Phosphogypsum is used in agriculture throughout the world [33], for example in Australia, Bangladesh, Brazil, Egypt, India, Kazakhstan [136], Pakistan, Spain [161], the Syrian Arab Republic and the USA [162]. In Brazil, 40% of all phosphogypsum produced is used in agricultural applications. In California, USA, a stack of phosphogypsum originating from phosphate rock from Idaho and Wyoming was utilized completely for agricultural purposes, prior to such use being restricted by regulations in 1989. A very small quantity of phosphogypsum with a low radium content produced in Florida, USA (representing only 0.03% of total TABLE 33. RADON EMISSION FROM PHOSPHOGYPSUM STACKS
Location
Mean Ra-226 concentration in phosphogypsum
(Bq/g)
Mean radon flux (Bq·m–2·s–1)
Radon flux per unit Ra-226 concentration (Bq·m–2·s–1 per Bq/g)
Central Florida, USA [150, 151]:
Stack, side slopes, access roadsa 1 1 1
Intermittently ponded areas 1 0.075 0.075
South-western Spain [159]:
Undisturbed surface 0.73 0.01–0.16 0.014–0.22
Surface disturbed by tilling 0.73 0.04–0.3 0.055–0.41
Agricultural soil (for comparison) 0.035 0.003–0.011 0.086–0.31
a The individual mean radon fluxes from the stack, side slopes and access roads were similar.
Florida production), continues to be used as a soil amendment in peanut farming. In the European Union, phosphogypsum is permitted for use as a soil amendment under the category ‘calcium sulphate’ [163, 164]. Phosphogypsum contributes to agricultural production in four principal ways:
(i) Reclamation of land such as estuarine marsh in order to render it agriculturally productive;
(ii) Remediation of saline and sodic soils (see Fig. 44);
(iii) Amendment of soil to prevent crusting and to enhance water retention;
(iv) Fertilization of soil for growing crops and pasture.
Phosphogypsum usually does not need to be tilled into the soil. Freshly generated phosphogypsum is unweathered and moist, and in this state it can be applied directly to the surface of the soil using conventional methods of distribution (see Fig. 45).
A large area of land near Huelva, Spain, has been progressively reclaimed by the application of phosphogypsum and returned to productive agricultural use.
An aerial view of the reclaimed land is shown in Fig. 46. This site is particularly FIG. 44. Soil near Huelva, Spain, prior to remediation with phosphogypsum (courtesy:
University of Seville, Spain).
FIG. 45. Direct application of phosphogypsum to soil (courtesy: University of Seville, Spain).
FIG. 46. Land reclaimed by the use of phosphogypsum (courtesy: University of Seville, Spain).
significant in terms of risk assessment since it is the only documented area in the world which can be used to study the impact of 40 years of continuous use of phosphogypsum on soils, a practice that still continues at this locality.
The benefits of applying phosphogypsum to saline and sodic soils are described in Refs [136, 161, 162] and include:
(a) Reduced sodium and aluminium toxicity of the soil;
(b) Increased calcium and sulphur dissolved from the phosphogypsum;
(c) Increased ammonia retention by the soil;
(d) Increased water retention of the soil through better conditioning;
(e) Greater water efficiency.
The use of phosphogypsum as a fertilizer, typically at application rates of 100–600 kg/ha, has been found to increase significantly the production rates of the following crops [33]:
Phosphogypsum is often used in its unmodified state. Depending on the specific crop application, however, it may be necessary to remove the acidic water (and even some of the other impurities) contained within it. For example, phosphogypsum ‘harvested’ for agricultural use from the inactive portion of a stack in northern Florida, USA, is first exposed to rainwater for about a year.
When the rainwater permeates the phosphogypsum, the acidic process water contained in the phosphogypsum is displaced, causing the pH of the harvested phosphogypsum to increase to a value greater than 5. In general, any removal of impurities has to be kept to a minimum to avoid making the use of phosphogypsum uneconomic compared with the use of relatively inexpensive natural gypsum.
