7
Journal of Colloid and Interface Science 267 (2003) 25–31 www.elsevier.com/locate/jcis Comparative study on Np(V) sorption on oxides of aluminum and silicon: effects of humic substance and carbonate in solution Weijuan Li and Zuyi Tao Radiochemistry Laboratory, College ofChemistry and Chemical Engineering, Lanzhou University, Lanzhou 730000, People’s Republic of China Received 4 November 2002; accepted 18 July 2003 Abstract The sorption of Np(V) (total concentration 10 5 mol/L) onto alumina and silica was studied by a batch technique under ambient aerobic conditions at 25 C. The effects of pH, ionic strength, humic substance (HS), and added carbonate in aqueous solutions on the sorption of Np(V) onto alumina and silica were investigated. The sorption isotherms of Np(V) on alumina and the relationships between the equilibrium concentrations after sorption onto silica and the initial concentration before sorption in the absence and presence of HS and added carbonate in solutions were determined. It was found that as compared with the sorption of Np(V) onto alumina, the sorption by silica on the basis of mass is tremendously less, negative sorption of Np(V) onto silica occurs, the relative rate of sorption onto silica is quicker the sensitivity of sorption onto silica to ionic strength is higher, the pH dependence of sorption onto silica is less, and consequently the characteristics of Np(V) sorption onto alumina and silica are distinctly different. The effect of addition of HS or carbonate in solution was studied. Little effect of addition of HS (20 mg/L) on sorption onto alumina and silica were found. The addition of carbonate (0.001 mol/L) increased Np(V) sorption onto silica at pH values below 10 and decreased it at pH values above 10. 2003 Elsevier Inc. All rights reserved. 1. Introduction The sorption of Np(V) onto γ -Al 2 O 3 as a function of pH under aerobic conditions was studied at very low concentra- tion (10 14 mol/L), very large ratio of solution volume (V ) to mass of sorbent (m), V/m = 5000 ml/g, and fixed ionic strength (0.1 mol/L NaClO 4 ). It was found that the sorp- tion begins at pH 5 and increases abruptly in the pH range 6–8, and in the pH range 9–11, the sorption percent remains constant and is equal to 95% (<100%) [1]. The sorption of Np(V) onto naturally occurring iron-containing minerals (hematite, magnetite, goethite, and biotite) as a function of pH (4–11) under aerobic conditions was studied at moder- ate concentration (10 7 –10 6 mol/L), V/m = 1000 ml/g, and fixed ionic strength (0.1 mol/L NaNO 3 ) [2]. In addi- tion, the sorption of Np(V) onto alumina as a function of pH under aerobic conditions was also studied at moderate concentration (10 6 mol/L), V/m = 1000 ml/g, and fixed ionic strength (0.1 mol/L NaNO 3 ) [2]. It was found that the sorption percentage increases gradually from 8% at pH 6 to * Corresponding author. E-mail address: [email protected] (Z. Tao). 25% at pH 11. The sorption of Np(V) on hydragillite as a function of pH was studied in CO 2 -free 0.1 mol/L NaClO 4 solutions and 0.1 mol/L NaClO 4 solutions containing car- bonate, 0.001 or 0.01 mol/L, at very low concentration (10 13 mol/L) and V/m = 166 ml/g [3,4]. It was found that from CO 2 -free 0.1 mol/L NaClO 4 solutions the sorp- tion begins at pH 6, and the sorption percentage is increased from 0% at pH 6 to 95% at pH 9, and that as compared with the sorption from CO 2 -free 0.1 mol/L NaClO 4 solutions, the sorption is decreased in the presence of carbonate and in the alkaline pH region and the decrease in sorption percent- age is proportional to the concentration of carbonate in the aqueous phase (0–10 2 mol/L). The similar Np(V) sorption edges on hematite and goethite from CO 2 -free 0.1 mol/L NaClO 4 solutions at total concentration 1.2 × 10 7 mol/L were obtained by Kohler et al. [5]. The sorption percent from CO 2 -free solutions reached 100%, while in the presence of CO 2 , the sorption percentage was abruptly decreased from maximum (<100%) at pH 7.5 to almost 0% at pH 9. In addition, Kohler et al. [5] determined the sorption edge of Np(V) on quartz from CO 2 -free 0.1 mol/L NaClO 4 solu- tions at total concentration 1.2 × 10 7 mol/L and found that the sorption begins at pH 8, the sorption percentage is increased from 0% at pH 8 to 80% at pH 12, and the 0021-9797/$ – see front matter 2003 Elsevier Inc. All rights reserved. doi:10.1016/S0021-9797(03)00735-5

