Soy flour–based biochars for enhanced removal of anionic and cationic dyes

The current study investigates the evolution of dye adsorption properties of protein-rich soy flour biomass converted to nitrogen-doped biochars. Biochars were synthesized from the single-step pyrolysis of soy flour at different temperatures, 450, 650, and 750 °C, determined from the TG/DTA/DTG studies. The 750 °C pyrolyzed sample was further treated with HNO 3 for demineralization. XRD, Raman, and FTIR results revealed that higher pyrolysis temperature led to increased amorphous graphitic carbon and mineral ash content and aromaticity. Treating the synthesized biochar with HNO 3 resulted in complete removal of mineral content, emergence of micropores, multifold increase in the surface area, higher oxidation of the surface, and lower pH pzc . From the adsorption study of anionic dye eriochrome black-T (EBT) and the cationic dyes methylene blue (MB) and crystal violet (CV), the best adsorption characteristics were observed for the sample pyrolyzed at 750 °C despite its very low surface area and porosity. The efficiency of adsorption was improved further by treating the sample with HNO 3. The adsorption results were explained on the basis of surface chemistry, pH pzc , and pore size of the samples. The maximum dye adsorbed ( q e ) onto the HNO 3 -treated sample for MB and EBT was 51.11 and 54.30 mg/g, respectively. For the HNO 3 -treated sample, the MB and EBT are both adsorbed via two-step chemisorption (pseudo-second-order kinetics) with MB forming multilayers on this sample and EBT forming monolayer as per Freundlich and Langmuir isotherm models, respectively. Graphical abstract