Co-transport of contaminants by disintegrated and residual superfine particles in biocarbon-based adsorbents

Activated carbon (AC) is widely used in water and wastewater treatment, while biochar (BC) is emerging as a cost-effective and environmentally sustainable alternative. However, the release of superfine AC/BC particles may impair treated-water quality and facilitate contaminant transport. This study investigated residual superfine particles and particle-associated contaminant transport in fixed-bed filters and stirred-tank suspensions followed by sequential coarse-sand, fine-sand, 0.45 μm membrane, and 0.02 μm membrane filtration. Particle characterization revealed apparent size-distribution peaks at 5 μm, 0.5–1 μm, and 100–200 nm. Lead (Pb), used as a tracer contaminant, was detected on residual AC/BC particles. Quantitative fractionation combined with acid digestion and ICP-OES showed that particle-bound Pb accounted for 84.5–90.2% of total Pb in fine-sand filtrates. Pb-bearing particles in the 0.02–0.45 μm fraction also remained after 0.45 μm membrane filtration. The apparent 5 μm fraction is more plausibly attributed to secondary aggregation of nanoscale and submicrometer particles during filtration, sample handling, or measurement than to direct passage of intact 5 μm particles through the membrane. These findings identify particle-facilitated transport as a potentially overlooked pathway in post-filtration systems, and demonstrate the need to evaluate both dissolved and particle-associated contaminants when assessing treatment performance. Graphical Abstract