Chain-Length-Selective Hydrolysis of Polyphosphate Fertilizer on Soil Goethite

The nutrient activity of ammonium polyphosphates (APPs, (NH₄)_<i>n</i>+2P_<i>n</i>O_3<i>n</i>+1, <i>n</i> < 20) in soils is closely associated with the minerals and polyphosphate (Poly-P) constitution. Herein, we investigate the differential performances of APP1 (P species of P₁-P₂) and APP2 (P species of P₁-P₇) mixed with goethite (O═Fe-OH). Goethite and dissociated Fe³⁺ both facilitate P-O cleavage, especially for the longer chains from APP2, with a hydrolysis constant (<i>K</i>) increase of 209% in a well-fitted first-order model. The terminal P-O bond lengths of Poly-Ps increase by 0.02-0.04 Å after being adsorbed. Subsequently, Poly-Ps promote goethite dissolution via loosening the solid Fe-O bond after a multi-dentate coordination with surface Fe³⁺. With hydrolysis accelerated by goethite, APP2 could quickly cleave to supply available P₁, and the derived P₂ would continue to spread Fe³⁺ out. These findings highlight the ability of Poly-Ps with abundant and long-chain P species to activate Fe and P in goethite-dominated soils.