanfordFuel Fabrication Facility near Richland, WA, that were collectedduring 19781981 ranged from 0.76 to 4.6g/g (0.513.1 pCi/g),with a median value of 1.8g/g (1.2 pCi/g);

the control samples for the Hanford Fuel Fabrication Studycontained uranium at concentrations of 0.321.128g/g(0.210.86pCi/g), with a median value of 0.73g/g (0.49 pCi/g);

uranium in the soil of the Paducah Gaseous Diffusion Plant inKentuckyranged from4.9 to 7.1g/g (3.34.8 pCi/g), whereas off-

Fig. 2. Activities with impact on soil contamination with uranium and uranium compounds[5560].
http://www.atsdr.cdc.gov/toxprofiles/tp150-c5.pdfhttp://www.epa.gov/superfund/sites/npl/nar1605.htmhttp://www.epa.gov/superfund/sites/npl/nar1605.htmhttp://www.atsdr.cdc.gov/toxprofiles/tp150-c5.pdf


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site samples taken as far as 12 miles away contained uranium atlevels of 6.49.0g/g (3.86.0pCi/g);

soil monitoring data from thearea surrounding theFeed MaterialProduction Center at Fernald, OH, showed that the uranium con-centrations within an 8 km2 area were between 4.5 and 34g/g(323 pCi/g) compared to a mean of 3.3g/g (2.2 pCi/g) for natu-ral background levels;

uranium levels in surface soils at the Fernald site as high as 50times natural background levels.

2.2. Depleted uranium

Depleted uranium results as a byproduct during the process-ing of natural uranium, which makes it suitable for use as fuel innuclear powerplants or as a component in nuclear weapons [4,61].Studies on the radiological and chemical properties of depleteduranium showed that they can be compared to those of naturaluranium, which is all over present in soil. Natural uranium hasthe same chemotoxicity, but its radiotoxicity is 60% higher[35,61].Some studies revealed that the external exposure to radiation fromdepleted uranium is generally not major concern, so that depleted

uranium is not a significant health hazard if it is not taken intothe body. Natural and depleted uranium is much more likely tobe chemical than radiation hazardous [35,39,54,55,62,63]. Somestudies have however shown that exposure to intact depleted ura-nium weapons systems, both munitions and armor, pose very littlerisk from external radiation[39].Also, it was found that the 234Uthat remains in depleted uranium emits a small amount of low-energy gamma radiation [32,35]. Even if allowed to enter the body,depleted uranium, like natural uranium, has the potential for bothchemical and radiological toxicity, with the two important targetorgans being the kidneys and the lungs [35,42,64]. Some publisheddata refer to the toxic effects of depleted uranium on reproductionanddevelopment,aswellasonriskofleukemiaandcentralnervoussystem[6569].The relative contribution of each pathway to thetotal uptake into the body depends on the physical and chemicalnatureof the uranium,as well as the level andduration of exposure[32,53,70,71].

2.3. Chemical forms of uranium

Uranium can be found in soil as sorbed (both on soil parti-cles and pore water), complexed, precipitated and reduced forms,all of which have various impacts on its mobility and fate inthe soil environment [72]. It can exist in many chemical forms[4,37,54,55,61,69,73,74].

In nature, uranium is generally found as an oxide, such as inthe olive-green-colored mineral pitchblende, which contains triu-ranium octaoxide(U3O8). In soil, uranium is typically in an oxidized

form, and in water, it is usually present as a uranyl hydroxyl car-bonate complex[37,75,76].Uranium in ores can be extracted andchemicallyconverted intouraniumdioxide(UO2)orotherchemicalforms usable in industry. When refined, uranium is a silvery-whitemetal with very high density (being 65% denser than lead).

Uranium dioxide (UO2) is the chemicalformmostoften used fornuclear reactor fuel. Uraniumfluorine compounds are also com-mon in uranium processing, withuraniumhexafluoride (UF6)beingthe form used in the gaseous diffusion enrichment process. Ura-nium tetrafluoride (UF4) is frequently produced as an intermediatein the processing of uranium. As noted above, in its pure form,uranium is a silver-colored metal. Because several of these com-poundsmightbe usedor producedduring theconversionprocess, abriefdescriptionof thephysicaland chemical properties is provided

below.

The mobility of uranium in soil and its vertical transport (leach-ing) to groundwater depend on properties of the soil such aspH, oxidationreduction potential, concentration of complexinganions, porosity of the soil, soil particle size and sorption proper-ties, as well as the amount of water available [57,77].Retention ofuranium by the soil may be due to adsorption, chemisorption, ionexchange or a combination of mechanisms[77].Any soil propertythat alters the sorption mechanism will also alter the mobility ofuranium in the soil.

Complexation and redox reactions control the mobility of ura-nium in the environment[78].Uranium can exist in the +3, +4, +5and +6 oxidation states. In aqueous media only U(IV) and U(VI)are stable. The primary abiotic and biological processes that trans-formuraniumin soilare oxidationreductionreactionsthat convertU(VI) (soluble) to U(IV) (insoluble)[79]. Further abiotic and bio-logical processes that can transform uranium in the environmentare the reactions that form complexes with inorganic and organicligands.

Some compounds, such as UCI4, decompose in aqueous mediatothe U(VI).In acid solution andin thebody, theoxygen-containingcation UO22+, where uranium has the oxidation state VI, is the pre-dominant form. In general, hexavalent uranium compounds are themost soluble.

The reduction of U(VI) to U(IV) by abiotic and biotic processes,as well as its re-oxidation has received considerable attentionbecause the oxidation state of uranium has a significant effecton its mobility in the natural environment. Uranium exists insolution predominantly as UO22+ and as soluble carbonate com-plexes (UO2)2CO3(OH)3, UO2CO3, UO2(CO3)22, UO2(CO3)34

and possibly (UO2)3(CO3)66 [8082]. Between pH 4.0 and 7.5,the pH range of most soils, U(VI) exists primarily in hydrolyzedforms. Uranium(VI), i.e., uranyl, uranium will exist in the +6 oxi-dation state under oxidizing to mildly reducing environments.Uranium(IV) is stable under reducing conditions and is consideredrelatively immobile because U(IV) forms sparingly soluble miner-als, such as uraninite (UO2). Dissolved U(III) easily oxidizes to U(IV)

under most reducing conditions found in nature. The U(V) aqueousspecies (UO3+) readily disproportionates to U(IV) and U(VI). Underreducing conditions, the speciation of U(IV) is dominated by theneutral aqueous species U(OH)40 (aq) at pH values greater than 2[83,84].

The estimates of the solubilities and the speciation of uranium(nature and concentration species) are predicted from thermody-namic data, taking into account the presence of inorganic ligandsin the groundwaters studied, mainly [OH], [HCO3], [CO3]2,[H2PO4] [HPO4]2, [PO4]3, [SO4]2 (in case of disposal in rock-salt formation) and theproperties of these waters (redox potential)[85].

