Sorption Is Not Biocatalysis: Reassessing Michaelis–Menten Kinetics and Active Site Models on Bulk Iron Oxides

Phosphorus plays a central role in both biological and geochemical systems, governed by the unique combination of thermodynamic stability and kinetic lability in phosphate ester bonds. Understanding phosphate hydrolysis at inorganic surfaces is crucial not only for phosphorus chemistry but also for clarifying how catalytic function emerges across different material scales. Iron oxide nanomaterials have been shown to promote abiotic phosphate ester hydrolysis with enzyme-like activity, exhibiting reproducible turnover and saturable kinetics under sorption-limited conditions. Recent studies have extended these interpretations to bulk mineral phases such as goethite and ferrihydrite from soil, applying Michaelis–Menten kinetics to describe reaction rates. However, these bulk systems lack structurally defined active sites and do not meet the mechanistic criteria required for biocatalytic modeling. Crucially, their experimental designs do not decouple hydrolysis from sorption, and product detection remains incomplete—particularly for sorbed inorganic phosphate and evolving organophosphorus intermediates, as assessed by phosphorus K-edge X-ray absorption spectroscopy. Calculated kinetic parameters such as kcₐₜ rely on speculative assumptions about active site densities, leading to model-fitting artifacts rather than mechanistic insight. We argue that the observed reactivity in bulk systems arises from surface-mediated transformations, not true biocatalysis. More broadly, this Perspective challenges the assumption that biocatalytic behavior at the nanoscale can be linearly extrapolated to bulk analogues. By distinguishing nanoscale-specific features—such as structural disorder, redox dynamics, and localized electron transport—from bulk-phase surface chemistry, we propose a more rigorous framework for interpreting phosphorus reactivity and catalysis across material scales.