Operational optimization of copper removal from real integrated circuit substrate wastewater by sodium sulfide precipitation

ABSTRACT Graphical overview of Na2S precipitation of real IC substrate wastewater. A optimized condition of pH 6.50, contact time 7.5 min, and Na2S:Cu ratio 1.71 achieved 99.29 plus or minus 0.22% Cu removal and produced a copper sulfide precipitate. Real integrated circuit substrate wastewater is a chemically complex treatment matrix in which elevated copper coexists with chloride, organic matter, and other dissolved constituents that can alter sulfide demand and precipitation behaviour. This study evaluated copper removal from combined real wastewater by sodium sulfide precipitation and optimized solution pH, contact time, and sodium sulfide-to-copper molar ratio using response surface methodology with a central composite design. The wastewater exhibited pH 3.95, copper 900 mg L−1, chloride 1,017 mg L−1, chemical oxygen demand 3,034 mg L−1, and biochemical oxygen demand 858 mg L−1. The reduced model showed that solution pH, sulfide dosage, and their interaction governed copper removal, whereas contact time was not significant within the investigated design domain. Response Optimizer analysis identified pH 6.50, a contact time of 7.50 min, and a sodium sulfide-to-copper molar ratio of 1.71 as a high-removal operating condition associated with a predicted copper removal of 99.90%; triplicate confirmation experiments achieved 99.29 ± 0.22% removal. Supporting X-ray diffraction, field emission scanning electron microscopy with energy-dispersive X-ray spectroscopy, and particle size analysis indicated dosage-dependent changes in precipitate characteristics. These findings provide experimentally supported operating conditions for sulfide-based treatment of real integrated circuit substrate wastewater.