EhpH-diagramsshow thethermodynamic stabilityareasof dif-ferent species in an aqueous solution. Stability areas are presented

as a function of pH and electrochemical potential scales. Usuallythe upper and lower stability limits of water are also shown inthe diagrams with dotted lines. Traditionally these diagrams havebeen taken from different handbooks [Pourbaix Atlas Handbook].In most handbooks these diagrams are available only for a limitednumber of temperatures, concentrations and element combina-tions[83,84,86,87].

The distribution of uranium species at 25 C as a function of pHfor both oxidizingand moderately reducing conditionsis illustratedin Fig. 3, which further emphasizes that hydrolysis species and car-bonate complexes are of primary importance. The major oxidationof U(IV) and U(V) as states of uranium is also apparent.

Uranium (IV) is very insoluble forming uraninite (UO2) or amixed valence oxide phase like UO2.25 or U02.33. Uranium(VI) is

much more soluble and mobile. Uranium(VI) also forms soluble


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Fig. 3. EhpH and uranium species distributions as a function on pH for oxidizing and mildly reducing conditions (adapted upon[88,89]).

complexes with carbonate anions in natural waters. The aque-ous speciation of U(VI) in carbonate-containing waters at nearneutral and basic higher pH values is dominated by a seriesof strong anionic aqueous carbonate complexes [e.g., UO2CO30

(aq), UO2(CO3)22 and UO2(CO3)34].Fig. 3shows that the aque-

ous complex [UO2(CO3)2]2

is the predominant form of uraniumbetween pH 7 and 8 in an oxidized environment. Numerous inves-tigations of the adsorption of uranium on soils and minerals haveshown that carbonate complexing appreciably reduces adsorptionof uranium leading to its release from soils [9093].EhpH dia-gram for uranium shows the presence of solid phase at low Eh andpredominance of dissolved uranium carbonate complexes at highEh values. The upper diagonal dashed line is the superior stabilitylimit of water and represents oxidizing conditions, while the lowerdiagonal dashed line represents the inferior limit of water stabilityunder reducing conditions. When Eh values are above 0.25V andpH between 7 and 8, uranium will be in the oxidized valence state(VI). Also, when Eh values are higher than 0.25V (usually for pHranging between 1 and 5), uranium is in the valence state (VI), as

uranyl ion [UO]2+

. In alkaline medium, carbonate is the most sig-

nificant ligand (in natural water) and the greater solubility of theU(VI) ion is in part due to its tendency to form anionic carbonatecomplexes[78,9093].

The dependency of thespeciation distributionon pH andcarbondioxide concentration in a closed system is shown in Fig. 4.The

formation of carbonate complexes can change the stability fieldof U(VI). These U(VI) complexes may exist in alkaline conditionsand high carbonate concentrations even in reducing conditions[9496].

Uranyl hydroxy complexes such as UO2(OH)+ and (UO2)3(OH)5+

are also formed, but generally in smaller amounts except at hightemperature or in carbonate-depleted alkaline water. In reducingwater, the U(IV) hydrolysis leads to U(OH)40 [83,84].The solubil-ity of reduced uranium is low and it has a strong tendency tohydrolyse, forming colloids, especially when environmental con-ditions change. High concentration of inorganic salts hinders theformation of colloids, while colloids already present may coagulate[95,97].

In dolomitic water EhpH diagram for uranium (0.010.5 mg

U/L) is more complicated due to the presence of calcium


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Fig. 4. The effect of pH and concentration of carbon dioxide (log C) on the speciation of uranium in ground water assumed as a closed system [94].

and magnesium ions, and other possible complex with sulfate

[98100].A diagram for a real dolomitic system is presented inFig. 5.pH-drop below 6 (under normal oxidizing conditions) might

allow for uranium to stay in solution as [(UO2)3OH]5+ instead ofprecipitation as a carbonate[99,100].

In addition to dissolvedcarbonate, uranium can also form stablecomplexes withother naturallyoccurringinorganic and organic lig-ands such as phosphate complexes [UO2HPO40 (aq) and UO2PO4](Fig. 6)[101].Complexes with sulfate (Fig. 7)[102],fluoride and

Fig. 5. EhpH diagram for uranium (0.010.5mg U/L) in dolomitic water (adapted

upon[100]).

Fig. 6. Calculated uranium speciation in the system UO2PO4CO3OHH2O atover-saturation att= 25 C[101].

possibly chloride are potentially important uranyl species whereconcentrations of these anions are high. However, their stabilityis considerably less than the carbonate and phosphate complexes[82].

Fig. 7. EhpH diagram and uranium speciation in present of sulfates at t= 25 C(concentrations ofU-ions:0.01 mg/L;concentrationsof sulfate-ions: 0.1 mg/L)[102].(1) UO2+2 ; (2) U(SO4)

2+;(3)U4+;(4)UO2(SO4)0; (5) U(SO4)02; (6) UO2;(7)UO2(SO)

2;

(8) UO2(OH)2H2O; (9) U3O8; (10) U4O9.


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EhpH diagram fromFig. 7shows that, if the pH of uranium-bearing aqueous medium is increased, an insoluble uranateprecipitate (namely yellow cake) is formed. Also, this diagramrepresents a useful guideline regarding the regions of pH and oxi-dation potential in which simple uranium oxide, ions in solutionsand insoluble uranates exist[102].

Organic complexes may also be important to uranium aqueouschemistry, thereby increasing their solubility and mobility in soil.The uncomplexed uranyl ion has a greater tendency to form com-plexes with fulvic and humic acids than many other metals with a+2 valence[103].In particular, the presence of organic substancesand/or colloids in the groundwater increases the complexity of thesystem. Humic substances formed by the degradation of plantsand animals constitute a heterogeneous category of compoundswith a complex forming capacity due to the presence of carboxylic,hydroxy and phenolic groups [4]. Dissolved humic substances(humic and fulvic acids) proved to be strong complexing agents formany trace metals in the environment, forming also stable com-plexes or chelates with radionuclides[104,105].These substancescan be found as dissolved in surface waters as well as in ground-waters, in concentrations ranging from less than 1 mg (TOC)/L tomore than 100 mg (TOC)/L. It has been shown that the binding ofmetals to humic acid apparently occurs at binding sites with rel-atively well-defined complex formation constants[104].Uraniummineral precipitation and co-precipitation processes may also beimportant during remediation for some environmental conditions,and several uranium (co)precipitates may form, depending on thegeochemical conditions [106,107]. Solubility processes may alsobe particularly important for the environmental behavior of U(VI)under oxidizing conditions in those soils that become partially sat-urated with water or completely dry, when the concentration ofuranium in the residue pore fluids may exceed the solubility limitsfor U(VI)-containing[84].

2.3.1. Uranium oxides

The most common forms of uranium oxide are U3O8 and UO2.

Both oxide forms are solids with a low solubility in water andrelatively stable over a wide range of environmental conditions[61,108].U3O8 is the most stable form of uranium and is the formfound in nature. The most common form of U3O8 is yellow cake,a solid named for its characteristic color that is produced duringthe uranium mining and milling process. UO2 is a solid ceramicmaterial and is the form in which uranium is most commonly usedas a nuclear reactor fuel [4]. At ambient temperatures, UO2 willgradually convert to U3O8. Uranium oxides are extremely stable inthe environment and are thus generally considered the preferredchemical form for storage or disposal[61].

