Dis ibu ion o na u al adionuclides in
sequen ially ex ac ed ac ions o sedimen s
om a ma sh a ea in Sou hwes Spain:
U iso opes
A. Ma ı´ nez-Agui e, R. Pe ian ez
Dep . Fı&sica Aplicada, EUITA, Uni e sidad de Se illa, C a. U e a km 1, 41013 Se illa, Spain
Abs ac
A s udy o he dis ibu ion o U in di e en ac ions o in e idal sedimen s collec ed in
a ma sh a ea enhanced by he adioac i e was es discha ged om a e ilize complex has been
ca ied ou . The s udy has shown ha U iso opes a e mainly loca ed in non- esidual ac ions o
he sedimen s. Cop ecipi a ion wi h amo phous e omanganese oxyhyd oxides is he main
p ocess o inco po a ion o his elemen om he wa e column in o he sedimen . The esidual
ac ions only con ain some 5% o he o al U in he sedimen . Mo eo e , all ac ions seem o
e lec he same pa e n o con amina ion, wi h in ac a qui e good ela ionship be ween he
U in mos ex ac ed ac ions. Thus, hose sedimen s loca ed along he Odiel i e basin ha e
highe U concen a ions in all he sequen ially ex ac ed ac ions. Concen a ion a ios o
Spa ina densi lo a ha e also been s udied. In his pape , i has been conside ed ha no all he
subs a e sedimen akes pa in he up ake o elemen s by he plan bu only he non- esidual
ac ions. Thus, a s udy o he dependence o hese concen a ion a ios on he concen a ion in
he non- esidual ac ion o sedimen s has been ca ied ou . 1999 Else ie Science L d. All
igh s ese ed.
1. In oduc ion
On a his o ical ime scale, sedimen s may be ega ded as 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. They became an en i onmen al hos o many
o he was e p oduc s discha ged by socie y. Once a subs ance has been inco po a ed
in o sedimen s, i s ul ima e a e depends on a numbe o e y complex ac o s. An
elemen can be locked pe manen ly in o a sedimen componen o i may sub-
sequen ly be eleased and ake pa in he a ious bio-geochemical eac ions. The
abili y o p edic he subsequen a e is one o he key ac o s in he assesmen o he
e ec o en i onmen al pollu ion. One o he undamen al dis inc ions in sedimen a y
geochemis y is ha be ween he la ice-held (i.e. esidual) and non-la ice-held (i.e.
non- esidual) ac ions o a sedimen . Elemen s loca ed in la ice si es can be con-
side ed o be en i onmen ally immobile, whe eas hose in non-la ice si es can be
conside ed 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 /in e s i ial wa e sys em (Ches e
& As on, 1981).
The dis inc ion o elemen s be ween esidual and non- esidual ypes is pa icula ly
app op ia e o he in es iga ion o adionuclide pollu ion. This is because he p o-
cesses which a e mainly in ol ed in he inco po a ion o hese elemen s om pollu ed
wa e s esul in hei loca ion in he non- esidual ac ions o he sedimen s (Ches e
& As on, 1981). The esidence o he elemen s in hese deposi s 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
hese eleases occu depends la gely on he manne in which he elemen is bound o
he sedimen and on how he pa icula kinds o bonds eac o a ious physico-
chemical condi ions. Thus, i is desi able o know he way(s) in which he elemen s a e
bound o he solid componen s in pa icula sedimen popula ions. The chemical
sepa a ion o a ious componen s in o di e en ac ions o e s he mos e ec i e
app oach o he s udy o elemen pa i ioning in sedimen s because he p ocesses by
which mos adionuclides a e inco po a ed in o he a ious componen s a e chemical.
Du ing ecen yea s se e al s udies ha e been ca ied ou on sedimen /soil o
mine al specia ion. Some o hese we e cen e ed on a i icial adionuclides, mos ly
in ol ing hose eleased by he Sella ield ep ocessing plan (B own e al., 1997) and
some o he s on na u al adionuclides in i e sedimen s o ocks (Pla e e al., 1992;
C espo e al., 1996). The aim o hese s udies was mainly o e alua e he mobili y o
hese adionuclides in he solid ma ix and hei inco po a ion om he liquid phase
in con ac wi h he solid.
