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Chapter 2. Phosphogypsum in Agriculture: A Review

Chapter 2. Phosphogypsum in Agriculture: A Review

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56



I. S. ALCORDO AND J. E. RECHCIGL



result of pollution control systems processes, such as in the neutralization

of waste sulfuric acid and in flue-gas desulfurization. Phosphogypsum is

the term used for the gypsum by-product of wet-acid production of phosphoric acid from rock phosphate. It is essentially hydrated CaSO, with

small proportions of P, F, Si, Fe, Al, several minor elements, heavy metals,

and radionuclides as impurities. Rock phosphate deposits are found

throughout the world, and on these deposits the phosphoric acid industries

are built. Countries with no natural phosphate deposits import the rock to

produce phosphoric acid for their industry and agriculture. Therefore, the

production of by-product phosphogypsum is more widely distributed

around the world than are the natural deposits of rock phosphate. Thus,

among the gypsum by-products, only phosphogypsum is of worldwide

importance in quantity and distribution.



B. WORLD

PRODUCTION

AND UTILIZATION

OF PHOSPHOGYPSUM

The three basic conventional processes used in wet-acid manufacture of

phosphoric acid are the dihydrate, the hemihydrate, and the hemidihydrate

processes. For each megagram (Mg) of P produced, the hemihydrate process yields about 9.8 Mg of dry phosphogypsum, whereas the dihydrate

and hemidihydrate processes yield about 1 1.2 Mg (Kouloheris, 1980).

Worldwide production of phosphoric acid, estimated at 1 1 million Mg of P

annually (Lin et al., 1990), also results in the production of approximately

125 million Mg of phosphogypsum. With only about 4% of the world's

phosphogypsum production being used in agriculture and in gypsum

board and cement industries, about 120 million Mg of phosphogypsum

accumulates annually; most of this excess is piled in stacks, and some is

stored in abandoned quarries or, in certain countries, dumped into waterways.

Australia produces 940,000 Mg of phosphogypsum annually, of which

200,000 Mg is used as soil conditioners or fertilizers. The rest is stockpiled

on land and in abandoned quarries. The stockpile in 1990 had reached 8

million Mg. Australia discontinued the use of phosphogypsum for making

plaster products in 1983 (Beretka, 1990).

India produces about 2.8 million Mg of phosphogypsum annually, and it

is used primarily as a soil amendment or conditioner for sodic soils

(Mishra, 1980).

Since 1970 phosphogypsum production in Japan has stabilized at 2.5 to

3.0 million Mg annually, almost all of which is used in the cement, gypsum

board, and plaster industries. The amount of phosphogypsum being used

as fertilizer ranges from 25,000 to 48,000 Mg annually. As a result, Japan

has no stockpile of phosphogypsum (Miyamoto, 1980). Full utilization of



PHOSPHOGYPSUM IN AGRICULTURE



57



Figure 1. One of the 20 phosphogypsum stacks located in Florida.



phosphogypsum in Japan is made possible by the Nissan hemidihydrate

phosphoric acid process that produces high-quality phosphogypsum suitable for the construction industry (Goers, 1980).

In 1979, Canada produced 4 million Mg of phosphogypsum and had

some 50 million Mg in large containment areas. On the basis of the 1979

production, Canada could have almost 100 million Mg of phosphogypsum

in stock at this time. Collings ( 1980) reported that producers of phosphogypsum had used the material as an additive to clay soils and as fertilizer.

According to Khalil et al. (1990), 14% of the total production of phosphogypsum in Iraq was reprocessed, 58% was stored or stockpiled, and

2890 was dumped into waterways. The Netherlands, with an annual production of 2 million Mg, simply discharges its phosphogypsum into its

surface waters (van der Sloot and de Groot, 1985).

In 1988 the wet-process phosphoric acid plants in the former USSR

produced 23.5 million Mg of phosphogypsum. Phosphogypsum production is expected to increase by the year 2000 to 43.6 million Mg annually.

The total quantity of the material in stock in January, 1990, was estimated

at 300 million Mg. Russia has been exploring various ways to use the

material in industry and agriculture. With 66 million ha of acid soil and

over 100 million ha of Solonetz and Solonetz-like soils that need to be

reclaimed or ameliorated, phosphogypsum is expected to play an important role in Russian agriculture. In 1988, 3.2 million Mg of phosphogypsum was used for chemical reclamation of Solonetz soils, and by the year

2000, phosphogypsum use for this purpose is expected to reach 19.2 million Mg annually (Novikov et d., 1990).

