Characterization of Radioactive Aerosols in Florida Phosphate Processing Facilities

Abstract

The health risks to workers in the Florida phosphate industry resulting from chronic inhalation of radionuclide-containing aerosols have not been adequately addressed. The present study establishes a database of information on the particle size distribution, density, shape, chemical composition, and radioactivity concentration for six phosphate facilities in the northern and central regions of the state. A seven-stage cascade impactor was employed to sample aerosols at various processing areas in these plants. Aerosol mass loadings are lowest mainly at shipping areas where they are approximately one order of magnitude less than at granulator areas within equivalent particle size intervals. Aerosol mass concentrations increase as the aerosol size increases for the majority of the plants and operational areas. The aerosol loading at each area varies widely depending on plant, and the variance is largest at the storage areas due to the variability of mechanical operations and patterns of building ventilation. The density of bulk dry product, settled dust, and airborne particles are between 1.6 to 1.7 g/cm ³ . Under electron microscopy, the particles appear as spheroids or rough spherical fragments across all plants, work areas, and sampled size intervals. The main elemental components of large-sized and medium-sized aerosols are similar to those found in the bulk dry product. For small-sized aerosols (0.2–0.4 μ m), the fraction of phosphorus is very small in comparison to elemental impurities such as silicon and sulfur. The ²³⁸ U decay series was found in both bulk dry product, settled dust, and airborne particles collected via high-volume samplers. The ²³⁸ U, ²²⁶ Ra, and ²¹⁰ Pb radioactivity concentrations in bulk dry product and settled dust from central Florida range from 63–112 pCi/g, 0.8–1.5 pCi/g, and 7–9 pCi/g, respectively. No significant differences in the radioactivity concentrations of ²³⁸ U and ²²⁶ Ra were found between dry product, settled dust, and sampled aerosols. However, ²¹⁰ Pb is highly concentrated in aerosols up to 25–87 pCi/g, most likely due to the deposition of ambient airborne radon decay products on workplace aerosols. The database of worker aerosol physicochemical characteristics established in this study for the Florida phosphate industry can be directly used for more realistic assessments of worker inhalation dose, thus providing a more firm basis for assessing the adequacy of existing respiratory and other radiological protection policies.

This work was supported by Grants #00-05-062R and #03-05-064 from the Florida Institute of Phosphate Research (FIPR). This article is dedicated to Professor William Emmett Bolch of the Department of Environmental Engineering Sciences at the University of Florida who passed away on December 26, 2003. The authors greatly appreciate his contributions, support, and devotion to this study and to the field of environmental health physics. The authors gratefully acknowledge Dr. Tony James (ACJ & Associates) and Dr. Alan Birchall (NRPB) for their assistance with the use of IMBA Professional v.3.0 for this research study.

Notes

^a International Commission on Radiological Protection;

^b Human respiratory tract model;

^c Activity median aerodynamic diameter;

^d Geometric standard deviation;

^e Activity median thermodynamic diameter;

^f Moderate absorption into blood;

^g Fast absorption into blood;

^h Slow absorption into blood.

^a Capital letters are plant identifiers. Plant A-a and Plant A-b refer to the same company plant (Plant A), but two different locations within the plant. A-b(1) and A-b(2) are two different sampling sites at the same location.

^a Data from material safety data sheet (CF, 2003a; CF, 2003b; Mosaic, 2003b; Mosaic, 2003a; PCS, 2003b; PCS, 2003a).

^a Monoammonium phosphate;

^b Diammonium phosphate.

^a Data from studies by Guimond (1978), Roessler et al. (1979), Owen and Hyder (1980), NCRP (1987), and Birky et al. (1998).