Responses of bioavailability and degradation of phenanthrene in soils with or without earthworms to the addition of mixed particles of biochar and polyethylene

In the natural environment, microplastics and biochar are normally coexistent. However, interactions between them on bioavailability and degradation of organic pollutants in soils are not clear. Here, wheat straw biochar produced at 400 °C (BC) and polyethylene (PE) with particle size ranges of 840–2000 and 104–178 μm were selected and named as LBC, SBC, LPE, and SPE, respectively. Their effects on bioavailability and degradation of phenanthrene in soils with or without earthworms were investigated. Sorption ability (lgKf) of PE-BC (1:1) mixtures for phenanthrene was higher than that of their corresponding individual PE or BC in general, which is mainly related to the change in DOC content in the equilibrium solution. Both initial desorbing fraction of phenanthrene extracted by n-butanol and degradation ratio of phenanthrene were more obviously decreased by the addition of 2% PE-BC mixtures than by the addition of 2% PE or BC in general. Correlation analysis results indicated that particle sorption played a key role in controlling phenanthrene bioavailability and then phenanthrene degradation no matter whether earthworm Pheretima guillelmi was present or not. Earthworm presence enhanced phenanthrene degradation in soils with SPE, LBC, or LPE but had no obvious effect on the degradation in soils with PE-BC mixtures due to the change in ingestion of SPE by earthworms or DOC solubilization. The enhanced sorption ability and the reduced SPE ingestion by earthworms by coexistence of PE and BC inhibited phenanthrene degradation, leading to higher persistence of phenanthrene.