Alfalfa Carrots Lemons Peaches Sugar beet
Apples Citrus Lentil Peanuts Sugar cane
Avocado Coffee Limes Pepper Sweet sorghum
Bahia grass Corn Lucerne Pineapple Tea
Barley Cotton Maize Rapeseed Tobacco
Beans Cover crops Mustard Rice Tomato
Beets Forage Niger Rye grass Turnip
Bermuda grass Frasier fir Onion Sorghum Upland rice Black gram Groundnuts Oranges Soya beans Vegetables Cabbage Guinea grass Pasture grass Squash Wheat
The use of phosphogypsum as a soil amendment has been studied extensively to determine the extent to which the introduction of additional heavy metals and radionuclides through the application of the phosphogypsum could lead to possible human health effects via the following pathways:
(a) Uptake of radioactivity and heavy metals from the amended soil by edible crops;
(b) Inhalation of radionuclides in airborne dust during the application of the phosphogypsum;
(c) External exposure rates due to the amended soil;
(d) Groundwater contamination;
(e) Radon emission from the amended soil.
In an evaluation of the application of phosphogypsum as a soil amendment and fertilizer [165], phosphogypsum with a 226Ra activity concentration of about 1 Bq/g was assumed to have been applied to the soil every second year for 100 years after an initial application of twice the biennial application rate. Six scenarios were considered, involving various rates of phosphogypsum application in the range 1.66–10 t/ha and tillage depths of 22–46 cm. After the 100 year period,
226Ra concentrations in the soil were calculated to be 0.03–0.12 Bq/g.
An investigation was carried out to determine the effect of phosphogypsum, when used as a calcium fertilizer for the production of crops such as cotton, on the resulting levels and behaviour of radionuclides [166]. Phosphogypsum with a
226Ra activity concentration of 0.51 Bq/g was applied at rates of 13 and 26 t/ha in conjunction with manure at a rate of 30 t/ha. The concentrations of 226Ra in the water draining from the fertilized areas were similar to those reported for non-treated areas (2.6–7.2 mBq/L).22 The results revealed that the activity concentrations of 226Ra in the crops were not affected by the phosphogypsum treatment and that there was no accumulation of radioactivity in the soil. Similar studies to investigate the buildup of radioactivity in soil or uptake by crops show no significant uptake in most cases [167, 168]. In a study carried out in five land parcels of the central Florida phosphate district, approximately 70 indidividual
22 While the 226Ra concentrations in the drainage water were not found to be elevated, the concentration of 238U was found to be 200 mBq/L; an order of magnitude higher than normal. However, the 234U:238U isotopic ratio in the uranium-enriched drainage water was 1.16, as opposed to a ratio of 1 in the phosphogypsum and other phosphorus fertilizers used, from which it was concluded that most of the additional uranium in the drainage water did not originate from the phosphogypsum but was thought to originate instead from the uranium naturally present in the soil (at a 238U activity concentration of 0.025 Bq/g) that had become desorbed.
food samples were collected and analysed for 226Ra, 210Pb and 210Po [62, 63].
Concentrations of 226Ra and 210Pb in food grown on mined phosphate lands were statistically higher than the concentrations of these nuclides in foods from unmined lands. However, an upper estimate of the likely incremental increase in annual ingestion dose was only 0.027 mSv. A radiological impact assessment of the application of phosphogypsum in agriculture was reported from Greece [169].
The concentration of 226Ra in rice and other agricultural products produced from soil tilled with phosphogypsum ranged from 0.0004 to 0.002 Bq/g and the estimated ingestion dose was 0.86 µSv/a.
