Methodological problems in determining TCAA in soils - The discovery of novel natural trichloroacetyl containing compounds and their interference with a common method for determining TCAA in soil and vegetation

摘要

Trichloroacetic acid (TCAA) is a pollutant with several sources and is also formed naturally in soil. We show that almost all investigated environmental compartments (soil, soil water, groundwater, spruce needles and throughfall, but not rain) contain compounds, which make false positives in the thermal decarboxylation method often used for determination of TCAA in environmental samples. The compounds are dominating quantitatively over TCAA in soil, soil water and groundwater, while TCAA is dominating in needle and throughfall samples. The compounds behave differently from TCAA with regard to the velocity and the pH-dependence of the chloroform release. We did not manage to reveal the whole chemical structure of the compounds, but a trichloroacetyl group seems to be the only plausible structure giving rise to CHCl_3 both upon heating and under alkaline conditions. Besides the trichloroacetyl group, the compounds did in general contain a carboxylic acid group, although in needle and throughfall samples, trichloroacetyl compounds with a neutral charge at pH 7.5 seemed to co-exist with the carboxylic acids. Trichloroacetyl groups in humic substances and possibly other macromolecular structures contribute to the major portion of the total trichloroacetyl-CHCl_3 in topsoil, but smaller molecules with less UV-VIS absorption seem to constitute the major part of trichloroacetyl-CHCl_3 in soil water and groundwater. The trichloroacetyl containing compounds are most likely naturally occurring compounds formed in the natural chlorination processes in soil, but additional studies are needed to substantiate this hypothesis.