U and Th speciation in river sediments

U and Th specia ion in i e sedimen s A. Ma inez-Agui e3, M. Ga cia-León3, M. I ano ich* b ªFacul ad de Fisica, Uni e sidad de Se illa, Apdo.1065, 41080 Se illa, Spain bAEA Technology, Ha well Labo a o,y, Oxon, OXll ORA UK Abs ac A s udy o he dis ibu ion o so ne na u al adionuclides in di e en chemical ac ions o i e bo om sedimen s has been ca ied ou . Toe s udy has shown ha he majo i y o he o al U in sedimen s is loca ed in he non- esidual ac ions, while Th is mo e likely o be p esen in he esidual ones. This di e ence be ween U and Th e lec s la gely a much highe mobili y o U ela i e o Th in su ace and nea -su ace en i onmen s. Cop ecipi a ion wi h amo phous e omanganese oxyhyd oxides is he main p ocess o U and Th inco po a ion om he wa e column in o he soil pa icles. Mo eo e , he dis ibu ion o he adionuclides and he analysis o he Th/U mass a ios in di e en chemical ac ions o sedimen s has made an unequi ocal connec ion be ween he enhanced U con en in i e sedimen s and he was es discha ged in o he i e s by he ope a ion o e ilize indus ies. Keywo ds: Radionuclide pollu ion; U anium (U); Tho ium (Th); Ri e sedimen s; Fe ilize indus ies l. ln oduc ion Ri e sedimen s may be ega ded as, a leas , a empo a y sink o much o he ma e ial which passes h ough he a ious aqua ic chemical and biological cycles ope a ing on he ea h's su ace. Thus, sedimen s become an en i onmen al hos o many o he was e p oduc s discha ged by socie y. Toe e ec s o hese man-made emissions, in so ne si ua ions, can be su icien ly s ong o a ec highly he composi ion o he deposi ed * Co esponding au ho . sedimen s. Once a subs ance has been inco  po a ed in o a sedimen , i s ul ima e a e depends on a numbe o e y complex ac o s. An elemen may be conside ed o be locked pe manen ly in o a sedimen componen o i may la e be eleased and ake pa in a ious biogeochemical eac ions. Toe undamen al dis inc ions ha ha e been made in sedimen s geochemis y is ha be ween ma e ial b ough o he si e o deposi ion in a solid o m and ha b ough in a dissol ed o m. In ha sense he dis inc ion o elemen s be ween esidual and non- esidual ypes is pa icula ly ap p op ia e o he in es iga ion o adionuclide poi- '.,,:{� h ion [1]. Thus, p ocesses in ol ed in he inco  po a ion o elemen s om pollu ed wa e s esul s j�:; hei loca ion in he non- esidual ac ions and hus, hey can usually be conside ed o be a leas po en ially mobile, i.e. en i onmen ally eac i e in he chemical and biological p ocesses which occu in he sedimen s/in e s i ial wa e complex. On he o he hand, elemen s loca ed in esidual ac ions can usually be conside ed, a leas o a i s app oxima ion, o be immobile, i.e. en i onmen ally un eac i e. Thus, he esidence o an ele men in a deposi may be only empo a y, and hei elease in o he ecosys em ep esen s a po en ial haza d. The ex en o which his elease occu s depends la gely on he manne by which his elemen is bound o he sedimen and on how pa icula kinds o bonds eac o a ious physico-chemical condi ions. The abo e conside a ions ha e led o he s udy o he dis ibu ion o u anium (U) and ho ium (Th) in di e en ac ions o sedimen s collec ed a he Odiel and Tin o i e s, which a e loca ed in Sou hwes Spain. Such sedimen s a e clea ly a  ec ed by he was es eleased by wo e ilize ác o ies loca ed in he le ma gin o he Odiel l e as well as by he phosphogypsum piles s o ed a he igh ma gin o he Tin o i e [2]. 