Design of porous activated carbon beads: Controlling size and structure for optimal dye adsorption

Hierarchically porous ACBs were synthesized from PANI/NMP precursor solution with adjustable sizes in a straightforward droplet extrusion that can be directly carbonized and activated for dye removal from wastewater efficiently. Three sizes of beads were synthesized and characterized by SEM, BET, FTIR, and XRD measurements, indicating that the mesoporous structures had surface area values of 450, 320, and 240 m²/g for small, medium and large ACBs, respectively. Adsorption tests with 3 g of ACBs and an initial methylene blue (MB) dye concentration of 15 mg/L revealed that smaller-sized beads had superior removal (96 %) at equilibrium within 3 h, which may be due to better pore accessibility and mass transfer. The Langmuir model exhibited the best fit for the equilibrium data (R²=0.9941, RMSE=0.0044), and the maximum adsorption capacity is equal to 10.81 mg/g, demonstrating monolayer adsorption on a homogeneous surface. Kinetic data fitted well to the pseudo-second-order model, which best described the adsorption behaviour through surface functional groups to control the adsorption rate. Meanwhile, the D–R model supported that the adsorption mechanism was primarily governed by physical interactions with minimal ion-exchange effect. This combined interpretation confirms that the overall dye removal mechanism was dominated by physisorption on the ACB surface.