RADIOACTIVITY IN FERTILIZERS AND ITS EFFECT ON ENVIRONMENTAL QUALITY

Demand for phosphorus application in agricultural production is increasing quickly throughout the globe. The bioavailability of phosphorus is distinctively low due to its slow diffusion and high fixation in soils, which makes phosphorus a key limiting factor for crop production. Applications of phosphorus-based fertilizers improve soil fertility and agriculture yield, but at the same time concerns over a number of factors that lead to environmental damage need to be addressed properly (Gupta et al. 2014).

Phosphate, nitrogen, and potassium fertilizers, which are used predominantly in order to increase crops in agriculture, provide basic nutrients to plants. In NPK fertilizers, gamma activity shows a wide variation because of the difference in the factories of manufactured fertilizers and the difference in the places from which the raw minerals for manufacturing the fertilizers were taken (Hussain and Hussain 2011). Radionuclides in phosphate fertilizer belonging to 232Th and 238U from phosphate rocks series as well as radioisotope of potassium (40K) are the major contributors of outdoor terrestrial natural radiation. The plants take some fractions of radioactivity, and radionuclides enter the food chain in this way (Bayrak et al. 2018). Phosphate rocks are largely used for the production of phosphoric acid, fertilizers, and gypsum (Jankovic et al. 2013; Sahu et al. 2014).

The natural radioactivity in phosphate rock depends on its origin. In sedimentary rock it is much higher than in volcanic rock. Granite rocks contain thorium in significant quantities. The main producers of phosphate rock are China, Morocco, Russia, and the United States. The radionuclide activity values differed among the districts, depending upon the geographic structures, rainfall amounts, and elevations of the districts. Some typical values of activity concentrations in phosphate rock are shown in Table 9.9. One reason for the increase in natural radiation involves the chemical fertilizers used in agriculture (Jankovic et al. 2013; Mir and Rather 2015; Durusoy and Yildirim 2017).

Phosphate ores typically contain about 1500 Bq/kg of uranium and radium, although some phosphates contain up to 20,000 Bq/kg of U,Os. In general, phosphate ores of sedimentary origin have higher concentrations of radionuclides of the uranium family. In 90% of cases, the ore is treated with sulfuric acid. The fertilizers become somewhat enriched in uranium (up to 150% relative to the ore), while 80% of the 226Ra. 30% of 232Th and 5% of uranium are left in phosphogypsum (Gaafar et al. 2016). Phosphoric acid is the starting material for TSP and ammonium phosphate fertilizers. Some typical values of activity concentrations in fertilizers are shown in Table 9.10.

TABLE 9.9

Some Typical Values of Activity Concentrations (Bq/kg) in Phosphate Rock

Location

226Ra

232Th

40R

Morocco

1600

1700

10

20

Togo

1100

1300

30

4

Western Sahara

900

900

7

30

Syria

300

1000

2

USA

1600

150

20

Tunisia

800

1000

20

30

India

1290

1340

90

10

Source: Sahu, S.K., et al„ J. Radial. Res. Appl. Sci., 7, 123—128, 2014.

TABLE 9.10

Some Typical Values of Activity Concentrations (Bq/kg) in Fertilizers in the World

Location

References

226Ra

232Th

40K

Iraq

Hussain and Hussain (2011)

13-89

1-27

12-2276

Saudi Arabia

Alharbi 2013

64

17

2453

India

Shahul Hameed et al. (2014)

2-396

5-39

33-93

Egypt

Uosifetal. (2014)

12-244

3-99

109-670

Serbia

Jankovic et al. (2013)

87

220

4860

Croatia

Barisic et al. (1992)

75

120

Egypt

Ghosh et al. (2008)

