Electrochemical Recovery of Copper from Model Wastewater Using Boron-Doped Diamond Electrodes

Introduction The recovery of Cu from wastewater is a key issue for environmental and economic aspects 1 . The wastewater treatment containing Cu has been carried out by various techniques such as chemical precipitation, coagulating sedimentation, and electrochemical methods. Although chemical precipitation method is the most commonly used technique due to relatively simple operation, the process needs to put additional chemicals and generates large amounts of sludge. On the other hand, electrodeposition is known as the “clean” method because it is able to achieve the recovery of Cu without requiring addition of chemicals and generating sludge. However, the current efficiency tends to decrease especially in a dilute solution, because hydrogen evolution and oxygen reduction reaction occur as competition reaction. Boron-doped diamond (BDD) electrode is a candidate for resolving the above problem because it has the excellent electrochemical properties 2 . Along these lines, we study on the recovery of Cu from dilute cupric sulfate solution as a model wastewater by electrodeposition method using BDD electrodes, and glassy carbon (GC) as a comparative carbon electrode. Experimental Two types of BDD films with different boron doping level (B/C = 0.1%, 1%) were deposited onto silicon wafer substrates by a microwave plasma-assisted chemical vapor deposition method. Electrochemical measurements were carried out using a conventional three-electrode system: BDD and GC as a working electrode, Pt plate as a counter electrode, and Ag/AgCl (saturated KCl) as a reference electrode. The surface of BDD electrodes was oxidized in 0.1 M H 2 SO 4 at 2.0 V for 10 min before electrochemical measurements. The electrochemical behavior of Cu ions was evaluated by cyclic voltammetry (CV) in an aqueous solution of 0.5 mM CuSO 4 and 0.1 M H 2 SO 4 with a scan rate of 0.3 V s −1 . Before CV measurements, the solution was deoxidized by bubbling with nitrogen for 10 min. Electrodeposition of Cu was carried out by chronoamperometry at various potentials from 0 to −0.8 V in 9-ml aqueous solution of 0.5 mM CuSO 4 and 0.1 M H 2 SO 4 . During the electrodeposition, nitrogen kept bubbling in the solution. After electrodeposition, concentration of residual Cu ions was measured by an inductively coupled plasma atomic emission spectroscopy. Results and Discussion Figure 1 showed the time-dependent Cu recovery rates and current efficiencies at the various potentials on BDD and GC electrodes. When the electrodeposition was carried out at 0 V on all electrodes, Cu could hardly deposit on the electrodes. The Cu recovery rates on 0.1%BDD increased with increasing the applied negative potential, whereas the Cu recovery rates on GC decreased at −0.8 V, compared to the other potentials. On 1%BDD, Cu recovery rates hardly changed at any potential except 0 V. From CV measurements, reduction potentials corresponding to Cu 2+ ions to metal Cu were −0.48 V, −0.28 V, and −0.20 V on 0.1%BDD, 1%BDD, and GC, respectively. Since the nucleation potential of Cu on 0.1%BDD is the most negative, Cu recovery rates at −0.2 V was lower than the other potentials. The nucleation overpotentials were different between 0.1%BDD and 1%BDD due to the difference of their conductive properties. As 0.1%BDD has a characteristic of a p-type semiconductor, a depletion layer occurs at the surface of the electrode with a band bending when the electrode contacts the solution. Therefore, cathodic reactions are inhibited due to increasing the depletion layer when applying the negative potential. On the other hand, 1%BDD has a characteristic of metal-like conductivity, so charge transfer from the electrode to Cu ions easily occurs. Also, the hydrogen evolution potential on 0.1%BDD was the most negative, followed in order by 1%BDD, and GC. Therefore, Cu recovery rates at −0.8 V on GC decreased due to the hydrogen evolution. The current efficiencies decreased with increasing the applied negative potentials on the all electrodes, because hydrogen evolution reaction easily occurs at more negative potential. Also, the current efficiencies on BDD were higher than GC at any potential. It is assumed that the current efficiency was improved using BDD electrodes due to inhibition of hydrogen evolution reaction, compared to GC electrode. Moreover, the current efficiency using 0.1%BDD was the higher than 1%BDD due to higher hydrogen overpotential, whereas 0.1%BDD also required high Cu deposition overpotential. Therefore, it was found that the recovery of Cu was achieved with high current efficiency using 1%BDD along with the lower power consumption. References (1) F. Fu and Q. Whang, J. Environ. Manage . 92 , 407 (2011). (2) Y. Einaga, J. Appl. Electrochem . 40 , 1807 (2010). Figure 1