THE REMOVAL OF MAGNETIC-MATERIALS FROM SURFACE SOILS - A SOLID-STATE C-13 CP/MAS NMR-STUDY

Five surface soils of differing chemical and mineralogical compositions were subjected to either a sequence of dithionite/citrate extractions in which the soil: citrate ratio was varied or to a sequence of 1% or 2% HF extractions. The 2% HF treatment resulted in the removal of the highest Fe concentrations (79-95%) while the dithionite/citrate extractions were less effective in removing Fe from the same soils (18-75%). The Fe remaining after HF treatment appeared to be mostly associated 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 of the samples as determined by CP/MAS C-13 n.m.r. spectroscopy. The total C-13 signal recovered after the various treatments was found to be correlated, in order of decreasing significance, with the mineral fraction present in the sample, the organic carbon/Fe ratio and the mass magnetic susceptibility. The expression (organic carbon/Fe) -0.147(mineral fraction present in the sample) +0.043(1/mass magnetic susceptibility), 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. spectra 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.