The microbial devulcanisation of waste ground tyre rubber using acidophilic microorganisms

Allan, Kathryn Mary (2018-03)

Thesis (MEng)--Stellenbosch University, 2018.

Thesis

ENGLISH SUMMARY: The structural stability provided by vulcanisation presents a major hurdle in the reclamation of tyre rubber. Devulcanisation processes aim to break the sulphur crosslinks in the tyre matrix, while preserving the rubber properties of the polymer. However, most mechanical, thermal and chemical devulcanisation techniques result in unselective breakage of both the sulphur crosslinks and the carbon chains in the rubber polymer. Disrupting the carbon chains causes deterioration of the mechanical properties of the rubber, resulting in a less valuable product. Conversely, microbial devulcanisation using sulphur oxidising acidophilic microorganisms has been reported to selectively break the sulphur crosslinks in tyre rubber without causing polymer degradation. Although promising, microbial devulcanisation is not yet industrially viable and the performance needs to be improved through increased toxin resistance and sulphur oxidation activity of the microbes. Acidithiobacillus ferrooxidans DSMZ 14882 and an acidophilic biomining consortium have been identified as promising cultures suitable for ground tyre rubber (GTR) devulcanisation. The sulphur oxidising, autotrophic acidophile, At. ferrooxidans has been investigated extensively in devulcanisation literature and displays good devulcanisation performance and toxin resistance. Furthermore, co-culturing with another sulphur-oxidising autotroph, At. thiooxidans, has been shown to further improve devulcanisation performance. Similarly, mixed cultures of acidophiles are known to improve sulphur oxidation activity of the autotrophs in biomining, particularly through symbiotic relationships with organic toxin consuming heterotrophic cells. However, the devulcanisation performance of complex acidophilic consortia containing heterotrophs has not yet been reported in literature. The devulcanisation and cell growth performance of active cultures of At. ferrooxidans DSMZ 14882 and a biomining consortium on GTR manufactured from waste tyre rubber is investigated. The sulphur oxidation activity of the devulcanisation cultures was maximised by developing culture preparation methods that ensured adequate biomass concentration and sulphur oxidising activity. Active cultures of At. ferrooxidans DSMZ 14882 and the biomining consortium are demonstrated to change the properties of industrial GTR within 15 to 30 days of incubation. At. ferrooxidans DSMZ 14882 is conclusively shown to achieve devulcanisation in the absence of polymer degradation, leading to a 1.09 ± 0.02 % GTR sol fraction increase. The biomining consortium devulcanisation performance is less conclusive, as microbial treatment only increased the GTR sol fraction by 0.56 ± 0.01 %. Additionally, polymer degradation and increased chemical additive extraction from the GTR is observed in biomining consortium GTR, and is attributed to the activity of heterotrophic microorganisms. Cell lysis is observed in devulcanisation cultures of both At. ferrooxidans DSMZ 14882 and the biomining consortium, and was attributed to acetone-extractable toxins in the GTR. Extensive cell death was caused by non-acetone-extractable toxins, and these are identified as the greatest challenge to improved growth performance. Attached cells in the non-sulphur-oxidising component of the biomining consortium demonstrated the greatest toxin resistance. At. ferrooxidans DSMZ 14882 was conclusively shown capable of devulcanisation of the industrial GTR used in this study. Despite the poorer devulcanisation performance of the biomining consortium, the increased toxin resistance compared to At. ferrooxidans DSMZ 14882 makes it a promising culture for further research. Additional investigation is required to improve understanding of the effect of the biomining consortium on GTR properties and the effect of GTR on the culture ecology. The toxicity of non-acetone-extractable-compounds suggests that further work should be conducted to increase toxin resistance of the microbes used in devulcanisation. Alternatively, detoxification techniques targeting the non-solvent-extractable- component of the toxins should be investigated to supplement solvent extraction methods usually used in microbial devulcanisation.

