SnO₂–Fe₂O₃ doped TiO₂ nanocomposites for reactive black 5 dye remediation: Dual-mechanism adsorption supported by kinetic, thermodynamic, and statistical analysis

Doped TiO₂ nanocomposites were synthesized using co-precipitation method with different Sn and Fe doping levels (1%, 3%, 5%, and 7%) to remove Reactive Black 5 (RB5), a common anionic azo dye. The nanocomposite containing 5% dopant exhibited the highest adsorption capacity with a q_max of 357.1 mg/g. Effects of pH (1–10), contact time (5–90 min), initial dye concentration (5–200 mg/L), and adsorbent dose (50–200 mgwere evaluated through batch adsorption experiments. At pH 3, 60 minutes of contact time, and 200 mg/L dye concentration, the highest removal (94.11%) was attained. Kinetic analysis shows that the pseudo-second-order model generated the best fit (R²=0.999), indicating chemisorption is the rate-limiting step. Isotherm modelling showed that adsorption is well explained by the Freundlich (R²=0.9832) and Langmuir (R²=0.9642) modelswhile also showing data support that the Temkin and double-layer statistical models could be useful in the overall analysis. Thermodynamic parameters (ΔG°=−2.80 to −4.98 kJ/mol; ΔH°=+0.008 kJ/mol; ΔS°=+0.033 J/mol•K) showed that the process was both spontaneous and endothermic. A regeneration study was carried oufor 10 cycles, and the nanocomposite was able to retain nearly 83% of its initial efficiency.