2. Samples and adiochemical p ocedu es Sampling was ca ied ou du ing he summe , 1989. Sedimen samples we e collec ed along he Odiel and Tin o i e basins and s o ed in plas ic b¡igs; hen, a he labo a o y hey we e d ied and 1'9W.de ed be o e he analysis. The loca ion o he S!lmpling s a ions a e shown in Fig. 1. Th ee sedi � s om he Odiel i e we e s udied: up s eam om he e ilize indus ies (S01), a ound o:n� o he e ilize indus ies (S02) and down s eam om hem (S03). In he Tin o i e , sedi men s om wo loca ions we e s udied: one ;; IQUnd he phosphogypsum pile a ea (STl) and ino he downs eam om his zone nea he con lu�nce wi h he Odiel i e (ST2). Addi ionally, a ió he sample (SOT) loca ed downs eam o he co' luence o bo h i e s was s udied. S:1bne sequen ial ex ac ion o six sedimen ac io'ns (Table 1) has been implemen ed o each sedimen sample (3,4]. The six sequen ially leached ac ions o he sedimen a e de ined in Table 1. F ac ions Fl -F4 a e e e ed o as sec onda y phases expec ed o ep esen he non- e sidual pa o he sedimen , being ei he an end p oduc de i ed om he i e wa e /solid in e  ac ion and exchange p ocesses o pa o he solid capable o di ec in e ac ion wi h i e wa e and hus sha ing so ne geochemical and iso opic cha  ac e is ics wi h he liquid phase. F ac ions F5 and F6 a e e med esidual phases and a e no ex pec ed o necessa ily in e ac on a sho ime-scale wi h he liquid phase, a leas no unde no mal condi ions ound in he su ace and nea -su ace en i onmen s. Once he ac ions we e sepa a ed, hey we e spiked wi h 236 U and 229Tb and he ex ac ion o U and Th iso opes was ca ied ou acco ding o he me hods gi en in [3,4]. U and Th ac i i ies we e measu ed by a-spec ome y wi h su ace ba ie o ion implan ed Si de ec o s. 3. Resul s and discussion Pe cen age dis ibu ion o U and Th in he di e en ac ions ( aking he con en in he un ac iona ed sedimen as 100% [2]) a e shown in Figs. 2 and 3. Wi h he excep ion o samples ST2 o U, he sum o he pe cen ages in he di e en ac ions ag ees well wi h 100%. Analy ical p ob lems in chemical sepa a ions o he e ogenei y in he sedimen sample canno be igno ed. 3.1. U anium Fig. 2 shows ha U is mainly associa ed wi h non- esidual ac ions, pa icula ly wi h F2 and F3 ac ions. Mo e han 40% o U is ound asso cia ed wi h he ac ion F3 (an excep ion is sam ple STl wi h 30% in his ac ion). This implies ha he main p ocess o U inco po a ion om he liquid phase ( i e wa e ) in o he sedimen s is he cop ecipi a ion wi h amo phous e oman ganese oxyhyd oxides. This is a ypical p ocess obse ed in es ua ine en i onmen s [5]. In sedimen S02 (Fig. 2b), ac ion F3 is s ill one o he main non- esidual ac ions con aining >40% o he o al U. Howe e , in his sample a o N � Phosphogypsum pi les *Fe lllze ac o les 3km 1 Fig. l. Map o he s udied a ea showing he loca ion o he sampling poin s, he e ilize ac o ies and he phosphogypsum s o age a ea emplacemen s. high p opo ion o U (- 50%) is also ound in ac ion F6 ( he p ima y mine al ac ion). This sample (Fig. 1) was aken in he icini y o a e ilize ac o y which eleases pa o i s solid was es di ec ly in o he Odiel