Jo. Skjemstad et al., THE REMOVAL OF MAGNETIC-MATERIALS FROM SURFACE SOILS - A SOLID-STATE C-13 CP MAS NMR-STUDY/, Australian Journal of Soil Research, 32(6), 1994, pp. 1215-1229
Five surface soils of differing chemical and mineralogical composition
s were subjected to either a sequence of dithionite/citrate extraction
s in which the soil: citrate ratio was varied or to a sequence of 1% o
r 2% HF extractions. The 2% HF treatment resulted in the removal of th
e highest Fe concentrations (79-95%) while the dithionite/citrate extr
actions were less effective in removing Fe from the same soils (18-75%
). The Fe remaining after HF treatment appeared to be mostly associate
d with ilmenite crystals which were only slowly attacked by the dilute
acid. During the 2% HF treatments, some organic carbon was lost (8-17
%), but this loss had no significant effect on the organic chemistry o
f the samples as determined by CP/MAS C-13 n.m.r. spectroscopy. The to
tal C-13 signal recovered after the various treatments was found to be
correlated, in order of decreasing significance, with the mineral fra
ction present in the sample, the organic carbon/Fe ratio and the mass
magnetic susceptibility. The expression (organic carbon/Fe) -0.147(min
eral fraction present in the sample) +0.043(1/mass magnetic susceptibi
lity), accounted for 85.3% of the variation in the relative visibility
of the C-13 signal. Prior to soil state CP/MAS C-13 n.m.r. analysis,
the recommended pretreatment for surface soils containing Fe involves
a sequence of 2% HF extractions in the order 5x1 h, 2x16 h and 1x64 h.
For soils high in Fe, this procedure allows CP/MAS C-13 n.m.r. spectr
a to be acquired that would otherwise be difficult to obtain. It also
results in a significant increase in sensitivity and in resolution of
the C-13 n.m.r. spectra of soils with moderate Fe contents.