Uranium and heavy metals in Phosphate Fertilizers

Agricultural applications of chemical fertilizers are a world wide practice. The specific activity of uranium-238 and heavy metals in phosphate fertilizers depends on the phosphate ore from which the fertilizer produced and on the chemical processing of the ore. Composite phosphate fertilizers samples where collected and the uranium-238 specific activity, in Bq/kg, and As, Cd, Cu, Pb, Se concentration, in ppm, were measured. The annual addition of these elements in soil due to fertilization were calculated and discussed. INTRODUCTION Since the 1950s, the application of plant nutrients, including phosphate fertilizers, has increased substantially. More than 30 million metric tons of phosphate fertilizers are annually consumed worldwide, which increase crop production and land reclamation (Lambert et al., 2007). The long-continued application of phosphate fertilizers can redistribute and elevate uranium and toxic heavy metals, such as As, Cd and Pb, in soil profiles and consequently their transfer to the food chain, mainly in acid soils. It can also raise these elements concentrations in irrigation runoff/drainage waters (da Conceicao, Bonotto, 2006). This work aims at estimating the concentration of uranium and heavy metals (As,Cd, Cu, Pb and Se) in phosphate fertilizers used in Saudi Arabia and to investigate the possible environmental hazards. MATERIAL AND METHODS Sampling and sample preparation Thirteen phosphate fertilizer samples of ten different brands were collected from the local market of Riyadh City. Five of those brands were of granular form, while the other five were leafy (water soluble) phosphate fertilizers. Samples were pulverized and homogenized. Analytical Techniques Gamma spectrometric analysis; the dried samples were transferred to polyethylene containers of 100 cm capacity Uranium-238 specific activity was measured using well calibrated gamma spectrometry based on hyper-pure germanium (HpGe) detectors. The HpGe detector had a relative efficiency of 40% and full width at half maximum (FWHM) of 1.95 keV for Co gamma energy line at 1332 keV. The gamma line transition 1001 keV was used for U specific activity calculation and the spectrometer was calibrated using reference uranium ore sample (IAEA-RG-U). Inductively Coupled PlasmaMass Spectrometer (ICP-MS); the concentration of As, Cd, Cu, Pb and Se were measured using ICP-MS after chemical dissolution and dilution. Standard solutions were used for calibration and quality control measures. RESULTS AND CONCLUSIONS Specific activity of U in Bq/kg and concentration of As, Cd, Cu, Pb and Se in ppm in phosphate fertilizers in Saudi Arabia and their statistical summary are given in table 1 and 2 respectively. The granule fertilizers are applied directly to the soil and dissolved slowly in the irrigation water. The leafy fertilizers (powder form) are dissolved in water that is spraying onto plants’ leaves. The average activities (range) of U was 1017 (173.8-2234) Bq/kg and <20 Bq/kg in granule and leafy phosphate fertilizers, respectively. In the locally produced fertilizer samples, the U, average (range) 1791 (1273 – 2234) Bq/kg, are much higher than Table 1: Specific activity of 238U in Bq/kg and concentration of As, Cd, Cu, Pb and Se in ppm in phosphate fertilizers in Saudi Arabia Type Sample U-238±EP As Cd Cu Pb Se PB2BOB5B % Ra 226P U-2 PH 1* 1774±3 7.6 7.6 20 <DL 29.15 46 0.00 PH 2* 2234±5 46 0.00 PH 3* 1884±2 7.5 7.51 22 <DL 28.04 46 0.00 PH 4* 1273±1 23 0.00 PH 5P 193±0.34 0.490.50 17 0.67 -0.82 17 0.6 PH 6P 233±0.32 0.510.50 14 0.66 17 0.5 PH 7P 368±0.42 1.0 1.0 11 1.6 23.92 18 0.7 G ra nu le fe rti liz er s PH 8P 174±0.23 4 4.1 111 0.63 0.79 17 0.2 PH 9 < 20 0.010.10 63 0.09 2.21 20 PH 10 < 20 0.210.21 9.2 0.11 1.71 44 PH 11 < 20 0.26 29 0.06 3.60 30 PH 12 < 20 0.02 26 <DL 0.14 20 Le af y Fe rti liz er s PH 13 < 20 0.040.04<DL<DL 3.61 52 P&P Error * locally produced + imported that in imported fertilizer samples, 242 (174 – 368) Bq/kg, and vice versa for Ra, 3.7 (2.7 – 4.9) and 145 (35.0 – 283) Bq/kg, respectively. Moreover, the specific activities of U and Ra in imported granule fertilizers are comparable with Ra/U activity ratios in the range of 0.2-0.8 and in the range 0.6-0.8 after excluding one imported phosphate sample (PH8). Also, their activities are strongly correlated with correlation coefficient (R) value of 0.95 (Khater et al., 2008). Generally, the specific activities of natural U series radionuclides in phosphate fertilizers depend on their levels in the used raw phosphate ore material. Radioactivity levels in phosphate ores are varied according to their geological origin (sedimentary, volcanic or biological origin) where U and its decay products tend to be elevated in phosphate deposits of sedimentary origin due to the accumulation of dissolved uranium, in the form of uranyl complex, in the sea water during geological formation of the phosphate rocks. A typical concentration of U in sedimentary phosphate deposit is 1500 Bq/kg (UNCEAR, 1993; Khalifa and El-Arabi, 2005). During phosphate fertilizers manufacture the phosphate ore is firstly attacked by sulfuric acid to produce the phosphoric acid (green acid) where uranium will be mainly concentrated in the phosphoric acid while radium, polonium, thorium and other insoluble radionuclides will be precipitated as sulfate salts and concentrated in the phosphgypsum by product. That could explain the high