The abo e conside a ions led o he s udy o U and Th iso ope concen a ions in
se e al chemical ac ions o bo om sedimen s collec ed along he Odiel and Tin o
i e basins (Ma ı´nez-Agui e e al., 1994b). As has been widely poin ed ou du ing
ecen yea s (Ma ı´nez-Agui e e al., 1994a, b), bo h i e s a e loca ed in an a ea
clea ly enhanced by non-nuclea indus ies, basically by he e ilize indus ies. In he
abo e s udy, mos U and Th iso opes we e ound o be co-p ecipi a ed wi h amo ph-
ous Fe and Mn oxyhyd oxides in o bo om sedimen s. Howe e , close o he ac o ies
he e was also a la ge con en o hese iso opes in he esidual ac ion o he sedimen s
(mainly in he case o Th). This was a ibu ed o he eleases o small esidual pa icles
om he phospha e ock p ocessing plan o he i e basin.
Recen ly enhancemen o U and Th iso opes was also ound in in e idal sedimen
samples collec ed in he ma sh a ea loca ed opposi e o he ac o ies along he Odiel
i e (Ma ı´nez-Agui e & Ga cı´a-Leo´ n, 1997). This enhancemen was a ibu ed o
wa e mo emen in o he a ea. Thus, he main objec i e o his pape is o iden i y he
loca ion o his con amina ion (U iso opes) in he a ious chemical ac ions o
sedimen s. This s udy would allow us o unde s and mo e abou he capaci y o his
adionuclide o mo e ac oss he a ea i any en i onmen al change occu s in he
sys em. Mo eo e , he ans e o his adionuclide om di e en chemical ac ions o
sedimen s o he plan Spa ina densi lo a has been also s udied. In a p e ious pape
(Ma ı´nez-Agui e e al., 1997), he ans e was s udied h ough he CR (concen a-
ion a ios), de ined as he a io be ween he concen a ion o an elemen in he plan
and he concen a ion o he same elemen in he subs a e sedimen . In ha s udy,
a clea non-linea dependence o he CR on he o al concen a ion in he sedimen o
he adionuclides o he U decay chain was ound. In his pape , he up ake o U by
he plan is s udied by conside ing only hose ac ions o he sedimen s which a e
mobile ( he non- esidual ac ion) and hus po en ially a ailable o plan up ake.
2. Samples and adiochemical p ocedu es
Sampling was ca ied ou in Feb ua y 1993. Se e al s a ions we e selec ed ac oss
he Odiel ma sh (see Fig. 1), which is clea ly a ec ed by he in low o he Odiel
i e wa e s. Samples (plan and subs a e) we e collec ed du ing low ide a he bo de
o he channel, in a eas co e ed wi h wa e du ing high ide. In he labo a o y,
sedimen samples we e d ied and powde ed be o e analysis. The bulk d y densi y
(g cm ) and pe cen age o o ganic ma e we e de e mined in each sedimen sample
and a e gi en in Ma ı´nez-Agui e e al. (1997). Spa ina densi lo a samples we e
collec ed a he same s a ions and a he same ime as sedimen samples. The oo s o
he plan s we e sepa a ed in o de o s udy only he abo e-g ound pa o he plan s.
Once sepa a ed, he samples we e washed wi h wa e o emo e any ace o solid
pa icles. Finally, hey we e ai -d ied and powde ed o analysis.
Two o h ee g ams o each homogenized sedimen sample we e used o sequen ial
ex ac ion. The me hod used o his sequen ial ex ac ion was o mula ed by Sho
(1986). The o iginal me hod (I ano ich e al., 1988) was modi ied ollowing C espo
e al. (1996). In he s udied sedimen s, he ion-exchangeable ac ion has been a oided
due o i s low U concen a ion in all cases. A e he ex ac ion o each ac ion, he esidue
was washed wi h dis illed wa e and he solu ion added o he p e ious supe na e.
(1) F1"º wi hin ca bona es: The o iginal sedimen sample is shaken o 6 h in
1 M sodium ace a e adjus ed o pH"5.0 wi h ace ic acid ( a io"1/20). The supe -
na e is ese ed o analysis (Nissembaum, 1972; Tessie e al., 1979).
(2) F2"adso bed (usually humic ma e ial and/o amo phous oxides): The esidue
om he p e ious ex ac ion is shaken o 1 h in 0.1 M sodium py ophospha e
solu ion adjus ed o pH"9.8 wi h 0.1 M NaOH ( a io"1/10). The supe na e is
ese ed o analysis (I ano ich e al., 1988).
(3) F3"º cop ecipi a ed wi h amo phous Fe and Mn oxyhyd oxides: The esidue
om he p e ious phase is shaken o 4 h in he da k in Tamm’s acid oxala e (0.2 M
ammonium oxala e and 0.2 M oxalic acid) ( a io"1/10). The supe na e is ese ed
o analysis (McKeague & Day, 1966).
Fig. 1. Loca ion o he sampling s a ions in he Odiel ma sh a ea in sou hwes Spain.
