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Title:Treatment of Spent Chemical Oxygen Demand Solutions for Safe Disposal
Author(s):Holm, Thomas R.
Contributor(s):Illinois State Water Survey
Subject(s):Mercury wastes -- Treatment
Sulfuric acid -- Treatment
Chromium compounds -- Treatment
Chemical laboratories -- Hazardous wastes -- Waste disposal
Abstract:The chemical oxidation demand (COD) test is used in numerous laboratories to characterize wastewater and effluent. The COD reagent contains high concentrations of sulfuric acid and mercury and dichomate salts. All treatment methods must first neutralize the sulfuric acid and, if necessary, reduce the dichromate to trivalent chromium. Neutralizing the acid and adjusting the pH to pH > 5 causes the precipitation of mercury oxide (HgO), which removes 96% of the mercury. This agrees very well with the amount of precipitation predicted by chemical equilibrium calculations. Adjusting the pH to a value greater than 7 causes chromium hydroxide precipitation, which reduces the soluble chromium concentration to less than 100 micrograms/liter (a reduction of greater than 99.99%), in fair agreement with chemical equilibrium calculations. A method for precipitating the mercury as mercury sulfide is reported. Excess sodium sulfide is added to precipitate mercury sulfide, then excess zinc is added to precipitate the excess sulfide. The mercury concentration in the filtrate is reduced to less than 3 micrograms/liter (a reduction of greater than 99.999%). There is no detectable odor of hydrogen sulfide. The mixed precipitate (mercuric sulfide, zinc sulfide, and chromium hydroxide) can be sent to a commercial mercury reprocessor.
Issue Date:1996
Publisher:Illinois Hazardous Waste Research and Information Center
Series/Report:Technical Research Report / Hazardous Waste Research & Information Center ; 20
Genre:Technical Report
Type:Text
Language:English
URI:http://hdl.handle.net/2142/2035
Sponsor:Illinois State Water Survey
Prepared for Illinois Hazardous Waste Research and Information Center HWRIC Project 93-112
Date Available in IDEALS:2007-09-08


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