Chemical Geology 360–361 (2013) 134–141

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Chemical Geology

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The dynamic role of natural colloids in enhancing plutonium transportthrough porous media

Jinchuan Xie ⁎, Jiachun Lu, Jianfeng Lin, Xiaohua Zhou, Mei Li, Guoqing Zhou, Jihong ZhangNorthwest Institute of Nuclear Technology, P.O. Box 69-14, Xi'an City, Shanxi Province 710024, PR China

⁎ Corresponding author. Tel.: +86 29 84767789; fax: +E-mail address: xiejinchuan@hotmail.com (J. Xie).

0009-2541/$ – see front matter © 2013 Elsevier B.V. All rihttp://dx.doi.org/10.1016/j.chemgeo.2013.08.016

a b s t r a c t

a r t i c l e i n f o

Article history:Received 8 March 2013Received in revised form 13 July 2013Accepted 10 August 2013Available online 19 August 2013

Editor: David R. Hilton

Keywords:PlutoniumColloidPorous mediaSpeciesTransport

Unexpectedly fast migration of Pu in aquifers at some nuclear sites was known as the result of enhancedtransport by natural colloids. Nevertheless, transport experiments still need to provide direct evidence for therole of colloids in promoting Pu transport. On the other hand, systematic laboratory studies should be carriedout to comprehensively evaluate Pu mobilization as affected by colloids that are varied in concentrations, sincenaturally occurring colloids have greatly different concentrations in aquatic environments. We isolated mineralcolloids with the diameter of 1 nm–1 μm from sandy soils and prepared the colloidal suspensions with theconcentrations ranged from c= 0 (solution) to c=2017.80 mg/L. The results of transport experiments showthat the mobile fraction of 239Pu (RPu= 8.62%) associated with the colloids of an extremely low concentration(c=0.5mg/L) is much larger than that not associated with the colloids (RPu=1.31%, c=0). The mobile fractionis defined by the percent recovery of Pu (RPu). This is strong evidence for the enhanced transport of Pu by thecolloids. Plutonium mobility (reflected by RPu) continually increases with the colloid concentrations ofc b 375.37mg/L and then declines at c N 375.37mg/L, which is demonstrated as a dynamic role of colloids in Putransport. The highest mobility (RPu = 52.48%), occurring at c = 375.37 mg/L, is referred to as the criticaltransport-enhanced concentration. Plutonium transport mechanism with a focus on its dynamic depositiononto the surfaces of porous media is explored.

© 2013 Elsevier B.V. All rights reserved.

1. Introduction

Plutonium released from nuclear tests (Tompson et al., 2002; Smithet al., 2003) and waste leakages (Dai et al., 2005; Parry et al., 2011) intoaquatic environments was generally considered as an immobile phasein terms of its low-solubility and strong sorption onto stationarymedia (McCarthy and Zachara, 1989), whereas it had traveled longdistances, e. g., ~1.3 km and ~4 km in aquifers of Nevada (Kerstinget al., 1999) andMayak (Novikov et al., 2006) nuclear sites, respectively.These observed distances are substantially larger than those of severalcentimeters that were calculated with predictive models by using thedistribution coefficient (Kd) determined by batch sorption experiments(McCarthy and Zachara, 1989; Ryan and Elimelech, 1996). Colloid-associated Pu defined by ultrafiltration measurements, was manifestedas the dominant species in the aquifers (Bates et al., 1992; Kersting et al.,1999; Novikov et al., 2006). Also, lab-scale transport studies of low-solubility radionuclides (e.g., Pu, Am, U) associated with colloids werecarried out under various conditions (Kurosawa and Ueta, 2001; Vilksand Baik, 2001; Perrier et al., 2005; Delos et al., 2008; Missana et al.,2008; Crançon et al., 2010), commonly suggesting that colloidsenhanced their mobilization. On the other hand, natural colloids

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including organic and inorganic colloidal particles with the diameterof 1 nm−1 μm are ubiquitous in aquatic environments (Pokrovskyet al., 2010; Allard et al., 2011; Schäfer et al., 2012). On the basis ofthese results, colloids are now widely acknowledged to play animportant role in enhancing transport of Pu in the geosphere.

However, such results cannot account for the enhanced transport ofPu (and other low-solubility radionuclides) by colloids. The colloid-enhanced transport is, in fact, an obvious implication that the mobilefraction of the contaminants associated with colloids (RPu−1) is largerthan that not associated with colloids (RPu − 2) (Honeyman, 1999;Turner and Fein, 2007), when the same contaminants having thedifference in species are transported under the same conditions suchas media, pore water velocity, and ionic strength. Therefore, aconvincing demonstration that colloids can enhance Pu transportmust lie on a comparative result: RPu− 1 (c N 0) N RPu− 2 (c=0), c isthe colloid concentration. This implication consistently tends to beignored in transport experiments (Kurosawa and Ueta, 2001; Deloset al., 2008; Missana et al., 2008). Until now, no attempt has beenmade to provide the evidence resulted from the comparative results.