Florida leads in the production of phosphogypsum in the United States.

Annual production has been placed at 32 million Mg. Over 400 million

Mg [Environmental Protection Agency (EPA), 19911 is stockpiled at 20

stacks (Fig. I). Annual use of phosphogypsum in agriculture is placed at



58



I. S. ALCORDO AND J. E. RECHCIGL



2% of production. For the entire United States, present annual production

has been placed at 45 million Mg (Arman and Seals, 1990),and there is a

stockpile of 8 billion Mg at stacks in 12 states (EPA, 1991). Because of

strict environmental regulations, phosphogypsum has not been used as raw

material for applicable industries to date. Research in this area, however, is

extremely active, not only in the United States but also worldwide (Boms

and Boody, 1980; Chang, 1990).



C. PHYSICAL

AND CHEMICAL

PROPERTIES

OF PHOSPHOGYPSUM

Table I shows the chemical analysis of phosphogypsum produced by

each of the three conventional processes used worldwide in the manufacture of phosphoric acid. Table I1 gives the chemical analysis of the major

constituents of phosphogypsum from several countries.

Phosphogypsum has a gypsum content ranging from 85 to 93Yo

(Appleyard, 1980). It has a silty feel, with particle sizes clustering around

0.05 mm in diameter. It may contain small amounts of calcium sulfite and

calcium carbonate (Nifong, 1988). Phosphogypsum may be contaminated

by as many as 50 elements originally present in various forms in rock

phosphate (Borris and Boody, 1980). May and Sweeney (1980, 1983)

detected 30 elemental impurities in Florida phosphogypsum using emission spectrographic analysis and 44 impurities using neutron activation

analysis. Some of the impurities, excluding the so-called EPA “toxicity

index” metals, which are presented separately, are given in Table 111.

Fluoride, also present originally in rock phosphate, may be found in

relatively high concentrationsin phosphogypsum. Australian phosphogypsum was reported to contain from 11 to 13 g F kg-I phosphogypsum

(Beretka, 1980, 1990). Indian phosphogypsum may contain 5 to 40 g F

kg-I (Mishra, 1980), which could be of environmental concern when the

phosphogypsum is applied at high rates for reclamation of sodic soils.

Florida phosphogypsum has 2 to 8 g F kg-’ (May and Sweeney, 1980,

1983). Leachates of certain Florida phosphogypsum samples had been

reported to contain F in excess of the EPA limit of 1.4 mg F liter-’ for

drinking water (Nifong, 1988). A large proportion of F in a Florida phosphogypsum sample dissolved readily in Mehlich I solution but not in water

(Alcordo and Rechcigl, 1992).

Aqueous equilibrium solutions of mined gypsum and phosphogypsum,

which contained 16.2 and 17.0 mmol Ca liter-’, respectively, reflect only

slight differences in solubility between the two types of gypsum (Shainberg

et al., 1989). Florida phosphogypsum dissolved at a constant rate of 0.26

and 0.43 g 100 m1-I in water and Mehlich I solution (0.025 A4 HCl+



PHOSPHOGYPSUM IN AGRICULTURE

Table I

Typical Chemical Analysis of Phosphogypsum Produced

by Three Different Processes in the Wet-Acid Production

of Phosphoric Acid"

Processb

Component

CaO



so3



PZO,

F

Si02

Crystal. H,O



Dihydrate Hemihydrate Hemidihydrate

322.0

465.0

2.5

5.0

4.0

0.5

3.0

200.0



369.0

503.0

15.0

8.0

7.0

I .o

3.0

90.0



325.0

440.0

6.5

12.0



5.0



1.o

1.O

190.0



From Kouloheris ( 1980).

Analytical results given in grams/kilogram.



