Efficient removal of methylene blue using Ca(OH)2 modified biochar derived from rice straw

This study investigated the adsorption performance of methylene blue using Ca(OH) 2 -modified biochar derived from agricultural and forestry wastes. The results showed that the modified rice straw biochar with 20% Ca(OH) 2 addition paralyzed at 500 °C exhibited the best adsorption performance for methylene blue. The adsorption data were fitted with four kinetic models and four isothermal adsorption models. Among which, the pseudo-second-order kinetic model and Langmuir isothermal model were the best fit, with high correlation coefficient of more than 0.99. The adsorption capacity of Langmuir fitting was 333.3 mg g −1 , which was 12.5 times higher than that for pristine rice straw biochar. The high adsorption capacity was maintained in a wide range of initial pH value of 4–12. SEM-EDX, organic element, surface structure, FTIR and XRD analysis indicated that Ca(OH) 2 modification significantly increased the polarity (higher O/C value), Ca content and the number of oxygen-containing functional groups (C-O and CO 3 2 − ${}_3^{2-}$ ) for pristine rice straw biochar, thereby substantially enhancing and improving the chemical adsorption of methylene blue. The adsorption mechanism involved for methylene blue included functional groups complexation, electrostatic interaction, ion exchange (mainly Ca 2＋ ) and π $\pi$ – π $\pi$ stacking. This study demonstrated that the modified rice straw biochar, as green and environmentally friendly raw material with cost-effectiveness, exhibited a great potential for broad application prospective subject to remediating printing and dyeing-contaminated wastewater.