2.3.2. Uranium hexafluoride

UF6is the chemical form of uranium, which is used during the

uranium enrichment process [65].Within a reasonable range oftemperature and pressure, it can be solid, liquid or gaseous[109].At ambient conditions, UF6 is a volatile, white, crystalline solid.Solid UF6 is readily transformed into the gaseous or liquid statesby the application of heat. All three phases solid, liquid and gas coexist at 64 C (the triple point). Only the gaseous phase existsabove 230 C, the critical temperature,at whichthe critical pressureis 4.61mPa. The vapor pressure above the solid reaches 0.1mPa at56 C, the sublimation temperature.

Solid UF6is a white, dense, crystalline material that resemblesrock salt. UF6does not react with oxygen, nitrogen, carbon dioxideor dry air, but it does react with water or water vapor (includinghumidityin theair) [110]. WhenUF6comesintocontact withwater,suchas water vapor in the air, the UF6andwater react,forming cor-

rosive hydrogen fluoride (HF) and a uraniumfluoride compound

called uranyl fluoride (UO2F2). For this reason, UF6is always han-dled in leak-tight containers and processing equipment. Althoughvery convenient for processing, UF6 is not considered a preferredform for long-term storage or disposal because of its relative insta-bility[61].

2.3.3. Uranium tetrafluoride

UF4

is often calledgreen saltbecause of its characteristic color. Itis generally an intermediate in the conversion of UF6to U3O8, UO2oruranium metalbecause it canbe readily convertedto any oftheseforms. UF4 is a solid composed of agglomerating particles with atexture similar to baking soda. It is non-volatile, non-hydroscopic,but only slightly soluble in water [61]. After exposure to water, UF4slowly dissolves and undergoes hydrolysis, forming any of severalpossibleuraniumcompoundsand hydrogen fluoride[110].Thetimefor hydrolysis can be lengthy. Although not as stable as the ura-nium oxides, several recent studies have indicated that UF4may besuitable for disposal.

2.3.4. Uranium metal

Uranium metal is heavy, silvery white, malleable, ductile andsofter than steel. It is one of the most dense materials known(19 g/cm3), being 1.6 times more dense than lead [37]. Uraniummetal is not as stable as U3O8orUO2because it is subjectto surfaceoxidation. It blurs in air, with the oxide film preventing further oxi-dation of bulk metal at room temperature. Water attacks uraniummetal slowly at room temperature and rapidly at higher tempera-tures. Uranium metal powder or chips will ignite spontaneously inair at ambient temperature[111].

Some characteristics of uranium compounds, most of themdeterminant for choosing the remediation technique, are summedup inTable 4[4,37,61,71,112117].

2.4. Uranium compounds toxicity

Thetoxic effects generatedby uranium exposureare based on its

chemical and radioactive characteristics. Toxicity is closely relatedto solubility, i.e., the more soluble the uranium compound is, themore toxic it becomes[118].

The permissible levels for soluble compounds are based onchemical toxicity, whereas the permissible body level for insolublecompounds is based on radiotoxicity.

Several possible health effects are associated with human expo-sure to radiation from uranium. Because all uranium isotopesmainly emit alpha particles that have little penetrating ability, themain radiation hazard from uranium occurs when uranium com-pounds are ingested or inhaled[118120].

The less water-soluble compounds (uranium trioxide, sodiumdiuranate, ammonium diuranate) were of moderate-to-lowtoxicity, while the insoluble compounds (uranium tetrafluo-

ride, uranium dioxide, uranium peroxide, triuranium octaoxide)were primarily pulmonary toxicants. Soluble uranium com-pounds are toxic both when breathed or ingested. Generally,hexavalent uranium, which forms soluble compounds, is morelikely to be a systemic toxicant than the less soluble tetrava-lent uranium [98]. The most soluble uranium compounds areUF6, UO2(NO3)2, UO2Cl2, UO2F2 and uranyl acetates, sulfatesand carbonates. Some moderately soluble compounds are UF4,UO2, UO4, (NH4)2U2O7, UO3 and uranyl nitrates. The rapid pas-sage of soluble uranium compounds through the body tendsto allow relatively large amounts to be absorbed. Soluble ura-nium compounds may (also) be absorbed through the skin. Theleast soluble compounds are high-fired UO2, U3O8, and uraniumhydrides and carbides. The high toxicity effect of insoluble com-

pounds is largely due to lung irradiation by inhaled particles. This


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Table 4

Physical characteristics of uranium compounds[61]

Compound Melting point (C) Density (g/cm3) Properties

Crystal particle Bulk

UF6 Uranium hexafluoride 64.1 4.68 4.6 Soluble in water, decomposes to UO2F2highchemically toxic

UF4 Uranium tetrafluoride 960 5 6.7 2.04.5 Very slightly soluble in water at ambientaltemperature

UO2F2 Uranyl fluori de Decomposes to U3O8at 300 6.37 2.6 Soluble in water at ambiental temperatureUO2Cl2 Uranyl chloride Decomposes Decomposes in the presence of light, exhibits

fluorescence, highly toxicU3O8 Triuranium octaoxide Decomposes to UO2at 130 0 8.30 1.54.0 Very stable, has a low solubility in waterUO2 Uranium dioxide 2878 20 10.96 2.05.0 Insoluble in water, highly toxic and

spontaneously flammable, used in weapons inplace of lead in the Gulf War (also calleduranium oxide, uranitite)

UO3 Uranium trioxide 200650 8.0 5.58.7 Insoluble in water, poisonous, decomposeswhen heated (also called uranyl oxide)

UO2(NO3)22H2O Uranium nitrate 118 (decomposes) 2.81 Dissolves in water to form a weak solution of nitric acid, the reaction is not hazardous,oxidizing and highly toxic compound

Uranium metal 1132 19.05 19 Insoluble

material is transferred from the lungs of animals quite slowly(http://web.ead.anl.gov/uranium/guide/ucompound/health/index.cfm).

Of the most important uranium compounds used industrially,UF6and UO2(NO3)26H20 are the most toxic, whereas UO3is onlymoderately toxic, and UO2, U308 and UF4 are considered low intoxicity.

A variety of materials on radiation effects, obtained from animalexperiments and studies in humans were published [3,119121]. Asit has already highlighted above, from literature data it results thatall uranium mixtures, natural, depleted or enriched, are consideredchemical toxins that may result in nephrotoxic effects [3,122,123].The majority of the uranium deposited in the kidney is removedwith a biological halftime of 6 days and the remainder with a 1500d half-time. No permanent effects have been observed in any expo-

sure case[3].The presence of high levels of uranium (U) compounds in the

human body has been reported to affect renal functions and, atvery high concentrations, lead to kidney failure[124,125].The pri-mary pathways of U entrance into thehumanbody are inhalation ofcontaminated dust or ingestion of contaminated water. The chem-ical toxicity of uranium as a heavy metal has raised public healthconcerns,especiallyin areas where contamination of local soils andgroundwaterfrom radioactive material has taken place. As a result,there is strong interest in remediation of uranium and depleteduranium laden areas[125].

