Radioactivity of phosphogypsum in the south-west of Spain

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 Downloaded om h ps://academic.oup.com/ pd/a icle/76/3/185/1631678 by Se illa Psych Lib B21 use on 15 Ap il 2024 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 Downloaded om h ps://academic.oup.com/ pd/a icle/76/3/185/1631678 by Se illa Psych Lib B21 use on 15 Ap il 2024 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 . Downloaded om h ps://academic.oup.com/ pd/a icle/76/3/185/1631678 by Se illa Psych Lib B21 use on 15 Ap il 2024 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 Downloaded om h ps://academic.oup.com/ pd/a icle/76/3/185/1631678 by Se illa Psych Lib B21 use on 15 Ap il 2024 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. REFERENCES 1. Bolı ´ a , J. P., Ga cı ´a-Teno io, R. and Ga cı ´a-Leo ´n, M. Fluxes and Dis ibu ion o Na u al Radionuclides in he P oduc ion and Use o Fe ilize s. App. Radia . Iso . 46, 717–718 (1995). 2. Bolı´ a , J. P., Ga cı´a-Teno io, R. and Ga cı´a-Leo´n, M. 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