Department of Chemical Engineering

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https://scholar.sun.ac.za/handle/10019.1/321
Department Process Engineering now has a new name, and will be known from March 2023, as Department of Chemical Engineering.

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Browsing Department of Chemical Engineering by Author "Allan, Kathryn Mary"

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    The microbial devulcanisation of waste ground tyre rubber using acidophilic microorganisms
    (Stellenbosch : Stellenbosch University, 2018-03) Allan, Kathryn Mary; Gorgens, Johann F.; Van Rensburg, Eugene; Danon, Bart; Stellenbosch University. Faculty of Engineering. Dept. of Process Engineering.
    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.