Furthermore, if changes in Pu mobilization may occur due tothe presence of colloids (i.e. c N 0), challenges arise again. Whatwill happen to Pu as the colloid concentration increases? Does itsmobility increase or decrease with c? And whether or not Pu hasthe highest mobility at one concentration. Since naturally occurring

135J. Xie et al. / Chemical Geology 360–361 (2013) 134–141

colloids in aquatic environments have the concentrations spanningseveral orders of magnitude (Kim, 1991; DeNovio et al., 2004), acomplete understanding of the role of colloids in Pu transport isrequired.

Studies on effects of the colloids with various concentrations on Pumobilization are lacking. As far as we know, only a few papers on Am(Artinger et al., 1998) and Pb (Papelis, 2002) exist. These resultsindicated the increasing mobilization of these low-solubility elementswith the concentrations. However, colloids that are assumed to havethe condition of infinite concentrations (c → ∞), would exhibit animmobile characteristic in porous media systems, due to the absenceof fluid drag. Accordingly, when the concentrations increased up to acritical value (cr), mobilization of the colloid-associated Am and Pbwould begin to decrease instead of continually increase. Then, theirmobile fractions might ultimately be reduced to zero at c→∞. It thusseems that colloids should have a dynamic role in the transport oflow-solubility radionuclides. The highest mobility (i.e., the largestpercent recovery) is therefore expected to exist at cr, referred to as acritical transport-enhanced concentration. This research is significantlyimportant for insight into Pu transport mechanism and hence foraccurately predicting the risk of Pu into subsurface water.

In this study, efforts were paid to provide the evidence for enhancedtransport of 239Pu in the presence of natural colloids by comparativetransport experiments. Experiments were further performed to studythe dynamic role of the colloids in Pu transport through porous media,and then discover the critical transport-enhanced concentration. Themineral colloids with the diameter of 1 nm–1 μm were isolated fromthe sandy soils collected at Lop Nor in northwestern China. This soilsample was used as the porous media packed into the columns.

2. Materials and methods

2.1. Porous media

Sandy soils were collected at Lop Nor (latitude 41° 30′ N, longitude88° 30′ E), and then air-dried, and sieved with stainless steel sieves.Their mineral constituents detected by X-ray diffraction (D/MAX-2400, Rigaku Co. Japan) are 50% quartz, 15% anorthose, 11% sericite(clay minerals), 6% orthoclase, 5% calcite, 4% dolomite, 4% chlorite(clayminerals), 2% amphibole, 1% gypsum, 1% pyrite and 1% undetectedminerals. Other characteristics include cation exchange capacity of0.0286mmol/g and pH of 8.5±0.1 (1:1 water–media).

About 500g of sandy soils having the 300–700μm in diameter wereadded to pure water (18.2 MΩ, Millipore) in one beaker and then thefine particles attached onto the soil grains were removed by ninewashings with pure water in order to avoid introducing unknownamounts of colloidal particles in transport experiments. This samplewas gradually dried at 50 °C and used as the stationary porous mediapacked into the columns.

2.2. Natural colloids

The sieved soil grains of 150g (b0.075mm in diameter) were addedto a glass beaker (4 L of pure water), and ultrasonically dispersed for15 min. The colloidal suspensions with Stoke's diameter of b1 μmwere siphoned from the upper suspensions into a polypropylene vesseland stored in the refrigerator (4 °C), which were used as the colloidalsource materials. Since these colloids are derived from the sandy soils,their mineral phases, including fine clay particles, aluminium or ironhydroxides, silica and silicates, are expected to be similar to thedetectedsoil sample.

To determine themass concentration of the parent suspensions, five25 mL aliquots of suspensions were transferred to concave Teflonmembranes, and dried at 50 °C by infrared light. The determinedconcentration was 751.27± 11.43mg/L. The parent suspensions werediluted with pure water to obtain final colloid concentrations ranged

from c = 0 (solution) to c = 751.27 mg/L. A higher concentrationof c = 2017.80 mg/L resulted from vacuum evaporation at roomtemperature.

2.3. Surface characterization of media and colloids

Both Brunauer Emmett Teller (BET) surface areas (As) and porevolumes (Ws) of the porous media and dried colloids were measuredwith N2 adsorption method at 77 K (ASAP 2020, Micromeritics).Adsorption/desorption isotherms of N2 are shown in Fig. S1, and themeasured results are given in Table S1 of the Supplementary data.

