Lipid humification by soil clays

dc.contributor.advisorClarke, C. E.en_ZA
dc.contributor.advisorHardie, A. G.en_ZA
dc.contributor.authorAdams, Adrian Richarden_ZA
dc.contributor.otherStellenbosch University. Faculty of Agrisciences. Dept. of Soil Science.en_ZA
dc.descriptionThesis (PhDAgric)--Stellenbosch University, 2019.en_ZA
dc.description.abstractENGLISH 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.en_ZA
dc.description.abstractAFRIKAANS 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.af_ZA
dc.description.sponsorshipNational Research Foundation (NRF)en_ZA
dc.format.extent201 pages : illustrationsen_ZA
dc.publisherStellenbosch : Stellenbosch Universityen_ZA
dc.subjecthumification, lipids, soil minerals, polyesters, complexation, RGB colour analysisen_ZA
dc.subjectPolymerisation of lipidsen_ZA
dc.subjectHumification in soilsen_ZA
dc.subjectSoil claysen_ZA
dc.subjectRed, green, blue (RGB) colour analysisen_ZA
dc.titleLipid humification by soil claysen_ZA
dc.rights.holderStellenbosch Universityen_ZA

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