Changes in hydrocarbons and elemental distribution in peloids during maturation processes (Sečovlje Salina Nature Park Slovenia)

Abstract

In Sečovlje Salina Nature Park, the therapeutic mud matured in the natural sedimentary environmental site. This work aimed to determine the influence of the peloid maturation process on the hydrocarbon and elemental distributions, as well as changes in morphology. For this purpose, the sample before and after maturation was examined using various methods. n-Alkanes were the most abundant among saturated hydrocarbons in both immature and mature peloid samples. The results showed that the maturation mainly influenced the change in distribution and concentration (from 378 to 1958 ppm) of n-alkanes. The organic matter (OM) of the immature peloid sample was characterized by a slight prevalence of long-chain and odd carbon-numbered n-alkanes, maximizing at n-C27. However, mature peloid's OM showed a similar share of short-, mid- and long-chain n-alkanes with a slight dominance of short-chain members, maximizing at n-C16. The origin of short-chain and even carbon-numbered n-alkanes was attributed to microbial precursors (e.g., Leptolyngbyaceae). Hopanes were considerably more dominant compared to steranes in both peloids. The hopane series of immature peloid was characterized by the dominance of 22,29,30-trinor-hop-5(6)-ene (C27 hopene), as well as the presence of C30-hop-22(29)-ene (diploptene), which are widespread in cyanobacterial species. The aromatic fraction of immature peloid pointed to the predominance of polycyclic aromatic hydrocarbons (PAHs). As peloid aging progressed, the sample was richer in methyl-branched alkanes, carboxylic acids, their methyl esters, and thermodynamically more stable hopanes and steranes. The presence of elements with toxicological relevance during maturation was reduced below the limits prescribed in most of the directives for cosmetic products. It specifically refers to: As, Ni and Se. A higher concentration of total sulfur in the mature peloid can be related to gypsum precipitation in the summer and/or more intensive microbial activity.