3. Methods and techniques for uranium removal

3.1. General description

The objective of any remedial action is to reduce the risks tohuman health, environment and property to acceptable levels byremoving or reducing the source of contamination or by blockingexposure pathways. Once the decision has been made that someremedial action is necessary, there are various potentialoptions forachieving that objective[5,9,126,127].

Radionuclides and heavy metals are retained by soil in threeways[11,128,129]:

adsorption onto the surface of mineral particles; complexation by humic substances in organic particles;

precipitation reaction.

As was highlighted above, the mobility of uranium in soilis mainly controlled by complexation and redox reactions[78,130,131]:

complexation leads to mobile species or precipitation of U-bearing minerals;

redox reactions changethe solubility between thetwo major oxi-dation states: U(IV)U(VI):o reduction of U(VI) to U(IV) immobilizes uranium;o oxidation of U(IV) to U(VI) mobilizes uranium because of the

dissolution of U(IV) bearing minerals.

These reactions are the basis for certain remediation tech-nologies, their combination determining the mobility and fate of

uranium. Furthermore, the techniques and methods for uraniumremoval from soil are selected according to the type of contami-nantspresent,thebehaviorofthecontaminantsintheenvironmentand the exposure pathways[5,9].For sites with mixed contamina-tion, it is often necessary to use several remediation technologies,sometimes in series, i.e., treatment trains, to effectively addressrisk from the radioactive, chemical and physical hazards that couldbe present. In addition, sites may have contamination in differentmedia[17]. It is not uncommon, for example, to have also ground-water contamination on sites with extensive soil contamination, sothat a rangeof technologiesis needed forremediation ofthe variouscontamination problems[5,132].

There arethree basic options for any intended remedial actions:monitored non-intervention, containmentor removal,summarizedinTable 5and various techniques/technologies are associated, suchas:

separation; concentration and/or volume reduction; immobilization/sequestration.

In contrast with organic compounds, radionuclides in generaland uranium in particular, cannot be commonly destroyed ordegraded.

Each one of the above fundamental technical choices will directdecision makers to substantiallydifferentpaths withregard to theirsubsequent choices, actions and potential results, making availablesignificantly different technological options for application, within

a remediation program, which involves multidisciplinary environ-
http://web.ead.anl.gov/uranium/guide/ucompound/health/index.cfmhttp://web.ead.anl.gov/uranium/guide/ucompound/health/index.cfmhttp://web.ead.anl.gov/uranium/guide/ucompound/health/index.cfmhttp://web.ead.anl.gov/uranium/guide/ucompound/health/index.cfm


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Table 5

Basic options for remediations of uranium-contaminated soils[3,133140]

Option for remediation Main actions involved Requirements andcharacteristics

Technique Goals Technology Description

Monitorednon-intervention

Establishing amonitoring scheme todetermine theevolution of the site,while it is maintainedundisturbed and an

alternative can beinitiated if required

In certain cases theradionuclide(s) presentmay have a sufficientlyshort half-life, allowingit/them first to decay toa level below that of

concern, i.e., applyingmonitorednon-intervention,before treating theother contaminants,thus reducing theproblem to a so-namedconventionalcontaminationproblem

Monitoring wells and/orinfiltration monitoringsystems

D ilution R adioactive madischarged routhe air and surfboth fresh and from nuclear faworldwide

The effectivenedilution in aqueis critically depthe speciation oradioelement uprevailing enviconditions

This will controsuch as solubiliadsorption to sbioavailability a

Natural attenuation Requires adequmonitoring, owevolution of nasystems with tiincomplete undof the processe

Containment Immobilizing thecontaminants insidethe area where theyalready exist, that iscontaining orrestricting themobility of theradioactivecontaminants

Aiming to reduce thepotential for furthermigration or entry intoactive pathways forexposure

Isolation Attempt to prevent thetransport of contaminantsby containing them withina designated area

Capping Provide an impbarrier to surfainfiltration tocontaminated sprevention of furelease of contathe surroundinwater or groun(simple single-and more compmulti-layer sys


11/36

Relatively simple andrapid implementationoften at lower cost thanalternatives thatrequire excavation;ability to treat largeareas and volumes ofwaste; potential for

successful containmentas the final action atthe site

Subsurface barriers Reduce the mocontaminated gfrom the site, othe flow ofuncontaminategroundwater thcontaminated s

Immobilization Reduce the mobility ofcontaminants by changingthe physical or leachingcharacteristics of thecontaminated matrix

Solidification/stabilization

Solidification information of a smatrix that phybinds the contamaterialStabilization refixation, usuallychemical reacticonvert the wamobile form

Vitrification Decreases mobmetal contaminhigh-temperattreatment of thcontaminated aresults in the fovitreous materan oxide solid


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Table 5 (Continued )

Option for remediation Main actions involved Requirements andcharacteristics

Technique Goals Technology Description

Removal Applying anappropriatetreatment scheme forcontaminantselimination

Aiming to extract,concentrate and thensafely dispose of thecontaminants atanother location

Toxicity and/ormobility reduction

Chemical and/or biologicalprocesses can be used toalter the form of metalcontaminants in order todecrease their toxicityand/or mobility

Chemical treatment Chemical reactinitiated that arto decrease themobility of con(oxidation, reduneutralization)

Permeable treatmentwalls

Remove contamgroundwater btransforming, por adsorbing thsolutes as the wthrough permetrenches contaireactive materithe subsurface iron, limestone

Biological treatment Exploits naturaprocesses that certain plants amicroorganismthe remediatiouranium (occurvariety of mechincluding adsoroxidation and rreactions, and m(bioaccumulatiphytoremediatphytoextractionphytostabilizatrhizofiltration, biochemical pr

Physical separation Attempts to separate thecontaminated materialfrom the rest of the soilmatrix by exploitingcertain characteristics ofuranium and soil; canoperate based on particle

size, particle density,surface and magneticproperties of thecontaminated soil

Screening Physical separatechniques are that operate baparticle size, padensity, surfacemagnetic propecontaminated s

Classification

GravityConcentrationMagnetic separationFroth flotation


13/36

Extraction Can be achieved bycontacting thecontaminated soil with asolution containingextracting agents (soilwashing andin situsoilflushing) or byelectrokinetic processes.The contaminated fractionof soil and/or process water

is separated from theremaining soil anddisposed or treated

Soil washing Requires soil exprior to treatmchemical treatminvolves additioextraction agenwith the contamleach it from thphysical treatmachieved by paseparation tech

adapted from mprocessing to cothe contaminanparticular size f(involves prelimscreening, secoscreening, chemtreatment, phytreatment, dewwater treatmen

Pyrometallurgicalextraction

Use elevated teextraction and for removal of mcontaminated s

Is usually precephysical treatm

provide optimusizeIn situsoil flushing Mobilize metal

leaching contamfrom soils so thbe extracted wiexcavating thecontaminated m

Electrokinetic

treatment

Apply a low den

current to contsoil in order to contaminants iof charged spec


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Fig. 8. Classification of remediation techniques by function[133].

mental research on characterization, monitoring, modelling andtechnologies for remediation (Fig. 8).

