Evaluating the abundance of two particle size ranges of polyethylene microplastics in pig manure under integrated black soldier fly and biochar-amended composting treatments

The transfer of microplastic (MP) pollution from biomass waste to soil presents a significant challenge for resource utilization and treatment technologies. This study investigated combining black soldier fly larval bioconversion with biochar co-composting to control MPs in biomass waste. In the initial bioconversion process, polyethylene (PE) MPs of two different sizes were added: approximately 150 μm (Series 150) and approximately 300 μm (Series 300). Subsequently, the resulting larval residue was mixed with biochar of three different sizes (3.00-4.00 mm, 1.00-2.00 mm, and 0.50-0.75 mm) for co-composting. These co-composting groups were named S1-S3 (for Series 150-derived residue) and B1-B3 (for Series 300-derived residue), respectively. Results indicated that Series 150 PE posed a risk of accumulation in the larval gut, with the distribution peak decreasing from 215,289 items/kg at 81-111 μm to 152,742 items/kg at approximately 141 μm. Conversely, Series 300 PE underwent a concentration effect, with its abundance increasing in the larval residue; the distribution peak rose from 8615 items/kg at approximately 351 μm to 19,127 items/kg at approximately 321 μm. Characterization using Raman spectroscopy, scanning electron microscope, and energy-dispersive spectroscopy revealed that chemical oxidation of smaller MP particles was stronger during co-composting with medium- and small-particle biochar. This was evidenced by a decrease in carbon content to 74.00-78.29 % and an increase in oxygen content to 11.60-12.21 %, although no evident fragmentation was observed. Furthermore, small-particle biochar promoted the succession of fungal microbial communities while inhibiting bacterial community succession. This study offers novel insights into controlling MP pollution during biomass resource utilization.