Congo red dye removal using green synthesized titanium oxide nanoparticles using saponin biosurfactant derived from <i>Sapindus mukorossi</i>

The present study focuses on an eco-friendly approach for dye removal using green-synthesised titanium dioxide nanoparticles stabilised with a saponin biosurfactant derived from Sapindus mukorossi. Saponins were extracted by maceration, Soxhlet extraction, microwave-assisted extraction, and ultrasonication in ethanol and with Soxhlet extraction (70°C for 4 h), yielding the highest biosurfactant activity. The synthesised biogenic TiO2 nanoparticles were characterised by Fourier Transform Infrared Spectrometer (FTIR), X-ray diffraction (XRD), and SEM-EDX. The highest Congo red removal efficiency (90%) was achieved with a 5 g/L biogenic TiO2 dosage over 120 min, following pseudo-second-order kinetics and monolayer adsorption (qe exp = 84.03 mg/g at 20°C, R2 ≈0.99). The CR dye uptake was increased from 19 mg/g to 49 mg/g at 20°C, 20 mg/g to 53 mg/g at 40°C, and 21 mg/g to 56 mg/g at 60°C with increase in initial concentration from 50 ppm to 150 ppm, indicating the endothermic nature of CR dye degradation (∆H° = 3.81 kJ/mol, ∆S° = 93.83 J mol−1k−1 and ∆G° = −23.68 kJ/mol at 20°C). This study highlights the potential of biosurfactant-assisted nanomaterials for sustainable wastewater treatment, offering a green alternative to conventional dye-removal methods.