Interaction mechanisms of organic contaminants with burned straw ash charcoal

Black carbons (e.g., charcoal) have a great impact on the transport of organic contaminants in soil and water because of its strong affinity and ubiquity in the environment. To further elucidate their interaction mechanism, sorption of polar (p-nitrotoluene, m-dinitrobenzene and nitrobenzene) and nonpolar (naphthalene) aromatic contaminants to burned straw ash charcoal under different de-ashed treatments were investigated. The sorption isotherms fitted well with Freundlich equation, and the Freundlich N values were all around 0.31-0.38, being independent of the sorbate properties and sorbent types. After sequential removal of ashes by acid treatments (HCl and HCl-HF), both adsorption and partition were enhanced due to the enrichment of charcoal component. The separated contribution of adsorption and partition to total sorption were quantified. The effective carbon content in ash charcoal functioned as adsorption sites, partition phases, and hybrid regions with adsorption and partition were conceptualized and calculated. The hybrid regions increased obviously after de-ashed treatment. The linear relationships of Freundlich N values with the charring-temperature of charcoal or biochar (the charred byproduct in biomass pyrolysis) were observed based on the current study and the cited publications which included 15 different temperatures (100-850 degrees C), 10 kinds of precursors of charcoal/biochar, and 10 organic sorbates.