i e [2,6]. Conse quen ly, his sedimen is likely o con ain ma e ial om he mine als used o he e ilize p oduc- Table 1 ion. This ma e ial is expec ed o o m a majo pa o ac ion F6 o he sedimen a his loca ion. Sample STl is also o so ne in e es . A his loca ion (Fig. 1) so ne ma e ial om he phosph gypsum s o age a ea [2] is discha ged di ec ly in o he Tin o i e . He e, he la ges ac ion o o al De ini ion o sedimen ac ions and eagen s used o hei ex ac ion F ac ion code Fl F2 F3 F4 F5 F6 F ac ion Readily ion exchangeable Adso bed ions in humic ma e ial and/o amo phous oxides Cop ecipi a ed wi h amo phous e omanganese oxyhyd oxides Ca bona es C ys alline e omanganese oxyhyd oxides Resis a e (clay) mine als Reagen Magnesium chlo ide Sodium py ophospha e Tamm's acid oxala e Sodium ace a e Co in's eagen .. u a 80....----------*U b 110---------.... �u e 80.-----------, .. u d*º .. u ·----------,, 80.----------- 40 Fl Fl F3 F4 F5 F6 Fl Fl 'J F4 FS F6 Fl F2 FJ F4 F/J F6 Fig. 2. Pe cen age dis ibu ion o U in each sequen ially ex ac ed ac ions o samples (a) S01, (b) S02, (e) S03, (d) STl, (e) ST2 and (O SOT. U esides in ac ion F2 ( > 50%) ollowed by ac ion F3 (- 30%). Thus, U is inco po a ed in o he local sedimen p edominan ly by adso p ion om he liquid phase. A di e en chemical asso cia ion in he was es mus be esponsible o he 'anomalous' U dis ibu ion obse ed in his s a ion and ela ed o he discha ges om he phos phogypsum s o age a ea [2, 7]. In Table 2, concen a ions o U in µ,g/ g con ained in he main ac ions o he sedimen s a e shown. I is in e es ing o obse e ha sedimen samples aken nea he phospha e was e dis- cha ge a ea o he Odie) i e (SOl, S02, S03 and ST2) ha e i ually iden ical U concen a ions in ac ions F2 and F3 and hese a e no ice ably highe han he es o samples (STl and SOT). This di e ence is much clea e in he case o ac ion F3. This can be aken as an e idence o he ex ension o he con amina ion o he Odie! i e basin (e en he lowes pa o he Tin o i e bed). Thus, he liquid was es dis cha ged om he e ilize indus ies a ec he i e bed ups eam and downs eam om he indus ies, p obably due o idal mo emen s o Table 2 U concen a ion in µ,g/ g in he main ac ions o bo om sedimen s om he Odie! and Tin o i e s F ac ion S01 S02 S03 F2 12.7 ± 0.8 12.1 ± 0.6 13.7 ± 0.9 F3 31.5 ± 3.6 37.5 ± 2.4 34.9 ± 3.5 F5 0.42± 0.04 2.86 ± 0.10 1.92 ± 0.06 F6 1.93 ± 0.07 40.8 ± 2.8 5.89 ± 0.15 he liquid discha ges. In he case o he esidual phases, i can be obse ed ha samples S02 and S03 ha e highe U concen a ions in ac ions F5 and F6 han he es o he samples. This di e  ence is much clea e in he case o ac ion F6 o sample S02. This sample is loca ed in on o he e ilize ac o ies and p esen s a U concen a ion > 1 o de o magni ude highe han he es o samples. The U concen a ion in his ac ion dec eases downs eam on he i e bed. This can be aken as e idence ha he solid pa icles om he e ilize p oduc ion cha ac e ised by a high U con en in ac ion F6 ha e eached s a ions S02 and S03 bu a e no sedimen ed u he downs eam in he es ua y. 3.2. Tho ium Fig. 3 shows he pe cen age dis ibu ion o Th (232 Th) in he six sequen ially ex ac ed ac ions o sedimen s. I can be obse ed ha Th has a highe associa ion han U o esidual ac ions F5 and F6, always wi h > 30% in ac ion