Long‐term stability of phosphate sorbed on an allophanic Andosol and a synthesized allophane

Allophane and ferrihydrite are the main hosts of phosphate in allophanic Andosols, which are vital soil resources that support high human population densities. However, the sorption mechanism of phosphate on allophane has not been elucidated, unlike that of ferrihydrite. In particular, the effects of residence time on phosphate sorbed on allophane remain unclear. Therefore, the objectives of this study were to (1) understand the effect of residence time on the stability of phosphate sorbed on allophanic Andosol and allophane by desorption experiments using arsenate and (2) elucidate the sorption mechanism of phosphate on allophane using solid‐state 31P nuclear magnetic resonance (NMR). Consequently, the slow sorption of phosphate onto allophanic Andosol, allophane, and ferrihydrite continued for approximately 150 days. The ratio of total desorbable phosphate to phosphate sorbed onto the allophanic Andosol and allophane decreased with increasing residence time. In other words, phosphate sorption on allophanic Andosol and allophane was more irreversible with increasing residence time. The NMR spectra and X‐ray diffraction patterns showed that the molecular environment of phosphate sorbed onto allophane and ferrihydrite did not change at any residence time. Therefore, the slow sorption and irreversibility of phosphate were caused not by surface precipitation but by internal diffusion. In addition, the NMR spectra showed that most of the phosphate sorbed on allophane was present as inner‐sphere complexes.