Release and stability of water dispersible biochar colloids in aquatic environments: Effects of pyrolysis temperature, particle size, and solution chemistry

Abstract

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 effects of pyrolysis temperature, initial particle size, and solution chemistry on WDBC formation in aqueous solutions and examined the formation and colloidal stability of WDBC in natural solutions. Results showed that pyrolysis temperature determined the abrasion resistance of pyrolyzed BC, and the submicron fragment rate decreased in the order 400 °C (BC400) > 700 °C (BC700) > 200 °C (BC200). The WDBC yield decreased in the order BC400 (77.5–331 mg g−1) > BC700 (33.5–173 mg g−1) > BC200 (16.8–125 mg g−1) depending on BC size at a solution ionic strength (IS) ≤ 1 mM, which was positively correlated with the submicron fragment rate of bulk BC. With the exception of BC200, increasing IS (0.1–20 mM) 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. Derjaguin-Landau-Verwey-Overbeek interactions properly explained the effect of IS on WDBC from BC400 and BC700, while the steric resistance of abundant dissolved organic carbon on BC200 was mainly responsible for the high formation of WDBC at high IS (20–50 mM). WDBC had high colloidal stability and could form and stabilize well in natural surface waters and soil solutions, suggesting the relevant risk of long-distance migration of WDBC in environments. These findings represent new knowledge regarding the physical decomposition and the fate of BC in the environment.