Enhancement of Cationic Dye Adsorption by Alkaline-Activated Sewage Sludge

Wastewater from street food activities is a major pollution source. In this study, sewage sludge (SS) from a treatment plant in Thailand was converted into a porous adsorbent via NaOH activation and calcination (SS-B-C600), while SS-C600 was used as a control. Characterization revealed that both samples were composed of SiO2 with minor kaolinite. FTIR confirmed Si–O–Si vibrations in both samples, while SS-B-C600 showed enhanced –OH (Si–OH) groups, indicating improved surface hydroxylation. Activation significantly enhanced the adsorption performance for methylene blue (MB) in laboratory-scale experiments. The equilibrium data were best fitted by the Langmuir isotherm model, indicating monolayer adsorption, with maximum capacities of 3.11 mg/g (SS-C600) and 7.56 mg/g (SS-B-C600). The kinetic results were well described by the pseudo-second-order model, suggesting that the adsorption mechanism is governed by a combination of porosity and surface interactions through physisorption. DFT calculations revealed that intermolecular hydrogen bonds between MB and aluminosilicate play a key role in the formation of the complex, while the calculated interaction energy (ΔE = −304.27 kJ/mol) further confirmed the presence of strong intermolecular interactions. Moreover, SS-B-C600 showed stable performance over three reuse cycles, highlighting its potential as a cost-effective and sustainable adsorbent.