185
Radia ion P o ec ion Dosime y TECHNICAL NOTE
Vol. 76, No. 3, pp. 185–189 (1998)
Nuclea Technology Publishing
RADIOACTIVITY OF PHOSPHOGYPSUM IN THE
SOUTH-WEST OF SPAIN
J. P. Bolı
´ a †, R. Ga cı
´a-Teno io‡ and J. Mas†
†Depa amen o de Fı
´sica Aplicada, E.P.S. La Rabida
Uni e sidad de Huel a, 21819-Palos de la F a. Huel a, Spain
‡Depa amen o de Fı
´sica Aplicada, E.T.S. A qui ec u a
A da Reina Me cedes s/n, 41012-Se illa, Spain
Recei ed June 24 1997, Amended No embe 25 1997, Accep ed Decembe 12 1997
Abs ac — Phosphogypsum is he by-p oduc o p oducing phospho ic acid by eac ing phospha e ock wi h sulphu ic acid. In
he sou h-wes o Spain, and nea he own o Huel a, he e is a big indus ial complex wi h some ac o ies de o ed o he
p oduc ion o phospho ic acid, which p oduce annually 3 ⫻10
6
me ic onnes o phosphogypsum. O his amoun 80% is s o ed
in nea by si es named ‘gyp-s acks’. The phospha e ock ea ed in hese ac o ies o he p oduc ion o phospho ic acid has
238
U
ac i i ies be ween 1000 and 1600 Bq.kg
⫺1
, being hei daugh e s in app oxima ely secula equilib ium. A ac ion o hese
adionuclides, and in di e en p opo ion, goes wi h he phosphogypsum. The adionuclide ac i i y concen a ions in a ious
phosphogypsum samples collec ed a di e en places in he gyp-s acks a e gi en, and an a e age dose a e in ai due o gamma
ays a a poin 1 m abo e hese a eas is also de e mined.
INTRODUCTION
In he sou h-wes o Spain, nea he own o Huel a,
and jus in he es ua y o med by he con luence o he
Odiel and Tin o i e mou hs, he e is a big indus ial
complex wi h se e al ac o ies de o ed o he p o-
duc ion o phospho ic acid om phospha ic ocks.
These sedimen a y ocks come om di e en places in
he wo ld (Mo occo, Senegal and Togo), and con ain
abou 1000–1600 Bq.kg
⫺1
o
238
U, wi h i s daugh e s
(
230
Th,
226
Ra,
210
Po, %) in nea secula equilib ium
(1)
.
This equilib ium is dis up ed when he phospha ic
ock is ea ed wi h sulphu ic acid o p oduce phos-
pho ic acid by he so-called ‘we acid me hod’
(2)
. In his
p ocess, in addi ion o he phospho ic acid, dy-hid a ed
calcium sulpha e is gene a ed as a by-p oduc , com-
monly known as phosphogypsum. Abou 3 ⫻10
6
me ic
onnes o phosphogypsum a e annually p oduced in he
ac o ies, 20% o his amoun being di ec ly eleased
in o he Odiel i e , while he emaining 80% is s o ed
in e y ex ensi e zones, named gyp-s acks, placed nea
he ac o ies.
F om p e ious wo ks i is known ha phospho-
gypsum con ains he same adionuclides, bu in di e en
p opo ions, as he phospha e ock
(2,3)
, and so he o me
has he po en ial o pollu e he en i onmen
adiologically
(4–6)
. In his wo k, he concen a ions o
se e al adionuclides om he U-se ies (
238
U,
234
U,
226
Ra and
210
Po) ha e been de e mined in samples
o phosphogypsum collec ed in di e en places o he
gyp-s acks, and a di e en dep hs, as a i s s ep o
s udy some o he p incipal adiological conce ns ela ed
o he phosphogypsum s o age: he possible con ami-
na ion o g oundwa e wi h
226
Ra
(7)
and/o
210
Po
(8)
. Sec-
ondly, and based on he in o ma ion ob ained, an a e -
age abso bed dose a e in ai a 1 m abo e hese zones
has been calcula ed, as a pa ial e alua ion o hei
adiological haza d. This calcula ed dose a e is com-
pa ed wi h measu emen s ca ied ou using an en i on-
men al dose a e me e a di e en ep esen a i e poin s
o he phosphogypsum piles.
