1
In e ac ion o Eu-iso opes wi h saponi e as a componen o he enginee ed ba ie
Ma ía D. Alba (a), Miguel A. Cas o (a), P. Chaín (a), San iago Hu ado (b), M. Ma O a (a),
M. Ca olina Pazos (a), Ma ía Villa (b)
a Ins i u o Ciencia de los Ma e iales de Se illa, CSIC-US, A da. Ame ico Vespucio, 49, 41092
Se illa, Spain
b Cen o de In es igación, Tecnología e Inno ación de la Uni e sidad de Se illa, CITIUS, A da.
Reina Me cedes, 4, 41012 Se illa, Spain
Abs ac
Ben oni e is accep ed as he bes clay ma e ial in he enginee ed ba ie o deep geological
eposi o ies (DGRs) o adioac i e was e disposal. In ecen yea s, he in e ac ions be ween a
wide ange o a e-ea h (REE) ca ions and smec i es ha e been s udied. A combined s udy o
s able eu opium and adioac i e iso opes is epo ed he e. Saponi e was subjec ed o
hyd o he mal eac ions wi h s able and adioac i e (152Eu) eu opium ions unde subc i ical
condi ions. The s uc u al changes o saponi e we e e alua ed by XRD and SEM. The e ec o
empe a u e and eac ion ime on he changes was quan i ied by measu ing 152Eu h ough
gamma spec ome y. The eac ion be ween eu opium and saponi e was a i s -o de eac ion.
The p esence o Eu in he p ecipi a e in an amoun much highe han he ca ion exchange
capaci y o saponi e con i med pa icipa ion o chemical eac ions o su ace adso p ion in he
eu opium immobiliza ion, e en a empe a u es as low as 150 °C. The eac ion a e cons an
indica ed ha an 8- o 9-mon h pe iod was needed o he comple ion, wi hou signi ican
changes, o he eu opium/saponi e chemical eac ion unde he subc i ical condi ions o 200
°C and 350 °C.
Keywo ds
Enginee ing ba ie ; Radionuclide; Ben oni e
1. In oduc ion
The managemen o adioac i e was e con aining ac inides is cu en ly a key en i onmen al
p oblem due o he need o i s long- e m, sa e and e icien s o age. The deep geological
eposi o y (DGR) concep in ol es he placemen o long-li ing high-ac i i y adioac i e was e
(HARW) in ooms exca a ed deep wi hin a s able, low-pe meabili y bed ock. In many
coun ies, he DGRs a e based on a passi e mul i-ba ie sys em app oach ha combines
was e packages, enginee ed seals, and bed ock whe eby he majo esponsibili y o sa e y
alls on he enginee ed ba ie sys em (EBS). Nowadays, ben oni e is accep ed as he bes clay
ma e ial o he enginee ed ba ie o DGRs (Bailey, 1980). In almos all eposi o ies o his
ype i is assumed ha he geosphe e su ounding he EBS is sa u a ed wi h wa e since
2
g oundwa e g adually seeps back in o he unnel and could be in con ac wi h he EBS. Unde
sa u a ion condi ions, ben oni e, due o i s high con en o smec i e, swells and seals he
unnel, es ic ing almos all low o wa e inside he EBS (Bailey, 1980).
The in e ac ion o 4 elemen s, such as La, Lu, Nd and Sm, wi h na u al and syn he ic clay
mine als du ing he mal and hyd o he mal eac ions was s udied in a sys ema ic and
undamen al way (Alba and Chain, 2005, Alba and Chain, 2007, Alba e al., 2009a and Alba e
al., 2009b). A ne e s udied eac ion mechanism, based on he in e ac ion be ween he
lan hanide ca ions and he clay mine al, was iden i ied (T illo e al., 1994). Unde subc i ical
empe a u e and p essu e condi ions (374.3 °C, 218 a m), an in e ac ion be ween smec i es
and a e-ea h elemen s (REE) was ound o exis (Chapman and Smellie, 1986) and an
insoluble disilica e, REE2Si2O7, was gene a ed (Bece o e al., 2003). The eac ion ex ends o
he whole se o smec i es, al hough hei eac i i y di e s, wi h saponi e as he mos eac i e
(Alba e al., 2001). These esul s implied an e icien mechanism o he enginee ed ba ie
(ben oni e ba ie ) o highly ac i e adioac i e was e (HARW) managemen when he
p ope ies o ben oni es (ca ion exchange and swelling capaci y), ha a e esponsible o he
physical e en ion, ail his e en ion.
