Sustainable Biochar Filters for Microplastic Removal: Continuous-Flow Performance, Regeneration, and End-of-Life Strategie

Biochar is increasingly proposed as a low-cost, low-carbon filter medium for microplastic removal in wastewater treatment. This review synthesizes 267 peer-reviewed studies on microplastic and nano plastic removal involving biochar and biochar-based composites. Across these well-reported studies, biochar filters typically achieve 80–95% removal of microplastics in the 1–500 µm range at empty-bed contact time values of 10–30 min, flow rates of 1–10 mL min⁻¹, and cumulative throughputs of roughly 100–500 bed volumes before significant breakthrough. Morphology-controlled retention mechanisms interception, straining and entanglement allow high capture efficiencies relative to sand filters with similar grain sizes. Removal increases with particle size and is generally higher for hydrophobic polymers such as polystyrene and polypropylene than for biodegradable or more polar materials such as poly(3-hydroxybutyrate-co-3-hydroxyvalerate) or functionalized microplastics. Evidence on regeneration and disposal is limited and fragmented. Magnetic or modified bio chars can retain 70–95% of initial removal capacity over several adsorption–desorption cycles, but regeneration is often energy-intensive and rarely evaluated for microplastic fragmentation, by-product toxicity or greenhouse-gas emissions. Overall, biochar filters demonstrate strong technical potential for microplastic removal under continuous flow; however, progress towards sustainable implementation is hindered by uneven geographical coverage, incomplete reporting of key parameters, limited regeneration data, and unresolved end-of-life impacts.