Journal of Environmental Protection, 2011, 2, 168-174
doi:10.4236/jep.2011.22019 Published Online April 2011 (http://www.SciRP.org/journal/jep)
Radiochemical Characterization of Phosphogypsum
for Engineering Use
Hanan Tayibi1, Catalina Gascó2, Nuria Navarro2, Aurora López-Delgado1, Mohamed Choura3,
Francisco J. Alguacil1, Félix A. López1*
1National Centre for Metallurgical Research (CENIM), CSIC. Avda. Gregorio del Amo, Madrid, Spain; 2Centro de Investigaciones
Energéticas, Medioambientales y Tecnológicas (CIEMAT). Avda. Complutense, Madrid, Spain; 3National Engineering School, Sfax
University, Sfax, Tunisia.
Received November 22nd, 2010; revised January 17th, 2011; accepted March 7th, 2011.
ABSTRACT
The new phosphogypsum (PG) waste management policy allowed to reduce the negative environmental impact of this
residue by finding better alternatives uses with an extremely limited radiological impact. Building material could be
one of these alternatives that could lead to the production of final products with good mechanical properties and very
limited radionuclides content. The optimization of the radioactive levels in the building materials when PG is used for
its production requires the previous knowledge of the content of naturally occurring radionuclides in the PG waste.
This article aims the radioactive characterization of two different PG sources (from Spain (Fertiberia S.A., Huelva) and
Tunisia (Sfax), before being incorporated in building materials. For this purpose, the natural selected radionuclides
content belongin g to uranium and thorium decay series and 40K was determined, by means of two different methods: i)
gamma spectrometry with high-purity germanium detectors and ii) laser-induced kinetic phosphorimetry (KPA-11
Chemcheck Instruments Inc., Richland, WA). Also, the semiquantitative chemical composition, the mineralogical study
and the morphological aspect of the PG samples were analysed. The results obtained from both techniques show that
226Ra and 210Po are the main source of the radioactivity in both studied PG samples. However, PG samples from
Tunisia present low natural radionuclide levels (30.7 Bq·kg–1 average value for 238U, 188 Bq·kg–1(226Ra), 163
Bq·kg–1(210Pb), 12.4 Bq·kg–1 (232Th) compared to the level of natural radionuclides in PG samples from Huelva (102
Bq·kg–1 average value for 238U, 520 Bq·kg–1(226Ra ), 881 Bq·kg-1(210Pb) and 8 Bq·kg-1 (232 Th). Both PG fulfil European
Commission Recomm endation (ECR) for the maximum activity con centrations of naturally-occurring rad ionuclides for
industrial by product used in building materials in the European Union.
Keywords: Phosphogypsum, Radionuclide s An alysi s , Phosphate Industry, Gamma Spectrometry, TENORM
1. Introduction
Phosphogypsum (PG) is an industrial residue from
processing phosphate rock using the “wet acid” process
to produce the phosphoric acid (H3PO4) in fertilizer
plants (Equation (1)), which currently accounts for over
90% of phosphoric acid production.
Ca5F(PO4)3 + 5H2SO4 + 10H2O
→ 3H3PO4 + 5CaSO4·2H2O + HF
(1)
This process is economic however it results in the
generation of a large amount of PG (for every ton of
phosphoric acid produced, about 5 tons of PG are yielded).
The worldwide generation is estimated to be around 100
- 280 Mt per year [1,2]. PG consists principally of
calcium sulphate (CaSO4·2H2O) but also contains a high
level of impurities such as phosphates, fluorides and
sulphates, naturally occurring radionuclides, heavy metals,
and other trace elements.
Nowadays, PG represents one of the most serious
problems faced by the phosphate industry, since com-
mercial uses, in manufacturing gypsum board and Port-
land cement and in agricultural fertilisers or soil stabili-
sation amendments, consume less than 15% of the
worldwide generation of PG. The remaining 85% is dis-
posed of without any treatment and usually dumped in
large stockpiles exposed to weathering processes, occu-
pying considerable land areas and causing serious envi-
ronmental contamination of soils, water and the atmos-
phere, particularly in coastal regions [3]. The main prob-

Radiochemical Characterization of Phosphogypsum for Engineering Use169
lem associated with the storage of PG is considered to be
the relatively high levels of natural uranium-series ra-
dionuclides, naturally present in the phosphate rock and
which provoke a negative environmental impact and
many restrictions on the use of this residue. Depending
on the quality of the phosphate rock source, PG can con-
tain as much as 60 times the levels normally found prior
to processing. Previous study performed by Bolivar [4]
showed that about 80% of the 226Ra, 90% of the 210Po
and 20% of the 238U and 234U originally present in the
phosphate rock remain in PG. Furthermore, the most
important source of PG radioactivity is reported to be
226Ra [5]. 226Ra produces radon gas (222Rn), which has a
short half-life of 3.8 days, an intense radiation capacity,
and causes significant damage to internal organs [6].
