Pyrolysis temperature shapes the structure and filtration capacity of spent-coffee-ground biochar for 1-μm microplastics and bacteria removal from wastewater

ABSTRACT Graphical abstract showing the study workflow from spent coffee grounds to biochar production at three pyrolysis temperatures, followed by rapid small-scale column filtration of secondary-treated wastewater containing bacteria and 1-µm microplastics, and final quantification by flow cytometry, highlighting optimal performance at 500 degree C. Small microplastics are an increasing environmental concern due to their high mobility, large surface area-to-volume ratio, and ability to transport co-contaminants and microorganisms. Yet, particles in the 1-μm range remain difficult to quantify and remove, and standardized approaches to evaluate treatment performance are still lacking. This study addresses this methodological gap by combining controlled microplastic spiking in real secondary-treated wastewater with high-throughput flow cytometry for rapid quantification of 1-μm fluorescent microplastics and SYBR Green I-stained bacteria. Spent coffee grounds biochar produced at 300, 500, and 700 °C was evaluated in rapid small-scale column tests (1 mL min−1; sampling every 2 min) to investigate how pyrolysis temperature influences structure and removal efficiency. Biochar produced at 500 °C achieved nearly complete removal of 1-μm microplastics and retained approximately 90% of bacteria, outperforming materials produced at 300 and 700 °C, which showed reduced performance consistent with lower surface area and pore development. Breakthrough data from a long-term test were fitted using a lag-corrected Yoon–Nelson model, accurately predicting 50% breakthrough at 500 min and an estimated sorption capacity of 200 mg g−1. These results demonstrate that tuning biochar production and integrating rapid analytical tools enable systematic optimization of sustainable filtration strategies for emerging micropollutants.