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 P-31 NMR spectroscopy. The peak width of the P-31 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 P-31 peak width is greater with fulvates and smaller HA fractions than with the larger HA components. (c) 2007 Elsevier B.V.. All rights reserved.