Hazardous Waste Chemicals from Dichromate Chemical Oxygen Demand Analysis: Toxicity Reduction, Recovery, Recycling, and Testing

This study aims to find a solution to a pressing hazardous waste problem involved in the determination of Chemical Oxygen Demand (COD) testing by the dichromate method. Specifically, it investigated the effects of chromium(VI) concentration and acidity on the reduction- based detoxification of this dangerous ion, the recovery of the mercuric sulfate, and the reuse of the recovered susbtance. The study adopted the experimental research design. For the reduction experiments, the supernatant liquid of the spent reagents containing the Cr(VI) was used as sample. Iron filings are used as the reductant. It was added at an amount equal to 1: 1 Fe to Cr(VI) stoichiometric mass ratio. For the recovery experiments, the precipitate consisting mainly of mercury-chloride complex was used as sample. Synthetic water solutions were used in all COD determinations employed for evaluating the masking effectiveness of the recovered mercuric sulfate. Results revealed that at decreasing initial Cr(VI) concentration, the efficiency of chromium reduction by iron filings decreases; while at decreasing initial acidity, the efficiency of the reduction increases. Furthermore, mercury in spent reagents can be eliminated and mercuric sulfate can be recovered but its masking ability is not known because, under the conditions used for COD analysis (spectrophotometric with closed-tube digestion), there is indication that chloride is not at all oxidized.