Application of nickel foam as an effective electrode for the electrochemical treatment of liquid hazardous wastes of COD analysis containing mercury, silver, and chromium (VI)

Abstract

One of the heavy metal-rich liquid wastes produced in the environmental laboratories is related to COD tests. This waste contains Hg 2 + , Ag + , and Cr 6＋ , and it is; therefore, hazardous to the environment, and it should be treated before being discharged. For this purpose, an electro-amalgamation process was used to remove Hg 2 + , Ag + , and Cr 6＋ from this waste simultaneously. A graphite sheet was used as the anode and the nickel foam as the cathode. Three different electrode arrangements were investigated to choose the best electrode arrangement for designing the batch-scale experiments. The monopolar-paralleled arrangement was selected due to low remaining concentrations and energy consumption. The effects of the initial total heavy metals concentration, current intensity, and operating time on the removal efficiency were studied. For optimizing the process, the response surface methodology with the Box–Behnken method was utilized. The results indicated that the maximum reduction efficiency was achieved under the optimum operating conditions at a current intensity of 0.94 A, the total heavy metals concentration of 1000 mg L-1, and an operating time of 29 min. Also, some experiments were carried out on two different forms of nickel electrode, foam, and plate. Moreover, the results showed that the removal rates were decreased by using the plate electrode and also the reduced metals released in solution as ultrafine particles (57.8 nm). Consequently, the results indicated that the application of nickel foam in an electrochemical reduction process for treating the COD laboratory waste water was successful. • Electro-amalgamation process was used for Ag + , Hg 2+ , and Cr 6+ reduction. • Nickel foam electrode was proposed as the cathode in an electrochemical cell. • Ni foam showed better metal removal performance compared to Ni plate. • Removal efficiencies were above 99.8% under the optimum condition. • Amalgam particles adhere to Ni foam, but Ni plate releases them in the solution.