These sys ema ic s udies we e pe o med by using lan hanide ions as he HARW chemical
analogous. I u anium (95% o he spen uel) we e dis ega ded, an a e age composi ion o he
spen - uel pelle s o e e ence would be app ox. 18 mass% o plu onium and 2 mass% o
equally dis ibu ed nep unium and ame icium, wi h he emaining ac inides as mino elemen s
(As udillo, 2001). The e o e, he easibili y o he chemical-in e ac ion immobiliza ion
mechanism o he main HARW p esen in he nuclea was e emains o be es ed.
Thus, in o de o unde s and he abo e-men ioned hyd o he mal eac ion, a combined s udy
was pe o med: On one hand, he s uc u al changes o saponi e by he s able iso opes should
be conside ed. On he o he hand, oge he wi h a s able iso ope, a adioiso ope can be added
in ul a- ace concen a ions, which makes i possible o quan i a i ely de e mine he
dis ibu ion o he REE ca ions be ween he solid and he liquid pa s o he eac ion.
Many s udies in he li e a u e a e dedica ed o he adso p ion o he s able eu opium ions by
clay mine als. The ca ion exchange p ocess is he main adso p ion mechanism a 25 °C–150 °C
and a a mosphe ic p essu e (Sánchez e al., 2006 and Te e e al., 2006). The s udy o he
beha io o adioiso opes in a ious ma ices is widesp ead, o example adso p ion and
deso p ion p ope ies in con amina ed soils ha e been s udied in Liu e al. (2003) and in
Spalding (2001), while soil- o-plan ans e p ope ies ha e been in es iga ed by Rigol e al.
(2008). S udies o he adso p ion o adioiso opes on clay mine als include hose by De i ie e
al. (2004) o 125I, Vejsada e al. (2005) o 134Cs, and Rabung e al. (2005) o ace
concen a ions o cu ium. The beha io o eu opium ions in clay mine als has been widely
s udied, o example, adso p ion s udies ha e been ca ied ou by Rabung e al. (2005), and in
Fan e al. (2009), whe eas Rahman e al. (2007) ocused on leaching s udies.
Depending on he empe a u e, REE2Si2O7 is o med, unde hyd o he mal condi ions a 300
°C, om a wide se o REE3+ ca ions, such as Sc3+, Y3+, La3+, Nd3+, Sm3+ and Lu3+, (Alba e
3
al., 2009a). In o de o e alua e he applicabili y o he long immobiliza ion p ocess in eal
eposi o ies, i is necessa y o explo e he easibili y o he eac ion below 200 °C. The s udy o
s uc u al changes below his empe a u e, howe e , is no always achie able (Allen and
Wood, 1988, Ma he e al., 1982 and Sa age and Chapman, 1982), and he e o e highe
empe a u es a e aken as app op ia e condi ions o he simula ion o deep geological
disposal condi ions and o an inc ease o he eac ion a es.
The use o s able iso opes enables an analysis o be ca ied ou o he s uc u al changes o he
ben oni e a e he hyd o he mal eac ion wi h REE. The use o adioac i e iso opes enables o
ob ain u he da a: a) he immobiliza ion capaci y can be quan i a i ely de e mined; b) he
kine ics o he p ocess can be analyzed; c) he p oposed eac ion mechanism can be es ed in a
wide ange o empe a u es, e en when he ex en o he eac ion is insu icien o enable
de ec ion o s uc u al changes; and d) an app op ia e me hodology can be es ablished o
ex end he s udy o ins able adionuclides.
