Activated carbon and biochar are widely studied for removing pollutants from water, but new research suggests that the materials doing the cleaning may also create an overlooked transport pathway for contaminants.
A study published in Biochar found that extremely small particles released from activated carbon and biochar can remain in treated water after sand filtration and even after filtration through a 0.45 μm membrane. More importantly, these particles can carry adsorbed contaminants with them.
In fine-sand filtrates, 84.5% to 90.2% of the total lead detected was associated with residual carbonaceous particles rather than remaining in the dissolved form.
“Activated carbon and biochar are excellent adsorbents, but adsorption alone does not necessarily mean that a contaminant has been permanently removed from the water,” said lead author Ziheng Wang of The University of Manchester. “Our results show that if very small carbon particles remain mobile, the contaminants attached to them may also continue moving through the treatment system.”
Activated carbon is commonly used in drinking water and wastewater treatment, while biochar is attracting growing interest as a potentially lower-cost material produced from biomass. Both materials capture pollutants on their surfaces. However, physical breakdown, erosion, or the presence of very fine particles can allow some carbonaceous material to escape downstream.
To examine this process, the researchers tested activated carbon and three biochars made from hardwood, wheat straw, and corn straw. Lead was used as a model contaminant. The team combined adsorption experiments with fixed-bed filtration, stirred suspensions, sand filtration, and membranes with pore sizes of 0.45 and 0.02 μm.
Residual particles showed size-distribution features around 100 to 200 nanometers, 0.5 to 1 micrometer, and an apparent fraction near 5 micrometers. The researchers caution that the larger apparent fraction found after 0.45 μm filtration probably did not represent intact 5 μm particles passing directly through membrane pores. Instead, smaller particles may have aggregated during filtration, sample handling, concentration, or measurement. Further experiments are needed to verify this process.
The most important finding emerged when the researchers separated dissolved lead from lead carried by particles. After fine-sand filtration, particle-bound lead represented 84.5% of total lead for activated carbon and as much as 90.2% for corn straw biochar.
Even after 0.45 μm membrane filtration, lead remained associated with particles in the 0.02 to 0.45 μm size range , showing that conventional filtration and dissolved-phase measurements may overlook part of the contaminant load.
This distinction matters because common analytical procedures often filter water samples before measuring dissolved metals. Removing particles before analysis can therefore remove the contaminants attached to those particles as well, potentially giving an incomplete picture of contaminant transport.
The findings suggest that water-treatment performance should be evaluated using both dissolved contaminants and contaminants carried by residual particles.
The researchers emphasize that the experiments were conducted under controlled laboratory conditions. Water chemistry, including pH, ionic strength, natural organic matter, and dissolved ions, could change particle stability and contaminant transport in real treatment systems.
Future work should test representative drinking water and wastewater matrices and evaluate practical measures such as adsorbent pre-washing, mechanical stabilization, granulation, improved coagulation and flocculation, and combined sand and membrane filtration.
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Journal Reference: Wang, Z., Sedighi, M. Co-transport of contaminants by disintegrated and residual superfine particles in biocarbon-based adsorbents. Biochar 8 , 137 (2026).
https://doi.org/10.1007/s42773-026-00654-4
About Biochar
Biochar (e-ISSN: 2524-7867) is the first journal dedicated exclusively to biochar research, spanning agronomy, environmental science, and materials science. It publishes original studies on biochar production, processing, and applications—such as bioenergy, environmental remediation, soil enhancement, climate mitigation, water treatment, and sustainability analysis. The journal serves as an innovative and professional platform for global researchers to share advances in this rapidly expanding field.
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Biochar
Experimental study
Co-transport of contaminants by disintegrated and residual superfine particles in biocarbon-based adsorbents
21-Sep-2026