MATERIAL AND METHODS
Collec ion
The samples o phosphogypsum we e collec ed om
some gyp-s acks (co e ing 4.5 ⫻10
6
m
2
) placed nea by
he ac o ies. A o al o se en poin s was chosen co e-
sponding o gyp-s acks o di e en ages depending on
hei o ma ion. In e e y poin wo samples o abou 1
o 2 kg we e collec ed, one co esponding o he uppe
10–20 cm, and he o he one a a dep h ⬎50 cm. The
illing o hese gyp-s acks was done o e long pe iods
o ime, and a he same ime phospha e ocks o e y
di e en o igins ha e been ea ed in hese ac o ies.
Wi h his sampling s a egy, he inhomogenei y o he
adionuclides dis ibu ion can be s udied ho izon ally
and e ically o e he wide a ea o he gyp-s acks.
The samples, a e collec ion, we e d ied and si ed
as a p elimina y s ep in adionuclide de e mina ion.
In si u measu emen s
An en i onmen al dose a e me e equiped wi h a
high p essu e ionisa ion chambe was used o measu e
expe imen ally he dose a es due o he gamma ays
emi ed by he ma e ial o he phosphogypsum piles.
The su ey me e eadings we e aken a 1 m abo e
g ound and in 15 loca ions dis ibu ed homogeneously
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J. P. BOLI
´VAR, R. GARCI
´A-TENORIO and J. MAS
186
in he gyp-s acks. A each loca ion a leas i e eadings
we e eco ded, and he a e age abso bed dose a e
calcula ed, sub ac ing a backg ound alue due o he
sensi i y o he sys em o cosmic adia ion and i s elec-
onic noise.
Radionuclide de e mina ion
Concen a ions o U iso opes,
210
Po ,
226
Ra,
228
Ra and
40
K ha e been de e mined in he di e en phospho-
gypsum samples. U iso opes and
210
Po we e de e mined
by alpha pa icle spec ome y, while he es we e
measu ed by gamma ay spec ome y.
F om 0.5 o 1 g o sample, he U iso opes and
210
Po
we e independen ly isola ed applying a liquid–liquid
ex ac ion p ocedu e wi h TBP
(5)
. Then, he U iso opes
we e elec odeposi ed on o s ainless s eel planche s
acco ding o he me hod o Has alldius
(9)
, while he
polonium was sel -deposi ed on o sil e discs in a 2M
HCl medium
(10)
. Chemical eco e ies we e de e mined
adding, p e iously o he ea men o he samples,
known amoun s o
208
Po and
232
U. The alpha pa icle
spec ome y was pe o med using ou ion-implan ed
silicon de ec o s o 350 mm
2
in chambe s whe e a
acuum o 苲10
⫺1
mm Hg is kep du ing he measu e-
men s. Sys ema ic backg ound uns we e made o con-
ol possible con amina ion.
Speci ic ac i i ies o
226
Ra,
228
Ra and
40
K we e
de e mined by gamma ay spec ome y, using a HPGe
coaxial de ec o , 14% ela i e e iciency, and sensi i e
in he ange o 100 keV-2MeV. The measu emen s we e
done using a ixed geome y (cylind ical con aine s o
heigh 5 cm and diame e 6.5 cm), and a special cali-
b a ion p ocedu e o his geome y was ollowed. This
e iciency calib a ion akes in o accoun he sel -
abso p ion co ec ions a di e en ene gies ha mus be
applied due o a ia ions o densi ies be ween he ana-
lysed samples and he calib a ion sample
(11)
.
