Section Twenty
HEAVY METAL WASTES
82    COMPARATIVE EVALUATION OF HEAVY METAL
IMMOBILIZATION USING HYDROXIDE AND
XANTHATE PRECIPITATION
R. Mark Bricka, Chemical Engineer
M. John Cullinane, Jr., Research Civil Engineer
U.S. Army Engineer
Waterways Experiment Station
Vicksburg, Mississippi 39180
INTRODUCTION
The U. S. Environmental Protection Agency estimates that there are over 13,000 generators of
metal plating and finishing wastewaters. Treatment of these wastewaters, usually accomplished using
some form of hydroxide precipitation, is estimated to generate approximately 11 million metric tons/
year of sludge (F006) requiring special handling and disposal. The generator of metal contaminated
wastewaters is faced with the dual problem of first removing the metal contamination from the
wastewater, and second, disposing of the residual materials resulting from the wastewater treatment
process. As wastewater treatment and residuals disposal requirements become more stringent, the
development and application of innovative treatment methods becomes more practical.
An alternative treatment method, developed by the U. S. Department of Agriculture,12 uses
insoluble starch xanthates for the removal of heavy metals from wastewaters. Closer investigation
reveals that an entire family of insoluble xanthates can potentially be used for waste treatment.
Xanthates are sulfonated organic compounds. The xanthate-metal precipitation process can be represented as follows:
ROCSSNa + M+ - ROCSS-M + Na+ (1)
NaOH
2(ROCSSNa) + M++ - ROCSS-M-SSOCR + 2Na+ (2)
NaOH
where M +  = metal ion.
Hydroxide precipitation is generally effective in a pH range of 9 to 12. Xanthate precipitation is
effective for a pH range of 3 to 12. A previous study3 showed that xanthates selectively remove metals
according to the following hierarchy.
Na < < Ca-Mg-Mn < Zn < Ni < Cd < Pb-Cu-Hg (3)
Thus, xanthate precipitation methods offers several advantages over hydroxide precipitation.
Unfortunately xanthate precipitation, like hydroxide precipitation, produces significant quantities of
sludges that must be handled in accordance with the Resource Conservation and Recovery Act
(RCRA).
One currently utilized disposal technique for RCRA sludges uses chemical stabilization/
solidification (S/S) to immobilize contaminants. In this process, sludges are chemically treated by
mixing a binder material to improve the physical and chemical stability of the sludge.
The purpose of this study was to evaluate the metal immobilization capabilities of hydroxide and
xanthate sludges, and to compare the effects of S/S on these different sludges. Because of space
limitations, only a portion of the results for this study could be presented in this paper. A comprehensive report is scheduled for publication later this year.4
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