Eu3+ seems o be he mos sui able REE ca ion o his s udy since i s physical p ope ies
(ca ion size, hyd oliza ion cons an , and oxida ion s a e) a e compa able wi h hose o Am3+
and Cm3+ (B adbu y e al., 2005 and Fan e al., 2009). I is a ailable as s able iso opes, 151Eu,
153Eu, and he adioac i e iso ope, 152Eu (T1/2 = 13.5 yea s). The e o e, in his expe imen al
s udy we add ess he a e and mechanism o Eu3+ ion as a model sys em, unde condi ions
simila o hose p edic ed o he nuclea was e s o age assessmen . In geochemical p ocesses
o was e deg ada ion and was e/ ock in e ac ion, he expec ed empe a u es each up o
app ox. 200 °C (Sa age and Chapman, 1982). Howe e , many s udies de o ed o simula ing
deep geological disposal condi ions used empe a u es o up o 350 °C o inc ease he eac ion
a es, (Allen and Wood, 1988, Ma he e al., 1982 and Sa age and Chapman, 1982).
2. Expe imen al
2.1. Saponi e
Saponi e was ob ained om he Sou ce Clay Mine als Reposi o y Uni e si y o Missou i
(Columbia) and had he ollowing chemical o mula: Na0.61K0.02Ca0.09 (Si7.2Al0.8)IV(Mg5.79
Fe2+0.15)VIO20(OH)4 (Alba e al., 2001). The in e laye ca ion concen a ion and he wa e
con en we e es ima ed o be 103.0 meq/100 g and 9.2 mass% (Chain, 2007).
2.2. Eu3+ solu ion
Two se s o s a ing solu ions o 7.9 · 10− 2 M Eu(NO3)3 (151Eu and 153Eu, wi h 52.2% 153Eu)
we e p epa ed: i) The i s solu ion con ained pu ely s able Eu iso opes and ii) he second
solu ion was en iched wi h he adioiso ope 152Eu. A olume o 1 ml om a dilu ed s anda d
solu ion (9.8 Bq/ml) up o a o al ac i i y o 9.8 Bq was added o 35 ml o he o me solu ion,
i.e. a comp omise be ween low adioac i i y concen a ions o sa e handling and high
coun ing a es o ge as measu emen s. Due o i s sho hal -li e (13.5 yea s) 10− 14 mol o
152Eu we e added.
4
The pH o bo h solu ions was adjus ed o pH = 6.0–6.5, by slowly adding 0.05 M ammonia
solu ion unde s i ing.
2.3. Hyd o he mal eac ions
300 mg o he powde ed samples we e dispe sed in 40 ml o he Eu3+ solu ions and we e
hea ed in a s ainless s eel eac o , (Pe digón, 2002), a he empe a u es and imes
summa ized in Table 1. The cells ma ked wi h e ical line co espond o he ea men wi h a
s a ing solu ion o 7.9 · 10− 2 M Eu(NO3)3, he g ay colo cells wi h a s a ing solu ion o 7.9 ·
10− 2 M Eu(NO3)3 en iched wi h he 152Eu iso ope. The eac ion p oduc s we e collec ed by
il e ing, washed wi h dis illed wa e , and d ied in ai a 60 °C. The eac ion solu ion and he
washing liquid we e kep o he quan i a i e analysis o eu opium by gamma spec oscopy.
Fo he e alua ion o possible a i ac s such as adso p ion on o he wall o he eac o , he
eac o essel wi h 40 ml o Eu(NO3)3 7.9 · 10− 2 M was hea ed a 350 °C o 4.5 days. The
152Eu con en o he emnan solu ion showed ha he Eu adso p ion on o he con aine was
negligible unde hese eac ion condi ions.
2.4. Cha ac e iza ion me hods
X- ay powde di ac ion (XRD) pa e ns we e ob ained wi h a B uke D8I ins umen , a he
CITIUS, Uni e si y o Se ille (Spain) (a Ni- il e ed Cu Kα adia ion, 40 kV, 40 mA, scan s eps o
0.05° 2θ, wi h a coun ing ime o 3 s). The c ys alline phases we e iden i ied by using he
compu e p og am X'Pe HighSco e (Philips Analy ical B.V. Almelo, The Ne he lands).
Scanning elec on mic oscopy (SEM) was used o obse e he ma e ials o which no
app eciable c ys alline phases we e obse ed by XRD. The mo phology and chemical
composi ions o he samples we e analyzed, a he Mic oscopy Se ice o CITIUS, Uni e si y o
Se ille (Spain), in a scanning elec on mic oscope (JEOL 6460LV) equipped wi h EDX.
