Interaction of Eu-isotopes with saponite as a component of the engineered barrier

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 Re e ences M.D. Alba, P. Chain In e ac ion be ween lu e ium ca ions and 2:1 aluminosilica es unde hyd o he mal ea men Clays and Clay Mine als, 53 (2005), pp. 39–46 M.D. Alba, P. Chain Pe sis ence o lu e ium disilica e Applied Geochemis y, 22 (2007), pp. 192–201 M.D. Alba, A.I. Bece o, M.A. Cas o, A.C. Pe digón Hyd o he mal eac i i y o Lu-sa u a ed smec i es: pa I. A long- ange o de s udy Ame ican Mine alogis , 86 (2001), pp. 115–123 M.D. Alba, P. Chain, M.M. O a Chemical eac i i y o a gillaceous ma e ial in enginee ed ba ie . Ra e ea h disilica e o ma ion unde subc i ical condi ions Applied Clay Science, 43 (2009), pp. 369–375 M.D. Alba, P. Chain, M.M. O a Ra e-ea h disilica e o ma ion unde Deep Geological Reposi o y app oach condi ions Applied Clay Science, 46 (2009), pp. 63–68 C.C. Allen, M.I. Wood Ben oni e in nuclea was e disposal: a e iew o esea ch in suppo o he basal was e isola ion p ojec Applied Clay Science, 3 (1988), pp. 11–30 J. As udillo 8 El almacenamien o geológico p o undo de los esiduos adiac i os de al a ac i idad. P incipios básicos y ecnología. Edi o ial ENRESA (2001) S.W. Bailey S uc u es o laye silica es G.W. B indley, G. B own (Eds.), C ys al S uc u es o Clay Mine als and Thei X- ay Iden i ica ion, Mine alogical Socie y, London (1980), pp. 1–123 A.I. Bece o, M. Na anjo, M.D. Alba, J.M. T illo S uc u e-di ec ing e ec o phyllosilica es on he syn hesis o y-Y2Si2O7. Phase ansi ions in Y2Si2O7 Jou nal o Ma e ials Chemis y, 13 (2003), pp. 1835–1842 M.H. B adbu y, B. Baeyens, H. Geckeis, Th. Rabung So p ion o Eu(III)/Cm(III) on Ca-mon mo illoni e and Na-illi e. Pa 2: su ace complexa ion modeling Geochimica e Cosmochimica Ac a, 69 (2005), pp. 5403–5412 Chain, P., 2007. Sín esis Hid o e mal de Disilica os de Tie as Ra as a pa i de Silica os Lamina es: Un Mecanismo Inmo ilizado de Residuos Radiac i os de Al a Ac i idad. Ph.D. Thesis, Uni e si y o Se ille, Spain. N.A. Chapman, J.A.T. Smellie Na u al analogues o he condi ions a ound a inal eposi o y o high-le el adioac i e was e Chemis y and Geology, 55 (1986), pp. 167–173 K. De i ie , I. De ol-B own, S. Sa oye S udy o iodide so p ion o he a gilli e o Tou nemi e in alkaline media Applied Clay Science, 26 (2004), pp. 171–179 9 Q.H. Fan, X.L. Tan, J.X. Li, X.K. Wang, W.S. Wuand, G. Mon a on So p ion o Eu(III) on a apulgi e s udied by ba ch, XPS, and EXAFS echniques En i onmen al Science & Technology, 43 (2009), pp. 5776–5782 R.E. G im Clay Mine alogy McG aw-Hill Book Company, New Yo k (1968) C. Liu, J.M. Zacha a, O. Qa oku, S.C. Smi h E ec o empe a u e on Cs+ a he Han o d Si e, U.S.A En i onmen al Science & Technology, 37 (2003), pp. 2640–2645 J.D. Ma he , N.A. Chapman, J.H. Black, B.C. Lin e n The geological disposal o high-le el adioac i e was e — a e iew o he Ins i u e o Geological Sciences Resea ch p og amme Nuclea Ene gy, 21 (1982), pp. 167–173 Pe digón, A.C., 2002. Es udio del sis ema saponi a/Lu(NO3)3/H2O en condiciones hid o é micas. Ph.D. Thesis, Uni e si y o Se ille, Spain. Th. Rabung, M.C. Pie e , A. Baue , H. Geckeis, M.H. B adbu y, B. Baeyens So p ion o Eu(III)/Cm(III) on Ca-mon mo illoni e and Na-illi e. Pa 1: ba ch so p ion and ime- esol ed lase luo escence spec oscopy expe imen s Geochimica e Cosmochimica Ac a, 69 (2005), pp. 5393–5402 Rahman e al., 2007 R.O.A. Rahman, A.A. Zaki, A.M. El-Kamash 16 Figu e 2 17 Figu e 3 18 Figu e 4