Table I1

Chemical Analysis of Phosphogypsum Produced in Different

Countries

Country"

Component Australia Canada Japan

CaO



so3



SiO,

A1203



Mi@

Na,O

K20

Total P 2 0 5

Total F

Total H,O



329.0

451.0

50.0

3.4

0.6

0.4

3.5

0.5

8.8

13.0

209.0



340.0

458.0



8.4

17.0

196.6



Iraq



304.0 329.0

435.0 449.0

40.5

10.5

10.5

1.1

4.0

0.4

4.6

0.1

0.8

I .6 1.8

2.9

2.4

6.0

190.0 182.0



United States

311.0

420.0

5.7

5.7

1.4

0.0

6.1

0.1

37.0

8.0

188.0



a Highest value in a range of values (in grams/kilogram) reported for different sources of phosphogypsum from within the

country or state. Sources: Australia (Beretka, 1990); Canada

(Collings, 1980); Japan (Miyamoto, 1980); Iraq (Khalil et al.,

1990);United States (May and Sweeney, 1983).



59



60



I. S. ALCORDO AND J. E. RECHCIGL

Table I11

Some Elemental Impurities in Florida

Phosphogypsum Excluding the EPA “Toxicity

Index” Metals’

Element



Concentration (mg kg’)



A 1uminum

Antimony

Bromine

Cerium

Cesium

Chlorine

Cobalt

Copper

Erbium

Gadolinium

Gold

Iron

Lanthanum

Magnesium

Manganese

Molybdenum

Potassium

Sodium

Strontium

Titanium

Uranium

Zinc



2000.0

0.2

<0.9

49.0

0.5

< 150.0

0.6

<82.0

<330.0

150.0



a



<0.01



930.0

39.0

<940.0

25.0

6.6

215.0

520.0

600.0

440.0

9.6

< 340.0



From May and Sweeney ( 1983).



0.0125 M H,SO,), respectively. The aqueous solution had an electrical

conductivity (E,) of 0.21 S m-’ and a pH of 5.2 (Alcordo and Rechcigl,

1992). Figures 2 through 5 show the solubility curves of the major elemental constituents of a Florida phosphogypsum in water and in Mehlich I

solution. The solubilities of the “toxicity index” metals are given in Table

IV.

Although gypsum is a neutral salt, phosphogypsum is highly acidic;

its pH in water (1 : 1) ranges from > 2 to < 5, mainly due to acid impurities, such as sulfuric, phosphoric, hydrofluoric, and fluosilicic acids

(Nifong, 1988). But it is the heavy metals and the radionuclides present in

phosphogypsum that give rise to environmental concerns in its use in

agriculture.



PHOSPHOGYPSUM IN AGRICULTURE



61



50



PG



--e-Ca



A



40



P



-0



&



a,



->

5

-m



30



K

&



v)



c.



c

0

. 20

h



0



8

10



0



1



2



3



4



5



6



Solubility run

Figure 2. Solubility curves of phosphogypsum (PG) and Ca, P, K, and Mg in PG in

Mehlich I. (From Alcordo and Rechcigl, 1992.)

25



PG

+



I1



Ca



A

P

-itMg



-eK

+



0



16

1



A

2



A

3



A

4



A

5



A

6



Solubility run

Figure 3. Solubility curves of phosphogypsum (PG)and Ca, P, K, and Mg in PG in

water. (From Alcordo and Rechcigl, 1992.)



62



I. S. ALCORDO AND J. E. RECHCIGL

80



*

PG

Al



A

60



Fa



'CI



\



->

Q,



0



\



u)

VI



.41Q 40



-scu

--+--



\



Zn

&



\



CI



0



CI



Mn

--t-



F



u-



0



8



20



0



A



m



1



2



3



4



5



6



Solubility run

Figure 4. Solubility curves of phosphogypsum (F'G)and Al, Fe, Cu, Zn, F, and Mn in PG

in Mehlich I. (From Alcordo and Rechcigl, 1992.)

25



20



'0



->

Q,



$

f



15



v)



Q



.c1



0 10



.c1



rc

0



8

5



0



du

1



m

2



m

3



m

4



c

4



5



m

6



Solubility run

Figure 5. Solubility curves of phosphogypsum (PG)and Al, Fe, Cu, Zn,F, and Mn in PG

in water. (From Alcordo and Rechcigl, 1992.)