AFRIKAANSE OPSOMMING: Die strukturele stabiliteit wat deur vulkanisering aan rubber verleen word, bied ʼn groot struikelblok tydens voertuigbuitebande herwinning. Devulkaniseringsprosesse het die breking van swaelkruisbindings in die rubbermatriks ten doel, terwyl die eienskappe van die rubberpolimeer bewaar moet word. Meeste meganiese, termiese en chemiese devulkaniseringstegnieke lei egter tot nie-selektiewe afbraak van die swaelkruisbindings asook die koolstofkettings in die rubber in die voertuigbuiteband. Ontwrigting van die koolstofkettings veroorsaak agteruitgang van die rubberpolimeer wat tot ʼn laer kwaliteit produk kan lei. Daarenteen is gevind dat wanneer devulkanisering deur middel van swael-oksiderende, asidofiliese mikroörganismes uitgevoer word, die swaelkruisbindings in buitebandrubber selektief gebreek kan word om sodoende rubberpolimeerafbraak te bekamp. Alhoewel belowend, is mikrobiese devulkanisering nog nie ʼn industrieel werkbare tegnologie nie, hoofsaaklik weens onvoldoende toksienweerstand asook lae swael-oksidasie aktiwiteit van mikroorganismes wat tot dusver bestudeer is. Acidithiobacillus ferrooxidans DSMZ 14882 en ‘n asidofiliese biomyn konsortium is as belowende kulture geïdentifiseer om devulkanisering op gemaalde buitebandrubber (‘ground tyre rubber’, GTR) uit te voer. Die swael-oksiderende outotrofiese asidofiel, At. ferrooxidans, is ekstensief in devulkaniseringsliteratuur ondersoek en daar is bevind dat dié organisme uitstekende devulkaniseringsvermoëns en toksienweerstand vertoon het (5-9). Verder is daar bewys dat gesamentlike kweking met ʼn ander swael-oksiderende outotroof, genaamd At. thiooxidans, die devulkaniseringsvermoë van At. ferrooxidans kon verbeter. Dit is ook bekend dat gemengde kulture van asidofiliese organismes die doeltreffendheid van swael-oksidasie deur outotrofiese organismes tydens biologiese verwerking van mynmateriaal verbeter, hoofsaaklik omdat daar ʼn simbiotiese verwantskap met organiese toksien-verterende heterotrofiese organismes bestaan. Die devulkaniseringsvermoë van komplekse asidofiliese konsortiums wat heterotrofiese organismes bevat is egter nog nie in die literatuur beskryf nie. Die devulkanisering- en selgroeivermoë van aktiewe kulture van At. ferrooxidans DSMZ 14882 en ʼn biomynkonsortium op GTR wat van afval buitebandrubber vervaardig is, is in hierdie studie ondersoek. Die swael-oksiderende aktiwiteit van die devulkaniseringskulture is geoptimeer deur kultuurbereidingsmetodes te ontwikkel ten einde voldoende biomassa-konsentrasie en swael-oksiderende aktiwiteit te bewerkstellig. Daar is getoon dat aktiewe kulture van At. ferrooxidans DSMZ 14882 en ‘n biomynkonsortium die eienskappe van industriële GTR binne 15 tot 30 dae van inkubasie kon verander. Daar word ook bewys dat At. ferrooxidans DSMZ 14882 devulkanisering in die afwesigheid van polimeerdegradasie kon bewerkstellig, wat in ʼn 1.09% toename in die GTR oplosbare fraksie waarneembaar was . Daarteenoor het die biomynkonsortium die GTR oplosbare fraksie met slegs 0.56% verhoog, maar het tot polimeerdegradasie gelei weens die heterotrofiese mikrobes se aktiwiteit. Sel lise is in devulkaniseringskulture van beide At. ferrooxidans DSMZ 14882 en die biomynkonsortium waargeneem en is aan asetoon-ekstraheerbare toksiene in die GTR toegeskryf. Ekstensiewe seldood is deur toksiese komponente, wat nie tydens asetoon-ekstraksie verwyder kon word nie, veroorsaak. Hierdie toksiene is gevolglik as die grootste uitdaging ten opsigte van verbeterde groeivermoë geïdentifiseer. Selle in die nie-swael-oksiderende komponent van die biomynkonsortium en wat aan die substraat aangeheg was, het die beste toksienweerstand getoon. Daar is bewys dat At. ferrooxidans DSMZ 14882, die tipestam van At. ferrooxidans, in staat was om die industriële GTR wat in hierdie studie gebruik is, te devulkaniseer. Ten spyte van die swakker devulkaniseringsvermoë van die biomynkonsortium vergeleke met At. ferrooxidans DSMZ 14882, is eersgenoemde ʼn belowende kandidaat vir toekomstige navorsing weens die hoer toksienweerstand wat die kultuur vertoon het. Verdere ondersoek is nodig om die effek van die biomynkonsortium op GTR eienskappe en die effek van GTR op die ekologie van die kultuur beter te verstaan. Die toksiese effek van die toksiene wat nie asetoon-ekstraheerbaar is nie, impliseer dat verdere werk gedoen ook moet word om toksienweerstand van die mikrobes wat in devulkanisering gebruik word, te verhoog. As alternatief behoort detoksifiseringstegnieke wat op die nie-oplosmiddel-ekstraheerbare komponent van die toksiene gemik is ondersoek te word ten einde oplosmiddel-ekstraksiemetodes wat gewoonlik in mikrobiese devulkanisering gebruik word, aan te vul.

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