The
226
Ra was de e mined by aking he pho opeak
o i s daugh e nuclide
214
Pb a 352 keV, while he
228
Ra
was measu ed h ough he 911 keV gamma emission
om i s daugh e
228
Ac, and he
40
K by i s emission a
1460 keV. To ensu e he exis ence o secula equilib-
ium be ween
226
Ra and i s daugh e s, he samples we e
sealed and s o ed a leas o h ee weeks be o e he
measu emen s.
All he me hods ha ha e been desc ibed we e ali-
da ed h ough he pa icipa ion in di e en na ional and
in e na ional in e compa ison exe cises wi h succes ul
esul s as, o example, he IAEA-135 sedimen sample.
RESULTS AND DISCUSSION
Radionuclide concen a ions
In Table 1 he ac i i y concen a ions o he di e en
adionuclides de e mined in he supe icial (S) and
deepe s (D) phosphogypsum samples a e compiled.
F om hese esul s, and as was expec ed, i is possible
o deduce he exis ence o high concen a ions o adio-
nuclides belonging o he U se ies. Con e sely, low con-
cen a ions o
228
Ra (membe o he Th se ies) and
40
K
we e de e mined, in ag eemen wi h p e ious esul s,
which indica es ha hei speci ic ac i i ies in he phos-
pha e ocks a e ei he simila o lowe han in ypical
soils
(2)
.
High
226
Ra/
238
U and
210
Po/
238
U ac i i y a ios a e
ound in mos samples, showing he endency o
226
Ra
and
210
Po o go wi h he phosphogypsum in he we -acid
me hod, while he u anium gene ally ends o emain in
he phospho ic acid ac ion
(2)
.
In ac , based on he ac ha he ideal s oichome ic
phospha ic ock/phosphogypsum mass a io is 0.59, i
can easily be deduced ha mo e han 80% o
226
Ra o ig-
inally p esen in he phospha e ock emains in he phos-
phogypsum. In addi ion, and due o he hal -li e o
210
Po
(134.8 days), he endency o i s p ogeni o ,
210
Pb, o go
wi h he phosphogypsum ac ion can also be deduced,
because he
210
Po/
226
Ra ac i i y a ios we e nea one in
he analysed samples, i espec i e o he age o phos-
phogypsum o ma ion and s o age.
A clea conclusion om he obse ed da a is also he
high dispe sion o speci ic ac i i ies o he adio-
nuclides o he U se ies (as an example,
238
U concen-
a ions ange om 100 o 600 Bq.kg
⫺1
). This ac can-
no be conside ed su p ising in iew o he di e en
ypes o mine als ea ed in he ac o ies, and he poss-
ible di e en e ec i eness o he acidula ion me hod,
du ing he yea s ha his p ocess has been ca ied ou .
In his sense, i is no su p ising ha he U iso opes
ac i i y concen a ions in sample 4D, al hough i
di e ges om he gene al endency o U ac iona ion
o phospho ic acid: i can be ela ed simply o he inco -
po a ion o un eac ed phospha e ock and/o o e sa u -
a ion and e-p ecipi a ion o calcium luo apa i e which
can occu in he acidula ion p ocess, as was es ablished
by Hull and Bu ne
(3)
.
Ano he in e es ing ea u e can be deduced om he
da a. In mos o he collec ion poin s he
226
Ra and
210
Po
ac i i y a ios be ween he supe icial and he deepe
samples is lowe o equal o one. E en aking in o
conside a ion he inhomogenei y o he adionuclide dis-
ibu ion, he gene al endency gi es e idence o he
exis ence o some
226
Ra and
210
Po en ichmen in he
deepe laye s. Th ough a mo e sys ema ic s udy in phos-
phogysum co es, now in p og ess, an a emp is being
made o explain he p ocesses (leaching, mig a ion,
inco po a ion om he wa e s used o conduc he esh
phosphogypsum o he piles, e c.) ha can be espon-
sible o his
226
Ra and
210
Po en ichmen in he deep
phosphogypsum samples.
