Paper
RADIOACTIVE ROCK PHOSPHATE: THE FEED STOCK OF
PHOSPHATE FERTILIZERS USED IN PAKISTAN
M. Tufail,* Nasim Akhtar,
†
and M. Waqas
‡
Abstract—The existence of natural radioactivity in phosphate
fertilizers originates from the rock phosphate that is used as a
feed stock for the manufacturing of various types of phosphate
fertilizers. Gamma ray activity mass concentrations have been
determined in phosphate rock and fertilizer samples collected
from different fertilizer manufacturing factories and suppliers
in Pakistan. The technique of gamma ray spectrometry was
applied using a HPGe (high purity germanium) gamma ray
detector coupled with a personal computer (PC) based mul-
tichannel analyzer (MCA). The specific activity values of
40
K,
226
Ra, and
232
Th were widely distributed in the rock and
fertilizer samples. Activity mass concentrations of
238
U(
226
Ra)
in the rock phosphate from Jordan and Pakistan were 428
11 and 799 10 Bq kg
1
, respectively, and that in the
triple-superphosphate fertilizer was 701 21 Bq kg
1
, which
was greater than the average values of
238
U(
226
Ra) for other
types of fertilizers under study. The concentrations of
40
K and
232
Th were of the order of background level on the Earth’s
crust. Chemical analysis for concentration of different constit-
uents in rock phosphate was also carried out. The higher the
concentrations of P
2
O
5
and K
2
O were, the higher the specific
activities of
226
Ra and
40
K were, respectively, in the rocks under
study. The amount of radioactivity in the rock phosphate is a
source of continuous exposure to radiation, which varies
widely depending upon the origin and grade of rock; therefore,
radiation dose rate has been estimated. The values of dose rate
and other derived quantities were many folds larger than the
limits specified for those quantities.
Health Phys. 90(4):361–370; 2006
Key words: dose assessment;
40
K;
226
Ra;
232
Th
INTRODUCTION
ROCK PHOSPHATE (P
2
O
5
) is a terrestrial source of NORM
(naturally occurring radioactive material) whose concen-
tration varies from one place to another on the Earth;
therefore, efforts are being undertaken to determine the
amount of radioactivity associated with P
2
O
5
throughout
the world. A substantial amount of radioactivity is
transferred to fertilizers manufactured of phosphate rock
(Bhatti and Malik 1994). The radionuclides that exist in
the fertilizers are
238
U,
235
U,
232
Th decay series, and
40
K.
Activity mass concentrations of natural radionuclides in
phosphate fertilizers have been reviewed in the UN-
SCEAR (1982) report where it has been concluded that
for a given radionuclide and the type of fertilizers, the
activity mass concentrations vary remarkably from one
country to another, depending on the origin of the
components.
Sedimentary rock deposits supply about 80 – 85% of
the P
2
O
5
as a raw material for manufacturing phosphate
fertilizers throughout the world. Igneous type rock phos-
phate deposits supply the remaining 15–20% of P
2
O
5
to
the fertilizer industry (Awadalla and Habshi 1985). Rock
phosphate of sedimentary origin contains on average
0.015% U
3
O
8
(Hussein 1994). The world consumption of
phosphate fertilizers is about 60 million tons of P
2
O
5
(UNSCEAR 1993). The application rate of fertilizers
depends on, among other things, the type of soil and the
type of crop. The average consumption of phosphate
fertilizers per unit area of agricultural land varied from
3.6 kg P
2
O
5
per hectare in the developing countries to
10.9 kg P
2
O
5
in the developed countries, the world
average being 6.7 kg per hectare (UNSCEAR 1993).
Human beings are exposed from many pathways to
radioactivity present in phosphate rock. Phosphate rock,
which is processed to make phosphate products, contains
relativity high concentrations of naturally occurring ra-
dionuclides, and the radioactivity in phosphate rock
propagates in various amounts to all the phosphate
products and byproducts (Hartley and Freeman 1986). It
has been reported by Habshi (1985) that about 80% of
rock phosphate is being used as a raw material for the
manufacturing of phosphate fertilizers including
phospho-gypsum, whereas the remaining 20% is being
utilized for production of phosphorus compounds such as
detergents, metal surface treatment reagents, chemicals
for food industry, etc. The external exposure is to gamma
* Pakistan Institute of Engineering and Applied Sciences, P.O.
Nilore, Islamabad, Pakistan;
†
Health Physics Division, NIAB, Jhang
Road, Faisalabad, Pakistan;
‡
Department of Earth Sciences, Quad-i-
Azam University, Islamabad, Pakistan.
For correspondence or reprints contact: M. Tufail, Pakistan
Institute of Engineering and Applied Sciences, P.O. Nilore, Islamabad,
Pakistan, or email at dr_mtufail@yahoo.com.
(Manuscript received 31 March 2005; revised manuscript re-
ceived 28 July 2005, accepted 19 November 2005)
0017-9078/06/0
Copyright © 2006 Health Physics Society
361