Chakraborty, SwaroopSwaroopChakrabortyDhumal, PanktiPanktiDhumalMikulska, IuliiaIuliiaMikulskaPham, SangSangPhamBradford, Laura-JayneLaura-JayneBradfordMenon, DhruvDhruvMenonMisra, Superb K.Superb K.MisraLynch, IseultIseultLynch2025-10-222025-10-222025-1010.26434/chemrxiv-2025-hrf4khttp://repository.iitgn.ac.in/handle/IITG2025/33388Metal–organic frameworks (MOFs) are entering water technologies on the premise that abiotic stability predicts ecological safety. We overturn this assumption by showing that UiO-66—often regarded as chemically and structurally robust- remains intact after 7-day ageing in natural borehole water yet undergoes rapid in-vivo transformation in Daphnia magna. Microfocus X-ray absorption spectroscopy revealed collapse of the ordered Zr–carboxylate coordination into disordered Zr–O environments within the gut; EXAFS showed loss of second-shell features, and TEM confirmed loss of crystallinity with nanoscale aggregates appearing within 24 h of ingestion. Although acute immobilisation was limited (48 h EC₅₀ ≈ 26.5 µg mL⁻¹), a sublethal, environmentally relevant exposure (10 µg mL⁻¹) caused pronounced chronic effects: brood initiation was delayed by 3–5 days and cumulative reproduction decreased by ~74% without mortality. We attribute these outcomes to gut-level transformation and associated energetic/physiological burdens, not captured by standard acute tests. Our results establish a general principle—abiotic stability ≠ biological inertness—and argue that environmental risk assessment for water-sector materials must integrate in-vivo transformation pathways with chronic endpoints. This provides a mechanistic basis for Safe-and-Sustainable-by-Design of MOFs before widespread deployment in water treatment.Biotic transformationMOFsDaphnia magnaEnvironmental healthE-Printhttps://doi.org/10.26434/chemrxiv-2025-hrf4kArticle