Tire Rubber Recycling and Bioremediation: A Review

The accumulation of, and environmental hazards associated with, tire rubber and its disposal are major global concerns. The complex composition and structure of tire rubber and its additives make it highly resistant to natural degradation, but recently, microbial methods of detoxification and degradation of tire rubber have been developed. The polymers in tire rubber can be devulcanized by the activity of aerobic and anaerobic microbes and both the metabolic pathways and associated enzymes involved are becoming better known. Combining and adapting different rubber treatment processes could ultimately lead to more efficient ways of degrading and recycling tire rubber waste.     

Biotechnological possibilities for waste tyre-rubber treatment

Every year large amounts of spent rubber material, mainly from rubber tyres, are discarded. Of the annual total global production of rubber material, which amounts to 16–17 million tonnes, approximately 65% is used for the production of tyres. About 250 millions spent car tyres are generated yearly in USA only. This huge amount of waste rubber material is an environmental problem of great concern. Various ways to remediate the problem have been proposed. Among these are road fillings and combustion in kilns. Spent tyres, however, comprise valuable material that could be recycled if a proper technique can be developed. One way of recycling old tyres is to blend ground spent rubber with virgin material followed by vulcanization. The main obstacle to this recycling is bad adhesion between the crumb and matrix of virgin rubber material due to little formation of interfacial sulphur crosslinks. Micro-organisms able to break sulphur-sulphur and sulphur-carbon bonds can be used to devulcanize waste rubber in order to make polymer chains on the surface more flexible and facilitate increased binding upon vulcanization. Several species belonging to both Bacteria and Archaea have this ability. Mainly sulphur oxidizing species, such as different species of the genus Thiobacillus and thermoacidophiles of the order of Sulfolobales, have been studied in this context. The present paper will give a background to the problem and an overview of the biotechnological possibilities for solutions of waste rubber as an environmental problem, focusing on microbial desulphurization.