Key factors controlling transport of biochar colloids in porous media and the mutual effects with coexisting contaminants.

Large-scale application of biochar (BC) has been considered as an effective strategy for carbon sequestration, soil quality improvement, and water/soil remediation. Once introduced into soils, BC can disintegrate or fragment through physical, chemical and biological processes, resulting in the formation of BC colloids (BCCs) with particle size ranging from 1 to 1000 nm. Due to the small size, large surface area, and high surface activity, BCCs exhibit stronger colloidal stability in aquatic environments and higher mobility in porous media than the bulk BC particles. In particular, BCCs have much higher adsorption affinity for various contaminants, and thus may serve as vehicles for facilitating contaminants transport, posing potential risks to the environment and human health. In this review, we summarize the current state of knowledge on the transport of BCCs influenced by (1) the intrinsic physicochemical properties of BCCs (i.e. pyrolysis temperature, size, aging effect), (2) solution chemistry (i.e. pH, ionic strength, ionic composition), (3) inorganic/organic colloids (i.e., minerals, humic/fulvic acid), (4) flow conditions (i.e. flow velocity), and (5) properties of porous media (i.e. grain size, media type and surface properties, moisture and mineral content). In addition, differing from previous works, this review provides a systematic analysis of BCCs transport through an integrated lens of governing mechanisms and mathematical models, with a dedicated focus on the mutual effects between BCCs and contaminants-a critical yet underexplored area determining their combined environmental fate. Finally, this work concluded with remarks on the importance of BCCs transport in porous media and directions for future research.