Dosime ic implica ions
As a way o show he adiological impo ance o he
gyp-s acks, and based on he esul s p e iously ob ained
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RADIOACTIVITY IN PHOSPHOGYPSUM IN SW SPAIN
187
abou adionuclide concen a ions, he a e age gamma
abso bed dose a e, dD/d , in ai a a poin 1 m abo e
hese zones due o hei gamma ay emissions has
been calcula ed.
To do hese calcula ions, we assume he piles o be
a semi-in ini e olume wi h adionuclides dis ibu ed
uni o mly
(12)
. Ob iously, he adionuclides om he U
se ies, Th se ies and
40
K a e he mos impo an con-
ibu o s o he gamma abso bed dose a e. Then:
dD
d =dD
d (U se ies) +dD
d (Th se ies) +dD
d (
40
K) (1)
The alues o he gamma abso bed dose a e, dD/d in
Gy.y
⫺1
, o e e y gamma emi e , will be de e mined
knowing i s concen a ion C(Bq.cm
⫺3
) in he piles by
he exp ession:
dD
d =C⫻DRF (2)
DRF being he dose a e ac o s, in (Gy.y
⫺1
)
(Bq.cm
⫺3
)
⫺1
, whose alues depend on he ene gies o
he gamma emissions, he geome y and he dis ibu ion
o adionuclides in he sou ce.
Koche and Sjo een
(13)
ha e published he alues o
dose a e con e sion ac o s o monoene ge ic sou ces
be ween 0.01 MeV and 3 MeV, unde he same hypo-
heses o ex ension and uni o mi y in he sou ce as in
his wo k. The alidi y o his model o calcula ing he
dose a e has been e i ied by se e al wo ke s whose
p edic ions a e e y simila o he dose a es ob ained
om expe imen al measu emen s
(14)
. Then, using hese
alues, and conside ing in e e y se ies he exis ence o
secula equilib ium be ween he daugh e s o sho hal -
li e and hei p ogeni o s, bu no he exis ence o o al
secula equilib ium be ween all he adionuclides o
each na u al se ies, as i can easily be obse ed om
Table 1. Ac i i y concen a ions (Bq.kg
ⴚ1
) o he di e en adionuclides de e mined in he supe icial (S) and deepe (D)
phosphogypsum samples.
Sample
238
U
235
U
234
U
226
Ra
210
Po
228
Ra
40
K
228
Th
1S 111 ⫾7 7.4 ⫾0.5 104 ⫾7 336 ⫾18 392 ⫾11 ⬍12 ⬍49 ⬍8
1D 117 ⫾10 11.7 ⫾2.1 125 ⫾11 506 ⫾26 645 ⫾13 ⬍9⬍43 ⬍3
2S 107 ⫾23 6.5 ⫾2.5 115 ⫾20 465 ⫾24 345 ⫾13 ⬍4⬍49 ⬍3
2D 270 ⫾14 13.5 ⫾0.7 253 ⫾14 957 ⫾50 793 ⫾70 ⬍17 ⬍67 ⬍3
3S 290 ⫾15 9.1 ⫾1.6 296 ⫾15 861 ⫾45 550 ⫾25 ⬍12 ⬍47 ⬍3
3D 196 ⫾17 8.5 ⫾2.2 226 ⫾20 483 ⫾25 697 ⫾34 ⬍9⬍44 ⬍3
4S 161 ⫾14 7.0 ⫾1.8 161 ⫾14 339 ⫾20 451 ⫾22 ⬍12 ⬍50 ⬍3
4D 619 ⫾25 20.7 ⫾2.3 620 ⫾26 1220 ⫾63 794 ⫾26 ⬍27 ⬍85 ⬍4
5S 138 ⫾14 6.2 ⫾1.1 136 ⫾14 430 ⫾22 544 ⫾64 ⬍12 ⬍51 ⬍9
5D 133 ⫾16 6.1 ⫾1.2 121 ⫾15 530 ⫾28 428 ⫾38 ⬍4⬍56 ⬍3