301

24

3

Egypt

125-239

446-882

Egypt

366

67

4

The radioactive content of the phosphatic fertilizers varies considerably and depends both on their concentrations in the parent mineral and on the fertilizer production process (Chaney 2012; Mortvedt and Beaton 2014). Uranium-238 concentrations can range from 7 to 100 pCi/g in phosphate rock (US EPA 2016) and from 1 to 67 pCi/g in phosphate fertilizers (Khater 2008; NCRP 1987; Hussein 1994). Where high annual rates of phosphorus fertilizer are used, this can result in uranium-238 concentrations in soils and drainage waters that are several times greater than are normally present (NCRP 1987; Barisic et. al. 1992). However, the impact of these increases on the risk to human health from radionuclide contamination of foods is very small (less than 0.05 mSv/y) (NCRP 1987; Hanlon 2012; Sharpley and Menzel 1987).

Most public and farmers were exposed to the natural radioactivity present in the sources used for fertilizer manufacturing or those applied through fertilization practices in the field. Some of these activities may be liberated to the underground water (Alcaraz Pelegrina and Martfnez-Aguirre 2001). Phosphate is used in the production of some chemical fertilizers. Since phosphate contains some natural radionuclides like 238U, 232Th, and 40K, fertilizers become the major contributor for outdoor terrestrial natural radiations. The radioactive content of phosphotic fertilizers varies considerably and depends both on their concentrations in the parent mineral and on the fertilizer production process (Chaney 2012; Mortvedt and Beaton 2014). Uranium-238 concentrations can range from 7 to 100 pCi/g in phosphate rock (US EPA 2016) and from 1 to 67 pCi/g in phosphate fertilizers

(Khater 2008; NCRP 1987; Hussein 1994). Where high annual rates of phosphorus fertilizer are used, this can result in uranium-238 concentrations in soils and drainage waters that are several times greater than are normally present (NCRP 1987; Barisic et al. 1992). However, the impact of these increases on the risk to human health from radionuclide contamination of foods is very small (less than 0.05 mSv/y) (NCRP 1987; Hanlon 2012; Sharpley and Menzel 1987). The phosphate material is very insoluble, and therefore in its original state is practically unavailable as a plant phosphors source (IAEA 1973). Among the constituents of agricultural phosphate fertilizers are potassium ores (potassium sulphate, potassium chloride) (Conceicao and Bonotto 2006). Samples of granular and leafy types of NPK in addition to urea type collected from common markets in different regions in Iraq recorded high levels of radium-equivalent value in leafy-type NPK fertilizers, while urea types had no radionuclide. At the same time, the maximum specific activity and absorbed dose rate at lm above the ground surface (nGy/h) after the agricultural application of NPK fertilizers was 0.15% of the world average outdoor exposure (Figure 9.8) due to terrestrial gamma radiation (Hussain and Hussain 2011).

The abovementioned naturally occurring radionuclides materials (NORM), including uranium and thorium series, are considered the largest contributor to radiation doses received by human beings. Components of 238U, 235U, and 232Th series along with other nonseries radionuclides are given in Table 9.11.

Radioactivity in rock phosphate as a source of NORM was found to be varying from one place to another. This activity was transferred to phosphate fertilizer manufactured from rock-P source (Tufail et al. 2006). For example, a study was conducted to compare activity concentration in Egyptian and Japanese phosphate fertilizers (Hassan et al. 2017). Table 9.12 shows the variation in activities between the two countries.

Absorbed dose rate 1 m above the ground surface

FIGURE 9.8 Absorbed dose rate 1 m above the ground surface (nGy/h) originated from the agricultural application of NPK fertilizers. (After Hussain. R.O. and Hussain. H.H.. BrazArch Biol Technol., 54, 777-782, 2011.)

