Efficient adsorption of methylene blue (MB) on WO3/TiO2 nanocomposite adsorbent: Process optimization, isotherm and kinetic modelling, and thermodynamic analysis

Methylene blue (MB) has become a threat to the environment despite being a suitable dye in textile industries. The WO 3 /TiO 2 composite adsorbent was synthesized through the impregnation method and annealed at 200°C for 2 h, where WO 3 remained amorphous on TiO 2 . Composite was characterized by XRD, ATR and Zeta potential. The composition of WO 3 /TiO 2 was optimized at 3 mol%WO 3 , demonstrating maximum adsorption activity compared to TiO 2 . Zeta potential of WO 3 /TiO 2 was estimated to be −10.41 mV. With this composition, ∼72% MB removal was achieved after 60 min of contact in dark at optimized pH 6. Isotherm studies indicated that adsorption followed multilayer physisorption, consistent with Freundlich model (R 2 =0.997, RSS=2.9051 ×10 −4 ). The Langmuir model (R 2 =0.978, RSS=0.0104) revealed a 22.51 mg/g monolayer capacity at 298 K, confirming chemisorption, while the Temkin constant [B (heat of sorption) = 5.06398 J/mol] supported the coexistence of both mechanisms. Kinetic studies indicated that the process followed the pseudo-first order kinetic (PFO) (R 2 =0.9991, RSS=0.00223) model, supported by chi-square (χ 2 =0.232) and Sum of Squares Error ( SSE =2.7889) test results. Thermodynamic analysis revealed an endothermic process, with negative ΔG°(-0.2108 kJ/mol) and positive ΔH° (6.014 kJ/mol) values, and the positive ΔS° (0.0207 kJ/mol K) suggested strong affinity and randomness between MB and WO 3 /TiO 2 . The activation energy (E a =3.72 kJ/mol) confirmed physisorption as the dominant mechanism. Efficient adsorption behavior demonstrated by the WO 3 /TiO 2 was attributed to amorphous WO 3 , surface -OH groups, and electrostatic interactions between the WO 3 /TiO 2 and MB. Intraparticle diffusion (IPD) and Boyd plots did not pass through the origin, implied film diffusion was the dominant rate-controlling step, while IPD also contributed to adsorption process. A detailed adsorption mechanism was proposed.