Thus the potential problem of PG piles is the emanation
of 222Rn from the alpha-decay of 226Ra, a radionuclide
classified by the USEPA as a Group human carcinogen,
whose common presence in PG led to the regulation of
PG disposal under the National Emission Standards for
Hazardous Air Pollutants (NESHAP) and the National
Emission Standards for Radon Emission from PG Stacks
[7]. The United States Environmental Protection Agency
(USEPA) classified PG as a “Technologically Enhanced
Naturally Occurring Radioactive Material” (TENORM)
[6] and PG exceeding 370 Bq/kg of radioactivity has
been banned from all uses by the EPA since 1992. The
maximum regulatory limit of 222Rn exhalation (the flux
density of 222Rn gas entering the atmosphere from the
surface of a 226Ra-bearing material) established by the
EPA [8] is 0.74 Bq/m2/s.
In Huelva (Spain), PG stacks located on salt marshes
contain about 100 Mt of PG (area of approx. 1200 ha
with average height of 5 m) and are generally not com-
pletely watertight or even covered with any inert material,
leading to a local gamma radiation level between 5 and
38 times the normal rate (0.74 Bq/m2/s) [9]. The same
situation is observed in Sfax (Tunisia), where PG is ac-
cumulated in two enormous warehouses situated at the
coastal strip of the urban area, the first one is 12 m high
and covers an area of 40 ha, and the other one, 30 m high,
covers an area of 60 ha.
The present study was conducted to determine the na-
tural selected radionuclides content belonging to uranium
and thorium decay series and 40K in the two different PG
sources mentioned above, using two different methods.
2. Material and Methods
The PG samples used in this study came from a fertiliser
factory in Sfax city, Tunisia and from Fertiberia S.A.,
Huelva, Spain.
The semiquantitative chemical composition of the PG
samples was identified by an X-ray fluorescence analyser
(Philips model PW-1404 sequential wavelength dispersion
unit). Mineral species were determined by X-ray diffrac-
tion (Siemens model D5000, with a Cu tube and LiF
monochromator). The morphological aspect of the PG
was analysed using scanning electron microscopy (SEM)
(Joel model JXA-840) with energy dispersive spec-
troscopy (EDS). Natural selected radionuclides be-
longing to uranium and thorium decay series and 40K
present in PG samples have been quantified as shown in
the Figure 1.
Non-destructive analysis
Phosphogypsum
Sample: 700 g
238
U,
235
U,
226
Ra,
232
Th,
210
PB,
40
K
Destructive analysis
Gamma spectrometry Wet digestion
(20 ml of 15.6 M HNO
3
)
Total Uranium:
Laser-induced kinetic
phosphorimetry (KPA-11)
210
Pb:
Alpha spectrometry
Sample: 1 g
Figure 1. Scheme of radionuclides quantification procedure.

Radiochemical Characterization of Phosphogypsum for Engineering Use
170
Uranium: the uranium content in the samples was
determined using two different methods: 1) Direct meas-
urements by gamma spectrometry with high-purity
germanium detectors and 2) Laser-induced kinetic
phosphorimetry. The direct measurements were carried
out on 700-g aliquots of the samples packed in standard
marinelli beakers. The 238U activity concentration was
determined through the photopeaks of its immediate de-
cay product, 234Th (63 and 92.5 keV), whereas 235U was
measured directly from its 143.8 and 163.4 keV gamma-
ray peaks. Concerning Laser-induced kinetic phosphori-
metry technique, 1 g of the sample was completely di-
gested in 15.6 mol·l−1 HNO3 and the measurements were
performed using a kinetic phosphorescence analyzer
(KPA-11) (Chemcheck Instruments Inc., Richland, WA)
[10]. In order to compare the results obtained by both
techniques, the total uranium concentration obtained by
Laser-induced kinetic phosphorimetry, expressed in
µg·g−1, were then converted to the activity concentration
of each uranium isotope. Theoretical values of the iso-
topic composition of natural uranium (99.3% 238U, 0.72%
235U, and 5.5·10−3% 234U) and the specific activities of
these isotopes in natural uranium (Bq·g−1) were used for
this purpose [11].
Polonium: the polonium activity concentration in the
both samples was determined by alpha spectrometry,
following the 210Po separation procedure described in the
Figure 2 [12]. An aliquot of 1 g was digested in a hot plate
at a controlled temperature (< 90˚C), using 8 mol·l−1 HNO3.
