Selective electrochemical reduction of mercury(II) from a simulated traditional gold mining wastewater contaminated with cyanide and heavy metals

Abstract

In traditional gold mining (TGM), the wastewater discharges are contaminated with high concentrations of cyanide and heavy metals. Understanding the effects of different cyanide species on the electrochemical behavior of mercury(II) is critical for its removal from TGM wastewater. Herein, we evaluated mercury electroreduction using a model solution simulating a TGM wastewater containing Fe(II), Cu(I), Zn(II), Hg(II), and CN– ions. According to speciation diagrams, the predominant Hg(II) species is Hg(CN)42–. Cyclic voltammetry was used to study the reduction processes from the simulated TGM wastewater solution and to select the potential for the potentiostatic deposition process of mercury. Scanning electron microscopy with energy-dispersive X-ray spectroscopy was used to study the morphology and elemental composition of the deposits. The free cyanide (CN–) concentration affects the electrochemical behavior of the anionic cyanide complex Hg(CN)42– in addition to the chemical stability of 316 stainless steel under open circuit conditions. Voltammetry and electrochemical impedance spectroscopy measurements show that the 316 stainless steel electrode becomes more resistive when exposed to the simulated TGM wastewater solution due to a passive surface oxide, while the passive layer on the titanium electrode inhibits Hg(II) reduction.Graphical