(a) Malonic acid and H2O2 concentrations: The response surface relationship between the concentrations of malonic acid and H2O2 presented in Figure reveals that the interaction of high malonic acid concentration with high H2O2 concentration had a positive effect on leaching efficiency. In other words, increases in both malonic acid and H2O2 concentrations increase leaching efficiency, and a maximum leaching efficiency of about 50% was attained. However, the interaction of high malonic acid concentration and low H2O2 concentration influenced the leaching efficiency negatively. Similarly, a negative effect on leaching efficiency was observed for the synergy between low malonic acid concentration and high H2O2 concentration. The reason for the positive effect on leaching efficiency at high malonic acid and H2O2 concentrations can be understood by the mechanism of Cu leaching from sources like PCB sheets. The mechanism is as follows (a) H2O2 oxidizes malonic acid to peroxymalonic acid (Equation ) , (b) the peroxymalonic acid oxidizes Cu(0) on the PCB surface to Cu(II), and malonic acid (Equation ) , and (c) the malonic acid can lose a proton to water to give a malonate (Equations 5) which coordinates to Cu(II) (Equations 6) or the malonic acid coordinates to Cu(II) to give a Cu hydrogen malonate in the presence of H2O2 (Equation ) . The solubility of Cu(II) in water is enhanced by its complexation .