Enhanced Immobilization of Polycyclic Aromatic Hydrocarbons in Contaminated Soil Using Forest Wood-Derived Biochar and Activated Carbon under Saturated Conditions, and the Importance of Biochar Particle Size

Leaching behavior of organic contaminants such as polycyclic aromatic hydrocarbons (PAHs) can be altered in the presence of organic amendments through enhanced sorption of PAHs to the amended soil. The aim of the present study was to investigate the influence of biochar in two forms (i.e., crushed and pulverized), and granular activated carbon on immobilization and leaching behavior of selected PAHs from contaminated soil into water using the column leaching test. The influence of biochar particle size on leaching behavior of PAHs in biochar-amended soil was also evaluated for the first time in this study. Results showed relatively high initial concentrations of naphthalene, fluorene, and pyrene for unamended column (B1). Strong sorption of all PAHs to soil was observed in this study. At the end of the experiment less than one percent of the solid phase content of the sum of PAHs was released into water in columns B3 and B4. Change in pH due to the application of organic amendments did not affect PAH leaching results. The addition of pulverized biochar and granular activated carbon to soil remarkably reduced mobilization and leaching of the studied PAHs in most cases; however, higher molecular weight PAHs (i.e., pyrene, benzo(b)fluoranthene, and indeno(1,2,3cd)) were mobilized in the presence of crushed biochar in soil. The controlling role of biochar particle size on mobilization and release of PAHs in soil was observed. The application of pulverized biochar and activated carbon increased colloid content of the column effluents while reducing their dissolved organic carbon (DOC) content, suggesting a more significant role of colloid-facilitated transport than DOC-associated transport of PAHs in their leaching from carbon-amended soils, particularly for higher molecular weight PAHs. Based on the obtained results, pulverized biochar and granular activated carbon demonstrated promising and comparable performance in immobilizing PAHs in soil and reducing their leaching from soil into water through enhanced sorption of PAH compounds, which is markedly favorable in terms of soil remediation.