(4) F4"º wi hin c ys alline Fe and Mn oxides: The esidue om he p e ious
phase is shaken o 1 h in Co in’s eagen (0.175 M sodium ci a e and 0.025 M ci ic
acid) ( a io"1/10). The esidue is ese ed o analysis (Ande son & Jenne, 1970).
(5) F5"º wi hin p ima y mine als and clays"Resis an phase: The inal esidue is
dissol ed wi h HNO, aqua egia and HClOusing a mic owa e o en. The supe na e
is ese ed o analysis.
Table 1
Pe cen age o U concen a ion in each sequen ially ex ac ed ac ion in soils om he Odiel Ma sh. In
he las column he o al U concen a ions in he soils a e also gi en
Code F1 (%) F2 (%) F3 (%) F4 (%) F5 (%) To al (mBq g )
M3 13.3 7.6 75.7 1.4 2.0 767$38
M23 21.6 15.2 58.9 4.3 261$10
M4 12.8 9.0 74.7 1.9 1.5 566$16
M22 22.5 11.6 45.9 8.2 11.7 35.5$1.3
M21 27.7 1.4 49.6 7.6 13.7 17.7$0.8
M20 18.9 13.2 62.6 1.7 3.6 248$6
M18 22.4 6.5 68.4 1.2 1.6 541$36
M17 22.4 29.0 31.2 3.3 14.1 28.2$3.0
M16 16.0 10.4 69.6 1.5 2.4 483$17
M15 19.3 7.7 67.8 1.7 3.5 91.3$3.8
M5 14.7 10.7 67.1 5.4 2.2 249$7
M6 22.7 12.5 58.2 2.6 4.0 130$4
M9 20.3 13.2 60.8 3.3 2.6 134$4
M11 27.3 10.1 55.5 4.4 2.8 83.3$2.5
M12 34.6 15.3 34.6 6.0 9.7 43.1$1.3
M14 39.4 13.2 34.1 5.5 7.6 43.1$1.5
F ac ions F1—F3 a e e e ed o as seconda y phases expec ed o ep esen he
non- esidual pa o he sedimen , being ei he an end-p oduc de i ed om he i e
wa e /sedimen in e ac ion and exchange p ocesses o pa o he solid capable o
di ec in e ac ion wi h wa e and hus sha ing some geochemical and iso opic cha ac-
e is ics wi h he liquid phase. F ac ions F4 and F5 a e de ined as esidual phases and
a e no expec ed o in e ac on a sho ime scale wi h he liquid phase unde no mal
en i onmen al condi ions and hus aking ela i ely mino pa in di ec in e ac ions
on medium o sho ime scales.
Once he ac ions we e sepa a ed, he supe na es we e spiked wi h well-known
U ac i i ies and an anion-exchangeable esin me hod (Dowex AG1-X8 chlo ide
o m) was used o U iso ope sepa a ion (Ma ı´nez-Agui e, 1994a). Once sepa a ed
and pu i ied, elec opla ing on o s ainless-s eel planche s was ca ied ou . The plan-
che s we e inally coun ed by a-spec ome y wi h ion-implan ed de ec o s.
The d ied plan samples we e dissol ed in HNOand he same adiochemical
me hod o U ex ac ion (as o sedimen ac ions) was used. Mo e de ails on he
chemical p ocedu es can be seen in Ma ı´nez-Agui e e al. (1997).
3. Resul s and discussion
The pe cen age o he ac i i y concen a ion o U in each sequen ially ex ac ed
ac ion om each sample a e p esen ed in Table 1. In he las column o his able, he
o al U concen a ion (mBq g ), ob ained by he addi ion o he U concen a-
ion in each ex ac ed phase, is also gi en.

Fig. 2. U concen a ions in mBq g  in non- esidual ac ions o soils om he Odiel ma sh. F3 (䊐), F2
(䊉) and F1 (䊊). The e o ba s a e 1pdue o coun ing s a is ics. E o ba s o ac ions F1 and F2 a e
included in he poin s.
The da a in Table 1 show ha U is mainly associa ed wi h he non- esidual
ac ions o he sedimen s, pa icula ly wi h he F3 ac ion. Wi h he excep ion o
samples M12 and M14, collec ed along he Pun a Umb ia i e , 50% o mo e o he
o al U is associa ed wi h ac ion F3. This implies ha he main p ocess by which U is
inco po a ed in o he sedimen s is by cop ecipi a ion wi h amo phous e oman-
ganese oxyhyd oxides, which is a ypical p ocess in es ua ine en i onmen s (Shol-
ko i z, 1976). O he o he wo non- esidual ac ions, he ca bona e ac ion (F1)
seems o con ibu e mo e ( ange 13—40%) o he o al U in he sedimen s han he
abso bed ac ion (F2) ( ange 2—15 %) wi h he excep ion o samples M17, M12 and M14,
in which he U seems o be mo e equally dis ibu ed among all he non- esidual ac ions.