The porous media of about 5 g were crushed in an agate mortar sothat the electrokinetic potentials could be measured. The crushedparticles larger than 10 μm in diameter were removed using gravitysedimentation. Electrokinetic potentials (ζ) of the remained particlesb10 μm and the colloids, as a function of Na+ (NaCl) concentrationfrom 0.001 to 1.0 mol/L, were determined using Nano ZS (Malvern)under the pH8.5 condition.

2.4. Pu suspensions

Small amounts of 239Pu stock solution (95ng/g) in 1.5% HNO3, weredrop by drop added to the stirred colloid suspensions of 0–2017.80mg/L in ten Teflon bottles. The atom ratios in the 239Pu stocksolution were 240Pu/239Pu = 0.0346, 241Pu/239Pu = 0.000355 and242Pu/239Pu = 0.0000323. After 10 min, overall pH of the suspensionswas adjusted to 8.5 by 0.5M NaOH. The 239Pu and Na+ concentrationswere C0≈1×10−9mol/L and 0.002mol/L, respectively.

Such prepared suspensions were the Pu suspensions, in whichPu may exist as the dissolved (Cd mol/L) and the colloidal species(Cc mol/L). The Cd and Cc defined by the filtrate and the retentionfractions, respectively, were experimentally determined by 10 kDultrafiltration (Amicon Ultra-4, Millipore), as a function of colloidconcentration. For Pu in the solution without colloids (i.e., c = 0),partial Pu may form the polynuclear species by hydrolysis ofmononuclear Pu(IV) (Rothe et al., 2004; Wilson et al., 2011) andthen retained on the membranes because of relatively large particlesizes (N2 nm) as reported by Powell et al. (2011). This formationprocess involved hydroxide or oxygen bridging of Pu nuclei (Waltheret al., 2009). The distribution of Pu oxidation state from Pu(IV) toPu(VI) was analyzed before and after carrying out the transportexperiments.

2.5. Column transport experiment

Theporousmediawere loaded into ten polypropylene columnswiththe dimensions of 1.9 cm in diameter and 12 cm in height. Aqueoussolution free of colloids was fed into the columns from the top, untilconstant water contents monitored by weighing the columns wereachieved. Subsequently, one pore volume of colloidal suspensionswith the concentrations of 0–2017.80 mg/L was introduced into thecolumns to displace the existing water in the porous media systems.The Pu suspensions (c = 0–2017.80 mg/L) of 1.5 pore volumes wereimmediately injected into the columns, followed by injection of col-loidal suspensions for four pore volumes. The effluentswere continuallycollectedwith a fraction collector (Shanghai Jingke Industrial Co. Ltd.) atthe column outlet.

These processes maintained one fixed inflow rate by the use of aperistaltic pump (Longer Precision Pump Co. Ltd.). Both the aqueoussolution and colloidal suspensions had the same pH (8.5) and Na+

concentration (0.002 mol/L) as the Pu suspensions. Transportparameters are the following: water content (θ=0.38 cm3/cm3), bulkdensity (ρb = 1.56 g/cm3), pore volume (Vp = 12.27 cm3), effectiveporosity (ε=0.44), and inflow rate (2.04 cm3/min).

To examine the effect of colloids on the transport of tritium as a con-servative tracer through the porous media, tritiated water (~800 Bq/g)

136 J. Xie et al. / Chemical Geology 360–361 (2013) 134–141

was added to colloidal suspensions with the represented concentrationsof 0, 5.00, 95.13, 375.37, and 2017.80mg/L. The experimental proceduresof tritium transport were similar to Pu. Tritium radioactivities weremeasured by the liquid scintillation counter (Wallac 1414).

Transport experiments were carried out under an atmospherecondition (0.03% CO2, v/v) and room temperature (25 ± 1 °C). Allchemicals used in the experiments were of analytical grade.

2.6. Analytical methods

Plutonium oxidation state distribution including both in colloidalsolid and in aqueous phases was determined using centrifugal ultra-filtration (10 kD, Amicon Ultra-4, Millipore) and solvent extractiontechnique (TTA and HDEHP, Sigma-Aldrich Co.) according to theanalytic flow diagram in Fig. S2 (see Supplementary data). The resultsshowed that Pu(IV) in the suspensions was the predominant oxidationstate (N93.7%). Marked changes of the distribution were not observedin the effluents, apart from the slightly increased Pu(V/VI) (b3%).

Plutonium-239 concentrations of all samples were measured byICP-MS (Element, Finnigan MAT) using the isotope dilution method,as follows. Plutonium-242 was used as a spike, and the atom ratio of239Pu/242Pu was 0.001148. After introducing known amounts of242Pu in the samples, HNO3 was added to adjust the solution to8mol/L HNO3. NaNO2 was used to adjust Pu valence to Pu(IV) at 90 °Cfor 15 min, and then the solution was cooled at room temperature.These solutions were fed into Dowex 1× 2 resins (Dow Chemical CO.)packed in glass columns with 3 mm Φ × 50mm length. U and matrixelements in the feeding solution were removed via washing thecolumns using 5mL of 8mol/L HNO3, 3 mL of 10mol/L HCl, and 3mLof 3 mol/L HNO3. The purified Pu was eluted from the columns with1.2 mL of 0.01 mol/L HNO3–0.01 mol/L HF solution, and determinedwith the ICP-MS.

