Vital Threshold and Underlying Mechanism for the Complete Remediation of Oil and Microplastic Co-Contaminated Soil by Fast Pyrolysis

The coexistence of microplastics (MPs) and oil contaminants in soil has led to a new pollution scenario around the oil-production region, yet how to cost-effectively remediate soil with combined pollutants has rarely been explored. Herein, we propose a fast pyrolysis technique to perfectly remediate MP_S-oil copresence soil (MPs-oil-soil). The experimental data showed that pyrolysis at 500 °C for 15 min is a key threshold for the complete removal of MPs and petroleum contaminants from soil. Above this threshold, seed germination and the growth of wheat in the soil increased, and the rhizosphere microbial population decreased with increasing abundance of beneficial microbial flora, such as Proteobacteria, Actinobacteria and Bacteroidetes (which promote the circulation of nutrients and help to strengthen plant resistance). Structural equation modeling revealed that temperature had a more significant positive effect on the remediation effect than did time. Two-dimensional correlation spectroscopy combined with synchronous fluorescence spectroscopy showed that the presence of MPs was the main factor affecting the pyrolysis threshold. Three-dimensional excitation–emission matrix and UV–visible absorption spectroscopy revealed large differences in the aromaticity and relative molecular weight of dissolved organic matter before and after the pyrolysis threshold. These findings shed light on the mechanistic understanding of the pyrolytic remediation of microplastics and oil-contaminated soils.