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  Evaluation of Chemically Bonded Phosphate Ceramics for Mercury Stabilization of a Mixed Synthetic Waste (EPA/600/R-03/113) March 2003

This experimental study was conducted to evaluate a stabilization and encapsulation technique, called the Chemically Bonded Phosphate Ceramics (CBPC) technology, for synthetic waste materials contaminated with mercury and mercuric chloride. The technology was developed by Argonne National Laboratory.

Leachability tests were carried out by the constant-pH leaching test, the Toxicity Characteristic Leaching Procedure (TCLP), and the TCLP "Cage" modification. X-ray diffraction and spectroscopic techniques (using a scanning electron microscope, an energy-dispersive spectrophotometer, and a wave-dispersive spectrophotometer) were used to identify the solid-state mineral phases.

Data obtained from this study showed that stabilization of wastes considerably reduced the leachability of mercury. TCLP results showed that leachability of mercury decreased by a minimum of two orders of magnitude and a maximum of five orders of magnitude. The variation in the decrease in leachability was dependent on the amount and state of mercury in the waste.

Maximum reduction in leachability of stabilized wastes was observed with wastes containing elemental mercury at 50 weight percent (wt%) loading, followed by wastes containing mercuric chloride at 50 wt% loading, mercuric chloride at 70 wt% loading, and elemental mercury at 70 wt% loading. The three test methods produced similar amounts of leached mercury, but the constant-pH leaching procedure samples released slightly higher levels (at pH=2) compared to the TCLP methods.

On comparing the results obtained with the standard TCLP and the TCLP "Cage" modification, it was observed that leachates from stabilized wastes containing 50 wt% loading of elemental mercury and mercuric chloride were within the Land Disposal Restrictions requirement. Moreover, leachability indices measured with the TCLP "Cage" modification procedure showed high leachability indices, which indicates that mercury was retained well within the solid matrices. However, wastes containing 70 wt% loading of mercury and mercuric chloride had leachate concentrations exceeding the 0.2 milligrams per liter treatment standard and therefore did not meet disposal requirements in the Resource Conservation and Recovery Act.

Typical cost data for several competing treatment technologies for mercury-contaminated hazardous wastes were compared. The cost estimate ranges from $2.88 per kilogram (kg) for sulfur polymer cement stabilization/solidification to $16.37 per kg for conventional Portland cement stabilization (including disposal). The total cost (including raw materials, labor, and disposal for the CBPC process at $15.45 per kg) was on the high end of the treatment cost scale.


Paul Randall

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