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* The level of metals removal in a mixed-metal waste stream is dependent uponseveral factors including the concentration of the metals, pH, and added sulfidedose.
* Control of the pH and selection of the proper sulfide dose is critical to achieve the conditions necessary for the desired degree of metal sulfide precipitation.
* Selective precipitation of multi-metal waste streams may not be viable at the pilot- scale or full-scale levels using sulfide as a reagent due to the strict control of sulfide addition necessary to achieve sufficient separation.
The present computer model takes into account only the hydrolysis products of the
soluble metals and the aqueous protonation of the free sulfide. Expanded modelling work should consider the possible formation of additional soluble metal species, such as CdHS'*''
and the possible presence of other complexing/chelating agents in metal-containing wastewaters. In addition, because the results of this study are theoretical in nature, future studies should include undertaking experimental work to determine if the theoretical equations established in this study are valid. Recommended studies include:
1. Experimental validation of the theoretical models developed in this report.
2. Experimental studies establishing the effects of chelating agents on the
precipitation of metal sulfides.
3. Kinetic studies to determine the extent to which oxygen interferes with the metal-sulfide precipitation reactions due to sulfide oxidation.
4. Kinetic studies to determine the effects of the leaching of metals and sulfide
REFERENCES
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17. Scott, M.C., "An EPA Demonstration Plant for Heavy Metals Removal by Sulfide
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