3. Results and discussion

3.1. Electrokinetic properties of colloids and media

Changes in ζ-potentials of the colloids and media particles as afunction of Na+ (NaCl) concentration are presented in Fig. 1. Theζ-potentials were obtained from the measured electrophoretic mo-bilities within a range of Na+ concentrations from 0.001 to 1.0mol/L.

Negative values of the ζ-potentials are an indication of negativelycharged surfaces on the colloids andmedia. The negative surface charge

0.0 0.2 0.4 0.6 0.8 1.0

-40

-30

-20

-10

0

Media crushed to < 10 m in diameter Natural colloids with 1nm-1 m in diameter

Zet

a po

tent

ial

(m

V)

Na+ concentration (mol/L)

Fig. 1. Zeta potentials of natural colloids with the diameter of 1 nm–1 μm and mediaparticles (b10 μm) respond to the variation in Na+ concentration. The porous mediawere crushed in an agate mortar, and the fraction larger than 10 μm in diameter wasremoved using gravity sedimentation.

emanates from acidic functional groups, dominating the surfaces ofmineral media under environmentally relevant pH 8.5 conditions(Swartzen-Allen and Matijević, 1974). The charge characteristics ofthe two surfaces became less negative with increasing Na+

concentrations. This suggests that the amounts of negative charge onthe two surfaces decreased while introducing the positive ions of Na+

(NaCl) to the suspensions. Large differences in ζ-potentials of thecolloids and media surfaces were not observed, presumably due to thefact that they are derived from the same in situ sandy soils.

The surface areas of the colloids (69.43m2/g) are much larger thanthose of the media (6.06m2/g), leading to higher affinity of the colloidsfor Pu. Hence, potential changes in transport characteristics of Puwouldtake place by the colloids as carriers of Pu in aquatic environments.

3.2. The mobile fraction of Pu

In this study, the mobile fraction of Pu (RPu), referred to the percentrecovery of Pu in themobile phase, may describe transport processes ofthe mobile species of Pu through stationary porous media. The Pususpensions having the increasing turbidity with colloid concentrationsand the Pu breakthrough curves plotted as the normalized effluentconcentration C/C0 of Pu versus the number of pore volumes areshown in Fig. 2a and b, respectively.

The dissolved and polynuclear fractions of Pu in the aqueoussolution with pH 8.5 (i.e., c = 0) account for 45.6% (Cd%) and 54.4%(Cc% = 100%− Cd%) as shown in Table 1, respectively. The dissolvedPu, consisting of hydroxide and carbonate complexes, and Pu4+, aswell as the polynuclear Pu formed by aggregation of the complexes(Rothe et al., 2004; Walther et al., 2009; Wilson et al., 2011) might becomplexed and sorbed with/on the surfaces of stationary media(Kirsch et al., 2011). This surface complexation and/or sorptionoccurring in the absence of colloids gave rise to significant retention ofPu in porous media systems, as shown in Fig. 2b (low C/C0 for c=0).However, Pu became highly mobile (large C/C0) when Pu suspensionswith an extremely low colloid concentration of c = 0.5 mg/L wereintroduced in the system. The accumulated mass recoveries of Pu(RA-Pu) at c= 0.5 mg/L are much larger than those at c= 0 (Fig. S3).These comparative results do indeed provide powerful evidence forthe enhanced transport of Pu by the natural colloids.

The rising mobile Pu with the colloid concentrations was observed.The mobile fraction (RPu) is increased from 1.31% at c=0 to 52.48% atc= 375.37 mg/L (Table 1), further confirming the key role of naturalcolloids in strongly enhancing the transport of Pu. The increasing RPumay be ascribed to the reduced Cd% with the increasing colloidconcentrations. As mentioned above, this fraction, readily retainedonto the media through surface complexation, ultimately becameimmobile in the systems. Since more sorption sites (`SO−, `SOH2+)available for Pu association were offered by the colloids with increasingconcentrations, the colloid-associated species hence facilitated Putransport in the systems.

