Investigation of biodegradability of polyethylene under anaerobic conditions and evaluation of its potential for biogas production

In recent decades, the increase in plastic pollution has been a growing environmental problem. One of the most resistant contaminants nowadays is polyethylene (PE); due to its high resistance to degradation is easily accumulated in nature. However, several techniques can be used to break it down and, there are recent studies on this issue. In the present study, the main objective was to explore the possibility of biodegrading PE under anaerobic conditions after being treated by three different techniques and evaluate the potential of the material for biogas production. The techniques (pre-treatments) used on PE were photo-oxidation with UV radiation exposure (POxUV), microwave-assisted oxidation with KMnO4 (MAOx) and thermo-oxidative degradation with K2S2O8 (TOD). The material of study was PE in two of its more common forms, low-density and, high-density polyethylene (LDPE and, HDPE, respectively) in the form of commercial plastic bags (films). After applying the pre-treatments on the samples of PE it was found that only MAOx had oxidized LDPE samples, reducing the hydrophobicity of the material and, thus, only these were fit for biodegradation. The anaerobic degradation was done for a total of 125 days at thermophilic conditions (55°C). Two out of eight samples were subjected to co-digestion using glucose and, acetic acid as co-substrates. After the experimental time was finished, the samples were retrieved and analyzed. It was found that pre-treated PE can indeed be biodegraded to some extent. The FTIR showed a general decrease in the transmittance values as well as the appearance or intensification of a signal at 1377 cm-1 that indicates CH3 formation and, consequently, some degree of chain scission. Regarding the biogas production, among the samples, there was a cumulative biogas production overall minimum of 536.8 mL and, an overall maximum of 1474.5 mL. At the end of the experimental time, the PE samples' cumulative biogas production was still increasing, which means that there was a potential for further degradation although at a very slow rate. Keywords: polyethylene, pre-treatment, oxidation, anaerobic, biodegradation, biogas.