Any measurable remediation objective has to consider severalfactors, which could induce an impact on the decision makingprocess have to be considered, like basic evaluation criteria thatinclude engineering and non-engineering reasons for ensuring theachievabilityof the cheaper, smarter and cleaner soil remediationphilosophy, such as[20,28,132,141143]:

cleanup goals; form and concentration of pollutants; volume and physical/chemical properties of the polluted soils; remediation effectiveness; designated use of the cleaned site; cost associated with the remediation program; occupational safety and health risks associated with the technol-

ogy; potential secondary environmental impacts (collateral damage); prior experience with the application of the technology; sustainability of any necessary institutional control; socio-economic considerations.

Fig. 9is a schematic representation of the relations betweenevaluation of remediation alternatives and remediation aims andoptions as a support for decisions making about implementation,so that the remediation performances be fulfilled.

The costs for implementing available technologies will vary sig-nificantly between sites because costs are influenced by a widevariety of factors.Fig. 10represents the ranges of operating coststhat have been observed for remediation of metals-contaminated

soils by a number of techniques.

Remediation technologies available for treating uranium con-taminated soils and groundwater could be applied as either ex situorin situtechniques[132,144146].

In situ techniques are generally preferred because they causeless site disturbance, less contaminant exposure to the environ-mental professionals and public in the vicinity, and they are oftenless complicated and more economical[141146]. Also, becauseradionuclides are not destroyed,ex situremediation requires sitesfor waste disposal, which have to meet special acceptance criteriaand are limited[147].

A series of bench-scale studies were conducted to evaluate theinfluence of some factors affecting uranium removal from soils,such as soil characteristics, time, temperature, attrition scrubbing,pH and reagent concentrations, oxidizing and reducing chemicalenvironments[143145,148].

Soil characteristics affect the remediation process, since soilbehaves as a complex sorbent. Uranium preferentially adheres to

soil particles, with a soil concentration typically about 35 timeshigher than that in the interstitial water. Concentration ratios areusually much higher for clay soils (e.g., 1600). The concentrationsand distributions of uranium among particle size fractions of thesoils vary significantly,as is shown in Table 6. Data in Table 6 areforparticle size fractions of the some soils after wet sieving and sepa-ration of theclayfractions 0.002 mm diameterfollowingmethodsaccording to some authors[148150].

When mining soils are remediated,it is necessaryto discern thaturanium mining wastes comprise several types of waste[134]:

overburden (soil and rock that is covering a deposit of ore, suchas uranium. It usually contains at least trace amounts of the ore

plus radioactive decay products);


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Fig. 9. Remediation alternative evaluation in relation with aims and performance [adapted upon 139143].

unreclaimed, subeconomicores(ores thathave too little uraniumto be profitable, called protores);

barren rock (rock containing no ore); drill cuttings.

The EhpH diagrams (Figs. 37)indicate that sorption onto soilcan be strongly influenced by the pH of the soil solution and, to alesser extent, by the presence of calcium, suggesting specific chem-ical interactions between U(VI) and the soil matrix, so that theremediation process will depend on the same factors like those

Fig. 10. Estimated operating costs of some available remediation technologies for

contaminated soils (http://www.clu-in.org/download/toolkity/metals.pdf).

influencing uranium solubility[151,152].Therefore, soil remedia-tion techniques will be chosen considering sorption, complexation

and redox reactions as the dominant mechanisms responsible forthe reduction of mobility, toxicity or bioavailability of radionuclidecontaminants [10,78,130,131,153,154].

The sorptive phases that could control U(VI) sorption onto thesoil are the iron and manganese oxide coatings and the clay frac-tion. Specifically, hydroxyl groups on the oxide surface, SOH, areexpected to be the dominant sorption sites[155].

Hydroxylated groups (SiOH and AlOH) situated along theedges of clay minerals can also be significant sorption sites [156].Sorption onto such variable charge sites depends on the pH of thesoil solution.

Some authors reported adsorption edges at pH 45 workingwith sorbents as goethite and ferrihydrite[90,157,158];the lowersorption edge could be caused by the heterogeneity of the soil: iron

and manganese oxides, clays and a small fraction of organic matterare all present.The pH dependence of the sorption process indicates that pro-

tons (H+) compete with U(VI) for sorption sites (surface hydroxylgroups, SOH); at low pH, H+ is the key sorbing species, formingpositively charged diprotonatedsites(SOH2+).AsthepHincreases,U(VI) ions displace H+ and bind to OH groups on the surface. TheU(VI) reaction with those surface hydroxyl groups is similar to thehydrolysis reaction observed in aqueous solution only.

Sorption of U(VI)CO32 complexes is responsible for the largedegree of U(VI) removal from the aqueous solution observed nearneutral pH [98100]. The identity of these U(VI)carbonate surfacespecies was analyzed by several authors: Hsi and Langmuir[158]have discussed sorption of [UO2(CO3)SO3]2 and [UO2(CO3)3]4

onto goethite; Payne and Waite [159] have suggested UO2CO3,
http://www.clu-in.org/download/toolkity/metals.pdfhttp://www.clu-in.org/download/toolkity/metals.pdf


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Table 6

Concentrations dry weight basis and distribution of uranium in soils[149,150]

Soil, uranium concentration (mg kg1) Size fraction (mm) Particle sizedistribution (wt%)

Uranium concentration(mgkg1)

Uranium contribution bysize fraction (wt%)

Y-12 landfarm

>2.00 1.8 442 4.32.000.075 17.5 361 33.80.0750.020 34.1 31 5.80.0200.002 30.4 180 29.3

0.002 16.2 312 26.9

Incinerator soil-538

2.000.053 12.5 1033 270.530.002 73.9 286 44

0.002 13.6 1019 29

Minning soil-4462.000.053 22.6 117 70.0530.002 56.5 239 37

0.002 20.9 983 56

[UO2(CO3)2]2 and [UO2(CO3)3]4 sorption onto amorphous fer-ric oxyhydroxide (Fe2O3H2O); Waite et al. [90] have proposedsorption of UO2CO3onto ferrihydrite; Duff and Amrhein[81]havefound that [(UO2)2CO3(OH)3] is the sorbing species in the pres-ence of goethite. There is a number of spectroscopic studies thatprovide direct evidence for the formation of ternary Ucarbonato

species [SO2UO2(CO3)x on mineral surface such as hematite andsilica[160,161].

Several investigations on the removal of uranium from artifi-cially contaminated soils as well as from anthropogenic sources,such as mine tailings and DU processing facilities, revealed thatthe application of any remedial technology for soils contaminatedwith uranium must take into account the potential of exposure toworkers (as a consequence of the presence of radionuclides andthe type and energy of radiation emitted) and the need to keep theexposure as low as reasonable achievable [147,162164].The ulti-mate goal is to develop technologies that can further reduce risks,reduce cleanup costs and reduce the volume of remaining contam-inated soil[15].These involve technologies assessment based onsome reasons, which can include[15,141,146,147]:

accounting of contaminant distribution, soil characteristics andadhesion/absorption characteristics of contaminants on soil par-ticles;

evaluation of physical, chemical and biological processes thatshould have potential to remediate radioactive contaminatedsoils;

ranking of technologies based on technical value, potential expe-rience and facility of implementation;

engineering evaluation of technologies to determine scale-uppotential and cost effectiveness;

identification of secondary waste treatment needs for full-scaleimplementation;

identification of difficulties and research needed to overcome

technology limitations.