The surface species, consisting of `SOPu(OH)3, `SOPu(OH)4−,`SOPu3+, `SOPu2+, `SOPuO2, and `SOPuO2

+, are assumed to beformed by complexation of Pu(III/VI) with the sorption sites on thecolloidal surfaces. The surface complexation reactions and equilibriumconstants (logK) are presented in Eq. (S1)/(S8), according to bothMarimon (2002) and Schwantes and Santschi (2010). The surfacecomplexation models incorporated into the software package PhreeQc(v 3.00) were employed to model the complexation reactions. Thetypical input parameters included site density 2.3/nm2 (Schwantesand Santschi, 2010; Dong et al., 2012), 10−9 mol/L Pu, pH 8.5, and0.002 mol/L NaCl. The results reported in Table S3 show that Pu(IV)had the highest complexation ability with the colloidal surfaces, andthat the dominant species was `SOPu(OH)3. The increasing tendencyin `SOPu(OH)3 with the colloid concentrations was almost consistedwith the colloidal fraction (Cc%) in Table 1.

a

0 1 2 3 4 50.0

0.1

0.2

0.3

0.4

0.5

0.6

c = 2017.80c = 751.27c = 375.37 c = 175.18 c = 95.13 c = 50.14 c = 15.01c = 5.00 c = 0.50 c = 0

Nor

mal

ized

effl

uent

con

cent

ratio

n of

Pu,

C/C

0

Nor

mal

ized

effl

uent

con

cent

ratio

n of

3 H, C

/C0

Number of pore volumes

I II III

Colloid concentraiton, c (mg/L)

b

0 1 2 3 4 50.0

0.2

0.4

0.6

0.8

1.0c = 0c = 5.00c = 95.13 c = 375.37 c = 2017.80

Number of pore volume

Colloid concentration, c (mg/L)

c

Fig. 2. (a) Plutonium suspensions with the natural colloid concentrations ranged from c= 0 to c= 2017.80 mg/L, were used to perform transport experiments under the equivalentconditions, such as pH, Na+ concentration, and initial Pu concentration (C0). Plutonium in the solution without colloids (i.e., c= 0) existed both in the polynuclear species formed byhydrolysis of Pu(IV) and in the dissolved species (see Table 1). (b) Plutonium and (c) tritium breakthrough curves through the porous media. Normalized effluent concentration (C/C0)influenced by the colloid concentrations is indicative of the mobile species of Pu, which was collected in the fraction collector.

137J. Xie et al. / Chemical Geology 360–361 (2013) 134–141

The breakthrough curves of tritium as an inert tracer in the sus-pensions with the represented colloid concentrations ranged from

Table 1The mobile fraction of Pu (i.e., percent recoveries of Pu, RPu), its colloidal fraction (Cc%),minimum deposition rate coefficient (i.e. kmin as determined by Eq. (1)), and relativeelectrostatic repulsion F (i.e., slopes of Fig. 2b II curves), as a function of colloidconcentration c.

c (mg L−1) Cc%a RPu (%) kmin (min−1) F (min−1) r2

0 54.4± 0.3 1.31 2.226 0.000639 0.8490.50 67.8± 0.6 8.62 1.210 0.00338 0.9525.00 73.4± 0.05 19.13 0.808 0.00779 0.73315.01 78.9± 0.06 24.72 0.679 0.0110 0.99450.14 79.9± 0.2 36.14 0.496 0.0150 0.87795.13 86.1± 0.07 41.80 0.423 0.0153 0.983175.18 93.4± 0.8 49.17 0.360 0.0203 0.918375.37 97.2± 0.3 52.48 0.336 0.0217 0.996751.27 99.5± 0.4 51.06 0.340 0.0213 0.9602017.80 99.3± 0.1 12.70 1.158 0.00641 0.961

a Cc% = (C0 − Cd) / C0 × 100%, here Cd (i.e. the fraction passing through the 10 kDmembrane) is the concentration of the dissolved Pu (mol/L), C0 (mol/L) is the totalconcentration of Pu in the suspensions. These experiments were performed in triplicate.

c=0 to c=2017.80mg/L are presented in Fig. 2c. Overall, normalizedeffluent concentrations of tritium approach one. This indicates that thecolloids had no effect on tritium transport, due to the lack of sorptionaffinity of tritium for the mineral surfaces.

Artinger et al. (1998) and Papelis (2002) also observed the positiverelationship between percent recoveries of Am/Pb and the colloidconcentrations having the maxima of about 100mg/L. We assume thatthe colloid concentrationmight continually increase until the suspensionscontained almost no water (i.e. the colloid concentration c→∞). Underthis scenario, it should be noted that these low-solubility contaminantswould be overall retained (i.e., lim

c→∞Recoveries ¼ 0) because they could

notmove downstreamdue to the lack of fluid drag inwater-free systems.Furthermore, no more Pu would be associated with the sorption sitesunder the conditions of extremely high colloid concentrations. The excesscolloids might to some extent promote grain–grain pore blocking andthus impede Pu transport in the porous media systems. This under-standing combined with the results above may reflect a potentialtransport phenomenon in the geosphere: a critical transport-enhancedconcentration for low-solubility contaminants.

