In the analysis of Chemical Oxygen Demand (COD) a liquid residue rich in heavy metals (Hg, Ag, Cr, Fe) is obtained. This work aimed to remove, in a selective way, such metals from the residue using chemical precipitation, also creating a possibility to recover and reuse the heavy metals,. Different precipitants were evaluated (Cl-, Br-, I- and S= for Ag and Hg, and NaOH, NH4OH and NaHCO3 for Cr and Fe). The best results for selective recovery of Ag and Hg were obtained using chloride followed by sulphide. Due to the presence of Hg(I) it is necessary the use of NH4OH to separate Ag and Hg(I) that are both precipitated as AgCl and Hg2Cl2. Removal of Cr and Fe that attends the local limits set by the official control agency (FEPAM) was only obtained when NaOH was used as precipitating agent.
Waste; physico-chemical treatment; Ag and Hg recovery
