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