Application of 31P and 27Al MAS NMR for phosphate speciation studies in soil and aluminium hydroxides:: promises and constraints

Magic angle spinning (MAS) nuclear magnetic resonance (NMR) was used to investigate the chemical environment of P in soil and soil components. P-31 and Al-27 MAS NMR spectra are presented of synthetic aluminium hydroxides (amorphous aluminium hydroxide and gibbsite), reacted with P under different conditions of P concentration, temperature and pH. The reaction product is amorphous octahedral aluminium phosphate, which transforms (partly reversibly) to tetrahedral aluminium phosphate upon drying. Results of several experiments on excessively fertilized sandy soil material are discussed. The soil particle fraction smaller than 50 mu m was used for NMR analysis. P-31 and Al-27 MAS NMR spectra show a Ca-P pool and an Al-P pool. A six-fold water extraction removes part of both P pools. Oxalate extraction removes all Ca-P and Al-P from the sample. Removal of the labile P pool by the HFO-DMT long-term P desorption technique, does not drastically change the P-31 MAS NMR spectrum. The formerly mentioned Ca-P and AI-P are thus stable P pools. The H-1-P-31 cross-polarization (CP) spectrum of the original soil sample revealed a third chemical environment, which was identified as labile Ca-P: this P pool does not appear in the H-1-P-31 CP spectrum of the soil sample from which the fast P pool had been removed. The combination of MAS and CP MAS NMR can thus reveal at least three different P species in soil, of which two pools were identified as stable, and one as labile. (C) 1997 Elsevier Science B.V.