The Microbial Devulcanisation of Waste Ground Tyre Rubber Using At. ferrooxidans DSMZ 14,882 and an Unclassified Sulphur-Oxidising Consortium

Purpose Sol fraction, a measure of the free polymers removed from a ground tyre rubber (GTR) by organic solvent, and crosslink density, a measure of the number of sulphur crosslinks in the GTR, are necessary to determine whether a microbial activity causes both devulcanization and carbon degradation. The suitability of two sol fraction measurement methods to assessing the devulcanisation performance of At. ferrooxidans and a sulphur-oxidising consortium on industrial GTR was investigated. Method The devulcanisation performance and the relation between two Sol fraction methods (American Standard Testing Method, ASTM D6814 and the altered method) were determined for Acidithiobacillus ferrooxidans (DSMZ 14,882) and a mesophilic, sulphur-oxidising acidophilic consortium (UCT-30), used to treat unleached ground tyre rubber (untreated GTR) for 30 days. Results Both cultures were able to devulcanise untreated GTR after 30 days of incubation, despite the negative impact of the untreated GTR toxins on growth performance. The sulphur-oxidising consortium displayed the greatest toxin resistance and attached cells were observed at the surface of the untreated GTR particles. At. ferrooxidans (DSMZ 14,882) increased the Sol fraction of the GTR by 1.09 ± 0.9% (0.46 ± 0.1% ASTM) without causing any polymer degradation, whereas the sulphur-oxidising consortium increased the sol fraction by 0.56 ± 0.82% (− 0.26 ± 0.1% ASTM), but also caused polymer degradation at the surface of the GTR particles due to the activity of the heterotrophic microorganisms. In the comparison of the Sol fraction methods, ASTM yielded smaller absolute values, but better precision than the altered method. The absolute values for ASTM method fell within the range for the altered method due to the latter’s large variance. In addition, the ASTM method produced a change in sol fraction (Δsol) that was more consistent across the unleached GTR tested than the altered method Conclusion The ASTM sol fraction method provides better precision than the altered sol fraction method, making it more likely to indicate a statistically significant difference, despite the small absolute values measured. The altered method’s more aggressive treatment leads to larger observed changes in the sol fraction, making it easier to identify qualitative changes in the GTR properties. However, the higher temperature method also introduces increased variability leading to poor statistical significance of the results. Therefore, t