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This study presents a biogenically synthesised TiO₂– Serratia marcescens nanocomposite (TNS) for the efficient adsorption of Mn (II) from aqueous solutions using Box–Behnken Design (BBD) optimisation. The TNS material was synthesised under controlled pH conditions and characterised using SEM–EDX, TEM, XRD, FTIR, BET, and TGA–DSC, confirming a mesoporous crystalline TiO₂ framework functionalised with biomolecular groups from S. marcescens . The BBD model identified optimal conditions of 52.5 mg/L Mn (II), 0.01 g adsorbent, pH 7, 65 °C, and 92.5 min, achieving a removal efficiency of 98.7%, in strong agreement with the predicted value (100.4%). Adsorption followed the Freundlich isotherm, indicating multilayer adsorption on heterogeneous surfaces, while the pseudo-second-order model best described the kinetics. Thermodynamic analysis showed the process to be spontaneous and exothermic, with ΔH = − 13.05 kJ/mol and a positive ΔS, indicating increased interfacial randomness during adsorption. The TNS material retained over 60% efficiency after five cycles, demonstrating good reusability. These findings highlight the potential of microbial–TiO₂ hybrid nanocomposites as green, high-performance adsorbents for Mn (II) remediation.