Comparative study on Np(V) sorption on oxides of aluminum and silicon: effects of humic substance and carbonate in solution

Embed Size (px)

Citation preview

con:

erobicorption ofuilibrium

d carbonatebasis of

ensitivityteristics ofle effect

Journal of Colloid and Interface Science 267 (2003) 25–31www.elsevier.com/locate/jcis

Comparative study on Np(V) sorption on oxides of aluminum and silieffects of humic substance and carbonate in solution

Weijuan Li and Zuyi Tao∗

Radiochemistry Laboratory, College of Chemistry and Chemical Engineering, Lanzhou University, Lanzhou 730000, People’s Republic of China

Received 4 November 2002; accepted 18 July 2003

Abstract

The sorption of Np(V) (total concentration 10−5 mol/L) onto alumina and silica was studied by a batch technique under ambient aconditions at 25◦C. The effects of pH, ionic strength, humic substance (HS), and added carbonate in aqueous solutions on the sNp(V) onto alumina and silica were investigated. The sorption isotherms of Np(V) on alumina and the relationships between the eqconcentrations after sorption onto silica and the initial concentration before sorption in the absence and presence of HS and addein solutions were determined. It was found that as compared with the sorption of Np(V) onto alumina, the sorption by silica on themass is tremendously less, negative sorption of Np(V) onto silica occurs, the relative rate of sorption onto silica is quicker the sof sorption onto silica to ionic strength is higher, the pH dependence of sorption onto silica is less, and consequently the characNp(V) sorption onto alumina and silica are distinctly different. The effect of addition of HS or carbonate in solution was studied. Littof addition of HS (20 mg/L) on sorption onto alumina and silica were found. The addition of carbonate (0.001 mol/L) increased Np(V)sorption onto silica at pH values below 10 and decreased it at pH values above 10. 2003 Elsevier Inc. All rights reserved.

tra-

-ngeains

lsn ofder-

ofrate

6 to

a

-

-asedith

nd inent-the

ome offromIn

of

tagethe

1. Introduction

The sorption of Np(V) ontoγ -Al2O3 as a function of pHunder aerobic conditions was studied at very low concention (10−14 mol/L), very large ratio of solution volume (V )to mass of sorbent (m), V/m = 5000 ml/g, and fixed ionicstrength (0.1 mol/L NaClO4). It was found that the sorption begins at pH 5 and increases abruptly in the pH ra6–8, and in the pH range 9–11, the sorption percent remconstant and is equal to 95% (<100%) [1]. The sorptionof Np(V) onto naturally occurring iron-containing minera(hematite, magnetite, goethite, and biotite) as a functiopH (4–11) under aerobic conditions was studied at moate concentration (10−7–10−6 mol/L), V/m = 1000 ml/g,and fixed ionic strength (0.1 mol/L NaNO3) [2]. In addi-tion, the sorption of Np(V) onto alumina as a functionpH under aerobic conditions was also studied at modeconcentration (10−6 mol/L), V/m = 1000 ml/g, and fixedionic strength (0.1 mol/L NaNO3) [2]. It was found that thesorption percentage increases gradually from 8% at pH

* Corresponding author.E-mail address: [email protected] (Z. Tao).

0021-9797/$ – see front matter 2003 Elsevier Inc. All rights reserved.doi:10.1016/S0021-9797(03)00735-5

25% at pH 11. The sorption of Np(V) on hydragillite asfunction of pH was studied in CO2-free 0.1 mol/L NaClO4solutions and 0.1 mol/L NaClO4 solutions containing carbonate, 0.001 or 0.01 mol/L, at very low concentration(10−13 mol/L) and V/m = 166 ml/g [3,4]. It was foundthat from CO2-free 0.1 mol/L NaClO4 solutions the sorption begins at pH 6, and the sorption percentage is increfrom 0% at pH 6 to 95% at pH 9, and that as compared wthe sorption from CO2-free 0.1 mol/L NaClO4 solutions,the sorption is decreased in the presence of carbonate athe alkaline pH region and the decrease in sorption percage is proportional to the concentration of carbonate inaqueous phase (0–10−2 mol/L). The similar Np(V) sorptionedges on hematite and goethite from CO2-free 0.1 mol/LNaClO4 solutions at total concentration 1.2 × 10−7 mol/Lwere obtained by Kohler et al. [5]. The sorption percent frCO2-free solutions reached 100%, while in the presencCO2, the sorption percentage was abruptly decreasedmaximum (<100%) at pH 7.5 to almost 0% at pH 9.addition, Kohler et al. [5] determined the sorption edgeNp(V) on quartz from CO2-free 0.1 mol/L NaClO4 solu-tions at total concentration 1.2 × 10−7 mol/L and foundthat the sorption begins at pH 8, the sorption percenis increased from 0% at pH 8 to 80% at pH 12, and

