Manganese oxide catalyzed hydrolysis of polyphosphates

Polyphosphate (polyP) is a group of important phosphorus (P) species that plays critical roles in the marine P cycle and potentially mediates phosphorus (P) burial in sediments. Revealing the transformation of polyP is important for understanding the P cycle in aquatic environments. This study systematically investigated the hydrolysis of polyP with different chain lengths by four representative types of manganese (Mn) oxides (α-MnO₂, β-MnO₂, Î-MnO₂, and birnessite) under varied solution conditions. All four Mn oxides can rapidly hydrolyze polyP, with the hydrolysis rate in the order of α-MnO₂ > Î-MnO₂ > birnessite > β-MnO₂. The hydrolysis rates for longer chained polyP were relatively higher than those of shorter chained ones. Results from kinetic studies and time-resolved ³¹P solution nuclear magnetic resonance (NMR) spectroscopy indicated that the reaction mechanism was through a terminal-only hydrolysis pathway via one-by-one cleavage of terminal P-O-P bonds. The presence of Ca²⁺ obviously enhanced both the hydrolysis rate and extent. The presence of other common metal cations (Mg²⁺, Cu²⁺, Zn²⁺, and Mn²⁺) also showed promotion on polyP hydrolysis by Î-MnO₂. Formation of cation-polyP ternary surface complexes is likely the dominant mechanism of cation promotion on polyP hydrolysis on Mn oxides. P K-edge X-ray absorption near edge structure (XANES) analysis indicated that solid calcium polyphosphate (Ca-polyP) granules can be hydrolyzed by α-MnO₂ and transformed into amorphous calcium phosphate (ACP) phase, with increasing ACP content as pH increased. This study highlights the rapid transformation of polyP at the mineral-water interface, and has significant implications for the alteration of P bioavailability in aquatic environments and for P burial in marine sediments.