Polonium-209 standard dissolution was added to the
dissolved samples as a tracer to estimate the recovery of the
whole process. The polonium isotopes were self- deposited
on silver disks following Flynn’s method [13].
Complete dissolution
HNO
3
, 8M
Phosphogypsum
Addition of
209
Po
Standard dissolution
a.- Evaporation to near dryness on the hot plate a
t
T°  90°C.
b.- Dissolution of the residue in HCl.
c.- Eva
p
oration a
g
ain to dr
y
ness.

d.- Dissolution of the residue in HCl.
e.- Addition of NH
2
OH, Bi(III), sodium citrate.
f.- Filtration if necessary.
g.- Addition of deionized water to fill up the cell
Self-deposit of Po isotopes on
silver disks in water bath at 90°C, 3h.
Po silver disk
Alpha
spec tromet ry
Alpha-spectrum of a real sample: quantification
Aspect of silver disks
Figure 2. Scheme of 210Po separation procedure.

Radiochemical Characterization of Phosphogypsum for Engineering Use171
226Ra, 232Th, 210Pb and 40K: the concentrations of
these radionuclides were quantified by gamma spectro-
metry analysis using an HPGe detector. The detector was
shielded from external radiation by an iron wall (15 cm
thickness). The emission gamma spectrum was analyzed
using Genie-2000 application software. To ensure radio-
active equilibrium between 226 Ra and its short lived de-
cay products, the samples (700 g aliquot) were packed in
standard marinelli beakers, hermetically sealed and
stored for about four weeks prior to counting. The con-
centration of 226Ra and 232Th was estimated from their
daughters gamma-ray photopeaks, 214Bi (609 keV) and
228Ac (911.2 keV, 969.0 keV) respectively. 210Pb and 40K
were measured directly from their gamma-ray emissions
at 46.5 keV and 1460.8 keV respectively. The minimum
detectable activity limit (DL) was also calculated.
3. Results and Discussion
3.1. Phosphogypsum Characterization
The chemical composition of both type of PG sample is
summarised in Table 1. The data shows that sulphate
(expressed as SO3), CaO, SiO2 and P2O5 are the major
elements (50.7, 41.24, 1.38 and 1.2%, respectively) for
Tunisian PG, and 52.6, 42.82, 2.72 and 0.7, respectively
for Spanish PG.
The Figure 3 reports the powder X-ray diffraction
pattern of PG samples. As shown, Spanish PG presents
two maximum intensity diffraction peaks corresponding
to gypsum (CaSO4·2H2O) (JCPDS 33-0311) and bassanite
(CaSO4·1/2H2O), while the main diffraction peaks of the
Tunisian PG corresponds to gypsum (CaSO4·2H2O).
The morphological study of PG sample using SEM,
illustrated in Figure 4, shows two different sections of
the sample. The micrographs reveal a homogeneous and
prismatic PG piling arrangement and a well-defined
crystalline structure with a majority of orthorhombic
shaped crystals [14]. Similar results can be observed in
the study performed by Miloš and Dragan [15], in which
they reported that the marked crystal structure of PG
indicates that PG presents a more complex composition
than natural gypsum (characterized by a poorly expressed
crystalline structure), which may eventually influence its
chemical behaviour.
3.2. Natural Radionuclide Concentrations
The results of the natural radioactivity concentration
analyses of each one of the PG samples by the two tech-
niques employed for this purpose (gamma spectrometry
and laser-induced kinetic phosphorimetry) are listed in
Tables 2 and 3. The average activity concentration of
238U, 226Ra, 210Po, 232Th and 40K in Tunisian PG samples
are 30.7, 188, 194, 12.4 and 13 Bq·kg−1, respectively,
while in Spanish PG samples are 102, 520, 820, 8 and 39
Bq·kg−1, respectively. We can deduce that 226Ra and
210Po are the main source of the radioactivity in both PG
Table 1. The major element composition (wt%) of PG samples.
Element (Wt%) CaO SiO2 Al2O3 Fe2O3 MgO SO3 Na2O P2O5 F-
PG (Tunisia) 41.24 1.38 0.11 0.09 0.02 50.7 0.59 1.2 4.9
PG (Spain) 42.82 2.72 0.40 0.22 0.07 52.6 0.24 0.7 -
(a) (b)
Figure 3. X-ray pattern of phosphogypsum: (a) from Tunisia and (b) from Spain. Intensities in arbitrary units (a. u.).

Radiochemical Characterization of Phosphogypsum for Engineering Use
172
Figure 4. SEM micrographs of two different sections of a PG sample (15 keV).
Table 2. Uranium activity ratio in PG from Spain and Tunisia, expressed in Bq·kg−1 (± 2 s), by means of phosphorimetry and
gamma spectrometry techniques.
