Biochar land application can potentially be used for carbon sequestration, improving soil quality, and reducing non-point source pollution. Understanding biochar mobility is important because its transport in soil greatly influences its stability, the dynamics of soil microbial communities and organic matter, and the movement of biochar-associated contaminants. Here, the transport of biochar particles was studied in saturated and unsaturated sand columns by measuring breakthroughs of biochar pulse under three pH and two ionic strength (IS) levels. Breakthrough curves (BTCs) were fitted to a convection–dispersion model with kinetic and equilibrium deposition sites to estimate the key transport parameters (e.g. biochar deposition rate coefficients). Biochar retention was enhanced by lowering pH and increasing IS, corroborating the trends of fitted deposition rate coefficients. Under both saturated and unsaturated conditions, effluent mass recoveries decreased, respectively, by a factor of 6·6 or 15 when pH decreased from 10 to 4 at 10 mM IS, and by a factor of 1·4 or 3·9 when IS increased from 10 to 100 mM at pH 7. Biochar retention was greater in unsaturated media, implying that saturated flow elutes more biochar particles. The particles larger than 5·4% of median grain diameter were filtered out of suspension during passage through the media; whereas, the retention of smaller particles was clearly dependent on solution chemistry. Similar to other types of colloids, this study highlights the importance of pH, IS, particle size, and soil water saturation in controlling biochar movement by soil matrix flow. Copyright © 2010 John Wiley & Sons, Ltd.