Phosphogypsum Trace elements

To establish the free water content, samples were dried at room temperature to constant weight and then at 45°C for an additional 2 h. The dried samples were then analyzed for chemical, radiological, and trace elements. The pH measured was that of moist phosphogypsum. Water was added to phosphogypsum to produce very thick slurries into which the pH and standard electrodes were immersed and measurements taken. Except for pH and densities, the chemical and radiological results were then calculated back to the weight basis of the samples as received. The particle size distribution was determined on the dried samples. Emission spectrographic results were reported on the basis of the dried samples. 

These results indicated that trace elements were uniformly distributed in the phosphogypsum stacks. A uniform distribution of trace elements in the stacks would occur if the same quantities of trace elements were added to the stacks as were removed through leaching. However, three stacks, C, E. and F, are inactive. Stack C has been idle nine years. Stack E has been idle several months and Stack F has been idle twelve years. In spite of about 100 cm of rainfall a year [9] for nine and twelve years. Stacks C and F also showed no significant difference in concentrations of trace elements with depth. Thus, the results indicated that trace elements were not only uniformly distributed in the stacks but were not leached from the stacks in any significant amount. This also applied to sodium, potassium, copper, and nickel whose sulfates are soluble. 

The first phase of the Bureau s research [1 ] showed that phosphogypsum was not corrosive by Environmental Protection Agency (EPA) criteria. The study also presented evidence that phosphogypsum would not be toxic by EPA criteria and that trace elements and radium would not be leached from the stockpiles. These conclusions were obtained from statistical analyses of extensive quantities of spectrographic and radiological data. More direct confirmation of these conclusions was needed to decisively answer the question of leaching of toxic elements and radium from phosphogypsum stockpiles. 

Table I lists the EPA contaminants and the criteria that EPA has established to constitute a hazardous toxic waste. Table 2 shows the concentrations of the inorganic contaminants in the extract from the phosphogypsum samples. All of the organic compounds listed by EPA as hazardous toxic waste.s were tested by the standard EPA procedure none were detected. These included endrin, lindane, methoxychlor, toxaphene, 2,4-D silvex, and 2,4,5-TP silvex. All of the metals listed in Table I were found to be present in the extract at concentrations lower than allowed by EPA (as shown in Table 2). Therefore, by EPA definition phosphogypsum is not a hazardous toxic waste material. This confirms earlier research conclusions 71 that the leaching of trace elements from phosphogypsum is not significant in introducing hazardous toxic waste materials into the environment.

As shown in Table 3, the moisture content of subsurface material under the phosphogypsum was greater than that of the phosphogypsum. This indicates that the permeability of the subsurface material was less than that of the phosphogypsum. Thus, if trace elements were leached from the phosphogypsum, their concentrations should be increased in the subsurface material. 

Analyses of the subsurface material, before the phosphogypsum was accumulated, were not available. However, the subsurface material below the phos phogypsum consists of sand, clays, and phosphate rock. The concentrations of trace elements in the clays and phosphate rock (Table 6) may be used to estimate the concentration of trace elements present in the soil before the pbos Aogypsum was placed on top of it. For example, from Table 4. it can be seen that phosphate rock comprises 20% of the subsurface material. The concentration of a trace element previously present in the soil, as a result of the presence of phosphate rock, can be estimated as 20% of the value of the trace element shown in Table 6 (Column 3). 

The X-ray diffraction analyses of water-soluble material from phosphogypsum showed only the presence of gypsum and a trace of hemihydrate. The sensitivity of this analysis was not sufficient to identify compounds of the trace elements. 

To help explain the results of this study and to aid in predicting the future stability of phosphogypsum stockpiles, the hydrology associated with the piles and the solubility and absorption of trace elements and radium were considered. How well the objectives of this investigation were met is also briefly considered. 

Evaporation of the leach water from 1000 g of phosphogypsum and X-ray diffraction of the resulting crystals did not identify any compounds of trace elements. Trace elements may exist as sulfates, such as mercuric sulfate, or as calcium salts, such as calcium selenate, in equilibrium with a saturated solution of calcium and sulfate ions from gypsum. Further isolation of trace elements by extraction, precipitation, ion exchange, or other means would so alter this equilibrium that the original amounts and types of compounds present would not be determined. Literature studies of phase equilibria in saturated gypsum solutions were used to identify possible trace element compounds. 

TABLE 13—Comparison of total and teachable trace elements in phosphogypsum. 

Trace elements are not leached from phosphogypsum stockpiles. 

Absorption of trace elements and radium by phosphogypsum is the major reason for their not being leached. 

For some applications it may be necessary to remove impurities such as fluorine (Table 5.3), but limited direct applications have included soil conditioning, sulphur source. In the case of phosphogypsum, the remainder contains many metals, including rare earth elements and trace metals such as U and Cd which are transferred from the original phosphate rock. 

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