Removal of heavy metals from COD analysis wastewater with an organic precipitant

Removal of heavy metals from COD analysis wastewater with an organic precipitant


English :
Conventional treatment of wastewater samples generated from Chemical Oxygen Demand (COD) analysis in environmental laboratories usually encounters difficulty due to its strong acidity and high concentrations of heavy metals. This research explores an alternative method using an organic precipitant, dithiocarbamates, to remove heavy metals in wastewaters generated from COD analysis. The optimum treatment conditions were determined using four synthetic wastewaters. Each wastewater contained a single heavy metal, Hg, Ag, Cr or Fe, at concentrations of 150, 200, 35 and 135 mg/1, respectively. The optimum conditions for Hg and Ag removal were obtained by adjusting the initial pH of the synthetic wastewater with NaOH to 4 and adding four and one stoichiometric equivalents of dithiocarbamates, respectively. In contrast, Cr and Fe can be readily removed by hydroxide precipitation. The heavy metal concentrations in the treated synthetic wastewaters were 0.005,1, 0.1 and 0.3 mg/1 for Hg, Ag, Cr and Fe, respectively. Consequently, the treatment conditions obtained from the synthetic wastewater experiments were tested with real COD analysis wastewater. The dosage of the organic precipitant added was 3.3 g/g Hg and 1.53 g/g Ag. Prior to experimentation, the real wastewater was diluted with tap water by 10 folds, to yield the concentrations of 201, 182, 46.4 and 138 mg/1 for Hg, Ag, Cr and Fe, respectively, and pH of 0.3. The concentrations in the treated supernatant effluent after 30-minute settling were 0.001, 0.07, 0.1 and 0.3 mg/1 for Hg, Ag, Cr and Fe, respectively, and the final pH value was 8.5. This lab-scale treatment method can remove the heavy metals in the effluent that complies with the Thai industrial effluent standard. The estimated cost of treatment including the expense for sludge treatment is 11,000- 16,000 baht/m3 or 0.9- 1.3 Baht/sample from COD analysis by an open reflux method.