TABLE 9.11

Principal Natural Radionuclides Decay Series

Nuclide

Half-Life

Major Radiation

Nuclide

Half-Life

Major Radiation

23SU

4.47 BY

a. X

2l,Ra

4.0 sec

a

:l4Th

24.1 D

Ay. X

2l5Po

1.78 m sec

a

234Pa

1.17 min

Ay

21.Pb

36.1 min

А У

234Ц

245.000 Y

a. X

2"Bi

2.13 min

a

2;'Th

77.000 Y

A «.y

20,Tr

4.77 min

А У

226Ra

1600 Y

a, у

2s,Pb

Stable

222Rn

3.83 D

a

2J2Th

14.1 BY

a. X

2isp0

3.05 min

a

22sRa

5.75 Y

P

214Bi

19.7 min

Ay

22SXh

1.91 Y

a. X

214Pb

26.8 min

A y.X

22»Ac

6.13 hr

A y.X

21JPo

164 ft sec

a

224Ra

3.66 D

я, у

210Pb

22.3 Y

A «. У

22l’Rn

55.6 sec

a

2i°Bi

5.01 D

P

2.6Po

0.15 sec

a

210Po

138 D

a

212Pb

10.64 hr

A y.X

2U6Pb

Stable

2l2Bi

60.6 min

a, А У

235 U

7.1 x 109Y

a

2.2Po

0.305 ц sec

a

23'Th

25.5 hrs

Ay

2o»Xa

3.07 min

Ay

23'Pa

3.25 x 10*Y

a

20Spb

Stable

227Ac

21.8 Y

Ay

Non-Series Radionuclides

227Th

18.5 sec

a

4°K

1.28 BY

Ay

TABLE 9.12

Activity Concentrations of 226Ra, 232Th, and 40K in Egyptian and Japanese Fertilizers

Sample

Code

Composition

226 Ra

232Th

40K

Е8УР‘

Japan

Egypt

Japan

Egypt

Japan

Egypt

Japan

Egypt

Japan

1

EF1

JF1

SSP(15%PA)

P. acid 10%

761 ±31

25 ± 1

67 ± 13

5 ±2

251 ±94

3909 ±21

2

EF2

JF2

SSP(12%P205)

P. acid 8%

557 ± 19

62 ± 1

15 ± 6

15 ± 1

175 ±27

3280 ± 17

3

EF3

JF3

SSP(16%PA)

P. acid 20%

782 ± 24

74 ± 1

14 ±8

6± 1

222 ± 24

48 ±2

4

EF4

JF4

NPK

P. acid 20%

443 ± 11

200 ±2

ND

12 ± 1

88 ±22

231 ±4

5

EF5

JF5

Triple

P. acid 17%

312 ± 14

1264 ±5

ND±

8 ± 2

175 ±31

31 ±5

Source: Hassan, N.M., et al., J. Chem., Volume 2017, Article ID 9182768, 8 pages, 2017.

Their data from Hassan et al. (2017) indicated that the radionuclide concentrations in Japanese fertilizer were less than those of Egyptian fertilizers except for potassium, as seen in Figure 9.9. The radionuclide concentration of 40K is much higher in Japanese fertilizer samples and especially sample JF-1. Both Egyptian and Japanese fertilizers maintain radionuclide concentrations less than the recommended limits by UNSCEAR (2008).

Comparison of the estimated radiological indexes values during their work (Hassan et al. 2017) and other values in previous studies in literature indicated that the radium equivalent in the Egyptian and Japanese fertilizers was less than its value for fertilizer used in Algeria and Brazil but was greater than its value for fertilizer used in Saudi Arabia and Bangladesh. The gamma indexes had the same trend as radium equivalent (Table 9.13).

Environmental pathways of natural radionuclides from phosphate rocks. (From Khater. A.E.M.. et al., J Environ. Radioact., 55, 255-267, 2001.)

FIGURE 9.9 Environmental pathways of natural radionuclides from phosphate rocks. (From Khater. A.E.M.. et al., J Environ. Radioact., 55, 255-267, 2001.)

