Lipid humification by soil clays

Adams, Adrian Richard (2019-04)

Thesis (PhDAgric)--Stellenbosch University, 2019.

Thesis

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.

AFRIKAANS OPSOMMING: Ons het drie aspekte van die natuurlike polimerisasie (humifikasie) van lipiede deur grond kleie bestudeer – naamlik, die produkte gevorm, reaksie meganisme en kinetika – teen omgewings-temperature (ong. 20–50°C). Verskeie kleie is met oleïen suur (ons gekose model lipied) gereageer. Die Mn-oksied birnessiet was die mees reaktief teenoor oleïen suur, deur dit te polimeriseer na kwasi-soliede poliësters. 'n Ondersoek na die birnessiet-oleïen suur reaksie meganisme toon dat die formasie van 'n oppervlaks-uitruilingskompleks tussen oleïen suur karboksiel groepe en birnessiet oppervlaks-liggings (>Mn(III)/>Mn(IV)) 'n noodsaaklike eerste stap van die reaksie is. Gevolglike chelasie en een-elektron reduksie van Mn(III) na Mn(II) form radikaal oleïen suur spesies wat koppel en daardeur polimeriseer. Kinetika studies toon dat die birnessiet-oleïen suur reaksie naby-liniêr afhanklik is op birnessiet massa-lading (tempo orde ~ 0.75) maar so te sê onafhanklik is van birnessiet oppervlaks-pH (tempo orde ~ 0.2). 'n Bepaalde aktiverings-energie vir dié reaksie van 12.8 ± 4.2 kJ/mol toon dat dit energiek meer spontaan is as die gewone outoksidasie roetes. Hierdie bevindings verbreed ons begrip van die rol wat grond kleie in lipied humifikasie in gronde speel.

Please refer to this item in SUNScholar by using the following persistent URL: http://hdl.handle.net/10019.1/105826
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