Radionuclides,trace elements,and radium residence in phosphogypsum of Jordan |
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Authors: | Robert A Zielinski Mohammad S Al-Hwaiti James R Budahn James F Ranville |
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Institution: | (1) US Geological Survey, MS 973, Denver Federal Center, Denver, CO 80225, USA;(2) Environmental Science and Engineering Division, Colorado School of Mines, Golden, CO 80401, USA;(3) Present address: Environmental Engineering Department, Al-Hussein Bin Talal University, Ma’an, Jordan;(4) US Geological Survey, MS 973, Denver Federal Center, Denver, CO 80225, USA |
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Abstract: | Voluminous stockpiles of phosphogypsum (PG) generated during the wet process production of phosphoric acid are stored at many
sites around the world and pose problems for their safe storage, disposal, or utilization. A major concern is the elevated
concentration of long-lived 226Ra (half-life = 1,600 years) inherited from the processed phosphate rock. Knowledge of the abundance and mode-of-occurrence
of radium (Ra) in PG is critical for accurate prediction of Ra leachability and radon (Rn) emanation, and for prediction of
radiation-exposure pathways to workers and to the public. The mean (±SD) of 226Ra concentrations in ten samples of Jordan PG is 601 ± 98 Bq/kg, which falls near the midrange of values reported for PG samples
collected worldwide. Jordan PG generally shows no analytically significant enrichment (<10%) of 226Ra in the finer (<53 μm) grain size fraction. Phosphogypsum samples collected from two industrial sites with different sources
of phosphate rock feedstock show consistent differences in concentration of 226Ra and rare earth elements, and also consistent trends of enrichment in these elements with increasing age of PG. Water-insoluble
residues from Jordan PG constitute <10% of PG mass but contain 30–65% of the 226Ra. 226Ra correlates closely with Ba in the water-insoluble residues. Uniformly tiny (<10 μm) grains of barite (barium sulfate) observed
with scanning electron microscopy have crystal morphologies that indicate their formation during the wet process. Barite is
a well-documented and efficient scavenger of Ra from solution and is also very insoluble in water and mineral acids. Radium-bearing
barite in PG influences the environmental mobility of radium and the radiation-exposure pathways near PG stockpiles. |
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