3.2. Natural attenuation

The long-term restorations of ecosystems and contaminatedsites have been attributed to natural attenuation processes.

Natural attenuation is the process by which the concentrationof environmental pollution is reduced to an acceptable level bynatural processes. According to the USEPA [47], natural attenu-ation is the use of natural processes to contain the spread ofthe contamination from chemical spills and reduce the concen-tration and amount of pollutants at contaminated sites. It canalso be termed as intrinsic remediation, bio-attenuation and intrin-sic bioremediation [164,165,166]. Natural attenuation is considered

to be the least invasive approach to environmental remediation.

Generally, these processes occur in soil, groundwater, and sur-face water systems at all sites at varying rates and degrees ofeffectiveness to decrease the concentrations of organic and inor-ganiccontaminants [152,166,167,168]. Theefficiencyofthismodeofremediation will vary based on the biological and chemical natureof the contaminated site. The physical, chemical and biological

processes, the rate and extent to which these natural attenua-tion processes occur are different for each contaminant and sitehydrologic and geochemical conditions. Under certain conditions(e.g., through sorption or oxidationreduction reactions), naturalattenuation could effectively reduce the dissolved concentrationsand/or toxic forms of inorganic contaminants in groundwater andsoil[169].

Natural attenuation of radionuclides can occur through a num-ber of sorption processes, including incorporation of contaminantsinto a mineral in the soil or an aquifer or by being entrapped ina rock pore [170]. Whereas natural attenuation of organic con-taminants means breakdown and elimination by microorganisms,natural attenuation of radionuclides involves their encasing in amineral where they will not escape unless chemical conditions

change dramatically[135,152,166,170,171].A number of investigations were performed concerning the

natural attenuation of uranium in a tailings disposal site, whichrevealed that a number of radionuclides exhibit significant migra-tion potential in the presence of aqueous, low molecular weightorganic compounds immobile organic matter in the form of peator organic-rich horizons in soils and sediments that may pro-vide excellent substrates for radionuclide retention[168,172176].When uranium is dissolved in groundwater, it can be attracted tonatural iron coatings located on walls of rocks through which thewater is flowing, and bind to the iron coating and then move intomicroscopic rock pore and then incorporated into the iron coating[176,177,178].

The monitoring of the process involves the study of the rate

and extent of irreversible adsorption of uranium, the fraction ofuranium in aquifers associated with the phenomenon, and thedegreeof irreversibleadsorption of uranium required to reduce theamount of uranium under the safety levels[179,180].Microorgan-isms appear to be excellent indicators of soil health because theyrespond to changes in the soil ecosystem quickly[181].

Phytostabilization strategies may be suitable to reduce the dis-persion of uranium and the overall risks of U-contaminated soils.Plants grown in soils with high carbonateU fractions can accu-mulate the most U in shoot sand roots, while in clay soils withhigh Fe, Mn anorganic fractions this process is low[148].Macro-phytes in natural and constructed wetlands can influence uraniumimmobilization either directly by uptake and accumulation and/orindirectly (biomass productionlitterfall and root turnover decom-

position).


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A number of investigations on the natural attenuation of theuranium (U) load in the surface water in various environmentalconditions (for example, within a humid forest in Japan) were per-formed [15,176,178,180,182]. Surface water and sediments that hadaccumulated behind dams in the area were investigated in termsof their mineralogy and chemistry. The results of this investiga-tion indicate that U, which within the study area is derived frompegmatites at a mine, is attenuated by uptake onto the surface oforganic material as well as by amorphous material that forms overtime within the dammed sediments. In most cases costs only occurin connection with monitoring[177,179].

3.3. Physical processes

Physical processes include soil capping, soil washing, soil aer-ation and heat flow, water storage and drainage, solidification,solubilization and solute transport[28,183,184].

Landfilling of radionuclides is difficult and expensive and landdisposal restrictions have to be taken into account, as well as othernational and international regulations[147,185,186].

Soilcappingis a strategyof radioactivewastecontainment, whenburied waste is capped with layers of materials (even imperme-able) mainly to prevent surface waterinfiltrations, although it doesnot fully solve the contamination problem and does not changethe wastes toxicity[184,187191].In fact, caps are covers placedover landfills and other waste areas, and are intended to isolate thewaste.

Impermeablecapscouldbesingleofmulti-layerclays,toppedbyplastic sheets, soil,etc. [184,187]. Capping process can be structuredin four phases[184,188190]:

mobilization(preparation of site for cap construction); operational(actual cap construction); closure (installation of monitoring wells, decontamination and

demobilization of equipment); post-closure(monitoring and long-term maintenance).

Capping technology has the advantage of little disturbance ofcontaminantsaswellaslowercosts [184]. Somedisadvantagesreferto caps degradation by cracking (especially in environments withfreeze andthaw cycles) or weakening by penetration of plant roots,landfill setting, etc.[191].

Soil-washingor soil-flushingsystems are designed to treat soilswhere the majority of the contaminants are concentrated in thefiner-grained materials or on the surfaces of the larger soil par-ticles. Soil washing entails extraction of unwanted contaminantsfrom soil with liquids, generally aqueous solution, when the con-taminants are separated from the soil matrix and transferred tothe washing solution and then the washing solution is extractedfrom the soil. Water or liquid solutions, whether injected or infil-

trated into the contaminated area, mobilize the contaminant andare then collected and brought to the surface for disposal, recircu-lation, on-site treatment or reinjection [178]. Some operational andperformance criteria were used for this analysis, such as [192,193]:

removal efficiency, as the ratio of uranium activity in the treatedsoil to that in contaminated soil;

ability to meet the standards; possibility to achieve the smallest volume of the treated soil so

as to ensure off-site disposal.

The majority of soil-washing processes involve screening pro-cesses in orderto separate thefine contaminatedparticles [28,183].

Solidificationthe binding of a waste/soil into a solid mass can

reduce its contaminant leaching potential. This process involves

the generation of blocks of waste, where the radionuclides are keptmechanically within a solid matrix[147].Vitrification of moltenglass is another solidification method which uses heat of up to1200 C to melt and convert waste into crystalline products. Also,stabilization reduces the solubility and/or chemical reactivity ofa waste/soil. Usually, it entails the addition of various binders(cement, silicates) in order to limit the solubility and/or mobility ofradionuclides [147]. Both can be done in situ or ex situ on excavatedmaterials by processing at a staging area either on site or off-site[28]. Physical treatment processes can be sequenced in a treatmentqueue, such as: excavation, transportation to a vitrification site andvitrification/sequestration[109].

3.4. Chemical methods

Chemical processes for remediation of radionuclide pol-luted soils include chemical degradation/transformation,volatilization, oxidation/reduction, solubility processes andadsorption/desorption.

A variety of chemical remediation techniques are availablefor remediation of radionuclide-contaminated soils that can begrouped as

chemical conversion into a water-soluble form; chemical immobilization.