26 W. Li, Z. Tao / Journal of Colloid and Interface Science 267 (2003) 25–31

edvelsrs.byetic

p-o-tionmpletateonandud-oniq-be-kscomhewadis-eeof

elyn in

ofto

(V)uchn or-omely

connds. Itd HS

om-and

ions

ndmula

osehase

e--

alu-ionbe

dy

inaheibleTAlentpha

ra-dtingnt-

dy.heon-n ofer-tion-

00ent

am-n-ex-om

itutele-is N-ter

era-

ndndtivelyntagenays

eousi-

significant sorption of Np(V) by quartz is only observat pH> 9, and that in the presence of atmospheric leof CO2, no significant sorption of Np(V) by quartz occuAs compared with hematite and goethite, Np(V) sorptionthe quartz was weakest. The Np(V) sorption onto synthamorphous iron oxyhydroxide (Fe2O3·H2O) as a functionof pH was investigated by Girvin et al. [6]. The Np(V) sortion onto crystallineα-FeOOH (goethite) from aqueous slution was studied by synchrotron-based X-ray absorpspectroscopy and results ruled out the sorption as a siorder neptunium oxide or hydroxide or as a coprecipiwith an iron oxide–hydroxide phase [7]. Np(V) sorptionvarious aluminum oxides and hydrous aluminum oxideon various iron oxides and hydrous iron oxide was stied by Tochiyama et al. [8,9] at Np(V) total concentrati10−12 mol/L and various pH values. High-performance luid chromatography was applied to study the migrationhavior of Np(V) in a quartz-packed column [10]. Two peawere found. One peak corresponded to the unretardedponent, which is not subject to interaction with quartz. Tother peak corresponded to the retarded component. Itfound that the unretarded component is colloidal and thetribution coefficient (Kd ) of the retarded component is in thrange 0.36–3.9 ml/g at pH 6.8–9.3. As compared with thstudies of Np(V) sorption onto iron oxides, the sorptionNp(V) onto oxides of aluminum and silicon was scarcreported; as compared with the studies of Np(V) sorptiothe concentration 10−7–10−14 mol/L, the sorption of Np(V)at relatively higher concentration (10−5 mol/L) was scarcelyreported. The toxicity, potential mobility, and half-life237Np render it one of the most important radionuclidesbe stored in high-level nuclear waste repositories. Npmigration and sorption onto minerals associated with srepositories must be understood at a fundamental level ider to reliably predict its hydrogeochemical behavior frthe source repository, where the Np concentration is likto be high. Elevated partial pressures of CO2 are possible inmany subsurface environments. Carbonate and HS aresidered the most important inorganic and organic ligawith respect to actinide complexation in ground watersis necessary to understand the effects of carbonate anon Np(V) sorption.

The objectives of this paper were to determine and cpare the sorption characteristics of Np(V) on aluminasilica at relatively higher concentration (10−5 mol/L) and toexplore the effects of added HS and carbonate in soluton the sorption of Np(V) onto alumina and silica.

2. Experimental

The samples of alumina and silica, their conditioning astorage, the procedures of batch experiments, the forused to calculate the distribution coefficients (Kd = Cs/Ceq,ml/g), and the sorption percentages were identical to themployed in the preceding papers [6,11–13]. The solid p

-

s

-

concentration (Cs ) was calculated from the difference btween the initial concentration (C0) and the equilibrium concentration (Ceq) (Cs = (C0−Ceq)V /m, mol/g). The surfacecharge densities as a function of pH for the samples ofmina and silica were determined by potentiometric titratand the points of zero charge (P.Z.C.) were found to<pH 4 and pH 7.5 respectively [12–14]. The kinetic stuwas conducted by the batch technique too.

