All leaching experiments were undertaken in 50 mL beakers placed in a fume cupboard. Malonic acid and H2O2 of known concentration were added to a single piece of the PCB (2 cm x 2 cm) and left for the pre-determined contact time. The malonic acid leaching was undertaken following the Box-Behnken experimental design. In the Box-Behnken experimental design, malonic acid concentration, H2O2 concentration, and contact time of leaching were selected as independent variables. Cu leaching efficiency was chosen as the dependent variable. The real values of the independent variables coded as -1, 0, and +1 were set at the low, center, and high levels, respectively (Table ). Consequently, the input factors were malonic acid concentration (X1), H2O2 concentration (X2), and contact time (X3) values. The three-factorial Box-Behnken design was performed with 15 runs with 3 center points (Table ). After the PCB pieces had been contacted with the leach solution for the required duration, the leaching solution was filtered, and the filtrate was prepared for the determination of Cu concentration. Leaching efficiency was determined by