TABLE 9.13

Comparison of Radiological Indexes in Egyptian and Japanese Fertilizers and Their Values in Literature

Country

Sample

Radium Equivalent (Bq/kg)

Gamma Index (/y)

References

Egypt

Fertilizer

613 ±33

2.06 ±0.11

Hassan et al. (2017)

Japan

Fertilizer

454 ±5

1.63 ± 0.08

Hassan et al. (2017)

Algeria

NPK

1168

9.6

Boukhenfouf and Boucenna (2011)

Brazil

NPK

1772

12.3

Becegato et al. (2008)

Saudi Arabia

NPK

275

Alharbi (2013)

Egypt (Qena)

Phosphate fertilizer

462

3.1

Ahmed and El-Arabi (2005)

Bangladesh

Triple superphosphate

374

Alam et al. (1997)

Phosphate ores in Egypt reflected a radium concentration in the same range as UNSCEAR’s typical value (Hassan et al., 2016). The radionuclide concentrations of 226Ra, 232Th, and 4(,K maintained in phosphate ores of several countries is consistent with those of Egypt (Table 9.14).

Another contribution is by Sahu et al. (2014), who reported the radioactivity in rock phosphate and phosphogypsum in India compared to other countries, as listed in Table 9.15. They indicated higher levels of natural radionuclides in the gypsum ponds and rock silo than other locations in the plant premises. The 238U levels in the phosphogypsum sample were higher than those compared worldwide. Values of the activities due to 226Ra, 232Th, and 40K were varied according to phos- phatic fertilizer types from the Pakistani market as shown in Table 9.16, after Khan et al. (2004). They found that the concentration of 226Ra in all phosphatic fertilizers as well as phosphate rock samples is much higher than 232Th and 40K. All the analyzed phosphatic fertilizers and phosphate

TABLE 9.14

Specific Activities of 226Ra, 232Th, and 40K in Phosphate Ores Used in Several Countries All over the World

Country

Activity Concentration (Bq kg-1)

References

Phosphate ore

226Ra

2,2Th

*K

Saudi Arabia

513

39

242

Al-Zahrani et al. (2011)

Egypt

840

395

398

El-Taher and Makhluf (2010)

South Korea

4.0

49

Chang et al. (2008)

Brazil

256

3238

1202

Conceicao and Bonotto (2006)

Nigeria/Sokoto

16

40

Ogunleye et al. (2002)

Tanzania (Arusha)

350

280

Banzi et al. (2000)

Sudan (Uro)

4131

7.5

62.3

Sam et al. (1999)

Jordan

1044

2

8

Olszewska-Wasiolek (1995)

Tunisia

821

29

32

Olszewska-Wasiolek (1995)

Tanzania (Arusha)

5022

717

286

Makweba and Holm (1993)

Morocco

1600

20

10

Guimond (1990)

Egypt

871 ±92

19 ± 2

176 ± 18

Hassan et al. (2016)

Source: Hassan et al..

J. Taibah Univ. Sci.

, 10,296-306,2016.

TABLE 9.15

Radioactivity (Bq/g) in Phosphate Rocks and Phosphogypsum

Phosphate Rock Activity (Bq/g)

Location

References

226Ra

238U

232Th

«к

Morocco

Guimond and Hardin (1989)

1.6

1.7

0.01

0.02

Taiba-Togo (Western Sahara)

1.1

1.3

0.03

0.004

0.9

0.9

0.007

0.03

Syria

Attar et al. (2011)

0.3

1.0

0.002

Florida

Guimond (1990)

1.6

1.5

0.02

Tunisia

Olszewska-Wasiolek (1995)

0.8

1.0

0.02

0.03

India

Sahu et al. (2014)

1.29

1.34

0.09

0.01

Phosphogypsum Activity

Florida

Olszewska-Wasiolek( 1995)

0.9

0.069

0.01

Brazil

Mazzilli et al. (2000)

0.6

0.04

0.1

0.02

Brazil

0.2

0.04

0.1

0.01

Syria

Attar et al. (2011)

0.3

0.03

0.002

Egypt

Ahmed (2005)

0.1

0.04

0.5

Spain

Jose et al. (2009)

0.8

0.08

India

Sahu et al. (2014)

0.3

0.03

0.01

0.005

rock samples have shown a higher amount of 226Ra due to deposits of uranium in the rock phosphate. It could result in significant radiation exposure if these fertilizers are handled in places with poor ventilation that could lead to radon accumulation.