237Np stock solution was a generous gift from the ChInstitute of Atomic Energy. The oxidation state of tneptunium was confirmed to be pentavalent using visand near-infrared spectroscopy and extraction with T(2-thenoyltrifluoroacetone). The tetravalent and hexavaneptunium was less than 0.5%. After evaporation, alcounting was used to determine the concentrations of237Npin all aqueous solutions (FJ414 typeα-counter with lowbackground, Beijing Nuclear Instrument Factory). Thedionuclide purity of the237Np stock solution was examineby α-spectrometry and was found to be 99.5%. The countime for theα-activity was chosen to ensure that the couing error was below 3%.

Polypropylene test tubes (10 ml) were used in this stuIt was found that the sorption of neptunium (V) onto ttube walls was negligibly small under the experimental cditions used here. No correction was made for sorptioNp(V) onto alumina and silica. After equilibrium, the supnatant was usually separated from the solid by centrifugaat 3,600g (4000 rpm) in a horizontal rotor for 20 min. Accidentally, the centrifugations were performed at 14,4g(16,000 rpm) for comparison between results at differcentrifugation speeds.

All experimental procedures were carried out underbient aerobic conditions at 25◦C. Chemicals used were aalytical reagent grade. Fulvic acid (FA) used here wastracted from loess from the C horizon at depth 200 cm frthe ground surface at the field test site of the China Instfor Radiation Protection (CIRP, Shanxi province). Its emental composition on a moisture- and ash-free basis(2.21%), C (44.8%), H (3.6%), S+ O (49.4%). The ash content is 2.4%, the moisture 6.5%. Distilled–deionized wawas used without further membrane filtration and deation.

3. Results

3.1. Kinetics

The results of sorption kinetics of Np(V) on alumina asilica are simultaneously illustrated in Fig. 1. It was fouthat the steady states for both adsorbents are respecreached at about 1 and 5 h, and that the sorption perceat steady state are respectively−16% and 81%. As showin Fig. 1, the sorption percentages on the silica are alwless than 0%; in other words, the concentrations of aqusolutions at equilibrium (Ceq) are always larger than the in

W. Li, Z. Tao / Journal of Colloid and Interface Science 267 (2003) 25–31 27

e

Ht of

1.onntly,sed

)the

with

al-

f

or-both

ic

pHen-

n-ined

the

ultsnlywn

Hcon-dic

ande

lu-

01,of

.tlytheoseilicalm

Fig. 1. Variation of Np(V) sorption percentage with contact timon SiO2 (!), pH 3.5, and Al2O3 ("), pH 6.7. V/m = 100 ml/g;C0

NpO+2

= 3.5× 10−5 mol/L.

tial concentration (C0). It was found that the results at p3.5 and 6.0 on alumina and silica are fairly independenthe centrifugation speed (3600g and 14,400g). The negativesorption of Np(V) onto the silica is demonstrated in Fig.Figure 1 shows that the relative sorption rate of Np(V)the alumina is slower than that on the silica. Consequethe equilibration time between two phases of 25 h was uin this paper.

3.2. The effects of ratio of solution volume to mass ofsolid (V/m, ml/g)

The effects ofV/m on the sorption percentage of Np(Von alumina and silica were investigated. It was found thatsorption percentage of Np(V) on alumina is decreasedincreasingV/m, 99% atV/m = 10 ml/g, 99% atV/m =20 ml/g, 97% atV/m = 50 ml/g, 88% atV/m = 100 ml/g,and 64% atV/m = 200 ml/g at pH 6.7,C0

NpO+2

= 3.5 ×10−5 mol/L, and in the presence of 0.01 mol/L NaNO3,and that the sorption percentage of Np(V) on silica ismost independent ofV/m and equal to about−15% atpH 3.5,C0

NpO+2

= 3.5× 10−5 mol/L, and in the presence o

0.01 mol/L NaNO3. Afterward, theV/m = 100 ml/g wasused in the sorption experiments on both adsorbents inder to facilitate the comparison between the results onadsorbents.

3.3. The effects of pH and ionic strength

In the absence of fulvic acid (FA) and at three ionstrengths (0.001, 0.01, and 0.1 mol/L NaNO3), theKd val-ues of Np(V) sorption on alumina also show a typicaldependence in the pH range 4–9 (Fig. 2). Similar pH depdence of Np(V) sorption onγ -Al2O3 from 0.1 mol/L NaCl

Fig. 2. Variation of logKd of Np(V) on Al2O3 with pH: (!) 0.1mol/L, (") 0.01 mol/L, (×) 0.001 mol/L. V/m = 100 ml/g; C0

NpO+2

=1.0× 10−5 mol/L.

