A solution for plastic wastes; biodegradation with gamma irradiation to induce photo-oxidation by endophytic fungi

Synthetics polymers derivatives of petroleum are produced all over the world. Approximately 140 million ton per year are introduced in the environment as industrial wastes products, which are accumulated because their degradation is carried out slowly. This has a negative impact in the ecosystems and organisms.

Considering the problematic of plastic wastes, in this study was applied the biodegradation in two types of polymers: low density polyethylene (LDPE) and polypropylene (PP). These polymers are extremely recalcitrant, and the most persistent plastics dumped in the environment. To carry out this biodegradation they used  endophytic fungal strains isolated from two endemic plants Psychotria flavida and Humboldtia brunonis which produce laccase enzymes that have the capacity to degrade these materials, the activity of enzyme was quantified in each species of fungi viz. Cunnighamella echinulata, Pestalotiopsis sp, Hypoxylon anthochroum, Paecilomyces lilacinus, Aspergillus sp, Lasiodiplodia theobromae (Fig.1,2).

To be possible the biodegradation, the microorganism has to join to the polymer forming biofilms and use it as a sole source carbon. In this case were produced biofilms with different dose of irradiated gamma which induce the photo-oxidation in two polymers, the dose was 0-1000 kGy for LPDE and 0-100 kGy for PP (Fig.3,4). This generated a large surface area and produced a great degree of hydrophobicity due to the introduction of carbonyl groups that was used for endophytic fungi in their metabolisms enhancement the biodegradation. The possible mechanisms is the transformation of carbonyl groups to carboxyl groups in presence of oxygen (Fig.5) which undergo to B- oxidation and subsequently undergo to cyclic acid citric, that results in the production of CO₂ and H₂O (Fig.6). The efficiency for degradation of the polymers was measured with the decrease in intrinsic viscosity and average molecular weight of gamma irradiated.       

Figure 1. Shows a) the formation for biofilm fungi in the polymer and b) the presence of enzyme laccase.

Figure 2 Shows the activity of enzyme laccase in each species of fungi where the production level of laccase in L. theobromae was high (10.70 ± 1. U/ml) and in Hypoxylon anthochroum quite low (1.18 ± 0.10 U/ml).

Figura 3 shows percentage weight loss in gamma irradiated fungal inoculated LDPE films.

Figure 4 shows percentage weight loss in gamma irradiated fungal inoculated PP films.

Figure 5 shows (a) control film unirradiated (b) showed the peaks that correspond to addition of oxygen and photoxidation compared with the control film. The gamma irradiated LDPE (1000 kGy) rendered wavenumber 1712 cm^-1 related to carbonyl group formation which was absent in control. The area of peak at wavelength (3100-3500) cm^-1 was broad and bended in irradiated (1000 kGy) fungal treated samples (c) shows the presence of O-H stretching band of hydroxyl groups and adsorbed water corresponding to decrease in hydrophobicity  and attributing to degradation.

Figure 6 show the possible mechanisms for biodegradation a) Gamma radiation and b) the efficiency for degradation by fungi.

The biodegradation through irradiated gamma is a biologic alternative for the problematic of plastic wastes. This study shows that the radiation gamma can increase the capacity of microorganisms for use the polymer as a sole carbon source in their metabolism producing enzymes that degrade these materials.

If you have interested about the subject you can search the next paper “Biodegradation of gamma irradiated low density polyethylene and polypropylene by endophytic fungi”

-Reference

Sheik, S., Chandrashekar, K.R., Swaroop, K., & Somashekarappa, H.M. (2015).Biodegradation of gamma irradiated low density polyethylene and polypropylene by endophytic fungi.International Biodeterioration & Biodegradation.