Continuous-Flow Electrocoagulation for Hexavalent Chromium Reduction in Nickel Mining Wastewater: Insights into Optimization and Mechanisms

Abstract Indonesia’s extensive laterite nickel reserves drive substantial nickel mining activities yet pose environmental risks due to the wastewater’s hexavalent chromium (Cr(VI)) contamination. This study evaluates a continuous-flow electrocoagulation reactor, using aluminum and iron electrodes to remove Cr(VI) from nickel mining wastewater. Optimization via a Box-Behnken design examined current intensity, flow rate, and reactor flow distance. Results revealed that iron electrodes achieved superior Cr(VI) removal efficiency of up to 99% at a flow rate of 0.99 L/min, current intensity of 2 A, and reactor flow distance of 165,6 cm, whereas aluminum electrodes attained a maximum removal efficiency of 83% under optimal conditions of 2.9 A, 0.93 L/min, and 338 cm. Iron electrodes also demonstrated lower specific electrical energy consumption (0.48 kWh/g Cr(VI)) than aluminum (1.39 kWh/g Cr(VI)). Kinetic studies indicated that Cr(VI) removal on aluminum electrodes aligns with a pseudo-first-order model, while iron electrodes follow a second-order model, highlighting a shift from physical to chemical adsorption mechanisms. Adsorption isotherms further supported these findings, with Freundlich isotherms fitting the aluminum electrodes (R 2 = 0.998), suggesting heterogeneous, multilayer adsorption, and Langmuir isotherms fitting the iron electrodes (R 2 = 0.988), indicating homogeneous, monolayer adsorption. This study underscores electrocoagulation’s potential as an energy-efficient and sustainable solution for Cr(VI) removal in mining wastewater treatment.