Research has shown that there are at least three different formsof uranium in the contaminated soil:

uranium(VI) phosphate minerals; reduced U(IV) phases; complexed U(VI) with soil organic matter; a small fraction of U(VI) sorbed onto soil minerals[72].

These methods are often expensive to apply and lack the speci-

ficity required to treat target metals against a background ofcompeting ions[27,194,195].In addition, such approaches are notapplicable to cost-effective remediation of large-scale subsurfacecontaminationin situ. Also, the extraction media and proceduresdesigned for a selective uranium removal have to preserve thephysico-chemical characteristics of soils and avoid the generationof secondary waste forms, difficult to manage or dispose.

The removal procedures must guarantee a good mobility of ura-nium so that it would pass from the environmental componentinto another system[183,195].Four approaches to increase metalmobility in heavy metal and radionuclide polluted soils have beensuggested: change in acidity, change in ionic strength, change inredox potential and formation of mobile complexes [27,28,196].They are discussed in the following paragraphs.

Table 7highlights some chemical processes used for extractingU from contaminated soils indicating that several highly efficientchoices exist for the extraction of U from contaminated soils andother materials[197].Under ambient oxidizing conditions, U(VI)should be easily removed using bicarbonate, a strong inorganicacid or a weak organic acid (ascorbic, citric). Ebbs et al.[198]haveexamined the role of acidification and chelating agents in the sol-ubilization of uranium (U) from contaminated soil and comparedthe two methods.

3.4.1. Chemical extraction

The chemical extraction process from contaminated soilsinvolves the conversion of uranium into a water-soluble form,which is then extractedfrom the soil. An adequate uranium extrac-

tion can be achieved if certain conditions are met[195,199,200]:


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Table 7

Selected DU chemical soil extraction methods[197]

Total DU removed U content (mg/kg) Material Extraction method Overall percentage Reference

420 1320 Contaminated soil and ash 0.10.5 M NaHCO3 80 [138]95 708a Acid/mixed/alkaline tailings, contaminated soil 0.1 M NaHCO3 2094 [162]

449 732 Contaminated soil 0.20.6 M citric acid 8599 [163] 2629 Radioactive waste 4 M HNO3+0.05M H3BO3 >99 [164]

a

Values interpolated from publication.

uranium solubilization by exposing it to the solution which con-tains the chelating ions (citrate, carbonate);

availability of complexing anions, by controlling the solutionchemistry, chemical environment and conditions (pH, time, tem-perature) and early uranium precipitation;

uranium oxidation to the hexavalent state (in the presence ofcarbonate).

Using chelating agents to extract uranium from contaminatedsoils is considered to be a chemical treatment method[201205].

U(VI) cationic complexes are abundant from low to alkaline pH

(about pH 8) (Eqs.(1)(5)):

4H+ + UO22+ + 2e 2H2O(l) + U

4+ (1)

UO22+ + 2e UO2(s) (2)

UO22+ + H2O(l) H

+ + UO2OH+ (3)

3UO22+ + 5H2O(l)(5H

+ + [(UO2)3(OH)5]+ (4)

3UO22+ + 7H2O(l)(7H

+ + [(UO2)3(OH)7] (5)

Chelating agents may be either organic or inorganic com-pounds. A numberof inorganicchelatorshave been investigated forremediation and some of complexation constants for those chela-tors are now available [78,9093,99102,201]. Polyphosphates

are considered as the most efficient inorganic chelators. Theirannual consumption is higher than that of organic chelating agents[200,201,202]. Chelating compounds may be represented eitherusingconventional empirical and structured formulas or some typeformula as shown inTable 8.

The strong chelators with target metals will have much greatersolubility and stability (i.e., stability constant pK) than other reac-tionswithmetalsintheaqueousphase [200203]. Thisstrengthcanbe shown as pKchelators/pKnatural ligands. The greater this ratio,the stronger this chelator will be. The stability of chelates is influ-enced by a number of parameters. Several of the stability factorscommon to all chelate systems are the size and number of rings,substituents on the rings, and the nature of the metal and donoratoms. The role of acidification and chelating agents in the solubi-

lization of uranium (U) from contaminated soil was examined in aseries of experiments.

3.4.1.1. Method(s) using sodium carbonate/bicarbonate.Carbon-ate/bicarbonate ions could lead to a rapid and greatly increasedleaching and mobilization of U(VI) from a contaminated soil,depending on site-specific conditions[72,162,197].

Sodium bicarbonate has been used in the mining industry toextract U from carbonate bearing ore material. The bicarbonate ionforms strong aqueous complexes with U(VI) according to reactions(6) and (7)and enhances the dissolution of UO22+.

UO22+

(aq) + 2HCO3

(aq) [UO2(CO3)2]2

(aq) + 2H+

(aq) (6)

[UO2(CO3)2]2

(aq)+ HCO

3(aq) [UO2(CO3)3]

4

(aq)+ H+

(aq) (7)

This stable water-soluble complex forms easily under ambientconditions.Fig. 11presents the dominating complexes of UO22+ asa function of [CO3]2 [206208].

Mason et al.[138]used NaHCO3 solution as an alkaline treat-ment for U contaminated soils from a processing facility in OH,USA. The authors were able to recover 80% of the total DU in theaqueous phase. Residual DU in the soil was determined to be com-prised of relatively insoluble minerals, including meta-autunite(Ca(UO2)2(PO4)2xH2O, log Ksp(25 C) = 48.5), uranium metaphos-phate (U(PO3)4) and uraninite (UO2). It was considered that theautunite was formed because the local soil had high phosphate

content from prior pollution to the site. Mason et al. [138]indi-cated that efficiencies of 7590% corresponding approximately tothe percentage of uranium in the oxidized state were achieved forthe removal of uranium from contaminated soils (Ohio in USA)using 0.5 M sodium bicarbonate as the dominant reagent.

Sodium peroxide,Na2O2, was also added to theleaching processto promote oxidation of U(IV) by the following reactions:

Na2O2 + 2H2O(l) 2NaOH + H2O2 (8)

UO2(s) + H2O2 + 2H+ UO2

2+ + 2H2O(l) (9)

Use of sodium peroxide (oxidizing agent), improved uraniumremoval due to oxidation of U(IV), enhancing the solubility of theuranium[28].The oxidation of U(IV) to U(VI) occurred by a two

electron transfer from U(IV) to H2O2[209]. The resulting uranyl ion(UO22+)wasthenavailableforsubsequentcomplexationwithHCO 3ions by reaction(1).A 10:1 molar ratio of oxidant to U enhancedthe extraction of DU by 20%[206].

Phillips et al.[162]applied a process for concentrating uraniumfrom contaminated soils in which uranium is first extracted withbicarbonate and then the extracted uranium is precipitated withU(VI)-reducing microorganisms. Their results demonstrate thatbicarbonate extraction of uranium from soil followed by microbialU(VI) reduction might be an effective mechanism forconcentratinguranium from some contaminated soils.

Fig. 11. Speciation of uranium depending on CO32 concentration (adapted upon

[204206]).


