Pyrolysis-Aided Microbial Biodegradation of High-Density Polyethylene Plastic by Environmental Inocula Enrichment Cultures

Abstract

Polyethylene plastics are a major source of industrial and household wastes, the majority of which end up in the environment or in landfills. These wastes pose challenges for microbial biodegradation due to their polymeric structure. There is a critical need for a process that aids in the breakdown and reuse of plastic compounds. Pyrolysis of high-density polyethylene (HDPE) has previously been used to induce chemical changes in plastic compounds, resulting in more structurally simplistic compounds. Here, we demonstrate the ability of pyrolysis to produce microbially biodegradable intermediate compounds. Biodegradation of pyrolysis-treated plastics has not previously been demonstrated. We found that enrichment cultures derived from six different environmental inocula were able to achieve extensive biodegradation of polyethylene pyrolysis products over the course of 5 days. We verified the biodegradation by quantifying residual compound concentrations of alkenes using gas chromatography/mass spectrometry (GC/MS). 16S rRNA gene amplicon sequencing results demonstrated that the most dominant taxa in the microbial community belonged to the phylum Proteobacteria. Many organisms in this phylum have previously been shown to metabolize hydrocarbons. Our results indicate that the coupling of chemical and biological processes can speed up the breakdown and conversion of polyethylene to bacterial biomass by microbial consortia.

Evaluation of Pyrolysis Wax as a Solvent in Polyolefin Pyrolysis Processing

“…In our previous study and pyrolysis pilot plant experiments, we demonstrated that the complete dissolution of polyolefin plastics in paraffin wax at a 1:1 (wt) ratio mixture is possible at temperatures over 200 °C, and their viscosities are suitable for flowing the mixture into a pyrolysis reactor. In addition, those prior experiments also demonstrated that both the plastics and the paraffin wax in the mixture were degraded into pyrolysis products (paraffins were converted to alkenes) . In this work here, pyrolysis wax was substituted for the paraffin wax and the results are compared.…”

“…A unique liquid-phase fast pyrolysis reactor has been designed and fabricated (invention disclosure at Michigan Technological University, Office of Innovation and Commercialization) to control the heat transfer and vapor residence times within the reactor. An earlier paper described the methods of operating the plastic pyrolysis pilot plant . The system is composed of two parts.…”

“…An earlier paper described the methods of operating the plastic pyrolysis pilot plant. 30 The system is composed of two parts. The first part consists of a dissolution tank, an agitating tank (Coating Atomization Technologies), and 2.5 gallons in volume, which were used to dissolve HDPE chips (1 cm in size) obtained from Idaho National Laboratory (research collaborator) in either pyrolysis wax or paraffin wax solvent at a 1:1 mass ratio.…”

“…In addition, those prior experiments also demonstrated that both the plastics and the paraffin wax in the mixture were degraded into pyrolysis products (paraffins were converted to alkenes). 30 In this work here, pyrolysis wax was substituted for the paraffin wax and the results are compared. Figure 2A,B shows the viscosity vs temperature of HDPE/ pyrolysis wax, HDPE/paraffin wax, PP/pyrolysis wax, and PP/ paraffin wax mixtures with a 1:1 ratio.…”

Comparative Analysis of Injection of Pyrolysis Oil from Plastics and Gasoline into the Engine Cylinder and Atomization by a Direct High-Pressure Injector

“…A unique liquid feed pyrolysis reactor (invention disclosure at Michigan Technological University, Office of Innovation and Commercialization, Houghton, MI, USA) was designed in order to have controllable and rapid heat transfer and tunable vapor residence time within the reactor (Figure 1) [27]. Experimental methods for the pyrolysis system have been described previously by Byrne et al [28] and Kulas et al [27]. The liquid feed was achieved by melting and mixing HDPE obtained from Idaho National Laboratory (research collaborator) with a pyrolysis wax solvent in the dissolution tank at a 1:1 ratio [29].…”