The adioac i e eu opium, 152Eu, was measu ed wi h an HPGe coaxial gamma de ec o , a he
CITIUS, Uni e si y o Se ille (Spain). To calib a e he coun ing e iciency, 152Eu- ee, solid
samples ob ained a e he hyd o he mal eac ion we e spiked wi h a known amoun o
152Eu. The ac i i y concen a ion o 152Eu was measu ed in he liquid and solid phases.
3. Resul s and discussion
3.1. S uc u al s udies
The XRD pa e ns o he saponi e and hose a e eac ion wi h 7.9 · 10− 2 M Eu(NO3)3 a 350
°C, 200 °C and 150 °C, a e displayed in Fig. 1. The pa e n o he o iginal saponi e (Fig. 1a)
showed he gene al and basal e lec ions. The hk bands we e composed o asymme ical
e lec ions wi h he cha ac e is ic “saw- oo h” shape o he wo-dimensional e lec ions
(Wa en, 1941). The basal e lec ions we e symme ical. The 12.30 Å d001 alue o un eac ed
saponi e co esponded o he monolaye hyd a e o Na-saponi e ( Alba e al., 2001, G im,
1968, Ra ina and Low, 1977 and Wa en, 1941).
A e he hyd o he mal eac ion a 150 °C o 47 days, (Fig. 1b) he basal spacing o saponi e
was inc eased o 14.30 Å, in ag eemen wi h da a epo ed o smec i es sa u a ed wi h
5
mul i alen ca ions (Ra ina and Low, 1977). Also, he posi ion o he 060 e lec ion, in he
ange o he ioc ahed al smec i es (G im, 1968), was sligh ly shi ed o d060 = 1.526 Å.
Addi ionally, small e lec ions in he 25–30° 2θ ange we e obse ed and co esponded o F-
Eu2Si2O7 (PDF 87–2475, ma ked wi h F) and Eu(OH)3 (PDF 83–2305, ma ked wi h h). The XRD
pa e n was noisy and showed a p ominen backg ound which indica ed he pa ial dis up ion
o he saponi e amewo k.
The hyd o he mal eac ion a 350 °C yielded simila XRD pa e ns (Fig. 1c and d) which we e
domina ed by e lec ions o F-Eu2Si2O7 (PDF 87–2475, ma ked wi h F), c ys alline Eu(OH)3
(PDF 83–2305, ma ked wi h h), Eu2O3 (PDF 34–392, ma ked wi h o) and EuONO3 (PDF 31–518,
ma ked wi h n). These phases coexis ed wi h he emnan clay mine al o a new mixed-laye ed
clay mine al which was cha ac e ized by a se o basal e lec ions wi h d002 = 14.6 Å and he
absence o he hk e lec ions. The only e ec o he eac ion ime was he be e c ys alliza ion
o he new phases (Fig. 1c and d). Thus, empe a u e played a g ea e ole han ime in he
o ma ion o he disilica e, as is expec ed due o he laws o he modynamics and kine ics.
The elec on mic og aphs and he EDX analysis o he saponi e eac ed wi h Eu3+ a 150 °C o
47 days (Fig. 2) showed lamella mo phology o he majo i y o he pa icles (Fig. 2a) wi h
some small b illian pa icles on hei su aces. The EDX spec um o his saponi e (Fig. 2c) was
cha ac e ized by he absence o Na+, low con en o Mg2+, and a ela i ely high amoun o
Eu3+ when compa ed o he o iginal saponi e (Fig. 2b). When saponi e was eac ed a 350 °C,
mos o he pa icles again showed he lamella mo phology (Fig. 3a and b). The EDX spec a
(Fig. 3d) we e cha ac e ized by he Kα1 lines o Si, Al and Mg and Mα, and Lα and Lβ lines o
Eu. The dec ease o he Mg con en and he absence o Na, in compa ison wi h he o iginal
saponi e (Fig. 3c), we e due o he leaching o Mg ions om he s uc u e and he exchange o
Na+ by Eu3+ in he in e laye space. This is in line wi h he XRD pa e ns ha showed a
mode a ely deg aded s a e o he laye s and an inc eased basal spacing (Fig. 1c and d). The
al e ed saponi e seemed o o m agg ega es and showed an inc easingly blocky mo phology
wi h inc easing eac ion ime. In addi ion o he lamella pa icles, some compac pa icles
wi h b illian appea ance unde he backsca e ing elec on beam we e also obse ed wi h a
chemical composi ion (Fig. 3d) compa ible wi h he eu opium c ys alline phases al eady
obse ed by XRD, e.g. Eu2Si2O7, Eu(OH)3, Eu2O3 o EuONO3. These b illian pa icles we e
mo e abundan a he longe eac ion imes (Fig. 3b).