In Fig. 2, he concen a ions o U in all samples ha e been plo ed o ac ions
F1, F2 and F3. As can be seen, all non- esidual ac ions beha e in he same way, wi h
maximum concen a ions in samples M16, M18, M3 and M4 and minimal concen a-
ions in samples M15, M17, M21 and M22. As was poin ed ou in a p e ious pape
(Ma ı´nez-Agui e e al., 1997a), he la e samples con ained a la ge amoun o sandy
pa icles. Rela i e maxima a e also obse ed in samples M20 and M23. The main
conclusion is ha all samples collec ed along he Odiel i e ma gin show highe
concen a ions in he o al sedimen s as well as in all non- esidual ac ions, whe eas
hose samples wi h a highe con en o sandy pa icles ha e he minimal U concen a-
ions in o al sedimen s and in each non- esidual ac ion.
The same dis ibu ion pa e n o all sedimen s in he non- esidual ac ions can be
clea ly obse ed in Fig. 3. In his igu e, he U concen a ion in each non- esidual
phase is plo ed e sus he o al concen a ion in he non- esidual ac ion o he
Fig. 3. U concen a ions in each non- esidual ac ion e sus he U concen a ion in he o al
non- esidual ac ion o soils om he Odiel ma sh. Fi ing unc ions a e also plo ed. The con inuous line is
he hype bolic i ing and he dashed line he linea i ing.
sedimen s, conside ed as he addi ion o concen a ion in F1#F2#F3. As can be
seen, he concen a ion o U in each phase is linea ly ela ed o he o al U con-
cen a ion in he non- esidual ac ion o he sedimen s, wi h eg ession coe icien s
abo e 0.9 in all cases.
Fig. 4. (con inued opposi e).
Mo eo e , each non- esidual phase and he o al non- esidual ac ion o he
sedimen s a e also linea ly ela ed qui e well o he o al concen a ion in he
sedimen s, as can be seen in Fig. 4.
Thus, i he slopes o hese lines a e conside ed as indica ing he con ibu ion o
each ac ion o he o al U concen a ion in he non- esidual ac ion o he
sedimen s, i seems ha he ca bona e ac ion con ibu es 13%, he abso bed ac ion
7.9% and he amo phous ac ion 77.4%. Indeed, he h ee ac ions (by conside ing
he slope o he lines) gi e 98.2% (p ac ically 100%) o he non- esidual ac ion o he
sedimen s. Mo eo e , as could be expec ed, he h ee non- esidual ac ions a e also
linea ly ela ed o each o he (see Table 2). Howe e , he abo e conclusions do no
hold o hose samples wi h low U concen a ions. This can be deduced because in
all cases he y in e cep di e s om ze o, e en conside ing i s e o .
As commen ed on abo e, e en conside ing ha hese lines a e e lec ing qui e well
he ela ionship be ween phases, in mos cases hose samples wi h low concen a ion
a e no so well i ed o he lines and in ac , in mos cases, he y-in e cep abo e uni y
Fig. 4. U concen a ions in each non- esidual ac ion e sus he o al U concen a ions in he soils
om he Odiel ma sh. Fi ing unc ions a e also plo ed. The con inuous line is he hype bolic i ing and
he dashed line he linea i ing.
Table 2
Resul s ob ained in i ing he U concen a ions in di e en soil ac ions by linea and hype bolic
unc ions. ¹is he o al soil concen a ion and R he o al esidual concen a ion
Linea i ing y"A#Bx Hype bolic i ing y"ax@
xyA B ab
F3 F1 12.7$2.9 0.163$0.013 0.961 1.82$0.41 0.630$0.052 0.959
F3 F2 7.2$2.5 0.101$0.011 0.926 1.10$0.35 0.623$0.069 0.928
F2 F1 4.74$2.72 1.44$0.09 0.980 3.50$0.56 0.774$0.053 0.975
RF4 !0.082$0.56 0.435$0.045 0.9414 0.321$0.104 1.105$0.143 0.9123
RF5 0.221$0.544 0.557$0.044 0.9651 0.671$0.136 0.934$0.090 0.9484
¹F4 1.81$43 0.012$0.001 0.9342 0.467$0.123 0.450$0.052 0.9399
¹F5 2.54$0.44 0.017$0.002 0.9575 0.745$0.258 0.415$0.073 0.8732
¹R4.72$0.61 0.026$0.002 0.9725 1.097$0.204 0.442$0.037 0.9603