Patterns of mono- and difunctional carboxylic acids in hydrothermal leachates of humified soil organic matter and peat: geological and environmental implications

Maturation dependent generation of C₁-C₅ mono- and dicarboxylic acids is investigated by hydrous pyrolysis of soil and peat humates. Evolution and extinction patterns of acid populations released during progressive thermal maturing of these humates are examined. Abundant carboxylic acids yields were obtained and two types of carboxylic acid assemblages were identified: an immature acid assemblage, characterised by an abundant C₂-C₄ diacid component and a mature acid assemblage which is nearly devoid of these acids, except for the C₄ diacid. C₁-C₅ monoacids are abundant in both types of acid assenblages, however acid populations characterised by a n-C₄/i-C₄ monoacid ratio greater than 3.0 and enriched in C₅ monoacids may also indicate a mature acid population. The trans and cis forms of the C₄ unsaturated diacid may also prove useful in defining these assemblages. Geologically these different assemblages imply that the composition of sediment expelled carboxylic acids in the subsurface environment may depend on the timing of expulsion and thermal maturity of the source rock. The abundant production of these acids from humates also implies that where industrial activities involve heating of soil or rock organic matter, waste carboxylic acid effluents may cause industrial and environmental problems.