Environmental Performance of a Waste-Derived Phenol–Formaldehyde/Glass-Fibre Composite: Leaching and Multispecies Evidence for Safe Circular Design

Industrial-residue valorisation through polymer-composite manufacturing can reduce reliance on virgin resources, yet the environmental safety of such materials is rarely evaluated beyond conventional physicochemical characterisation. This study provides a novel, integrated assessment of a waste-derived phenol–formaldehyde/glass-fibre composite by combining pH-dependent leaching analysis with a multispecies ecotoxicological test battery. The composite contained post-production glass fibres, paper flakes, and viscose fibres bound with phenol–formaldehyde resin. Under near-neutral extraction conditions, most analysed metals and metalloids showed limited release. In contrast, acidic conditions substantially increased the mobilisation of Ba, Cr, Zn, Cu, and Pb, while the phenol index increased approximately 28-fold, from 0.25 ± 0.06 to 6.9 ± 1.7 mg L⁻¹. Biological responses differed markedly among taxa. Hordeum vulgare and Eisenia andrei showed limited short-term effects, whereas the marine diatom Skeletonema marinoi exhibited pronounced concentration- and time-related inhibition, with no cells detected after three days of exposure to the highest extract loading. The Aliivibrio fischeri assay also revealed concentration-dependent acute toxicity, with the strongest effects observed during the initial exposure period. The pronounced algal response could not be fully explained by the targeted chemical analyses, indicating a possible contribution of unidentified leachable constituents or mixture effects. The novelty of this study lies in demonstrating that technical functionality, waste diversion, and low concentrations of selected regulated contaminants do not alone ensure the environmental safety of waste-derived polymer composites. Integrating stress-condition leaching with sensitive biological endpoints offers a practical framework for safe-by-design formulation, application selection, and end-of-life management. Further studies should address material ageing, repeated leaching, particle release, and non-target identification of bioactive constituents.