Two-stage leaching strategy for the selective recovery of base and precious metals from waste motherboard PCBs: Kinetic study and process optimization

The rapid growth of electronic waste (e-waste), particularly discarded motherboard printed circuit boards (PCBs), poses both a serious environmental challenge and a promising source of valuable metals. This study develops an innovative two-stage hydrometallurgical leaching approach for the selective recovery of base (Cu, Pb) and precious (Au, Ag, Pd) metals from waste motherboard PCBs. In the first stage, Cu and Pb were selectively leached using a low-concentration HNO 3 solution. A comprehensive kinetic assessment, applying both heterogeneous shrinking-core and homogeneous models, revealed that Cu dissolution was governed primarily by surface chemical reactions, whereas Pb leaching was controlled by product-layer diffusion, with corresponding activation energies of 22.46 and 37.28 kJ mol −1 , respectively. The second stage targeted the leaching of Au, Ag, and Pd using thiourea in the presence of Fe 3+ as an oxidant. A Central Composite Rotational Design (CCRD) was employed to optimize thiourea, Fe 3+ , and H 2 SO 4 concentrations. Statistical analysis identified the optimal conditions for maximizing Au recovery (up to 89.73 %). In contrast, the leaching models for Ag and Pd exhibited less statistically significant dependencies on the studied variables. Validation experiments confirmed the Au model reliability with an experimental efficiency of 87.82 ± 3.07 %, showing only a 2.13 % deviation from the predicted value, while Ag and Pd recoveries reached 29.99 ± 1.23 % and 45.97 ± 6.75 %, respectively. Structural and compositional analyses (SEM-EDX, XRF, XRD) confirmed effective removal of metals and morphological changes. This sequential leaching process demonstrates high selectivity, improved metal recovery, and reduced environmental impact, offering a sustainable and economically viable pathway for e-waste valorization.