Experimental and theoretical studies on methylene blue and methyl orange sorption by wheat straw-derived biochar with a large surface area

Wheat straw-derived biochar (BC-4-700) with a large surface area (2263.10 m2 g−1) has been synthesized using potassium hydroxide as an activating agent, and its sorption performance is evaluated by sorbing methylene blue (MB) and methyl orange (MO). The sorption behaviours of both MO and MB by BC-4-700 all follow pseudo-second-order and Langmuir models, and the driving forces for the two sorptions are attributed to the pi–pi interaction. The largest sorption quantity for MO by BC-4-700 is 3.24 mmol g−1, being 1.37 times larger than that for MB (2.36 mmol g−1). There is an inflection point in the isotherm sorption curve of MB. It is widely accepted that MB is a cationic dye and MO is an anionic dye. However, the two experimental phenomena cannot be explained well by the widely accepted MB+ form. The pH detection experiments on MB and MO solutions suggest that MO does not hydrolyze in water and the existing form in water is the well-accepted MO−, whereas MB hydrolyzes in water and the existing forms in water most likely include both the well-accepted MB+ and the hydrolyzed form of MB+. This finding can account well for the two experimental phenomena mentioned above, which is helpful to understand deeply the behaviour of organic cations in water and the sorption mechanism of organic cations by sorption materials.