Monday, 7 November 2005
9

Relating Sorption/Desorption to Som Structure Characterized by Quantitative Pyrolysis Gc-MS.

Naoko Watanabe and Thomas M. Young. Dep. of Civil and Environmental Engineering, University of California, Davis, One Shields Ave., Davis, CA 95616

A set of eight soils was studied for composition and sorption/desorption to investigate linkages between these factors. Composition of soil organic matter (SOM) was quantitatively characterized using solid-state 13C NMR and pyrolysis GC-MS. In pyrolysis GC-MS analysis, CO2 dominated the total ion chromatogram for all soils: however, each soil produced a distinctively different pyrogram and 1.4 to 8.6% of the soil carbon was quantified as one of 205 pyrolysis marker compounds using external standards. Amorphous and free iron and aluminum contents were determined as potential indicators of the reactivity of the mineral phase, which may influence the configuration and accessibility of SOM domains within soil. Positive correlation was observed between NMR alkyl-C group and pyrolysis alkanes, alkenes, and alkyl substituted benzenes. Positive correlations between amorphous aluminum content and NMR aryl-C and carbonyl/carboxyl C-groups may indicate complexation between these mineral/organic phases. However, polar groups from pyrolysis showed negative correlations with iron and aluminum contents. Neither solid-state NMR or pyrolysis GC-MS were able to identify carbon structural groups that are responsible for causing hysteresis and desorption resistance represented as hysteresis index HI, organic carbon normalized distribution coefficient KOC or two-site model parameters fast fraction f, fast rate constant kf of slow rate constant ks. It may be because neither solid-state NMR or pyrolysis GC-MS are capable of distinguishing OM domains or because multiple domains collectively contribute to cause hysterisis and desorption resistance. In contrast, the hysteresis index HI (at Ce = 100ėg/L) seems to be strongly influenced by complexation of free and amorphous iron/aluminum and OM, suggesting that minerals may play an important role in establishing desorption rates either by determining which SOM components are preserved and/or by influencing the SOM conformation on the minerals.

Handout (.pdf format, 373.0 kb)

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