Domain mobility of humic acids investigated with one- and two-dimensional nuclear magnetic resonance: Support for dual-mode sorption model

Dual-mode sorption model of soil organic matter (SOM) postulates that SOM consists of condensed and expanded domains; sorption of organic compounds including a pesticides in the latter is linear and noncompetitive whereas sorption in the former is nonlinear and competitive between solutes. Although the model has been cited numerous times in the literature, spectroscopic data is needed to support the domain heterogeneity of SOM in terms of mobility difference. In this study several humic acid (HA) samples were investigated using one-dimensional proton wideline and two-dimensional WISE (wideline separation) solid-state nuclear magnetic resonance (NMR) techniques. One-dimensional proton wideline could only estimate the overall domain mobility of HA. The results from this technique showed a gradual mobility change (from rigid to mobile) of structural components of HA samples, Two-dimensional WISE experiments clearly indicated the mobility difference for different segments and even for the same functional groups. These results are consistent with the postulation of the dual-mode sorption model for SOM.