APPLICATION OF P-31 AND AL-27 MAS NMR FOR PHOSPHATE SPECIATION STUDIES IN SOIL AND ALUMINUM HYDROXIDES - PROMISES AND CONSTRAINTS

Magic angle spinning (MAS) nuclear magnetic resonance (NMR) was used t o investigate the chemical environment of P in soil and soil component s. 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, whic h transforms (partly reversibly) to tetrahedral aluminium phosphate up on drying. Results of several experiments on excessively fertilized sa ndy soil material are discussed. The soil particle fraction smaller th an 50 mu m was used for NMR analysis. P-31 and Al-27 MAS NMR spectra s how 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 fro m 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 spe ctrum. The formerly mentioned Ca-P and AI-P are thus stable P pools. T he H-1-P-31 cross-polarization (CP) spectrum of the original soil samp le revealed a third chemical environment, which was identified as labi le Ca-P: this P pool does not appear in the H-1-P-31 CP spectrum of th e soil sample from which the fast P pool had been removed. The combina tion 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.