Preparation of copper doped walnut shell-based biochar for efficiently removal of organic dyes from aqueous solutions

Abstract Malachite green has been demonstrated to be highly toxic, causing mutagenic side effects. To solve this problem, Cu-doped walnut shell-based activated carbon (Cu-WS-AC) was prepared and tested as a competitive adsorbent in various dyes removal. Compared with AC (1415 mg g−1), a 2477 mg g−1 experimental adsorption capacity of MG using Cu-WS-AC was obtained at 298 K. The universality performance showed that Cu-WS-AC was an effective adsorbent in the removal of other organic dyes including acid fuchsin (669.5 mg g−1), Congo red (248.2 mg g−1), methylene blue (414.5 mg g−1), alizarin red (1358 mg g−1), basic fuchsin (753.4 mg g−1). The samples were characterized using SEM, XRD, XPS, FTIR, N2 adsorption–desorption and pHpzc characterization. The adsorption mechanism was proposed. The electrostatic attraction, the hydrogen bonding, the π-π stacking between the graphite layer structure and the aromatic ring in the MG molecule are the main reason to bring about high removal capacity. Also, the surface complexation interaction between Cu0/Cu2+ and cationic dye MG occurring simultaneously help to achieve remarkable adsorption capacity. Intraparticle diffusion model was applied to further explain the adsorption process. Moreover, the adsorption kinetics showed that the adsorption process was most suitable for the pseudo-second order model (R2 = 0.9997), inferring the chemisorption process. The adsorption equilibrium data were in good agreement with the Langmuir model (R2 = 0.9995), giving a calculated maximum monolayer adsorption capacity of 3546 mg g−1. This work provides a new idea for the practical application of Cu-doped adsorption materials in organic dye containing water treatment.