6S 171 ⫾15 8.1 ⫾1.5 179 ⫾15 471 ⫾25 149 ⫾14 ⬍5⬍66 ⬍4
6D 185 ⫾14 7.0 ⫾1.5 196 ⫾15 475 ⫾25 681 ⫾23 ⬍9⬍46 ⬍3
7S 170 ⫾14 6.0 ⫾1.5 176 ⫾14 481 ⫾25 448 ⫾21 ⬍10 ⬍46 ⬍3
7D 154 ⫾14 6.4 ⫾1.6 150 ⫾14 651 ⫾34 1577 ⫾170 ⬍5⬍67 ⬍4
he expe imen al alues shown in he p e ious sec ion,
we can ob ain:
dD
d (
238
U se ies) =1.71 ⫻10
⫺5
C(
238
U)
+1.05 ⫻10
⫺7
C(
230
Th) +2.97 ⫻10
⫺3
(3)
C(
226
Ra) +4.62 ⫻10
⫺7
C(
210
Pb)
dD
d (
232
Th se ies) =3.87 ⫻10
⫺8
C(
232
Th) +1.51 ⫻10
⫺3
(4)
C(
228
Ra) +2.30 ⫻10
⫺3
C(
228
Th)
dD
d (
40
K) =2.69 ⫻10
⫺4
C(
40
K) (5)
Bu om he gene al concen a ions de e mined in phos-
phogypsum, i is known ha he concen a ions o
234
U,
210
Pb and
230
Th a e ei he lowe o , in ex eme cases,
sligh ly highe han he concen a ions o
226
Ra. In he
same way, i can be expec ed ha bo h ac i i y concen-
a ions o
232
Th and
228
Th in he phosphogypsum a e
going o be e y simila . These ac o s allow us o sim-
pli y he Equa ions 3, 4 and 5 o:
dD
d (U se ies) =1.71 ⫻10
⫺5
C(
238
U)
+2.97 ⫻10
⫺3
C(
226
Ra) (6)
dD
d (Th se ies) =1.51 ⫻10
⫺3
C(
228
Ra)
+2.30 ⫻10
⫺3
C(
228
Th) (7)
dD
d (
40
K) =2.69 ⫻10
⫺4
C(
40
K) (8)
being he DRF exp essed in (Gy.y
⫺1
)(Bq.cm
⫺3
)
⫺1
.
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J. P. BOLI
´VAR, R. GARCI
´A-TENORIO and J. MAS
188
Then, i we assume an a e age densi y o he phos-
phogypsum sample o 1.25 g.cm
⫺3
and as a e age and
ep esen a i e alues o he needed concen a ions in he
phosphogypsum he alues shown in Table 2, he
gamma abso bed dose a e, dD/d , o e e y se ies, and
he o al will be:
dD
d (U se ies) =2.18 mGy.y
⫺1
(9)
dD
d (Th se ies) =3.23 ⫻10
⫺2
mGy.y
⫺1
(10)
dD
d (
40
K) =1.86 ⫻10
⫺2
mGy.y
⫺1
(11)
dD
d (To al) =2.23 mGy.y
⫺1
(12)
This las alue is abou six imes highe han he a e age
ob ained om a wo ldwide ypical soil.
The alidi y o he model used, and consequen ly he
e i ica ion o he a e age calcula ed dose a e, has been
pe o med by measu emen s in si u using he en i on-
men al dose a e me e . The alues ob ained a di e en
places in he gyp-s acks a e shown in Table 3, being
he a e age dose a es in o al conco dance wi h hose
calcula ed om he model. Also, i is possible o
obse e a ela i ely high dispe sion o he indi idual
alues, which migh be expec ed due o he e i ied non-
homogeneous dis ibu ion o he adionuclides in he
phosphogypsum piles.
In Table 4 he con ibu ion is shown o he gamma
abso bed dose a e om he di e en adionuclides (and
i s sho -li e daugh e in secula equilib ium) o he pho-
sphogypsum piles deduced om he model calcula ion.
