Recovery and Recycling of Silver from Residual Solution Derived from Determination of Chemical Oxygen Demand

Abstract

Analytical methodologies that generate wastewater rich in potentially recoverable silver ions are commonly used to determine chemical oxygen demand (COD). The chemical residue resulting from these assays is classified not only as dangerous, containing high concentrations of heavy metal ions, Ag+, Hg (2+), Cr3+ and Cr(VI), but also is highly acid. The disposal of the crude residue in the environment will result in negative environmental impacts and financial loss, since silver has significant value. Considering the aforementioned, the risk of silver shortage, as well as the fact that silver sulfate is one of the most expensive inputs used in these assays, in this paper we proposed procedures for the recovery of silver present in residual solution from the COD determination and converting the silver recovered in an acid solution of silver sulfate. In order to do so, the silver present in the residual solution was precipitated as AgCl, which, after being separated from the liquid, was converted into metallic silver, using sucrose as reduction agent. The recovered metallic silver has greater than 99 %(w/w) purity, and the acidic solution of silver sulfate prepared from the recovered material was tested in COD determination, providing comparable results to those obtained with acid solution prepared from commercial Ag2SO4.