The removal of magnetic materials from surface soils - a solid state ¹³C CP/MAS NMR study

Abstract

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 13C n.m.r. spectroscopy. The total 13C 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 13C signal. Prior to soil state CP/MAS 13C n.m.r. analysis, the recommended pretreatment for surface soils containing Fe involves a sequence of 2% HF extractions in the order 5 x 1 h, 2 x 16 h and 1 x 64 h. For soils high in Fe, this procedure allows CP/MAS 13C 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 13C n.m.r. spectra of soils with moderate Fe contents.