SPECTROSCOPIC EVIDENCE FOR CONDENSED DOMAINS IN SOIL ORGANIC MATTER

Abstract

Soil organic matter (SOM) has previously been proposed as a dual-mode sorbent for hydrophobic organic compounds (HOC), i.e., partitioning takes place in both expanded (rubbery) and condensed (glassy) domains, whereas competitive sorption takes place only in condensed domains. The objective of this paper was to provide some spectroscopic evidence of condensed domains in SOM. Six humic acids (HA) extracted from different depths of a single soil profile were analyzed using X-ray diffraction (XRD) and solid-state 13C nuclear magnetic resonance (NMR) with cross-polarization (CP) and magic angle spinning (MAS) techniques. The XRD results showed a strong peak at 0.35 nm for the condensed aromatic structure of the HA from mineral horizons and a dominant peak at 0.43 nm for less tightly packed side chains of the HA from surface organic horizons. The 13C CP/MAS NMR results indicated further that HA becomes more aromatic from surface organic to mineral horizons. This increase of aromatic structure in HA was confirmed by the increase of nonprotonated carbon signals at 130 ppm using a dipolar-dephasing technique and by the increase of atomic C/H and C/O ratios. These spectroscopic and elemental data corroborate further the dual-mode model for the sorption of HOC in SOM.