In terms of c N 375.37mg/L, we note that normalized effluent con-centrations of Pu slightly decrease at c = 751.27 mg/L (Fig. 2b). The

138 J. Xie et al. / Chemical Geology 360–361 (2013) 134–141

trend towards decreasing RA-Pu at c N 375.37mg/L is easily observed inFig. S3. Themobile Pu (RPu=51.06%) at c=751.27mg/L is thus smallerthan RPu = 52.48% at c = 375.37 mg/L. To study whether RPu wouldfurther reduce at cN751.27mg/L, we carried out an additional transportexperiment. One aliquot of the suspensions of c = 751.27 mg/L wasconcentrated to 2017.80 mg/L via vacuum evaporation at roomtemperature. An observed fact that themobile Pu RPu is steeply declinedto 12.70% at c=2017.80mg/L, resulting from its breakthrough curve inFig. 2b, is reported in Table 1.

As expected, the results in Table 1 demonstrate the presence of thecritical transport-enhanced concentration of c = 375.37 mg/L, wherePu exhibited the highest mobility. This finding reveals that naturalcolloids played a dynamic role in Pu transport through the porousmedia. The nonmonotonic dependence of RPu on colloid concentrationsc is presented in Fig. 3a.

The colloid concentrations are usually much lower (b5 mg/L) inmost underground water (Kim, 1991; Schäfer et al., 2012) andsometimes slightly higher in the water surrounding detonation cavities(55 mg/L) (Buddemeier and Hunt, 1988). Accordingly, the colloid-enhanced Pu transport may occur in the aquifers, as supported by ourlab-scale transport experiments with colloid concentrations from c=0 to c = 50.14 mg/L. In contrast, the pore water in the vadose zonesoils can have the much larger colloid concentrations during the periodof the infiltration events such as rain falls (Ryan et al., 1998; El-Farhanet al., 2000), and the concentrations in excess of 1 g/L have been oftenreported (see the reviews by DeNovio et al. (2004)). Therefore, thecritical Pu transport phenomenon (i.e. the dynamical role of the naturalcolloids) most likely takes place in field sites located at vadose zone.This transport may be thought as the worst-case scenario: the porewater encounters the contamination risk reaching its greatest extent,due to association of Pu with the colloids of the critical concentration.No studies have found this critical phenomenon, and hence the relevantexperimental results have not been involved in the transport models(Cvetkovic et al., 2004; Šimůnek et al., 2006; Bekhit and Hassan,2007). To more accurately predict the risk of low-solubility radio-nuclides into subsurface environments, the dynamic role of colloids, inparticular its critical transport-enhanced concentration, should beincorporated in the widely used transport models.

3.3. Insight into the transport mechanism

Transport of colloidal particles in porousmediawas accompanied bytheir retention in the systems via the physical mechanisms such as

0 500 1000 1500 20000

10

20

30

40

50

60

Concentration of natural colloid, c (mg/L)

The

mob

ile fr

actio

n of

Pu,

RP

u (%

)

0.0

0.5

1.0

1.5

2.0

2.5

RPu

kmin

Dep

ositi

on r

ate

coef

ficie

nt, k

min

(m

in-1

)

Critical transport-enchanced concentration 375.37 mg/L

a

Fig. 3. (a) Responses of the mobile fraction of Pu (i.e., the percent recovery of Pu, RPu) and(b) Responses of the relative electrostatic repulsion (i.e., the slopes of Fig. 2b II curves, F) and kthe breakthrough curve data (C/C0) of Fig. 2b II.

diffusion, interception, pore straining, and gravity settling, which havebeen thoroughly discussed since several decades ago (McDowell-Boyer et al., 1986; Bradford et al., 2007; Johnson et al., 2011). Filtrationtheory into which these physical processes are partially incorporatedmay help us to better understand the transport of actinides associatedwith the colloids in real environments. Some researchers studied thecritical time (Redner and Datta, 2000) and concentrations (Pandyaet al., 1998), when/where the particles started plugging in the filtrationbeds due to the trapping of suspended particles as they passed throughthe porousmedia. These findings are useful for explaining the existenceof the critical transport-enhanced concentration, though the increasedcolloid sizes at higher concentrations (see Fig. S5) could not cause theobserved plugging in our transport experiments. Roussel et al. (2007)further suggested that possible attractive forces between the particleand the solid surfaces may have an important impact on particletrapping. Here we attempt to describe the dynamical role of the naturalcolloids by attractive and repulsive interactions as a chemicalmechanism.