Thus, in he ini ial s ep o he eac ion a 150 °C, he Na+ ions o he saponi e we e exchanged
by Eu3+. As he eac ion p og essed up o 350 °C, he saponi e s uc u e was decomposed
o ming c ys alline F-Eu2Si2O7. O he mino eu opium phases we e also obse ed a 150 °C
and 350 °C.
3.2. The ex en o he eac ion
The pa ame e s used o he quan i a i e e alua ion o he ex en o he eu opium and silica e
eac ion as a unc ion o ime and empe a u e a e summa ized in Table 2. The amoun o
eu opium bound by saponi e was calcula ed om he measu ed adioac i i y in he liquid
(non- eac ed) and he solid ( eac ed) phases a e he hyd o he mal eac ion. The con en o
silicon in ol ed in he chemical immobiliza ion mechanism indica ed he p og ess o Eu2Si2O7
o ma ion.

6
E en a he lowes empe a u e and eac ion ime, he amoun o bound eu opium ions was
mo e han h ee imes he ca ion exchange capaci y o saponi e (103.0 meq/100 g). This
clea ly indica ed ha in addi ion o he ion exchange, eu opium phases we e o med.
The amoun o bound eu opium ions a e he eac ion o 20 days changed almos linea ly
wi h he empe a u e (Fig. 4): 390 ± 20 meq/100 g a 150 °C, o 990 ± 40 meq/100 g a 200 °C,
and o 1170 ± 30 meq/100 g a 350 °C, whe e 390 and 990 a e he a e age alues ob ained
om wo measu emen s. The dependence on he eac ion ime could no be clea ly seen a
150 °C, since, a ha empe a u e, he eac ion imes we e oo sho .
The esul s we e i ed o an exponen ial unc ion whe e he exponen co esponded o he
eac ion a e cons an , k. I0 was in ag eemen wi h he a e age 152Eu eco e ed in he liquid
and in he solid. The i ing indica ed ha he eac ion be ween eu opium and saponi e was a
i s -o de eac ion.
Fo 350 °C and 200 °C, he bes i was exponen ial bu wi h a a e cons an ha sligh ly
dec eased be ween 350 °C and 200 °C (1.33 · 10− 2 s 1.19 · 10− 2). The eac ion was almos
h ee imes as e a 350 °C han a 200 °C. Using he calcula ed eac ion a es, i can be
in e ed ha he chemical immobiliza ion o Eu ion would be comple ed a e 8–9 mon hs o
eac ion unde subc i ical condi ions a 350 °C and a 200 °C. Since he eac ion imes a 150 °C
we e no long enough, i ing could no be ca ied ou a his empe a u e.
4. Conclusions
The amoun o Eu in he eac ed saponi e in much highe amoun s han co esponding o he
ca ion exchange capaci y o saponi e was he esul o he ca ion exchange and he
neo o ma ion o he eu opium phases, e en a empe a u es as low as 150 °C. The kine ic
pa ame e s o eu opium immobiliza ion we e e alua ed. Mo eo e , hese esul s may be used
o p edic he beha io o HARW ca ions (Am, Cm, Th, U, and Pu) wi hin he ben oni e ba ie s
when s uc u al analysis is un easible.
Acknowledgmen s
We g a e ully acknowledge inancial suppo om Minis e io del Medio Ambien e y Medio
Ru al y Ma ino p ojec no. 300/PC08/3-01.1 and DGICYT p ojec no. CTQ2010-14874. Finally,
we hank ENRESA o i s inancial suppo .
7
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9
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16
Figu e 2

17
Figu e 3
18
Figu e 4