63



PHOSPHOGYPSUM IN AGRICULTURE

Table IV



EPA “Toxicity Index” Metal Concentration Limits and Conteuts in Florida Phosphogypsum

and Phosphogypsum Leachate

Concentration (mg liter-’)

EPA “toxicity

index” metal

AS

Ba

Cd

Cr



0.85

105.00

0.59



Pb

Hi3



I .30

0.50

1.40

0.69



Se

Ag

a



Analysis in



Florida PG” (mg kg-’)



6.00



Percentage of

total leached”



Florida PG



leachateb



EPA limit in

leachatec



28

3



0.0 13

0.200

0.010

0.040



5.0

1100.0

1.o

5.0



0.001



5.0

0.2



47



13

28

4

4

6



0.00 1

0.003

0.060



1.o

5.0



From May and Sweeney (1983).

From Nifong (1988) and May and Sweeney ( I 982).

From the Federal Register (1980).



The United States EPA has set certain criteria to define corrosive, hazardous, and toxic waste (Federal Register, 1980). The EPA criterion for

corrosivity is a pH 5 2.0 or 2 12.5. Its criterion for toxicity ofwaste is based

on the kind of contaminants that are likely to leach into ground water.

These are extracted from the waste material according to EPA extraction

procedures (Federal Register, 1980). The hazardous nature of the waste is

judged by the concentrations of specific contaminants in the extract. Table

IV lists the contaminants and the EPA concentration limits in drinking

water together with the mean concentrations in the leachates of Florida

phosphogypsum.

Radioactivity is the major concern in the use of phosphogypsum in both

industry and agriculture. Radium-226, the decay product of uranium-238

in rock phosphate, is the major source of radioactivity in phosphogypsum.

In the manufacture of phosphoric acid, 86% of 238Ugoes with the phosphoric acid and 8OYo of the 226Rastays with the phosphogypsum (Roessler,

1990; EPA, 1991). Radiation levels of Florida phosphogypsum range from

296 to 1406 Bq kg-’ with an average of 747 Bq kg-l (May and Sweeney,

1980, 1983). The higher the radioactivity in the raw material, the higher it

is in the phosphogypsum (Table V). Some countries have established the

following radiation characterization limits for monitoring phosphogypsum



I. S . ALCORDO AND J. E. RECHCIGL



64



Table V

Computed and Measured Radiation in Phosphogypsum Produced from

Rock Phosphate of Various Origins"



Rock origin



Measured radiation in

rock (Bq kg-')



Florida



Morocco

Togo

Kola

Natural gypsum



Radiation in

phosphogypsum (Bq kg-I)

Computed



Measured



1295.0

1387.5

1295.0

1 1 1.0



869.5

925.0

85 1.O

74.0



740.0

1147.0

740.0

74.0



-



37.0



-



From Kouloheris (1980).



(Kouloheris, 1980):



Radiation (Bq kg-')



< 370

370-925

>925



Regulations/characterization

Free use

Compulsory declaration

Compulsory registration



The EPA regulations issued on December 18, 1978, placed phosphogyp

sum on the list as a hazardous waste because of its radioactivity. To be

excluded from the list, a solid waste material must contain less than 185 Bq

of 226Rakg-', or, for a single discrete source, the total z2aRacontent must

be less than 37 X 104 Bq (Federal Register, 1978).

The EPA rule on phosphogypsum (Federal Register, 1989) placed the

regulation of phosphogypsum stacks under the National Standards for

Hazardous Air Pollutants. The final EPA rule on the removal of phosphogypsum from the stack for use in agriculture has been formulated only

recently (Federal Register, 1992). The rule is based on a biennial application of phosphogypsum to agricultural land, which land is later converted

to residential use after 100 years of phosphogypsum application. Under

such an assumption the acceptable maximum individual risk (MIR) of

lifetime exposure from indoor radon inhalation and y-radiation of 3 X



PHOSPHOGYPSUM IN AGRICULTURE



65



would not be exceeded if the phosphogypsum used contained not

more than 370 Bq kg-' and the rate of application was approximately

3.0 Mg ha-'. Such a rate covers the upper 95th percentile of the application rates used in the United States. Removal of phosphogypsum from

stacks for use in research and development is now also allowed but the

amount cannot exceed 3 18 kg, the amount of phosphogypsum that can be

contained in a 55-gallon drum. Other uses of phosphogypsum are presently

prohibited without prior approval of the EPA.