The pathways of natural radionuclides from phosphate fertilizers to the environment and finally to the public (Figure 9.9) demonstrated the need to recognize and calculate the exposure rate comparing to Egypt and the world average (Table 9.17). Khater et al. (2001) listed the calculated

TABLE 9.16

Specific Gamma-Ray Activities Due to 40K, 226Ra, and 232Th in Different Brands of Phosphatic Fertilizers Available in Pakistan

Fertilizer

Specific Gamma-Activities (Bq kg-1)

40K

22bRa

232Th

Single Superphosphate

221.2

556.3

49.7

Triple Superphosphate

142.5

558.6

84.8

Nitrophos

205.7

389.4

79.9

Mono Ammonium Phosphate

137.7

560.9

85.1

Di Ammonium Phosphate

237.5

545.3

65.5

Phosphate Rock

207.3

439.5

50.4

Source: Khan, K., et al., Geol Bull., 37,59-64,2004.

TABLE 9.17

Calculated Exposure Rate (nGy/h) at 1 m above the Ground Due to Natural Radionuclides in Wet Phosphate Rock and Soil

226 Ra

232Th

40R

Total

nGy/h per Bq/kg

0.461

0.623

0.041

Wet rock

132

14.8

0.89

148

Soil

11.8 (8.8-15.IP

18.1 (10.5-27.5)

5.34(3.1-6.8)

35.3 (22.3-49.4)

Egypt

32(8-93)

World

55

Source: Khater, A.E.M., et al., J Environ. Radioact., 55,255-267, 2001. a Mean (range).

exposure rates (nGy/h) at 1 m above the ground due to natural radionuclides in wet phosphate rock and soil and recorded that for soil the exposure rate (35 nGy/h) is comparable to the Egyptian average (32 nGy/h) and less than the world average (55 nGy/h) (UNSCEAR 1993).

It is of interest to follow up the phosphate manufacture processes and its by-products. In this respect, the phosphate processing operations comprise the mining and milling of phosphate ore and then the manufacture of phosphate products by either the wet or the thermal process. More than 70% of the ore being beneficiated in several process steps to increase the P205 concentration before delivery are wet processes. The main route for more than 90% is then acidulation with sulfuric acid, besides nitric and hydrochloric acid in minor extent with the main by-product of gypsum sulfate (phosphogypsum), of which 4-5 tons are received when 3 tons of ore are turned into 1 ton of P205 (Figure 9.10).

The activities of radionuclides may have an impact on soil and plant upon fertilization with such phosphatic fertilizers. In this regard, Alsaffar et al. (2016) found that the radioactivity produced by the addition of fertilizers to pots planted with rice was apparently insignificant compared with that of soil alone. They also indicated that 226Ra concentrations in rice grains were increased with increasing urea. Additionally, 226Ra concentrations in grains also slightly increased with increasing both NPK and NPK+Mg rates, but 226Ra concentration was found to be lower than that of urea. Therefore, risk assessment due to application of such fertilizers should be taken into consideration in comparison with the average annual ingestion dose worldwide.

Flow of radionuclides in thermal process of phosphorus production

FIGURE 9.10 Flow of radionuclides in thermal process of phosphorus production. (From Scholten, L.C., Approaches for regulating management of large volumes of waste containing natural radionuclides in enhanced concentrations, Official Publication of the European Communities, 1996; Penfold, J.S.S., et al.. Establishment of reference levels for regulatory control of workplaces where materials are processed which contain enhanced levels of naturally occurring radionuclides, NRPB report on contract number 95-ET-009, 1997.)

 
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