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Abstract
238U and 232Th concentrations in phosphate fertilizers are of critical importance due to the concerns that via several pathways these radionuclides will reach and potentially affect man. These radionuclides are introduced in the environment because phosphate fertilizers contain natural radionuclides in relatively large quantities and enter agricultural land during cultivation. Also, phosphate fertilizer industry is one of activities leading to environmental pollution with fluoride and heavy metals. Wastewater produced from this industry and discharged into the aquatic environment is one of the sources of environmental pollution with these radioactive and stable elements.
Therefore, the main aim of this study was to assess the environmental pollution caused by a production plant of phosphate fertilizers through investigation various types of environmental samples taken from around the outlet of the wastewater discharge pipes of this plant. Such information would lead to an accurate evaluation for the impact of such industry on environment.
Results of the study
Characteristics of samples and their fluoride content
The pH values of fertilizer samples were ranged from 6.8 to 8.0 (with a mean value of 7.5) and from 6.7 to 7.4 (with a mean value of 7.0) for phosphate and other types of fertilizers, respectively. These pH values are considered a weak base. The range of pH values for sediment samples was from 5.1 to 6.8 with an average value of 6.5, which is considered as weak acid. The water samples had a range of 7.3-8.0 for the pH values with a mean of 7.6, which is considered a weak base. The sediment samples had TOM values between 2.0 and 4.1% with an average value of 2.9%. The TOM values showed that the sediment was a littlie rich in total organic matter content.
The phosphate fertilizer showed higher values of fluoride concentration (mean = 0.16 mg/g) than those for other types of fertilizers (mean = 0.004 mg/g). The results showed that the lowest fluoride content found in Abu-Tartor phosphate raw materials. The fluoride content of sediment samples had a range from 0.42 to 1.88 mg/g with a whole mean of 1.1 mg/g. The results indicated that the wastewater polluted with fluoride produced from the fertilizer company may be affecting the environment.
Heavy Metals contents
The contents of Sr, Sn and Ba were ranged from 397 to 1091, 16.7 to 235 and 471 to 1645 mg/kg for phosphate fertilizer samples, respectively. The concentrations of Sr and Ba in urea fertilizer were found to be under the detection limit of the used X-ray fluorescence spectrometer. A low content of Sn (16.3 mg/kg) was determined in urea fertilizer. In the case of sediment samples, the contents of Sr, Sn, Ba and Fe were ranged from 192 to 435 mg/kg, 33.8 to 117 mg/kg, 1210 to 4284 mg/kg and an under detection limit to 452 g/kg, respectively. The results indicated that there is no significant correlation between the concentrations of any two elements.
No clear results were deduced from the present data to allow testing of the effect of wastewater discharged from the fertilizer company upon the environment due to the limited number of samples. A comparison study of the determined heavy metals in fertilizer and sediment samples - collected from different parts of the world - showed that the existence of differences between their contents is due to the different geological origin of the collected samples.
Radioactivity contents
The results showed that the fertilizer, sediment, water and plant samples had concentration of 226Ra in the range of 0.6-1312 Bq/kg with a mean of 382 Bq/kg, 15.4-33.8 Bq/kg with a mean of 21.5 Bq/kg, 0.06-1.3 Bq/l with a mean of 0.58 Bq/l and 8.9-17.3 Bq/kg with a mean of 13.1 Bq/kg, respectively. The activity concentrations of 232Th are in the ranges of 0.75-24 Bq/kg with a mean of 8.3 Bq/kg, 10.4-19.3 Bq/kg with a mean of 14.2 Bq/kg, 0.015-0.16 Bq/l with a mean of 0.05 Bq/l and 2.0-29.8 Bq/kg with a mean of 13.6 Bq/kg for fertilizer, sediment, water and plant samples, respectively. Also, the activity concentrations of 40K are ranged 2.1-5313 Bq/kg with a mean of 549 Bq/kg, 128-281 Bq/kg with a mean of 219 Bq/kg, 0.14-0.6 Bq/l with a mean of 0.4 Bq/l and 686-977 Bq/kg with a mean of 867 Bq/kg for fertilizer, sediment, water and plant samples, respectively. The results also showed that low content of 137Cs was determined in only two phosphate fertilizer samples (F2 and F5). Most of sediment samples contained a low concentration of 137Cs with a range of 1.0-2.4 Bq/kg. By the analysis of the radioactivity contents results of the samples collected from different locations around the outlet of wastewater discharge pipes of the phosphate fertilizer company, it could be concluded that the environment may be slightly affected with low concentrations of 226Ra and 232Th isotopes due to the discharged wastewater.
In the case of 226Ra concentration, the results of the present work are somewhat comparable for all types of super phosphate samples with the values of the other studies. The observed differences may be due to the different geological origin of the phosphate ore. The concentrations of 232Th and 40K in the present study were the lowest compared with the other studies, the only exception being for 232Th concentration in the triple super phosphate sample. On the other hand, the wide ranges of concentrations of 226Ra, 232Th and 40K are most probably due to the differences of the geological origin of phosphate raw as mentioned before. Also, the results showed that the 226Ra, 232Th and 40K concentrations in all types of studied environmental samples are lower compared with the results of other studies with a few exceptions.
The results of the correlation between the 226Ra and fluoride contents in fertilizer and sediment samples may be supporting to some extent the assumption that the environment may be slightly affected with the discharged wastewater from the phosphate fertilizer industry. The radium equivalent (as a radiation hazard index) ranged from 284 to 1316 Bq/kg with an average value 664 Bq/kg for phosphate fertilizer samples and from 9.6 to 432 Bq/kg for other types of fertilizer. One of phosphate fertilizer samples (F2, super phosphate raw material) has Raeq lower than the maximum which is 370 Bq/kg. Also, one of other types of fertilizer samples (F9, Crystal Nasser) has Raeq higher than this maximum due to the high concentration of 40K.
Recommendations
The treatment of the wastewater produced from the phosphate fertilizer industry is an important way to reduce the impact of this industry on the environment.
Using raw materials that contain low levels of natural radioctivities, heavy metals and fluoride for the phosphate fertilizer industry is preferable.
A good ventilation of the industrial area of the phosphate fertilizer plant and the storage places of the end products and by-products is necessary to avoid radon accumulation.