Dissolved metal cations promote the hydrolysis of tripolyphosphate in aqueous solution: systematic kinetics and mechanistic insights.

Abstract

Tripolyphosphate (TPP) is a short-chain polyphosphate (polyP) that enters aquatic systems through industrial and consumer applications, yet the kinetics of its abiotic hydrolysis in the presence of dissolved metal cations remain poorly quantified. This study systematically investigates the hydrolysis of TPP in the presence of common cations. The results demonstrate that dissolved cations promoted TPP hydrolysis in a concentration- and pH-dependent manner. At pH 6, the promoting effect decreased in the order Cu²⁺ > Al³⁺ > Zn²⁺ > Ca²⁺ > Mn²⁺ > Mg²⁺, a trend that directly correlates with the stability constants of the corresponding metal-TPP complexes, supporting a Lewis-acid-type promotion mechanism. Apparent rates increased with metal dose, whereas ionic strength had no effect, and excess EDTA suppressed the reaction to the metal-free background, indicating that metal-TPP association, rather than outer-sphere electrostatics, governs the rate. Time-resolved ³¹P NMR spectroscopy resolved the sequential conversion of TPP to pyrophosphate and then orthophosphate, consistent with exo-cleavage. Regression of the apparent rate constants on cation descriptors across the full pH grid (pH 4-8; n = 18) linked the activity ranking to metal electronegativity (partial Spearman ρ = 0.74, p < 0.001, controlling for pH), interpreted as statistical consistency with Lewis-acid promotion. Extrapolation of the dose-response data to realistic dissolved metal concentrations indicates that this pathway is unlikely to dominate polyP turnover in pristine waters but may be relevant in metal-rich, particle-poor systems. This work provides an internally consistent kinetic baseline and mechanistic constraints for cation-promoted abiotic hydrolysis of short-chain polyphosphate.