(c) H2O2 concentration and contact time: A positive influence on leaching efficiency was achieved at high H2O2 concentration and prolonged contact time (Figure ). A negative effect on leaching efficiency was obtained both at high H2O2 concentration and short contact times and at low H2O2 concentration and lengthy contact times. The highest leaching efficiency of 50% was obtained at an H2O2 concentration of about 0.8 M and a contact time of 120 min. The rationale for the observation is the same as that described for the interaction between malonic acid concentration and contact time. 3.4 Computational study. Density functional theory (DFT) calculations were performed using Maestro (Schrödinger Release 2021-4: Maestro, Schrödinger, LLC, New York, NY, USA) to gain insight into the interaction between malonic acid and Cu. The optimized structure of malonic acid was obtained, and the corresponding free energy was recorded (Table , entry 1).