F6. How e e , he non- esidual ac ion F3 appea s o be ano he a ou ed ac ion o Th. Thus, i seems ha , as o U, cop ecipi a ion wi h amo phous e omanganese oxyhyd oxides is he main p ocess o Th inco po a ion om he i e wa e o he sedimen . Basically, we can make he same com men s as we did in he case o U. Thus, sample S02 p esen s a di e en pe cen age dis ibu ion han he es o samples, wi h > 60% o Th in he mine al ac ion F6. Mo eo e , ac ion F2 does no seem o be a a ou ed ac ion o Th. Only sample STl has -15% o Th in his ac ion whe eas he es con ain < 5% in he same ac ion. In Table 3, Th concen a ions in µ,g/ g in he main ac ions o he sedimen s a e shown. I can be obse ed ha , excep ing sample S03, he Th ST1 ST2 SOT 8.69 ± 0.54 11.2 ± 0.8 6.86 ± 0.30 4.51 ± 0.26 33.3 ± 2.1 14.1 ± 0.7 0.35 ± 0.02 0.90± 0.04 0.92 ± 0.06 1.68 ± 0.13 2.77 ± 0.11 2.44 ± 0.14 con en in he non- esidual ac ions F2 and F3 is qui e simila in all s a ions. S a ion S03 has a highe concen a ion o Th in bo h non- esidual ac ions. This is mo e e iden in he case o ac ions F3, wi h - 1 o de o magni ude highe concen a ions in sample S03 han in he case o he es o samples. I seems ha he dissol ed Th is quickly cop ecipi a ed on he i e bed. Thus, Th con ained in he liquid discha ges seems o a ec only he sedimen s closes o he e ilize indus ies. In he case o he esidual ac ions, samples S02 and S03 p esen highe concen a ions in bo h ac ions han he es o samples, whe eas he es ha e simila Th concen a ions. Thus, i can be concluded ha he i e sedimen s nea he e ilize indus ies ha e inco po a ed Th de i ed om he liquid and solid was es bu i does no each s a ions u he ups eam o downs eam along he i e . 3.3. Th / U mass a ios In e ac ion o non- esidual ac ions wi h solu ion is essen ially e lec ed in he Th/U mass a io. I is well known ha Th is a pa icula ly insoluble elemen in na u al wa e s and i is usu ally ound associa ed wi h solid ma e . Thus, his a io is ypically much below uni y in wa e s and abo e uni y in solid pa icles. These ac s can be clea ly ob ained by s udying he alues o such a io in he main ac ions o sedimen s om he Odiel and Tin o i e s which a e p esen ed in Table 4. In he case o non- esidual ac ions F2 and F3, and excep ing ac ion F3 o sample S03, he Th/U mass a io is much below uni y and ypical o liquid phases. In ac , hese alues a e simila o hose ound in wa e samples om he same i e s [2). Toe excep ion o phase F3 o sample S03 seems o be ela ed o he ac ha Th is , TII a 80,------------. ,n d 801------------, 20 F F2 F3 F4 F5 F6 ,Th b 80.----------- % Tlo e. ao---------- Fl F2 F3 F4 F5 F6 ,T11 C 80.-----_.;;..----....... ..,n 80...---------- FI F2 F3 F4 FS F6 Fig. 3. Pe cen age dis ibu ion o Th in each sequen ially ex ac ed ac ions o samples (a) S01, (b) S02, (e) S03, (d) ST1, (e) ST2 and (0 SOT. Table 3 Th concen a ion in µ.g/g in main ac ions o bo om sedimen s om he Odie! and Tin o i e s F ac ion S01 S02 S03 STl ST2 SOT F2 0.79 ± 0.08 0.74 ± 0.06 1.31 ± 0.08 1.10 ± 0.12 0.59 ± 0.05 0.70 ±0.05 F3 7.71 ± 0.51 3.97 ±0.22 38.4 ± 4.0 2.02 ± 0.14 5.05 ±0.39 5.88±0.36 F5 1.89 ± 0.13 1.41 ± 0.07 6.22 ±0.33 0.68±0.08 1.44 ± 0.11 0.96±0,07 F6 4.83 ± 0.22 12.7 ± 1.1 15.8 ± 0.80 4.36± 0.31 4.21 ± 0.33 6.61 ±0.63 Table 4 To/U mass a io in main ac