This study aims to clarify the removal mechanism and to calculate the effective current of electrocoagulation (i.e., EC) for treating wastewater containing Ni(II), Cu(II), and Cr(VI). The adsorption behavior of various heavy metals onto Al(OH)₃ coagulant generated by the EC process was investigated and the estimating method of the corresponding current was established. Results indicate that adsorption of single Ni(II) and Cu(II) by Al(OH)₃ coagulant can be simulated by the Langmuir isotherm, while Cr(VI) adsorption fits the Freundlich isotherm better. As treating single heavy metal of wastewater, the removal mechanism of the EC process is the adsorption reaction. Under the coexisting condition, the Ni(II) and Cu(II) will compete for the same active sites on the Al(OH)₃ surface and Cu(II) suppresses Ni(II) adsorption. As treating the coexisting heavy metals, Ni(II) removal not only associates with adsorption but also with the coprecipitation. In contrast, Cr(VI) does not compete with other metal ions for the same type of adsorption sites. Whether single or coexisting conditions, the adsorption capacity of heavy metals onto Al(OH)₃ coagulants can be used to compute the necessary current to effectively remove heavy metals in the EC system.

In this work, nano V/TiO₂ catalysts at different molar ratios were prepared and fabricated as the electrocatalytic electrodes for electrocatalytic degradation. The effect of the vanadium doping on the surface morphology, microstructural, and specific surface area of V/TiO₂ catalysts was probed by field emission scanning electron microscope (FESEM) x-ray diffractometer (XRD), and Brunauer–Emmett–Teller (BET), respectively. Afterward, the solution of Acid Red 27 (AR 27, one kind of azo dye) was treated by an electrocatalytic system in which the nano V/TiO₂ electrode was employed as the anode and graphite as the cathode. Results demonstrate that AR 27 can be effectively degraded by the nano V/TiO₂ electrodes; the highest removal efficiency of color and total organic carbon (TOC) reached 99% and 76%, respectively, under 0.10 VT (molar ratio of vanadium to titanium) condition. The nano V/TiO₂ electrode with high specific surface area facilitated the electrocatalytic degradation. The current density of 25 mA cm⁻² was found to be the optimum operation for this electrocatalytic system whereas the oxygen was increased with the current density. The electricity consumption of pure TiO₂ and nano V/TiO₂ electrode in this electrocatalytic system was around 0.11 kWh L⁻¹ and 0.02 kWh L⁻¹, respectively. This implies that the nano V/TiO₂ electrode possesses both high degradation and energy saving features. Moreover, the nono V/TiO₂ electrode shows its possible repeated utilization.

A series of experiments with different NaCl concentrations added to the PCB (printed circuit board) wastewater were prepared to investigate the chloride effect on the formation of aluminum floc and removal efficiency of Cu and Ni. The effects of pH, current density, and different concentration of NaCl were studied and the results are discussed. Results imply that chloride ions are favored to avoid the passivation of the aluminum anode in the EC (electro-coagulation) system. Chloride ions used as the electrolyte can facilitate the release of Al³⁺, which results in the current efficiency of over 100% in the EC system. For the PCB factory’s wastewater, the EC system could remove Cu²⁺ and Ni²⁺ effectively (both Cu and Ni concentration of treated wastewater was less than 1.0 mg/L within three minutes). PCB wastewater’s pH value could maintain stably about 9.0 in the EC system when the initial pH value was around 2.5. The estimated electricity consumption for treating PCB wastewater by the EC process was about 0.894 kWh for each meter of cubic wastewater.