Remediation of ternary heavy metal-polyethylene microplastics co-contaminated soil using co-modified biochar with deep eutectic solvent and warm patch: competitive interactions, mechanistic insights, and microbial community response

The accumulation of heavy metals (HMs) and microplastics in soil has emerged as a major environmental challenge worldwide. In this study, a series of biochars were developed using hollyhock straw as the raw material to remediate soils co-contaminated with Pb, Cr, and Cd. Additional experiments were conducted to assess the impact of polyethylene microplastics (PE-MPs) on the efficacy of biochar in soil remediation. In this study, we utilized the inexpensive and readily available warm patches, which are composed of iron, vermiculite, and activated carbon, in combination with a green and economical deep eutectic solvent (MAG) as modifying agents for biochar. The warm patch and deep eutectic solvent-co-modified biochar (MAG-WIM/B) exhibited superior performance in immobilizing HMs, improving the soil physicochemical properties, enhancing the functional enzyme activity, and increasing the microbial community diversity of soil. Soil incubation experiments showed that the addition of MAG-WIM/B reduced DTPA-extractable Pb, Cr, and Cd contents by 50.8 %, 46.7 %, and 24.2 %, respectively, by day 82. Monte Carlo simulations further revealed a preferential adsorption of Pb in the ternary HM system. Moreover, the introduction of MAG-WIM/B led to a significant improvement in the urease activity and shifted the dominant microbial species from Proteobacteria to Actinobacteria. PE-MPs significantly limited the effectiveness of biochar in remediating HM-contaminated soil by altering its physicochemical properties and competing with HMs for the active sites present on biochar. This work provides a strategic framework for the remediation of soils co-contaminated with multiple HMs and PE-MPs.