Adsorption characteristics of Direct Red 23 from aqueous solution by biochar

Abstract Biochar, produced by the pyrolysis of pig manure at 800 °C, was characterized and investigated as an adsorbent for the removal of Direct Red 23 from aqueous solution. The adsorption kinetics was best described by a modified Freundlich model and intra-particle diffusion models. The value of the apparent adsorption constant k (modified Freundlich model) decreased from 0.133 to 0.076 L/(g·min 1/m ), when the amount of biochar increased from 1 to 8 g/L. Values of effective particle diffusion coefficient (D p ) and liquid film diffusion coefficient (D f ) were calculated to further determine the rate-limiting step of the adsorption process. The value of activation energy 5.558 kcal/mol indicates that the adsorption is a physical process controlled by film diffusion and the adsorption process is endothermic. When pH  ZPC (8.71),the surface of biochar is positively charged; the positive charge promoted the anionic dyes attracted onto the biochar by electrostatic force; and a hydrogen band was formed with the protonated OH functional groups on the active sites. The adsorption equilibrium study indicates that the adsorption was in a multilayer mode, and the maximum adsorption capacity was 2.19 × 10 − 5  mol/g, at pH 2.20 and temperature 25 °C. Considering the widely availability and high removal efficiency, biochar is deserved further investigations for applications in more hazardous substances removal.