A completely randomized greenhouse experiment was carried out in Spain growing tomato (Lycopersicum esculentum Mill L.) using a reclaimed marsh soil amended with various rates of phosphogypsum up to 20 t/ha to investigate the transfer of 238U, 226Ra, 210Po, Pb and Cd to the crop [170]. The experimental set-up is shown in Fig. 47. The concentrations of 238U and 226Ra in the crops were below the detection limit in all cases and the concentration of 210Po was 0.0004–0.0007 Bq/g on a dry weight basis. The results also showed that the FIG. 47. Greenhouse studies involving various crops (courtesy: University of Seville, Spain) [170].
application of phosphogypsum as a calcium amendment for agricultural purposes could lead to some uptake of cadmium in the plants.
In the Cerrado region of Brazil, where significant amounts of phosphogypsum are used for agricultural purposes, a field study explored the impact on two typical soils of the region, one clayey and the other sandy. These analyses included an evaluation of the mineralogical composition, the organic matter content and the concentrations of P, K, Ca, Mg and Al. The concentrations of radionuclides and metals in the phosphogypsum and soil samples were also measured. The organic matter content of the soil samples was low and the potential acidity high. The mean 226Ra activity concentration in the phosphogypsum samples was 0.252 Bq/g. In addition, this study verified that the concentrations of radionuclides and metals in the phosphogypsum were lower than the background concentration in the clayey oxisol soils of Sete Lagoas, Minas Gerais, Brazil. These results indicated that the application of phosphogypsum as a soil amendment in agriculture would not have a significant impact on the environment [171].
A three year field study in Florida, USA, involving the application of phosphogypsum at relatively low rates (up to 4 t/ha), showed no statistically significant increases in radionuclide concentrations in soils and groundwater or in the levels of airborne radon and gamma radiation measured 1 m above the plots [172]. A subsequent study, which has become a benchmark because of its comprehensive methodology and scope [173], developed data to assist in the assessment of the environmental impacts of the application of phosphogypsum at higher rates (up to 20 t/ha) to an established bahia grass pasture, primarily in terms of the radionuclides in phosphogypsum and secondarily in terms of the heavy metal impurities. The results can be summarized as follows:
(a) Exposure from the inhalation of radon progeny was determined from measurements of the 226Ra activity concentration in the soil, the radon flux from the soil and the radon concentrations in the air. For the application of phosphogypsum at a rate of 0.4 t/ha over a 100 year period, the incremental radon flux from the amended soil was projected to be about 40% of the mean value for undisturbed land (with no phosphate mineralization) in the region. For a house constructed on the amended soil, the phosphogypsum in the soil was estimated to increase the indoor radon concentration by about 1–10 Bq/m3 (representing, for a typical house, an increase of about 2–20%).
For a cumulative treatment of up to 40 t/ha, the phosphogypsum was predicted to contribute less than 3.7 Bq/m3 to the radon concentration over the field.
(b) Exposure to external gamma radiation was determined from dose rate measurements made following a single treatment of phosphosgypsum of up
to 40 t/ha. After the first year, no gamma exposure attributable to the phosphogypsum could be detected. It was concluded that the radionuclides from the phosphogypsum had penetrated the soil or had been removed by weathering or harvesting. The incremental annual effective dose received by an individual remaining permanently on the treated land was projected to be 0.028 mSv after 100 years.
(c) Exposure from the ingestion of radionuclides in water and food was determined from measurements of the activity concentrations of 226Ra,
210Pb and 210Po in samples of soil, water and forage. The results suggested that the radionuclides contained in the phosphogypsum had limited mobility in surface water and groundwater during the first two years after application to the soil. However, it is possible that they may have been gradually mobilized, appearing in the groundwater at a later date. The activity concentrations of 226Ra in shallow groundwater after 100 years of phosphogypsum use were projected to be about 0.1 Bq/L. Levels of 210Pb were projected to be similar to the baseline levels in runoff and shallow groundwater (<0.04 Bq/L). Doses to humans from the ingestion of animal products that had been contaminated with radionuclides taken up by forage appeared to be within the range of variation in a normal diet.