The fate and transport of Pu through stationary media are largelydetermined by the rate at which it deposited onto media surfaces.Deposition of the Pu not associated with the colloids was controlledby the surface complexation and sorption. In another case that colloidsas carriers of Puwere present in the porousmedia systems, the chemicalinteraction between colloid and media surfaces would impact aninfluence on deposition of the colloid-associated Pu. The interactionforces may exhibit as either repulsion or attraction, depending onmany factors such as solution chemistry, colloid concentrations, andmedia surface heterogeneity. Accordingly, the transition from the directcontact of Pu with media surfaces to the indirect contact via colloidsinvolved more complicated processes. The deposition rate coefficient kis determined by Eq. (1) (Tufenkji and Elimelech, 2004; Walker et al.,2004) and the results are presented in Fig. 4.

k ¼ − UθL

lnCC0

� �ð1Þ

where U (0.72cm/min) is the approach (superficial) velocity i.e., Darcyvelocity; θ (0.38cm3/cm3) is the water content; L (9.8cm) is the heightof media packed into the columns; C/C0 is the normalized effluentconcentration of Pu in Fig. 2b. Usually, the calculation of the depositioncoefficients considers only one point within the plateau of the break-through curves. Here, we analyze the kinetic deposition process.

0 500 1000 1500 20000.000

0.005

0.010

0.015

0.020

0.025

Fk

min

Concentration of natural colloid, c (mg/L)

Rel

ativ

e el

ectr

osta

tic r

epul

sion

, F (

min

-1)

0.0

0.5

1.0

1.5

2.0

2.5D

epos

ition

rat

e co

effic

ient

, km

in (m

in-1

)

The maximum F = 0.0217 min-1

b

minimum deposition rate coefficient (kmin) to the variation in colloid concentrations c.min to the variation in colloid concentrations c. The kmin was determined by Eq. (1) using

c = 0 c = 0.50c = 5.00 c = 15.01 c = 50.14 c = 95.13 c = 175.18c = 375.37c = 751.27 c = 2017.80

2 4 6 8 100

1

2

3

4

5

6

Colloid concentraiton, c (mg/L)

Dep

ositi

on r

ate

coef

ficie

nt, k

(m

in-1)

Elapsed time, t (min)

I II

kmin

Fig. 4. Plutonium deposition rate coefficient (k) on the stationarymedia surfaces heaved as the kinetic characteristics. Natural colloids with the concentrations spanning several orders ofmagnitude played a dynamic role in the deposition rates. The represented results of k, i.e., the minimum deposition rate coefficient (kmin), are listed in Table 1.

0.000 0.005 0.010 0.015 0.020 0.025

0

10

20

30

40

50

60 Observed Fitted

T h

e m

obile

frac

tion

of P

u, R

Pu

(%)

The relative electrostatic repulsion, F (min-1)

RPu

(%) = 2472.52F - 0.64, r 2 = 0.989

Fig. 5. The correlation between the mobile fraction of Pu (i.e., the percent recovery of Pu,RPu) and the slopes F of breakthrough curves (Fig. 2b II) was found as the linear first-orderequation under the unfavorable attachment conditions. The slope F reflects the repulsiveinteraction between Pu and stationary media, as affected by colloid concentrations.

139J. Xie et al. / Chemical Geology 360–361 (2013) 134–141

Small amounts of Fe, Mn and Al (Table S2), generally present asanisopachous coatings on mineral media surfaces, may create thesurface charge heterogeneity (Ryan and Elimelech, 1996; Kretzschmarand Sticher, 1998; Del Nero et al., 2004; Drelich and Wang, 2011). Thecolloids as carriers of Pu, initially introduced in the porous mediasystems, may attach to these surface patches under unfavorableattachment conditions. The kinetics of Pu deposition in Fig. 4 showsthat Pu experienced rapid declines in deposition rates, attributed tothe presence of repulsive electrical double layer forces after filling ofthe carriers on the patches. The subsequent contact of colloid-associated Pu with the media walls was therefore hindered by theenergy barriers created from the deposited colloids, which causedmuch slow declines in the deposition rates. The represented results ofk, suggested as the minimum deposition rate coefficient (kmin), arereported in Table 1. However, the rates of the rapid declines decreaseat c N 375.37 mg/L, in particular c = 2017.80 mg/L, implying that therepulsive forces blocking Pu deposition reduced. The represented kminsare used in Fig. 3a tomore clearly describe the dynamic effects of colloidconcentrations on deposition kinetics of Pu.

As shown in Fig. 3a, strong retention of the free Pu (c=0) onto thestationary media was responsible for its highest deposition rate. However,the great decline in kmin at the extremely low colloid concentration (c=0.5 mg/L) occurs in response to the repulsive interaction between thetwo surfaces of the introduced colloids and media. The deposition ratescontinually decreasewith colloid concentrations (cb375.37mg/L), ascribedto the transformation in the species of Pu in its suspensions (i.e. Cc% inTable 1). The prevailing repulsion over the surface complexation, resultingfromthis transformation, couldmakePumuchmoremobile in the systems.With the further increase in colloid concentrations (c N 375.37 mg/L),however, the deposition rates increase. This suggests that the depositioncondition of the higher colloid concentrations promoted to some extentattractive forces.

