Wet removal of elemental mercury by acid-assisted electrochemical oxidation method

As a global pollutant, mercury emission is increasingly restricted in recent years. It is urgent to explore a new and efficient mercury removal technology for coal-fired power plants. A new acid-assisted electrochemical oxidation (AEO) technique for mercury removal was proposed using platinum plate as cathode and fluorine-doped tin dioxide (FTO) glass as anode. The effects of acid type, acid concentration, applied direct current (DC) voltage, electrolyte type, SO₂, NO and O₂ on the Hg⁰ removal efficiency were carried out. The results indicated that the mercury removal efficiency increased with the increase of DC voltage and nitric acid concentration. When the concentration of nitric acid increased to 0.15 mol/L, the mercury removal efficiency remained unchanged. SO₂ and NO inhibited the removal of Hg⁰ in AEO system, but the inhibition was reversible. Compared with the mercury removal efficiency under single experimental conditions, the mercury removal efficiency of electrochemical oxidation can reach 96% under the experimental conditions of 0.1 mol/L nitric acid and 4V DC voltage, suggesting that the synergistic effect of nitric acid and DC voltage plays a key role. According to the experimental results, the mechanism of Hg⁰ removal in AEO system was analyzed. At the anode, Hg⁰ was oxidized by hydroxyl radical (^.OH) generated by the oxidation reaction on the anode surface. At the cathode, dissolved oxygen or O₂ adsorbed on the surface of Pt is reduced to form anionic superoxide radicals (^.O⁻₂). Moreover, parts of ^.O⁻₂ would produce ^.OH with the aid of electron at acidic condition. Free radicals capture experiments showed that ^.O⁻₂ and ^.OH were the main active substances for the removal of Hg⁰ by acid-assisted electrochemical method. The research is helpful for the development of effective electrochemical techniques for industrial mercury removal and recycling of industrial acid waste.