Residence time effects on sorption and desorption mechanisms of phosphate and myo-inositol hexakisphosphate on allophane

Abstract

Understanding the molecular-scale mechanisms of phosphate (PO₄) and inositol hexakisphosphate (IHP) interactions with allophane is critical for predicting phosphorus (P) retention in volcanic ash soils. This study investigated the sorption and desorption behavior of PO₄ and IHP on allophane over residence times of 60 days at pH 4 and 6, integrating macroscopic kinetics and isotherms with solid-state ³¹P and ²⁷Al NMR spectroscopy. Allophane showed strong sorption affinity for both PO₄ and IHP, with PO₄ sorption increasing markedly at lower pH (from 0.89 mmol g⁻¹ at pH 6 to 1.36 mmol g⁻¹ at pH 4), while IHP sorption was less pH-dependent. Sorption followed a biphasic kinetic pattern: an initial rapid phase driven by ligand exchange, followed by a slower phase involving minor structural reorganization. PO₄ exhibited faster sorption kinetics than IHP, with rate constants nearly four times higher at both pH levels. NMR spectroscopy revealed that both PO₄ and IHP initially formed inner-sphere surface complexes on allophane, rapidly initiating surface precipitation of AlPO₄ and Al-IHP, particularly under acidic conditions, indicating that the surface precipitation processes are initiated durting the early stages of sorption. These surface precipitates became increasingly dominant with longer residence times and coincided with reduced phosphate desorption. These findings highlight the critical roles of residence time and pH in regulating PO₄ and IHP binding mechanisms on allophane, providing molecular-level insights into legacy P dynamics and substantiating allophane’s function as a long-term sink for both inorganic and organic phosphorus in Andisols.