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.