Hierarchical Porous Catalytic Pyrolysis Char Derived from Oily Sludge for Enhanced Adsorption

A novel pyrolysis char (PC), prepared by H₃PO₄ catalytic pyrolysis of oily sludge (OS), was presented to remove methylene blue (MB) dye from aqueous solution for the first time. The optimal preparation conditions (catalytic pyrolysis temperature of 411 °C, H₃PO₄ impregnation ratio of 2.44, and catalytic pyrolysis time of 59 min) were predicted by the response surface methodology. The optimal PC exhibited favorable hierarchical porous properties, which brought a large adsorption capability (322.89 mg/g). The adsorption process fitted well with the Langmuir model and pseudo-second order model. In addition, thermodynamic parameters showed that the adsorption process was endothermic (ΔH ⁰ > 0) and spontaneous (ΔG ⁰ < 0). The adsorption capability was strongly influenced by coexisting metal ions due to the competitive adsorption effect. The inhibition for MB adsorption was arranged in the following order: Al³⁺ > Fe³⁺ > Mg²⁺ > Ca²⁺ > K⁺ > Na⁺. The adsorption mechanism of MB onto the OS-derived PC includes pore filling, π-π interactions, and electrostatic interactions. The as-obtained PC adsorbent exhibited good reusability performance, which leads to great potential in practical application for wastewater treatment.