Synthesis of magnetic activated carbons from black liquor lignin and Fenton sludge in a one-step pyrolysis for methylene blue adsorption

Black liquor lignin and Fenton sludge were used to prepare magnetic activated carbons (MACs) by one-step pyrolysis using KOH as an effective activator. Brunauer-Emmett-Teller (BET) analysis showed that the addition of Fenton sludge increased the mesopore volume of MACs compared with the control AC, and the addition of KOH not only improved the porosity but also enhanced the formation of high ferromagnetic particles (alpha-Fe). X-Ray Diffraction (XRD), X-ray Photoelectron Spectroscopy (XPS), transmission electron microscopy (TEM), and vibrating sample magnetometer (VSM) analysis revealed that the iron-based materials in the MACs were Fe3O4 and alpha-Fe nanoparticles, indicating good magnetic properties. The textural properties and the magnetic properties of the MACs can be modified by controlling the Fenton sludge/lignin ratio. The adsorption process of MAC(0.5-2) (m(Fenton sludge): m(lignin): m(KOH)=0.5:1:2) and MAC(1-2) (m(Fenton sludge): m(lignin): m(KOH)=1:1:2) were investigated. The two MACs exhibited good adsorption capacity (q(MAC0.5-2) = 307.2 mg g(-1) and q(MAC1-2) = 184.1 mg g(-1)) for methylene blue (MB) dye. The adsorption kinetics of MB dye onto two MACs followed the pseudo-second order model and the equilibrium data was fitted best with the Sips isotherm model. The thermodynamic study revealed that the adsorption process was endothermic and spontaneous in nature. The recovery efficiency of MAC(0.5-2) and MAC(1-2) remained 84.0% and 92.1% respectively after five-cycle adsorption-desorption. The synthesized MACs with high magnetic separation efficiency, good recovery ability, and high adsorption efficiency can be promising and effective adsorbents for MB dye or other dyes.