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Table 8

Examples of different types of chelating compounds for uranil and corresponding chelats [55,201,202]

Active group in chelators Chelator agents Corresponding chelats

Proton acid groups

Partially dissociated acid

Alcohol/phenol groups that loseprotons


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Table 8 (Continued )

Active group in chelators Chelator agents Corresponding chelats

Entire multidentate ligand molecule ora donor atom that l ose protons

Bench scale experiments described by Kulpa and Hughes [197]showed that a certain soil could be treated effectivelyusing a 0.2Msodium bicarbonate solution at a temperature of approximately320 C and a retention time of 1.5 h and concluded that chemicaltreatment using carbonate extraction achieved removal efficien-

cies of up to 90%. A pilot plant designed to process 2-ton batches ofcontaminated soil indicated that chemical extraction soil washingwould result in contaminant removal efficiencies of approximately82% and volume reductions of 95%[197].

Based on these results, a soil washing facility of 10 ton/h pro-ductionwas designed and constructed, leading to important costsavings comparative to shipment and disposal procedure. A blockdiagram of this system is presented inFig. 12.This process whichuses a relatively mild concentration of sodium carbonate to form acarbonate complex with uranium is approximately 8595% effec-tive depending on properties of the soil and source term of theuranium material contained in the soil.

The feasibility of the carbonate extraction process in a full-scaleoperationdesignedtoleachuraniumfromcontaminatedsoilsusing

sodium carbonate/bicarbonate solution was also determined byseveral researchers[192,210,211].

3.4.1.2. Method using citric acid. Weak organic acids or their saltscan be used as environmentally compatible compounds. Citrate isused as a complexing agent to mobilize sorbed and precipitateduranium in both in situand ex situextraction of soils and nuclearreactor components. Various researches revealed that citric acidis highly effective in uranium mobilization and the efficiency ofextraction fromcontaminated soilsincreasedwith the acidconcen-tration[202,213].However, care should be taken with the quantityof citric acid used in such systems, because additional quantitiesmayresult in uranium migrationwhich contaminates groundwater.

Laboratory and bench-scale experiments were conducted by

Kantar and Honeyman[212]to determine the efficiency of citric

acid as an agent to mobilize and extract uranium from contami-nated soils.Some results indicatedthat citricacid is highlyeffectivein removing uranium, and that the extraction efficiency increaseswith increasing citric acid concentration, especially under slightlyacidic to alkaline conditions[212,214,215].

The enhanced U(VI) desorption in thepresenceof citrate maybeexplained through several processes, including the complexationof U(VI) with citrate and extraction of secondary coatings (e.g., Fe),together with the liberation of Fecitrate complexes into solution[212,215217].In batch washing systems, the presence of 10 3 Mcitric acid enhances the extraction of uranium 2.8 times greaterthan water alone for the conditions of the experiment. Huang etal.[218]found a close correlation between the U and the Fe and Alconcentrations in the soil solution after the addition of citric acidwas found, explained by the dissolution of Fe and Al sesquioxidesand hence release of U from soil material to the soil solution. Forexample,a removal efficiency of up to98% was achieved with 10 mLof 103 M citric acid in batch systems,whereas it is required150mLof 0.1M citric acid to accomplish similar extraction efficiencies in

column soil flushing systems[212].Citric acid (C6H8O7, H3Cit) has also been used to treat DUcontaminated soil from various locations (OH, USA, Serbia) withremovals efficiencies ranging from 85 to 99% [125,163,217]. TheacidformedanaqueouscomplexwithU(VI)underacidicconditions(below pH 5.0) by the reaction(10)[219]:

UO22+ + Cit3 UO2Cit

(10)

An importantadvantage of Ucitrate complexes for remediationaccomplishments is their biodegradability. This may depend on thepH of the system, initial U:citrate molar ratios, contact time. Forexample, Huang et al. [218] and Gramss etal. [213] have found that,in solutions buffered at pH 67, limited biodegradation of citrateoccurred within 10 days with initial U:citrate molar ratios rang-

ing from 1:2 to 1:8, while over 99% of the citrate is biodegraded


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Fig. 12. Soil washing process simplified flow diagram[195].

rapidly at pH 89. They explained this behavior by the fact that theincrease of pH shifted the equilibrium speciation of uranium from(UO2citrate)to (UO2)3(OH)7and, consequently, the bioavailabilityof citrate was increased. AtpH 67, a significant amountof uraniumis also observed to associate with biomass, whereas only a neg-ligible amount is observed at pH 89 [218].The biodegradabilityof Ucitrate complexes is an important control over the potential

migration of residual uranium after the extraction process is com-plete[216].The experimental results of Huang et al. [218]showthat the residual concentration of uraniumcitrate complexes leftinthetreatedsoilscanbereducedrapidlyifthesoilwaterpHrangedbetween 8 and 9 after the extraction process. The biodegradationof Ucitrate complex does not cause uranium release[220].

3.4.1.3. Two-stage acid leaching procedure. Another method of DUextraction was reported by Jonathan et al. [221] and Nirdosh [164].A two-stage acid leaching procedure was developed for treatmentof radioactive soil waste or aqueous system containing around2.1 104 M (50 mg/L) U.

In the two-stage leaching procedure, uranium is first removedfrom the particulate matter by forming coarse particulate slurry

in an aqueous medium containing acid, hydrogen peroxide andiron. The coarse particulate matteris separated from the slurry andprocessed for subsequentrelease to the environment. Residual ura-nium is further leached fromthe remaining fine particulate fractionin a second-stage leaching by forming slurry of the fine particulatefraction in an aqueous mediumcontaining acid, hydrogen peroxideand iron. The fine particulate matter is separated from the slurryand processed for subsequent release to the environment as cleansoil[221,222].

Uraniumisrecoveredfromtheleaching-stepssupernatantsolu-tions in either a two-stage precipitation or by passage over anion-exchange resin. Water recovered from the uranium removalsteps is recycledfor use in the upstream processing steps. In partic-ular, the water requirements are reduced by utilizing a closed loop

system for process water which is reclaimed, clarified and recircu-

lated[221].The overall processing scheme of the method is showninFig. 13.

3.4.1.4. Extraction with solvents and supercritical fluids. A variety ofhazardous organic solvents could be replaced with supercriticalfluids, which are hybrid solvents with properties between thoseof gases and liquids, with a low viscosity, high diffusion rates

and no surface tension[221,223,224],CO2 and water are the bestknown solvents applied for extraction in supercritical conditions[225227].

By changing the temperature and pressureand, perhaps, addingsmall amounts of co-solvents, these two environmentally benigncompounds can span a remarkably wide range of solvating power[226,228,229]. Supercritical CO2(SC-CO2) can be used with chelat-ing agents to replace organic solvents for the extraction of uraniumfrom aqueous solutions produced in the processing of the ore, aswell as for the removal of heavy metals from soils and sludges[225,226,228]. The removal of uranium from acidic waste solutionsin the nuclear industry using solvent extraction is currently per-formed with tri-butylphosphate (TBP)[230].There are numerouspublished papers which highlight the use of supercritical fluids

for uranium extraction [226,229,231235]. Direct dissolution ofrecalcitrant uranium oxides usingnitric acidand metal-complexingagents in supercritical fluid carbon dioxide has also been reported[236238].

Meguro et al. [229] applied a new method referred to assupercritical

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