Heavy metals or emerging organic contaminants: Which drive zebrafish embryo toxicity in printed circuit board manufacturing wastewater?

Printed circuit board (PCB) manufacturing wastewater contains complex mixtures of legacy metals and emerging organic contaminants, yet its aquatic toxicity and main contaminants driving it remain largely unknown. Here, we integrated zebrafish embryo toxicity testing, chemical analysis, and toxic-unit assessment to characterize wastewater risk across treatment stages at PCB manufacturing facilities in Guangdong, China. Untreated influents exhibited pronounced acute toxicity (LC50: 0.142%‒18.2%), and most pretreated effluents still induced high lethality, whereas most final effluents showed substantially reduced mortality, revealing treatment-stage-dependent attenuation of acute toxicity. Sublethal developmental and functional effects varied across wastewater streams, with hatching impairment frequently among the most sensitive endpoints. Metal(loid)s (0.0160-9,950 mg/L), particularly Cu, dominated the measured contaminant burden and were effectively removed during treatment. By contrast, emerging organic contaminants, including per- and polyfluoroalkyl substances (PFAS), polybrominated diphenyl ethers (PBDEs), and organophosphate esters (OPEs) occurred at much lower concentrations (0.274‒50.0, 0.655‒144, 81.3-399,000 ng/L, respectively), with PFAS and PBDEs showing limited removal. Toxic-unit analysis identified heavy metals, especially Cu, as major drivers of acute toxicity. Quantified organic contaminants contributed little to acute lethality, yet their potential contribution to chronic sublethal effects should not be overlooked. Unexplained toxicity in one final effluent suggests contribution from unmeasured toxicants, highlighting the need for effect-based nontarget screening. Overall, these findings offer process-specific bioassay evidence to broaden the scope of chemical screening and to support toxicity-oriented wastewater management in emerging industries.