Visualization of biochar colloids transport and retention in two-dimensional porous media

Biochar colloids (BCs) have received considerable attention in recent years due to the unique environmental behavior and potential risks. In this work, a real-time visualization system to reveal the transport and retention mechanisms of biochar colliods in two-dimensional (2-D) porous media under the ionic strength (IS), pH and media grain size was developed. The system consists of charge coupled device (CCD) microscope camera and flow cell (connected to a pump and sample collector). The breakthrough curve of flow cell effluent concentration was used to quantitatively analyze the migration behavior of BCs. BCs transport in 2-D porous media was simulated with a simplified Double Monod model. The results show that BCs had a strong migration capacity in 2-D porous media. Electronegativity of BCs and quartz sand increases with the decrease of IS and the increase of pH, and the electrostatic repulsion between them then increases, resulting in the reduction of the adhesion of BCs on the sand surface, which promotes the migration and reduces the retention of BCs in the 2-D porous media. In addition, the reduction of media grain size had an inhibitory effluence on the transport of BCs, and the finer sand could provide more deposition sites for BCs. Meanwhile, the Double Monod model also fitted the transport behavior nicely (R2 > 0.94). The visualization method in this study properly explains the migration behavior of BCs in 2-D porous media, which provides reasonable support for more comprehensive evaluation environmental risk.