Demineralisation of a crop soil by mild hydrofluoric acid treatment: Influence on organic matter composition and pyrolysis

Abstract

A recently developed demineralisation treatment, using 2% hydrofluoric acid, was applied to the <50 μm fraction of a silty loamy soil typical of crop soils from northern France. The material thus isolated was compared with an untreated control through elemental analysis and thermal degradation (Rock-Eval pyrolysis; analysis by combined gas chromatography–mass spectrometry of the pyrolysates obtained by off-line pyrolyses at 300 and 400 °C). It appeared that: (i) efficient removal of the minerals was achieved by this treatment while only limited losses of organic carbon occurred; (ii) large retention of the pyrolysis effluents by the mineral matrix took place in the untreated sample; (iii) the extent of this retention, for the different types of pyrolysis products, is controlled by their molecular weight and polarity; (iv) through these off-line pyrolyses information can also be obtained on the origin of the components of Soil organic matter (SOM) pyrolysates (thermovaporized products vs. pyrolysis products formed via cracking reactions; lignin-derived phenolics vs. melanoidin-derived ones); and (v) pyrolytic studies limited to the untreated sample would have provided highly biased quantitative and qualitative information on SOM so that the presence of some important constituents would have passed unnoticed.