Selective adsorption and precipitation of pyrophosphate from water-soluble ammonium polyphosphate by calcite

Polyphosphate (poly-P) with linear phosphoanhydride (P-O-P) structure is increasingly being used as an alternative phosphorus (P) fertilizer. Understanding the interfacial reactions of poly-P on minerals is essential for sustainable P management and water bodies protection. In this study, we investigated the selective adsorption and precipitation of two water-soluble ammonium polyphosphates (APP1 containing two P species and APP2 containing seven P species) by calcite via batch adsorption experiments and molecular dynamics (MD) simulations, mono-ammonium phosphate (MAP) as a comparison. Calcite presented the strongest fixation for APP1 at low P concentration and/or initial reaction time, followed by APP2 and MAP. However, calcite presented the strongest fixation for MAP at high P concentration, followed by APP1 and APP2. Calcite selectively fixed pyrophosphate (P 2 ) in APP and formed Ca 2 P 2 O 7 precipitate and/or Ca 2 P 2 O 7 complex. The dissolution of Ca 2 P 2 O 7 and/or Ca 2 P 2 O 7 at high APP concentration contributed to the decline of the APP fixation by calcite. MD simulations showed that electrostatic interaction and hydrogen bonds played a key role in the water-phosphates-calcite systems. The results develop new insights regarding the selective adsorption and precipitation of various P species coexistence in calcite-rich environments. • P fixation in APP by calcite decreases with the increasing number of P species. • P fixation in APP versus MAP by calcite depends on P concentration and reaction time. • Calcite selectively fixes pyrophosphate (P 2 ) in APP and forms Ca 2 P 2 O 7 and ≡Ca 2 P 2 O 7 . • Ca 2 P 2 O 7 / ≡Ca 2 P 2 O 7 dissolution reduces P fixation by calcite at high APP concentration. • Hydrogen bonds play a key role in the water-phosphates-calcite adsorption systems.