Formation and stability of water dispersible biochar colloids in soil-water systems

Pathways for the physical disintegration of biochar (BC) and the release of water dispersible BC colloids (WDBC) have received much attention due to their unique impacts on carbon loss and contaminant. However, the current understanding of the mechanisms involved in WDBC formation and associated influencing factors is rather limited. This study systematically explored the formation and colloidal stability of WDBC in various solutions. Results showed that the WDBC yield decreased in the order BC400 (400℃)> BC700 (℃)> BC200(200℃) at a solution ionic strength (IS) ≤ 1 mM （NaCl）. With the exception of BC200, increasing IS (0.1–20 mM NaCl) and decreasing pH (3.0–10.0) significantly inhibited WDBC yield. Release and sedimentation dominated the WDBC formation processes with the former being more susceptible to solution chemistry. The test results of 22 soil solutions showed that the yields of WDBC from BC400 were considerable, while the yield of WDBC from BC700 could be neglected. Principal component analysis showed that the yield of WDBC in soil solutions was closely related to the contents of dissolved organic carbon, Fe and Al ions of soil solutions. WDBC had high colloidal stability and could stabilize well in natural surface waters and soil solutions. These findings represent new knowledge regarding the physical decomposition and the fate of BC in the environment.