solution was found by Righetto et al. [1]. However, in cotrast with the finding that the sorption percentage rema95% in the pH range 9–11 [1], in this paper, at pH> 10.3and ionic strengths 0.01 and 0.001 mol/L NaNO3, the sorp-tion percentage are larger than 99%. As compared withpH dependence of Np(V) sorption onγ -Al2O3 by Righettoet al. [1], in this paper, the sorption of Np(V) on Al2O3begins at lower pH and the increase inKd value with in-creasing pH is slower. The differences between the resby Righetto et al. [1] and by our laboratory may be mairelated to the difference in Np(V) concentrations. As shoin Fig. 2, in the pH range 6.5–8, where theKd value abruptlyincreases with increasing pH, theKd values at the same pand three ionic strengths are very close to each other. Intrast in the pH range 3–6.5, theKd value is slowly increasewith increasing pH, theKd values at the same pH and ionstrengths 0.1 and 0.01 mol/L NaNO3 are similar, while theKd values at the same pH and higher ionic strengths (0.10.01 mol/L NaNO3) are obviously larger than that at thsame pH and lower ionic strength (0.001 mol/L NaNO3).The effect of ionic strength on the Np(V) sorption onto amina is demonstrated in the pH range 3–6.5.

In the absence of FA and at three ionic strengths (0.00.01, and 0.1 mol/L NaNO3), the sorption percentagesNp(V) on silica as a function of pH atC0

NpO+2

= 1.1 ×10−5 mol/L and V/m = 100 ml/g are shown in Fig. 3The pH dependence of Np(V) sorption on silica is distincdifferent from that on alumina mentioned above. First,sorption percentages on silica are obviously lower than thon alumina, and the negative sorption percentages on sare found at lower ionic strengths, 0.01 and 0.001 mo/LNaNO3; i.e., the concentrations of Np(V) at equilibriu(Ceq) are larger than the initial concentrations (C0). Sec-ond, the positive sorption of Np(V) onto silica at 0.1 mol/L

28 W. Li, Z. Tao / Journal of Colloid and Interface Science 267 (2003) 25–31

lu-n-at

n onxi-

ion

urve–pH

-

ions

i-

ylin-ec-

n-

qui-)

-

e.,H

tivei-

s-

ndAermga-

(V)

n, at

onal

Fig. 3. Variation of sorption percentage of Np(V) on SiO2 with pH:C0

NpO+2

= 1.1 × 10−5 mol/L, (") 0.1 mol/L, (!) 0.01 mol/L, (×) 0.001

mol/L NaNO3; C0NpO+

2= 8.8 × 10−6 mol/L, (2) 0.01 mol/L NaNO3;

C0NpO+

2= 8.8 × 10−6 mol/L, (Q) 0.01 mol/L NaNO3, 0.001 mol/L

Na2CO3.

NaNO3 ionic strength begins at pH 7 instead of pH 3 on amina in Fig. 2. Third, the sorption onto silica is not monotoically increased with increasing pH. There is a maximumabout pH range 9–10; before the maximum, the sorptiosilica is increased with increasing pH, while after the mamum, the sorption is decreased with increasing pH.

The sorption percentage Np(V) on silica as a functof pH at C0

NpO+2

= 8.8 × 10−6 mol/L, V/m = 100 ml/g,

and ionic strength 0.01 mol/L NaNO3 is shown in Fig. 3too. The similar shape of the sorption percentage–pH cwas obtained. As compared with the sorption percentagecurve atC0

NpO+2

= 1.1×10−5 mol/L, V/m = 100 ml/g, and

ionic strength 0.01 mol/L NaNO3, the curve at lower concentrations of Np(V) (8.8 × 10−6 mol/L) in the pH range7.5–11 is obviously higher than that at higher concentrat(1.1×10−5 mol/L) and the same ionic strength, 0.01 mol/LNaNO3. The negative sorption at higher concentration (1.1×10−5 mol/L) and 0.01 mol/L ionic strength becomes postive sorption at lower concentration (8.8× 10−6 mol/L) and0.01 mol/L ionic strength, in the pH range 7.5–11.

