Doctoral Degrees (Soil Science)

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Lipid humification by soil clays
(Stellenbosch : Stellenbosch University, 2019-04) Adams, Adrian Richard; Clarke, Catherine E.; Hardie-Pieters, Ailsa G.; Stellenbosch University. Faculty of Agrisciences. Dept. of Soil Science.
ENGLISH ABSTRACT: We studied three aspects of the natural polymerisation (humification) of lipids by soil clays – namely, the products formed, reaction mechanism and kinetics – at environmental temperatures (c. 20–50°C). Various clays were reacted with oleic acid (our chosen model lipid). The Mn-oxide birnessite was the most reactive toward oleic acid, polymerising it into quasi-solid polyesters. A probing of the birnessite-oleic acid reaction mechanism revealed that the formation of a surface exchange complex between oleic acid carboxyl groups and birnessite surface sites (>Mn(III)/>Mn(IV)) is a crucial first step of the reaction. Subsequent chelation and one-electron reduction of Mn(III) to Mn(II) forms radical oleic acid species which couple and thereby polymerise. Kinetic studies revealed that the birnessite-oleic acid reaction was near-linearly dependent on birnessite mass-loading (rate order ~ 0.75) but virtually independent of birnessite surface pH (rate order ~ 0.2). A determined activation energy for the reaction of 12.8 ± 4.2 kJ/mol revealed that it is energetically more spontaneous than the usual autoxidation pathways. These findings broaden our understanding of the role soil clays play in lipid humification in soils.