Precipitation and hydrolysis of water-soluble ammonium polyphosphate on calcite surface depend on the number of P species

Abstract

Water-soluble ammonium polyphosphate (APP) containing various phosphorus (P) species has been increasingly used as an alternative P-fertilizer. Understanding the geochemical reactions of APP on calcite is significant for P management and environmental protection. In the current study, we investigated the precipitation and hydrolysis of APP1 containing two P species and APP2 containing seven P species on calcite, monoammonium phosphate (MAP) containing single orthophosphate (P1) as a comparison. With an increasing number of P species, the P fixation capacity declined and the hydrolysis rate of APP increased on the calcite. Overall, the hydrolysis kinetics of APP in the presence and absence of calcite were better fitted with a first-order model. APP favored the calcite dissolution to release Ca2+, which selectively precipitated with pyrophosphate (P2) in APP. Calcite slowed the APP1 hydrolysis due to the formation of Ca2P2O7 with crystal transformation from monoclinic to triclinic over time. However, calcite favored the APP2 hydrolysis due to the formation of Ca-poly-P complexes, which were catalyzed hydrolysis by Ca2+ and released P1 into the solution. The results from density functional theory showed that longer-chained poly-P was preferentially hydrolyzed via cleavage of terminal PO43- on calcite surface.