Effect of KOH activation on the structural properties of durian peel biochar and its performance for PET microplastic removal

The persistence of microplastics (MPs) in aquatic environments challenges conventional water treatment systems, increasing demand for sustainable removal materials. This study investigated the influence of KOH activation on the structural properties of durian peel biochar and its performance for polyethylene terephthalate (PET) microplastic removal from aqueous systems. Batch experiments evaluated biochar modification, contact time, and solution pH. Physicochemical characterization revealed that KOH activation substantially enhanced surface area, pore development, and surface structural complexity. Post-adsorption FE-SEM showed PET retention on rough surfaces, cracks, and structural cavities, while XPS revealed an increase in the O/C atomic ratio from 0.13 to 0.21 and the appearance of an ester-related O-CO component, supporting PET retention on the biochar surface. Under optimum conditions (4 g L-1, pH 5, 180 min), KOH-activated biochars achieved PET removal efficiencies of 86.83-90.50%, substantially higher than unactivated biochars (51.00-51.17%) and comparable to commercial activated carbon (88.50%). PET removal decreased with increasing pH, indicating that solution chemistry influenced PET-biochar interactions. The pseudo-first-order model provided the best empirical description of the time-dependent PET retention behavior under the investigated conditions. Considering the large PET particle size (20-53 μm) relative to the predominantly microporous structure, enhanced removal was attributed primarily to accessible external-surface retention and physicochemical interactions rather than direct micropore filling. These findings demonstrate that KOH activation effectively improves the structural properties and PET retention performance of durian peel biochar, highlighting its potential as a bio-based material for microplastic mitigation.