Simultaneous removal of Cr(VI) and Cu(II) from acid wastewater by electrocoagulation using sacrificial metal anodes

Abstract

• In MP-P mode, EC with Al-Al-Al-Al combination outperformed other Fe and/or Al combinations. • Al EC was highly efficient for simultaneous removal of Cr(VI) and Cu(II) from acid wastewater. • Appropriate concentration of Cu(II) promoted removal of Cr(VI) and total Cr, and vice versa. • Removal mechanisms were elucidated via characterization of scums, precipitates, and cathode deposits. Removal of heavy metals from acid wastewater is an important concern in the field of water treatment. Here, electrocoagulation (EC) technology with Fe and/or Al anode was investigated for the simultaneous elimination of Cr(VI) and Cu(II) from wastewater (initial pH: 3.5–4.0). In the energy-efficient monopolar parallel mode, the EC with Al-Al-Al-Al combination (anode–cathode-anode–cathode) showed the highest removal efficiency among different electrode combination modes, whereas the EC with Fe-Fe-Fe-Fe combination showed the lowest removal efficiency. The main electrochemical parameters of Al EC were further optimized based on cost-effectiveness analysis. Operating cost, including energy consumption, sludge treatment, and anode loss was estimated. The sludge treatment cost accounted for the largest percentage of the operating cost (82.4%) whereas energy consumption cost was the lowest (4.8%). At the initial pH of 3–6, Al EC displayed desirable removal efficiency towards Cu(II) (77.6%–97.9%), Cr(VI) (84.2%–96.4%), and total Cr (83.1%–94.9%). At the conductivity of 899–2300 μS/cm, excellent pollutant removal efficiency was maintained (90.2%–99.8%). Appropriate concentration of Cu(II) promoted the removal of Cr(VI) and total Cr, whereas too high concentration of Cu(II) inhibited their removal and vice versa. Through SEM-EDS and XRD analyses of the scums, precipitates, and cathode deposits, possible mechanisms of Cr(VI) and Cu(II) removal were proposed.