31P NMR peak width in humate–phosphate complexes

The mobility of inorganic phosphate (P) attached to solid humic acid (HA) and fulvic acid (FA) via a metal “anchor” was investigated by 31P NMR spectroscopy. The peak width of the 31P resonance was monitored as an indicator of the degree of attachment of the element to the humic matrix. The concept was demonstrated by contrasting peak widths of thoroughly dry M–HA–P complexes with those that had been allowed to absorb different amounts of moisture. It was shown that the presence of moisture, which enhances the mobility of P, results in a significant reduction of chemical shift peak width. The work was extended to comparisons between dry systems with and without metal anchors; systems with anchors consisting of different metals; systems comprising different humates and fulvates; and systems with different size fractions of a humate. It was shown that both the type of humate/fulvate, and the metal anchor used lead to different degrees of mobility within the humic matrix. It was also found that the effect of metal addition on 31P peak width is greater with fulvates and smaller HA fractions than with the larger HA components.