ions o bo om sedimen s om he Odie! and Tin o i e s F ac ion S01 S02 S03 F2 0.062 ± 0.007 0.061 ± 0.006 0.096 ± 0.009 F3 0.245 ± 0.032 0.10 ± 0.009 1.10 ± 0.16 F5 4.5 ± 0.5 0.493 ± 0.030 3.24 ± 0.20 F6 2.50 ± 0.15 0.311 ± 0.034 2.68 ± 0.15 quickly cop ecipi a ed on he su ace o sus pended ma e which a e deposi ed downs eam a such a poin and no u he downs eam. I is also in e es ing o obse e ha his a io is much lowe in ac ion F2 han in F3. This could be in e p e ed by he ac ha Th is no easily adso  bed on o solid pa icles compa ed wi h U. In ac all s a ions p esen simila alues o his a io in ac ion F2, independen ly o he concen a ion o U and/o Th in he ac ion. In he esidual ac ions F5 and F6 he Th/U mass a io o all samples, excep sample S02, has alues abo e uni y and close o he ypical alue o nea  su ace ocks o 3.5 [8]. Toe excep ion o bo h esidual ac ions o sample S02 con i ms he U en ichmen ela i e o Th ypical o he phos pha e mine al u ilised in he e ilize p oduc ion [7]. 4. Conclusions Toe U and Th dis ibu ion in di e en ac ions o bo om sedimen s om he Odiel and Tin o i e s has p o ided much in o ma ion on hei en i onmen al beha iou . Unde es ua ine condi ions U and Th occu mainly in he non- e sidual ac ions al hough Th ends o eside mo e in esidual ac ions han U. These di e ences be ween U and Th e lec la gely he much highe solubili y (and hence mobili y) o U ela ed o Th in su ace and nea -su ace en i onmen s. Toe sedimen ac iona ion and analysis p esen ed he e has made an unequi ocal connec ion ST1 ST2 SOT 0.127 ± 0.016 0.053 ± 0.006 0.102 ± 0.009 0.448 ± 0.040 0.152 ± 0.015 0.417 ± 0.032 1.94 ± 0.25 1.60 ± 0.14 1.04 ± 0.10 2.60 ± 0.27 1.52 ± 0.13 2.71 ± 0.30 be ween he enhanced U con en and espec i e Th/U mass a ios in i e sedimen s close o he e ilize ac o ies and he was es discha ge poin in he Odiel and Tin o i e s. Re e ences [1] R. Ches e and S.R. As on, The pa i ioning o ace me als and ansu anics in sedimen s, in Techniques o Iden i ying Specia ion in Aqua ic En i onmen s, IAEA, Vienna, 1981, pp. 173-193. [2] A. Ma inez-Agui e, M. Ga cia-León and M. l ano ich, Toe dis ibu ion o U, Th and 226Ra de i ed om he phospha e e ilize indus ies on an es ua ine sys em in Sou hwes Spain, J. En i on. Radioac i i y, 22 (1994) 155-177. [3] M. I ano ich and R.S. Ha mon, U anium Se ies Dise quilib ium: Applica ions o Ea h, Ma ine and En i on men al Sciences, Cla endon P ess, Ox: o d, UK, 2nd edn, 1992. [4] A. Ma inez-Agui e, Radiac i idad na u al en di e ses compa imen os na u ales de Andalucía, PhD hesis, Se ille Uni e si y, Spain, 1991 (in Spanish). [5] E.R. Sholko i z, Floccula ion o dissol ed o ganic and ino ganic ma e du ing he mixing o i e wa e and seawa e , Geochim. Cosmochim. Ac a, 40 (1976) 831-845. [6] A. Ma inez-Agui e and M. Ga cia-León, Na u al a dioac i i y enhancemen s by human ac i i ies in i e s o he Sou h o Spain, J. Radioanal. Nucl. Chem. Le ., 155 (1991) 97-106. [7] A. Ma inez-Agui e, M. Ga cia-León and M. I ano ich, Iden i ica ion and e ec s o an h opogenic emissions o U and Th on he composi ion o sedimen s in a i e /es ua ine sys em in Sou hem Spain, J. En i on. Radioac i i y, 23 (1994) 231-248. [8] J.J.W. Roge and J.A.S. Adams, Handbook o Geochem is y, Wedepohl Sp inge Ve lag, Be lin, 1969.