Influence of natural organic matter on the aggregation dynamics of biochar colloids derived from various feedstocks.

Abundant biochar colloids (BCs) produced from a wide range of feedstocks, resulting from forest fires, agricultural production, and environmental restoration, exhibit varying aggregation behaviors influenced by feedstock type and natural organic matter. However, the impact of natural organic matter on the colloidal stability of BCs derived from different feedstocks remains poorly understood. In this study, six selected biochars were derived from various feedstocks as follows: sewage sludge (SS), rice husk (RH), oil seed rape straw pellets (OSR), wheat straw pellets (WS), miscanthus straw pellets (MS) and softwood pellets (SW). The colloidal stability of BCs, with the exogenous addition of organic matter, was further determined. The order of critical coagulation concentrations (CCCs) of BCs with the presence of humic acid (HA) was as follows: RH (989.48 mM) < MS (1084.69 mM) < SS (1149.76 mM) < WS (1338.99 mM) < OSR (2402.98 mM) < SW (3151.32 mM). This order was significantly positively correlated with the specific surface area and negatively correlated with the ash content of the bulk biochar. Compared to HA, bovine serum albumin (BSA) more effectively inhibited the aggregation behavior of BCs due to steric hindrance. The initial aggregation rate constant (k) of BCs at 3000 mM NaCl was as follows: MS (0.238 nm/s) > OSR (0.142 nm/s) > WS (0.128 nm/s) > SS (0.126 nm/s) > RH (0.118 nm/s) > SW (0.112 nm/s). The stabilizing effects of BSA on biochar colloids were independent of the physicochemical properties of bulk biochar. In the presence of BSA, a thin layer of protein corona significantly enhanced the stability of biochar colloids, particularly the BCs derived from MS. Our results underscore the importance of considering feedstock resources and natural organic matter type when assessing the aggregation and potential risks of BCs in aquatic systems.

Carbon content determines the aggregation of biochar colloids from various feedstocks.

The aggregation kinetics of biochar colloids (BCs) play a crucial role in the fate and transport of contaminants, as well as the carbon (C) cycle in the environment. However, the colloidal stability of BCs from various feedstocks is very limited. In this study, the critical coagulation concentration (CCC) of twelve standard biochars pyrolyzed from various feedstocks (municipal source, agricultural waste, herbaceous residue, and woody feedstock) at 550 °C and 700 °C were investigated, and the relationship between the physicochemical characteristics of biochar and the colloidal stability of BCs was further analyzed. The CCC of BCs in the NaCl solution followed the trend of municipal source < agricultural waste < herbaceous residue < woody feedstock, which was similar to the order of C content in biochar. The CCC of BCs showed a strong positive correlation with the C content of various biochars, especially pyrolyzed at a higher temperature of 700 °C. The BCs derived from lignin-rich feedstock (e.g., woody feedstock) had the highest colloidal stability, followed by cellulose-rich feedstock (e.g., agricultural waste and herbaceous residue). The BCs derived from organic matter-rich feedstock (municipal source) were easy to aggregate in the aqueous environment. This study quantitatively provides new insights into the relationship between BCs stability and biochar characteristics from various feedstocks, which is critical to assess biochar environmental behavior in aqueous environments.