Influence of dissolved organic matter, kaolinite, and iron oxides on aggregation and transport of biochar colloids in aqueous and soil environments

The aggregation and transport of biochar colloids (BCs) in the soil and groundwater are critical for applying biochar in the field and assessing long-term environmental risk. This research aimed to study the influence of dissolved organic matter (DOM) with different molecular weights (including humic acid, HA; bovine serum albumin, BSA; deoxyribonucleic acid, DNA) and three minerals (including kaolinite, goethite, and hematite) on the aggregation and transport behaviors of BCs. The adsorption of DOM on the surface of BCs increased the stability, inhibited aggregation, and promoted the transport of BCs. As the molecular weight of DOM increased, the thicknesses of the adsorption layer of HA, BSA, and DNA on BCs surface were 2.2 nm, 5.3 nm, and 5.6 nm, respectively, resulting in increasing steric hindrance and improving the stability and mobility of BCs. Kaolinite also significantly enhanced the stability and mobility of BCs by increasing the electrostatic repulsion. Goethite and hematite quickly combined with BCs through electrostatic attraction, resulting in stronger aggregation and retention of BCs. Compared to hematite, goethite provided more adsorption sites for BCs due to its needle-like shape, so goethite caused a larger heteroaggregation rate. Overall, the presence of DOM with different molecular weights and the minerals with varying surface charges in the soil environment had a significant and distinct impact on the stability, aggregation, and transport of BCs, which advances the knowledge of colloidal biochar fate in the soil and groundwater.