Transport and retention of biochar nanoparticles in a paddy soil under environmentally-relevant solution chemistry conditions

Land application of biochar has been increasingly recommended as a powerful strategy for carbon sequestration and soil remediation. However, the biochar particles, especially those in the nanoscale range, may migrate or carry the inherent contaminants along the soil profile, posing a potential risk to the groundwater. This study investigated the transport and retention of wood chip-derived biochar nanoparticles (NPs) in water-saturated columns packed with a paddy soil. The environmentally-relevant soil solution chemistry including ionic strength (0.10–50 mM), electrolyte type (NaCl and CaCl₂), and natural organic matter (0–10 mg L⁻¹ humic acid) were tested to elucidate their effects on the biochar NPs transport. Higher mobility of biochar NPs was observed in the soil at lower ionic strengths, with CaCl₂ electrolyte being more effective than NaCl in decreasing biochar NPs transport. The retained biochar NPs in NaCl was re-entrained (∼57.7%) upon lowering transient pore-water ionic strength, indicating that biochar NPs were reversibly retained in the secondary minimum. In contrast, negligible re-entrainment of biochar NPs occurred in CaCl₂ due to the primary minimum and/or particle aggregation. Humic acid increased the mobility of biochar NPs, likely due to enhanced electrosteric repulsive interactions. The transport behaviors of biochar NPs can be well interpreted by a two-site kinetic retention model that assumes reversible retention for one site, and irreversible retention for the other site. Our findings indicated that the transport of wood chip biochar NPs is significant in the paddy soil, highlighting the importance of understanding the mobility of biochar NPs in natural soils for accurately assessing their environmental impacts. - Highlights: • Transport of wood chip biochar nanoparticles is significant in a paddy soil. • Humic acid enhanced the transport of biochar nanoparticles in the soil. • Transport of biochar nanoparticles in the soil should take into account the surface charge heterogeneity and roughness of soil. • Transport of biochar nanoparticles in the soil can be well interpreted by a two-site kinetic retention model. - Transport of biochar nanoparticles is significant in soil and the relevant environmental impacts must be of concern.