3.4. Sorption equilibria in the absence of FA and carbonateadded in solutions

The sorption isotherms of Np(V) on alumina at pH 4.5±0.1, pH 4.6 ± 0.1, and pH 5.9 ± 0.1 are simultaneouslshown in Fig. 4. The three isotherms are approximatelyear. The averageKd values of the three isotherms are resptively denoted in Fig. 4. TheKd values at pH 4.5± 0.1 and4.6± 0.1 are equal within experimental error. TheKd valueat pH 5.9 ± 0.1 is roughly double that at pH 4.5 ± 0.1; the

Fig. 4. Np(V) sorption isotherms on Al2O3 under different pH.V/m = 100 ml/g; I = 0.01 mol/L NaNO3.

strong effect of pH on the Np(V) sorption is also demostrated in Fig. 4.

Figure 5a shows the relationship between the Np(V) elibrium concentration (Ceq) after sorption and the Np(Vinitial concentration (C0) before sorption at pH 7.4 ± 0.1,V/m = 100 ml/g, and 0.01 mol/L NaNO3 ionic strengthin the absence of FA or Na2CO3. It was found that the experimental points at initial concentrations less than 1.5 ×10−5 mol/L are located above the diagonal line; i.Ceq> C0. The negative sorption of Np(V) on silica at p7.4± 0.1, V/m = 100 ml/g, and 0.01 mol/L NaNO3 ionicstrength is clearly demonstrated in Fig. 5a. The negasorption in Fig. 5a is consistent with Fig. 3, while at intial concentrations larger than 1.5× 10−5 mol/L, except thelast experimental point, theCeq is either equal to or slightlyless than theC0.

3.5. Effects of FA and carbonate added in solutions

The Np(V) sorption isotherm on alumina in the preence of FA (initial concentration,C0

FA = 20 mg/L) atpH 5.8 ± 0.1, V/m = 100 ml/g, and 0.01 mol/L NaNO3ionic strength is shown in Fig. 4 too. This isotherm athe isotherm at pH 4.5 ± 0.1 and in the absence of Fmerge into a single one. As compared with the isothat pH 5.9 ± 0.1 and in the absence of FA, the small netive effect of FA on the Np(V) sorption at pH 5.8 ± 0.1 isdemonstrated. After the addition of 0.001 mol/L Na2CO3 insolutions, the pH value of aqueous solution and the Npsorption are obviously increased (Fig. 4,P). For silica, afterthe addition of 0.001 mol/L Na2CO3 or 20 mg/L FA in so-lution, the relationships atV/m = 100 ml/g and 0.01 mol/LNaNO3 ionic strength betweenCeq andC0 at pH 7.3± 0.1,pH 7.0 ± 0.1, and pH 9.1 ± 0.1 are respectively shown iFigs. 5b, 5c, and 5d. As shown in Figs. 5b, 5c, and 5dlower initial concentrations (<2 × 10−5 mol/L), the exper-imental points are located either above or on the diag

W. Li, Z. Tao / Journal of Colloid and Interface Science 267 (2003) 25–31 29

Fig. 5. The relationship between the Np(V) equilibrium concentrationCeq after sorption on silica and the initial concentrationC0.

s

inon

lrp-11

-

eenlid

on.in

well]Hatives

r-ption

x-suche9.5

of

sil-ingfour

le 1.lu-byob-d insedeen2,6](V)

.g.,a-se.

fleher

line, i.e., Ceq > C0, or Ceq = C0; at higher initial con-centrations (>2 × 10−5 mol/L), the experimental pointare close or below to the diagonal line, i.e.,Ceq = C0 orCeq < C0. No obvious effects of FA and carbonate addedsolutions were found. The sorption percentage of Np(V)silica as a function of pH in the presence of 0.001 mo/LNa2CO3 added in solutions is shown in Fig. 3 too. The sotion percentage is increased monotonically from pH 7 toat C0

NpO+2

= 8.8 × 10−6 mol/L, 0.01 mol/L NaNO3 ionic

strength, andV/m = 100 ml/g. As compared with the sorption percentage–pH curve in the absence of Na2CO3 addedin solution, the negative effect at pH< 10 and the positiveeffect at pH> 10 are demonstrated.

4. Discussion

The distinct difference in sorption characteristics betwalumina and silica implies that the properties of the sophase surface are primarily controlling the Np(V) sorptiFirst and foremost, we should deal with the differenceelectrical charges at surfaces of both solid phases. It isknown that the pHpzc of silica ranges from 1.8 to 3.5 [13,14and the pHpzc of alumina used here is 7.5 [12]. In the prange used here, the surface of silica always carries negcharges, while at pH< 7.5, the surface of alumina carriepositive charges, at pH> 7.5 the surface of alumina caries negative charges. Consequently, the negative sor

of Np(V) on silica is not surprising, so long as Np(V) eists in negatively charged species in aqueous solutions,as NpO2CO−

3 and NpO2(CO3)3−2 . However, the fact that th

sorption on silica increases with increasing pH from 7 tocannot be solely explained by the exclusion of NpO2CO−

3and NpO2(CO3)3−

2 from the negatively charged surfacesilica.

