Highly-efficient nitrogen self-doped biochar for versatile dyes’ removal prepared from soybean cake via a simple dual-templating approach and associated thermodynamics

Biochar has very low adsorption capacity to anionic dyes due to negative charge on the surface. Herein, a dual-templating strategy by mild activators (K 2 C 2 O 4 ·H 2 O and CaCO 3 ) was applied for synthesis of soybean cake nitrogen self-doped biochar (NSB) to enhance the adsorption capacity for dyes. The results of element analysis showed NSB had N contents of 3.51%. Its specific surface area was 2413.00 m 2  g −1 and maximum adsorption amount of Rhodamine B and Congo Red reached up to 839.97 mg g −1 and 882.64 mg g −1 , respectively. The NSB retained high adsorption capacity after 5 cycles. By linear fitting of full Van't Hoff equation , lower enthalpy value shown in Congo Red adsorption in comparison with Rhodamine B adsorption, indicated the efficient anionic dye removal by NSB. Negative heat capacity change value indicated water molecules adsorbed on the NSB surface were replaced by dyes molecules. This paper provides a framework to prepare excellent NSB for the efficient removal of dyes from wastewater and shed lights on the mechanism of how NSB work for respective cationic and anionic dye from thermodynamics.