Correlations and adsorption mechanisms of aromatic compounds on biochars produced from various biomass at 700 °C

Knowledge of adsorption behavior of organic contaminants on high heat temperature treated biochars is essential for application of biochars as adsorbents in wastewater treatment and soil remediation. In this study, isotherms of 25 aromatic compounds adsorption on biochars pyrolyzed at 700 degrees C from biomass including wood chips, rice straw, bamboo chips, cellulose, lignin and chitin were investigated to establish correlations between adsorption behavior and physicochemical properties of biochars. Isotherms were well fitted by Polanyi theory-based Dubinin-Ashtakhov (DA) model with three parameters, i.e., adsorption capacity (Q(0)) and adsorption affinity (E and b). Besides the negative correlation of Q(0) with molecular maximum cross-sectional areas (sigma) of organic compounds, positive correlations of Q(0) with total pore volume (V-total) and average diameter of micropore (D) of biochars were observed, indicating that adsorption by biochars is captured by the pore-filling mechanism with molecular sieving effect in biochar pores. Linear solvation energy relationships (LSERs) of adsorption affinity (E) with solvatochromic parameters of organic compounds (i. e., alpha(m) and pi*) were established, suggesting that hydrophobic effect, pi-pi interaction and hydrogen-bonding interaction are the main forces responsible for adsorption. The regression coefficient (pi(1)) and intercept (C) of obtained ISERs are correlated with biochar H/C and R-micro, respectively, implying that biochars with higher aromaticity and more micropores have stronger pi-pi bonding potential and hydrophobic effect potential with aromatic molecule, respectively. However, hydrogen-bonding potential of biochars for organic molecules is not changed significantly with properties of biochars. A negative correlation of b with biochar H/C is also obtained. These correlations could be used to predict the adsorption behavior of organic compounds on high heat temperature treated biochars from various biomass for the application of biochars as sorbents and for the estimating of environmental risks of organic compounds in the present of biochars. (C) 2017 Elsevier Ltd. All rights reserved.