*DL. Detection Limit
Table 3. Natural radionuclide activity ratio of the 238 Uranium-series (226Ra, 210Pb and 210Po in this case in radioactive
equilibrium), 232 Th and 40 K expressed in Bq·kg−1 (± 2 s) in PG from Spain and Tunisia.
Sample 226Ra(214Bi) 210Pb 210Po 40K 232Th(228Ac)
PG (Tunisia) 188 ± 9.5 163 ± 81 194 ± 78 < 13.5 12.4 ± 1.4
PG (Spain) 520 ± 23 881 ± 58 820 ± 43 < (39) * 8 ± 2
*DL. Detection Limit
samples. The results obtained were compared to those
reported in other world regions (Table 4) [5,9,16,17].
It is found that a special attention is drawn to the low
natural radionuclide levels present in the PG samples
from Tunisia (30.7 for 238U, 188 (226Ra), 194 (210Po), 12.4
(232Th) and 13 Bq·kg−1 (40K)). Furthermore, these values
are lower than those of the average worldwide activity
concentration of 238U, 232Th, and 40K (50, 50 and 500
Bq·kg−1, respectively) [18]. This different content may
be attributed to the nature of the phosphate rock, the
depth of sampling [19] and differences in the industrial
process applied to obtain phosphoric acid. Natural ra-
dioactivity in the different phases of the production
system has recently been analysed by Bolivar et al. [20]
showing that Pb, Ra and to a certain extent Th isotopes
are exclusively supplied by the phosphate rock and
remain associated to the PG particles, while uranium
decreases according to the number of washings of the PG.
The data showed that the both studied PG samples
(Tunisian and Spanish) fulfil European Commission
Recommendation for the maximum activity concentra-
tions of naturally-occurring radionuclides in common
buildings materials and industrial by-products used for
building materials in the EU [21]. This finding could
make possible the use of the studied PG as building
material.
Concerning the measurement techniques used in this
study, it found that the use of gamma spectrometry for
uranium determination allows the analysis of a more
representative aliquot of the whole sample than in the
case of the KPA technique. KPA has a lower detection
limit (sensitivity) and better uncertainty (6%), but due to
the limitations of wet digestion until total dissolution and
chemical interferences, only 1 g can be analysed. Never-
theless, the results obtained from both techniques are in
good agreement.
U (Phosphorimetry) U (Gamma spectrometry) (DL)*
Sample 238U 234U 235U 238U(234Th) 235U
PG (Tunisia) 30.7 31.6 1.4 27 ± 4.9 (40)* < 6.5 *
PG (Spain) 102 ± 1 105 ± 5 4.7 ± 0.2 81 ± 28 8 ± 3 (20) *

Radiochemical Characterization of Phosphogypsum for Engineering Use173
Table 4. Activity concentrations of different types of phosphogypsum samples analyzed, in Bq kg-1 [5,9,16,17].
Origen 238U 226Ra 210Pb 210Po 230Th
Spain [9] 220 670 520 - 8.2
China [16] 15d 85 82 82 -
Indonesia [16] 43 473 480 450 -
India [16] 60 510 490 420 -
Egypt [17] 140 459 323 - 8.3
Florida [5] 130 1140 1370 1030 113
Australia [5] 10 500 - - -
Sweden [5] 390 15 - - -
4. Conclusions
The sensitivity of the analytical methods is good enough
to detect the radionuclides existing in both types of sam-
ples.
For uranium quantification, the gamma spectrometry
allows the analysis of a more representative aliquot of the
whole sample than in the case of the KPA technique.
However; KPA has a lower detection limit (sensitivity)
and better uncertainty (6%).
The PG samples from Tunisia present a low natural
radionuclide levels (30.7 Bq·kg−1 for 238U, 188 Bq·kg−1
(226Ra), 163 Bq·kg−1 (210Pb), 12.4 Bq·kg−1 (
232Th)) com-
pared to the level of PG samples from Huelva (102
Bq·kg−1 for 238U, 520 Bq·kg−1 (226Ra), 881 Bq·kg−1 (210Pb)
and 8 Bq·kg−1 (232Th). In both cases, the values are below
the maximum activity concentrations limits of naturally-
occurring radionuclides in common buildings materials
and industrial by-products used for building materials in
the EU, recommended by the European Commission.
5. Acknowledgements
The authors are grateful to both, Spanish National R & D
& I Plan (Project CTQ2008-02012/PPQ) and AECID
(Project Nº A/5537/06) for the financial support of this
study. Furthermore, Hanan Tayibi is grateful to CSIC
(Spanish Council for Scientific Research) for an I3P con-
tract (I3PDR-6-01).
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