Looking a his able i can be seen ha he main p ob-
Table 2. A e age alues o ac i i y concen a ions (Bq.kg
ⴚ1
)
o he adionuclides used in he es ima ion o he a e age
gamma abso bed dose a e in ai a 1 m abo e he g ound.
238
U
226
Ra
40
K
228
Ra
228
Th
210
Pb
202 587 55 11 4 607
Table 3. Gamma abso bed dose a es (mGy.y
ⴚ1
) a 1 m heigh abo e g ound measu ed using an en i onmen al dose a e
me e in he 15 selec ed zones om he gyp-s acks.
D1 D2 D3 D4 D5 D6 D7 D8
3.09 3.35 2.93 2.79 3.46 3.14 2.26 2.07
D9 D10 D11 D12 D13 D14 D15
2.18 1.83 1.93 1.98 1.76 2.56 1.88
A e age dose =2.48 ⫾0.15 mGy.y
⫺1
lem associa ed wi h he phosphogypsum s o age is i s
con en o
226
Ra, and i s decay o
222
Rn ha can emana e
om he piles in o he a mosphe e inc easing i s po en-
ial adiological haza d. Conce ning his las poin i is
in e es ing o no e ha Hull and Bu ne
(3)
in one o hei
s udies done abou Flo ida gyp-s acks ha e de e mined
ha he loss o
222
Rn om he phosphogypsum in he
ield is less han 10%, much less impo an han ha
deduced om labo a o y measu emen s. These au ho s
explain his ac indica ing ha he
222
Rn emana ion
om he gyp-s acks is p obably impeded by su ace
c us , which esul s om compac a ion and ec ys alis-
a ion o phosphogypsum. These su ace c us s we e
obse ed in he ex ensi e piles si ed in Huel a when he
sampling in his wo k was ca ied ou .
CONCLUSIONS
High concen a ions o adionuclides belonging o he
U se ies (specially
226
Ra and
210
Pb-
210
Po) ha e been
de e mined in samples collec ed in phosphogypsum
piles, o med as a way o s o age o he main by-p oduc
associa ed wi h he phospho ic acid p oduc ion.
These gyp-s acks, which co e an a ea o 4.5 ⫻10
6
m
2
in he su oundings o some e ilise ac o ies in SW
Spain, cons i u e a clea adiological anomaly. As a way
o show he adiological impo ance o hese phospho-
gypsum piles, he abso bed dose a e a 1 m abo e he
Table 4. Con ibu ion o he gamma abso bed dose a e
(mGy.y
ⴚ1
) a 1 m abo e g ound om he na u al adio-
nuclides in secula equilib ium wi h hei daugh e s o he
phosphogypsum piles.
Radionuclide Con ibu ion Pe cen age
(mGy.y
⫺1
) o o al
238
U 0.00 0.0
226
Ra 2.18 97.8
228
Ra 0.02 0.9
228
Th 0.01 0.4
40
K 0.02 0.9
To al 2.23 100.0
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RADIOACTIVITY IN PHOSPHOGYPSUM IN SW SPAIN
189
zones due o i s gamma emissions ha e been de e mined
ei he expe imen ally (using an en i onmen al dose a e
me e ) o by calcula ion om he adionuclide ac i i y
concen a ions applying a simple model. The esul s
ob ained by bo h me hods a e in ag eemen , he gamma
abso bed dose a es being six imes highe han alues
ob ained om a wo ldwide ypical soil.
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ACKNOWLEDGEMENTS
The au ho s a e g a e ul o M Jesu
´s Gonza
´lez, M
Juan Luis Aguado, M En ique Gu ie
´ ez and M
San iago de Paz om Uni e si y o Huel a o hei
assis ence in ca ying ou he alpha spec ome ic de e -
mina ions. Financial suppo by he Agencia de Medio
Ambien e (Jun a de Andalucı
´a) is also specially
acknowledged.
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