Four different sorption characteristics on alumina andica cannot be solely explained by the difference in havelectrical charges at surfaces of both adsorbents. Thedifferent sorption characteristics are summarized in TabFurthermore, the sorption characteristics of Np(V) on amina and silica in this paper are different from thoseprevious authors [1,2,6]. The sorption characteristicstained by previous authors [1,2,6] are also summarizeTable 1. As compared with the experimental conditions uby previous authors [1,2,6], the main differences betwthe experimental conditions used by previous authors [1,and in this paper are that (1) the total concentration of Npused in this paper is highest (∼10−5 mol/L) and that (2) inthis paper, membrane filtration and ultracentrifugation (e55,000 rpm [1]) were not used in the purification of wter and in the separation of colloids from the liquid phaThough under aerobic conditions (Eh > 300 mV), Np(V) isstable within a wide range of pH [15], and NpO2CO−

3 andNpO2(CO3)3−

2 exist at pH> 7.5 and atmospheric level oCO2 (PCO2 = 10−3.5 atm), the Np(V) species may possibbe in some colloidal forms at higher pH values and hig

30 W. Li, Z. Tao / Journal of Colloid and Interface Science 267 (2003) 25–31

as

ly [6]

withe range

Table 1Four different sorption characteristics

Sorption onto alumina Sorption onto silica Sorption onto alumina Sorption onto silicin this paper in this paper by previous authors by previous author

Sorption % Positive sorption only Negative sorption at ionic strength Positive sorption only [1] Positive sorption on0.001 and 0.01 mol/L NaNO3, positivesorption at 0.1 mol/L NaNO3

Dependence of Inversely proportional Almost independentsorption onV/m

Effect of pH Monotonic increase Increase with increasing pH in the Monotonic increase with Monotonic increasewith increasing pH in range 7.5–9.5 and decrease with increasing pH in the range increasing pH in ththe range 2–11 increasing pH in the range 9.5–11 5–11 [1,2] 7–12 [6]

Effect of Weak Strongionic strength

atnore,ol-ro-po-nerhose

esar-2,6]

(V)ion

. 3.

n-on

ent,,

aareom-Alen-s ofmbethanom-hecenionil-

-en-

ly

achwithsur-n aac-

(V)ub-ate,te.

bicen

and-r-ent.r ason ofults

co-ds forup-archhend

concentrations, since the minimum solubility of Np(V)pH 9–10 is 1× 10−5 mol/L [10], just less or larger thathe Np(V) concentrations used in this paper. Furthermthe migration behavior of Np(V) in a quartz-packed cumn was studied by using high-performance liquid chmatography [10]. It was found that the unretarded comnent is colloidal at pH 6.2–6.9, 0.1 ml of Np(V) solutio(5× 10−4 mol/L) at the injection port, and deionized watas eluent. These experimental conditions are similar to tused in this paper. Hence, NpO+

2 , NpO2OH, NpO2(OH)−2 ,NpO2CO−

3 , NpO2(CO3)3−2 , and colloids are possible speci

of Np(V) in this paper. And the difference in sorption chacteristics in this paper and in the previous papers [1,may be related to the form of the colloids.

Nero et al. [4] reported a continuous decrease of Npsorption onto hydragillite with increased total concentratof carbonate from 0.001 to 0.01 mol/L in the alkaline pHrange. This effect of carbonate is different from that in FigThe negative effect of carbonate at pH< 10 and the positiveeffect of carbonate at pH> 10 are simultaneously demostrated in Fig. 3. This difference of effect of carbonatesorption of Np(V) may be related to the nature of adsorbto the ratio of Np(V) concentration to CO2−

3 concentrationand to the species of Np(V) in liquid phase.

