Organic compound-mineral interactions: Using flash pyrolysis to monitor the adsorption of fatty acids on calcite

Abstract

Fatty acids are near ubiquitous organic compounds in living organisms in the Earth's biosphere. Following death of an organism in the marine environment its fatty acids may survive descent to the sea bed where they can be juxtaposed with minerals. The aim of this study was to investigate the interaction of fatty acids with the common marine mineral calcite. Adsorption of tetradecanoic acid (C14) on calcite results in a sigmoidal or "s" isotherm. Flash pyrolysis experiments were conducted on samples of fatty acid adsorbed onto calcite and were compared with similar experiments on pure fatty acid and on salts of a fatty acid. Flash pyrolysis of pure tetradecanoic acid generated unsaturated and saturated hydrocarbons and a series of unsaturated and saturated low molecular weight fatty acids.Flash pyrolysis of free tetradecanoic acid salt produced saturated and unsaturated hydrocarbons, an aldehyde and a homologous series of saturated and unsaturated ketones, one of which was a symmetrical mid chain ketone (14-heptacosanone). Flash pyrolysis data from adsorbed tetradecanoic acid samples suggested that adsorption is analogous to the formation of the calcium salt of tetradecanoic acid. A key characteristic of the flash pyrolysis products of adsorbed fatty acids and fatty acid salts was the production of ketones with higher molecular weights than the starting fatty acids. Ketonisation was not observed from the flash pyrolysis of pure acid which implied the catalytic significance of the calcite mineral surface.The abundance of hydrocarbons relative to ketones in the pyrolysates negatively correlated with the proportion of fatty acids adsorbed to the surface of calcite. The ability to use flash pyrolysis to diagnose the nature of fatty acid interactions with mineral surfaces provides a valuable tool for monitoring the fate of these important lipids at the Earth's surface as they pass into the geosphere and are subjected to diagenetic processes.

Adsorption of tetradecanoic acid on kaolinite minerals: Using flash pyrolysis to characterise the catalytic efficiency of clay mineral adsorbed fatty acids

“…Pyrolysis of tetradecanoic acid shows distinct products (Fig. 3), including alkenes which are the products of alkyl chain scission and alkanes such as tridecane which is the decarboxylation product of tetradecanoic acid (Zafar et al 2017). Along with that the saturated and unsaturated low molecular weight fatty acids are also produced as a result of alkyl chain scission and showed that chain scission is preferred over decarboxylation for the free fatty acid.…”

“…Previously, our work showed that the symmetric ketone, 14-heptacosanone, formed from calcium tetradecanoate, further degraded to produce a homologous series of ketones and hydrocarbons (Zafar et al 2017). Pyrolysis products of a ketone-kaolinite mixture are significantly different from the chromatograms of fatty acid adsorbed kaolinite (Figs.…”

“…The number of these studies is limited especially in aqueous solution due to the solubility limitations of carboxylic acids in water. Most adsorption studies have been conducted on minerals, such as calcite (Suess 1970, Meyers and Quinn 1971b, Lahann and Campbell 1980Zafar et al, 2017) and montmorillonite (Meyers and Quinn 1971a), however, the adsorption of fatty acids on kaolinite is relatively unexplored.…”

“…Previously, studies reported the flash pyrolysis of fatty acid sodium salts (Lappi and Alen 2009) and carboxylic acids (Maher et al 2008) suggest mechanisms such as decarboxylation, decarbonylation, and dehydration for the formation of different organic compounds (Lappi andAlen 2009, Frety et al 2014). Yet, despite much work done on flash pyrolysis of carboxylic acids, the compounds evolved after the adsorption of fatty acids on clay minerals remains poorly understood although a recent study by Zafar et al, (2017) studied this process on calcite.…”

“…4 a & b) which means that the 14-heptacosnone cannot be considered as a stand-alone compound for the formation of all classes of organic compounds from fatty acid adsorbed kaolinite. One prominent difference in the pyrolysis products of fatty acid adsorbed calcite (Zafar et al 2017) and kaolinite is that the later shows the formation of homologous series of ketones and symmetric ketone only at high fatty acid adsorption. This difference gives us a clue that the excess acid after the monolayer formation might be involved in the formation of symmetric ketones in the fatty acid adsorbed kaolinite.…”