Molecular Mechanism of Linear Polyphosphate Adsorption on Iron and Aluminum Oxides

Polyphosphate is a polymeric P species with environmental and industrial significance. Understanding their interaction with natural minerals is fundamental for predicting their transport and fate in the environment. This study investigates the molecular mechanism of interaction between linear polyphosphates with varied chain length (15P, 60P, and 130P) and Fe/Al oxides using quartz crystal microbalance with dissipation (QCM-D), 31P solid-state nuclear magnetic resonance (NMR) spectroscopy, and attenuated total reflection-Fourier transformed infrared (ATR-FTIR) spectroscopy. QCM-D results show that all three polyphosphates irreversibly adsorb on Fe/Al oxides at pH 4-10 with similar mass-based adsorption amounts despite the large difference in chain lengths. ATR-FTIR and NMR spectroscopy results suggest that terminal phosphate groups of the polyphosphate molecules may form bidentate binuclear surface complexes, and a fraction of middle phosphate groups may form monodentate mononuclear surface complexes on Fe/Al oxides. The interaction modes persist for both minerals and all tested pH conditions. A combination of these complementary techniques helps gain a mechanistic understanding of polyphosphate interaction with Fe/Al oxides, and the results fill a knowledge gap on polyphosphate cycling in natural environments.