Radionuclides, trace elements, and radium residence in phosphogypsum of Jordan
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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.
KeywordsRadium Rare earth elements Phosphogypsum Barite Radiobarite Jordan
This research was conducted over 9 months in 2004–2005 while the second author, M.S. Al-Hwaiti, was a visiting Fulbright Scholar at the Colorado School of Mines, Golden, Colorado. We thank the personnel of the Aqaba Industrial Complex and the Indo-Jordan Chemical Company for site access and for support of field sampling. R. L. Driscoll of the US Geological Survey (USGS) performed X-ray diffraction analysis and fine-particle separations. H.A. Lowers of the USGS provided assistance with the operation of the scanning electron microscope. Any use of trade, product, or firm names is for descriptive purposes only and does not imply endorsement by the US government.
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