It is noted that the rate of approach to total breakthrough (C/C0=1)in Fig. 2b II is highly relevant to colloid concentrations. Liu et al. (1995)suggested that this rate was an indication of the relative excluded areasblocked by the previously deposited latex particles. Accordingly, itseems that this rate may reflect the changes in interaction betweenPu and stationary media: a steeper slope of the breakthrough curvecorresponds to larger repulsion. This interaction is considered as thesum of two contributions, including the electric double layer repulsionand the van der Waals attraction. The curves of Fig. 2b II are enlargedin Fig. S4 and the slopes F defined as the relative electrostatic repulsion

are presented in Table 1. The relative electrostatic repulsion F rises, andthen declines with c. This is highly consistent with the observeddynamic role of colloids in transporting Pu, as manifested by both RPuand F curves in Fig. 3a/b. Typically, the maximum F of 0.0217 min−1

arises at c = 375.37 mg/L where c is the critical transport-enhancedconcentration. The correlation between F and RPu is found as thelinear first-order equation, and the result is presented in Fig. 5.Also, the declining F (Fig. 3b) with the higher colloid concentration(c N 375.37 mg/L) indicates that the attractive forces are favorablefor Pu deposition increase.

To further illuminate the declined RPu at c N 375.37 mg/L, here wecalculate the interaction potential ФT between the colloid and mediasurfaces, as supplied in the Supplementary data. The colloid diameters(average dc, Fig. S5) were measured with the dynamic light scatter(Marlven NanoZS). For the Pu suspensions with the variation in colloidconcentration c, collision among colloids via Brown motion (diffusion)exhibited high probability at the larger c, where the number density ofcolloids was higher. The fraction of collisions that succeeded in weakintercolloid aggregation was presumably increased on the unfavorable

140 J. Xie et al. / Chemical Geology 360–361 (2013) 134–141

attachment condition. Increasing the colloid concentration thus led to afaster growth of aggregates, as reported by Heidmann et al. (2005) andCzigány et al. (2005). Consequently, we observe a change in colloid size:from dc = 486.8 nm (c = 375.37 mg/L) to dc = 578.4 nm (c =2017.80 mg/L), considered as potential effects on the interactionpotential and plugging of colloid-associated Pu in pore spaces. Nomarked changes in dc were found at cb375.37mg/L.

The repulsive energy barrier (Фmax) is up to several hundreds of kBT(Table S4), indicating the presence of unfavorable attachment con-ditions for the primary energy minima (Фmin1). Therefore, the colloid-associated Pu could not be pushed through the barrier to deposit inФmin1, as shown in Fig. S6. However, the shallow second energyminima(Фmin2) appear and their depth increase (more negative) with thecolloid concentration, in particular at c=2017.80mg/L. This increasedattractive interaction Фmin2 might to some extent promote depositionof colloids as carriers of Pu and hence had a contribution to the declinedmobile fraction of Pu at the higher colloid concentrations.

4. Conclusions

Natural colloids have strong impacts on Pumobility in porousmedia.Our results of transport experiments show that the mobile fraction of239Pu associated with the mineral colloids having the extremely lowconcentration (RPu=8.62%, c=0.5mg/L) is much larger than that notassociatedwith the colloids (RPu=1.31%, c=0). This is strong evidencefor the enhanced transport of Pu by the colloids. Plutonium mobilitycontinually increases with the concentrations of c b 375.37 mg/L, andthen declines at c N 375.37 mg/L, demonstrated as a dynamic role ofthe colloids in Pu transport. The increasing mobility is attributed tothe transformation in species from the not associated Pu to theassociated Pu with the colloids. The relative large attraction occurringin the shallow second energy minima is partially responsible for thedeclining mobility at c N 375.37 mg/L. The highest Pu mobility (RPu =52.48%) at c = 375.37 mg/L is referred to as the critical transport-enhanced concentration.

Further work is necessary to understand the effects of particle size,grain size, ionic strength and pH in the suspensions, and pore watervelocity on the dynamic role of the natural colloids in transport of Pu.

Acknowledgments

We thankQuanlin Shi, Zhiming Li, andHaijunDang for their valuablecontributions to this research.We also acknowledge the participation ofQichu Xu, Lili Du, Haitao Zhang and Yanmei Shi for their assistance inthe experiments. This research was supported by the National DefensePre-Research Foundation of China.

Appendix A. Supplementary data

Supplementary data to this article can be found online at http://dx.doi.org/10.1016/j.chemgeo.2013.08.016.

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