The small effects of FA on Np(V) sorption onto silic(Figs. 5b and 5c) and onto alumina at pH 5.8 (Fig. 4)expected and may be attributed to the relatively strong cplexation of NpO+

2 with carbonate. The small effect of Fis consistent with the argument made by Lieser and Muhweg [15]. They pointed out that as the stability constanthumic substances calculated on the basis of the mole nuof carboxylate groups are generally somewhat smallerthose of the monocarbonate complexes, it follows that cplexation of humic substances will only be important if tconcentration of HS is high and at the same time the contration of carbonate ions is low. The effect of FA on sorptof U(VI), Zn, Yb, I, and Se(IV) onto oxides of alumina, sica, and iron was studied in detail by our laboratory [16].

r

-

5. Conclusions

(1) Under aerobic condition (Eh > 300 mV), in the presence of atmosphere and at relatively higher Np(V) conctration (∼10−5 mol/L), because Np(V) exists in positivecharged species NpO+2 , neutral species NpO2OH, negativelycharged species NpO2(OH)−2 , NpO2CO−

3 , NpO2(CO3)3−2 ,

and colloids, and these species are in equilibrium with eother in the liquid phase and these equilibria are shiftedchanging pH and adding carbonate, many reactions withfaces are responsible for the overall sorption of Np(V) osurface. The sorption of Np(V) is very complicated; the ftors affecting the sorption process are numerous.

(2) Effect of humic substances on the sorption of Npmay be not important at low concentrations of humic sstances and relatively higher concentrations of carbonbecause of the relatively strong complexation of carbona

(3) The sorption of Np(V) onto alumina under aerocondition is obviously stronger than that onto silica; evnegative sorption onto silica occurs.

(4) The results of this paper are conducive to understing the retardation of Np(V) migration in a geological fomation containing quartz or alumina as a major componAt the same time, we look upon the results of this papea severe challenge to greater exertions. The expectatifinding a satisfactory explanation for all experimental resis unrealistic at present.

Acknowledgments

This work was supported by CIRP as a part of aoperative research project on safety assessment methoshallow land disposal of low-level radioactive wastes sported by CIRP and the Japan Atomic Energy ReseInstitute (JAERI). We thank Professor Hu Jingxin from tChina Institute of Atomic Energy for valuable comments adiscussion on an early version of the manuscript.

W. Li, Z. Tao / Journal of Colloid and Interface Science 267 (2003) 25–31 31

.

57.76

im.

9)

n-

rks,n-

6)

05–

a-

rti-

9–

cl.

15

References

[1] L. Righetto, G. Bidoglio, B. Marcandalli, I.R. Bellobono, RadiochimActa 44/45 (1988) 73–75.

[2] S. Nakayama, Y. Sakamoto, Radiochim. Acta 52/53 (1991) 153–1[3] M.D. Nero, B. Made, G. Bontems, A. Clemenl, Radiochim. Acta

(1997) 219–228.[4] M.D. Nero, K.B. Said, B. Made, A. Clement, G. Bontems, Radioch

Acta 81 (1998) 137–141.[5] M. Kohler, B.D. Hongmen, J.O. Leckie, Radiochim. Acta 85 (199

33–48.[6] D.C. Girvin, L.L. Ames, A.P. Schwab, J.E. McGarratl, J. Colloid I

terface Sci. 141 (1991) 67–78.[7] J.-M. Combes, C.J. Chisholm-Brause, G.E. Brown Jr., G.A. Pa

S.D. Conradson, P.G. Eller, I.R. Triay, D.E. Hobart, A. Meijer, Eviron. Sci. Technol. 26 (1992) 367–382.

[8] O. Tochiyama, H. Yamazaki, T. Mikami, Radiochim. Acta 73 (199191–198.

[9] O. Tochiyama, S. Endo, Y. Inoue, Radiochim. Acta 68 (1995) 1111.

[10] S. Nakayama, H. Arimoto, N. Yamada, H. Moriyama, K. Higashi, Rdiochim. Acta 44/45 (1988) 179–182.

[11] T.W. Chu, J.Z. Du, J.R. Lu, Z.Y. Tao, J. Radioanal. Nucl. Chem. Acles 210 (1996) 197–205.

[12] T.W. Chu, J.Z. Du, Z.Y. Tao, Adsorpt. Sci. Technol. 15 (1997) 34360.

[13] X.K. Wang, W.M. Dong, H.X. Zhang, Z.Y. Tao, J. Radioanal. NuChem. 250 (2001) 491–496.

[14] Z.Y. Tao, H.X. Zhang, J. Colloid Interface Sci. 252 (2002) 15–20.[15] K.H. Lieser, V. Muhlenweg, Radiochim. Acta 43 (1988) 27–35.[16] Z.Y. Tao, T.W. Chu, J.Z. Du, X.X. Dai, X.J. Gu, Appl. Geochem.

(2000) 133–139.