11. USES OF PHOSPHOGYPSUM IN AGRICULTURE

A. SOURCEOF S AND Ca FOR CROPS

1 . Sulfur Deficiency and Need for Ca Source Other Than Lime



Sulfur is essential to plant nutrition. In general, plants contain as much S

as P, the usual range being from 0.2 to 0.5% on a dry-weight basis. Sulfur

ranks in importance with N as a constituent of the amino acids cysteine,

cystine, and methionine in proteins that account for 90% of S in plants. It

is also involved in the formation of oil in crops such as peanut (Arachis

hypogaeu L.), soybean [Glycine max (L.) Merr.], flax (Linum usitissimum), and rapeseed (Brassica campestris) (Tisdale et al., 1985).

In the past three decades, S deficiencieshave been reported with increasing frequency throughout the world (The Sulphur Institute, 1982; Tisdale

et al., 1986). Sulfur status reports for the United States and Canada (Tabatabai, 1986), Brazil (Jones, 1967), the Australasian region (McLachlan,

1979, India (Tandon, 1986; Takkar, 1986), China (Zhongqun, 1986),

Indonesia (Ismunadji, 1986), Thailand (Keerati-Kasikorn, 1986), and

Bangladesh (Mazid, 1986; Islam et al., 1986) indicate a growing need to

meet S deficiency in these countries.

The reasons given for the increasing S deficiencies worldwide are (1) the

shift from low-analysis to high-analysis fertilizers containing little or no S,

(2) use of high-yielding crop varieties that remove greater amounts of S

from the soil, (3) reduced industrial S emission into the atmosphere due to

pollution-control measures and decreased use of high-S-content fossil fuels,

(4) decreased use of S in pesticides, and ( 5 ) declining S reserves in soil due

to erosion, leaching, and crop removal. Increased consumption of sulfur-



66



I. S. ALCORDO AND J. E. RECHCIGL



free, high-analysis fertilizers is seen as the most important reason for the

increasing S deficiency worldwide (Jordan, 1964; Moms, 1986).

Calcium, with concentrations ranging from 0.2 to 1.OYo in plant tissues,

is also essential to plant life. Calcium deficiency manifests in the failure of

terminal buds and apical tips of roots to develop. Also, lack of Ca results in

general breakdown of membrane structures, with resultant loss in retention

of cellular diffusible compounds. Disorders in the storage tissues of fruits

and vegetables frequently indicate Ca deficiency (Tisdale et al., 1985).

The need for Ca by plants may be readily satisfied by liming materials

such as calcitic and dolomitic limestone. However, lime application in

large amounts on certain soils can be detrimental to plant growth. Kamprath (1971), in a review of the effect of lime on Oxisols and Ultisols,

reported that lime application that raised the soil pH to 7 resulted in

reduced rates of water infiltration, reduced availability of P, B, Mn, and

Zn, and reduced growth of sudangrass(Sorghum vulgare var. Sudanese L.),

corn (Zea mays L.), and soybean. Therefore, for certain soils that require

large amounts of Ca to support commercially viable crop yields, or for

crops such as peanut that need large amounts of readily soluble Ca, a Ca

source other than lime may be necessary.

Thus, with increasing S deficiencies worldwide and the need for a Ca

source other than liming materials, phosphogypsum deserves serious consideration for agricultural applications where mined gypsum has traditionally been used. Because phosphogypsum and mined gypsum are chemically similar, studies on mined gypsum are also cited throughout this paper

when they may be of assistance in evaluating phosphogypsum for a particular agricultural application.

The present review highlights a scarcity of studies on phosphogypsum.

Amacher and Miller (1 987), in assessing the use of phosphogypsum in

agriculture, cited only eight papers on its use as a source of S and Ca for

crops, all of which were conducted outside the United States. Shainberg et

al. (1989), in their extensive review of the use of gypsum on soils, cited 10

studies on phosphogypsum published from 1983 to 1986, all of which were

conducted in Brazil, and all pertained to its use in alleviating subsoil

acidity rather than as a source of S or Ca for crops. The scarcity of studies

on phosphogypsum use in agriculture worldwide could be due to the

availability of other sources of S and Ca. In the United States, environmental concerns and government regulations hindered the use of phosphogypsum even for agricultural research (Federal Register, 1989). However, results of on-going environmental studies on phosphogypsum use in

Florida (Rechcigl et al., 1992a) and in other southeastern states (Mullins

and Mitchell, 1990) may help change existing regulations to allow for the

use of phosphogypsum in commercial agriculture.



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