Dual-stage treatment of waste printed circuit boards for Cu2+, Pb2+, and Zn2+ recovery: A precipitation-adsorption hybrid process

Sustainable treatment of metal - rich effluents generated during printed circuit board (PCB) recycling is complicated by their high metal concentrations and strong competition in multicomponent systems. This study develops and evaluates a dual - stage precipitation - adsorption hybrid process for selective recovery of Cu 2+ , Pb 2+ , and Zn 2+ from nitric - acid PCB leachates. In the first stage, sulfide precipitation using Na 2 S achieved near - quantitative Cu 2+ recovery (97.71 ± 0.40%) at an optimized Cu/S molar ratio of 1:1.8, while Pb 2+ and Zn 2+ largely remained in solution. Structural analyses (SEM - EDX, XRD, XRF) confirmed the formation of high - purity crystalline CuS with minimal co - precipitation of other metals. In the second stage, residual metals were captured via adsorption onto an olive stone - derived KOH - activated carbon (OSACK). Single - metal adsorption experiments at different concentrations revealed a clear affinity order of Pb 2+ > Cu 2+ > Zn 2+ , exhibiting the highest uptake capacities of 159.58 ± 3.12, 64.41 ± 5.24, and 28.09 ± 4.56 mg g −1 , respectively, obtained at 3.4 g L −1 adsorbent dosage and initial concentrations of 1300 mg L −1 (Pb 2+ ), 1200 mg L −1 (Cu 2+ ), and 800 mg L −1 (Zn 2+ ). Binary - metal systems (Cu 2+ - Pb 2+ , Cu 2+ - Zn 2+ , Pb 2+ - Zn 2+ ) demonstrated pronounced competitive effects, significantly suppressing Cu 2+ and Zn 2+ adsorption in the presence of Pb 2+ , while Pb 2+ retention remained largely unaffected. Adsorption followed Freundlich isotherms and Elovich kinetics, indicating heterogeneous, predominantly chemisorptive interactions. Regeneration studies showed stable carbon performance over multiple adsorption - desorption cycles. Validation with real PCB leachates highlighted the robustness and scalability of the proposed hybrid process for integrated metal recovery and effluent treatment.