Experimental and DFT insights into the adsorption mechanism of methylene blue by alkali-modified corn straw biochar

Abstract

As an efficient and cost-effective adsorbent, biochar has been widely used in the adsorption and removal of dyes. In this study, a simple NaOH-modified biochar with the pyrolysis temperature of 300 °C (NaCBC₃₀₀) was synthesized, characterized, and investigated for the adsorption performances and mechanisms of methylene blue (MB). NaCBC₃₀₀ exhibited excellent MB adsorption performance with maximum removal efficiency and adsorption capacity of 99.98% and 290.71 mg g^-1, which were three and four times higher than biochar without modification, respectively. This might be attributed to the increased content of -OH and the formation of irregular flakes after NaOH modification. The Freundlich isotherm suggested multilayer adsorption between NaCBC₃₀₀ and MB. Spectroscopic characterizations demonstrated that multiple mechanisms including π-π interaction, H-bonding, and pore-filling were involved in the adsorption. According to density functional theory (DFT) calculations, electrostatic interaction between NaCBC₃₀₀ and MB was verified. The highest possibility of the attraction between NaCBC₃₀₀ and MB was between -COOH in NaCBC₃₀₀ and R-N(CH₃)₂ in MB. This work improved our understanding of the mechanism for MB adsorption by modified biochar and provided practical and theoretical guidance for adsorbent preparation with high adsorption ability for dyes.