Thermalanalysis of mineral soils before and after oxidationwith sodium hypochlorite

The nature of recalcitrant organic carbon (OC) in soil is a matter of great debate and various chemical treatments exist for its isolation. We compared calorimetric properties from silt+clay fractions of eight mineral soils by means of DSC before and after chemical oxidation with sodium hypochlorite (NaOCl) to find out, whether recalcitrance of soil organic matter against chemical oxidation coincides with its thermal stability. NaOCl oxidized around 75% of the OC, which corresponded well to a mean loss in heat of reaction of 80%. Peak temperatures and 50% burnoff temperatures did not change systematically after oxidation showing that the thermal stability of NaOCl residues was similar to that of untreated samples. Three samples revealed peaks at >520°C after oxidation indicative for the presence of pyrogenic carbon.