Biodegradability of cellulose polymers

  • Biodegradablity of Cellulose and its derivative

Many types of microorganisms are known to biodegrade cellulose. Bacteria and fungi are of particular interest because they are the most widely available degrading microorganisms.
  • Cellulose Degradation by Fungi

There are variety of observed ways through which fungi breaks down cellulose. The exact mechanism depends on the exact species of the fungi that is doing the degradation. Here we look at one of the more common ways through which fungi can degrade cellulose.[1]

One way in which fungi breaks down cellulose chain is by generating radicals that can react with the cellulose via a free radical mechanism. Fungi produces hydrogen peroxide (H2O2) in an aerobic environment, hydrogen peroxide dissociates into hyroxyl radicals. The hydroxyl radicals react with the cellulose chain and break it down.

Fungi also produce three principle types of enzyme to biodegrade cellulose through hydrolysis.[1]
  1. endo-1,4-B-glucanases: An endoenzyme, this enzyme randomly attacks the internal connecting bonds of cellulose to generate polymers of lower weight.
  2. exo-1,4-B-glucanases: An exoenzyme, this enzyme selectively attacks the terminal end of the cellulose and forms monomers like glucose and two, three or four unit long oligomers.
  3. 1,4-B-glucosidases: this enzyme catalyzes the breaking of oligomers (produced by the exo-1,4-B-glucanases) to glucose, which is used by the fungi as a nutrient.

Some fungi also have enzymes that catalyze the oxidation reaction of a large cellulose chain or the lower molecular weight oligomers produce by enzymatic hydrolysis. For example, if peroxide radicals (from hydrogen peroxide, see above) are present in an environment then they can break the C2-C3 bond in cellulose bond to provide aldehyde cellulose. The aldehyde cellulose is very reactive and reacts with water to break into small fragments.[1]
Below is a schematic representation of the oxidation reaction.


Figure 1[1] : Schematic representation of breaking of C2-C3 bond by free radical attack by peroxidases generated by hydrogen peroxide. The alcohols on cellulose form aldehyde cellulose, which is very reactive and breaks down to form lower molecular weight fragments upon contact with water.
  • Cellulose Degradation by bacteria

Bacteria can degrade cellulose via endoenzymatic or exoenzymatic mechanism or a combination of both. The mechanism by enzymes degrade cellulose are very complicated and can vary depending on the environment, multiple enzymes are involved in the process. Bacterias can degrade cellulose both aerobically and anaerobically. Variety of products, like CO2, CH4, H2, H2S, are evolved in degradation through both aerobic and anaerobic degradation.

  • Degradation of Cellulose Acetate, a derivative polymer of cellulose

For a variety of applications, cellulose needs to be modified to cellulose acetate, which has many applications, such as films and cigarette filters.[2] Varoious studies have demonstrated the degradability of cellulose acetate. It is important to remove the acylate on cellulose acetate before it can be attacked by microrganisms that can degrade cellulose. Enzymes, that can remove the acetyl group, are common in many microorganisms.[3] Northop and Rowe (1987) showed that Cellulose acetate is significantly deteriorated in moist soil after 2 months.[3] This means that moist soils have microorganisms necessary to degrade cellulose.
Degree of substitution is important for CAs to be biodegradable. Komareck et al. used the evolution of CO2 from in vitro samples of cellulose acetate to compare the rate of degradation of CA with different degrees of substitutions, 1.85, 2.07 and 2.57.[4] The biodegradability reduced as degree of substitution increased, but was not inhibited at any degree of substitution.[4]

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  1. ^ Lenz RW. 1993. Biodegradable polymers. Advances in Polymer Sciences.107:1-40.
  2. ^ Buchahnan CM, Gardner RM, Komarek RJ. 1993. Aerobic biodegradation of cellulose acetate. Journal of Applied Polymer Science. 47(10): 1709-1719.
  3. ^ Puls J, Wilson SA, Holter D. 2011. Degradation of cellulose acetate-based materials: a review. Journal of Polymers and the Environment. 19: 152-165
  4. ^ Komarek RJ, Gardner RM, Buchanan CM, Gedon S. 1993. Biodegradation of radiolabelled cellulose acetate and cellulose propionate. Journal of Applied Polymer Science. 50(10): 1739-1746.