Evaluation of NaOH leaching techniques to extract humic acids from palaeosols

It can be shown quite conclusively that the often used technique (AAA) of l eaching palaeosol samples with hot or boiling dilute HCl solution, followed by treatment with up to 2% NaOH solution and a further treatment with dilu te HCl solution may not remove all of the so-called humic acids from palaeo sol samples. In fact, in many cases it does not remove any of the humic aci ds, only carbohydrates, amino acids and other organic compounds. This can b e shown by examination of the visible spectrum (400-700 nm) of the "humic a cid" extract in alkaline solution. Hence, many of the reported C-14 results from the "humin" fraction of palaeosols are actually from the humic acid f raction which is tightly bound to the clay component of the palaeosol. The NaOH leach also has the potential to introduce carboxylic acid groups from atmospheric CO2 into phenol groups. A reevaluation of humic acid extracts o f palaeosols collected from the Loess Plateau in China has been made, with analyses comparing the properties of the various organic extracts. Also, an extraction technique based on recommendations from the International Humic Substances Society, incorporating removal of impurities by chromatography plus a decarboxylation step is proposed. (C) 2000 Elsevier Science B.V. All rights reserved.