Disposal of plastic mulching film through CO2-assisted catalytic pyrolysis as a strategic means for microplastic mitigation

Abstract

Conventional disposal processes (incineration and landfilling) of agricultural plastic wastes release harmful chemicals and microplastics into our ecosystems. To provide a disposal platform not releasing harmful chemicals, pyrolysis of a representative agricultural plastic waste was proposed in this study. Spent plastic mulching film (SMF) was used as a model waste compound. To make pyrolysis process more environmentally benign, CO₂ was used as a raw material in pyrolysis of SMF. H₂ and hydrocarbons were produced from pyrolysis of SMF under the inert (N₂) and CO₂ conditions, because SMF is composed of polyethylene. To enhance conversion of hydrocarbons into H₂, catalytic pyrolysis of SMF was conducted over Ni/SiO₂. Compared to non-catalytic pyrolysis, total concentration of pyrolytic gases was enhanced up to 3.1 and 11.3 times under N₂ and CO₂ conditions, respectively. The gas phase reactions between CO₂ and hydrocarbons led to formation of CO, which enhanced production of pyrolytic gases under the CO₂ condition. Moreover, gas phase reactions resulted in less production of pyrolytic oil from CO₂ condition (15.9 wt%) in reference to the N₂ condition (22.6 wt%). All experimental results confirmed that CO₂ and SMF can be used as useful feedstocks to produce value-added products. ENVIRONMENTAL IMPLICATION: Plastic waste used from a sector of agriculture is incinerated or/and landfilled, generating hazardous microplastic and volatile compounds into the environment. Thus, an environmentally friendly process for plastic waste materials in the agricultural industry is required. This study converted a spent plastic mulching film (SMF), broadly used for plastic greenhouse, into value-added syngas through catalytic pyrolysis. CO₂ was used as a reactant. We found that concentration of CO₂ was key to improve syngas formation from pyrolysis of SMF. Thus, this study suggested that CO₂/SMF are used as useful feedstocks through catalytic pyrolysis, while they were previously discarded as waste materials.

Keywords: (micro)plastic disposal; Carbon dioxide; Catalytic pyrolysis; Circular economy; Waste valorization; Waste-to-energy.