Surface and colloid properties of biochar and implications for transport in porous media

Biochar contains a large amount of colloidal particles. Biochar colloids are generated (1) during pyrolysis of the feedstock, where some of the biochar formed is in the colloidal size fraction, and (2) by the breakdown of larger biochar particles. Here, we review the surface and colloid properties of biochar and the implications for transport of biochar colloids in porous media. We discuss the origin of biochar colloids and their physical and chemical properties relation to feedstock and pyrolysis temperature. We then review the transport of colloidal biochar in sand columns and soils and relate the transport behavior to surface characteristics. We finally examine the effects of aging, i.e., natural and simulated environmental weathering, on the surface properties of biochar colloids. Biochar colloids tend to have lower zeta potential and lower aromaticity than the bulk biochar. Due to their smaller size, biochar colloids are more stable in aquatic environments and more mobile in the subsurface than the parent biochar particles. Like the parent biochar material, pristine biochar colloids contain a variety of hydrophobic functional groups (e.g., alkyl, aromatic), and become more hydrophilic as the biochar ages. At environmentally relevant pH, biochar colloids are negatively charged.