<p>This study aimed to fabricate an affordable biosorbent from the biomass of <i>Pleurotus ostreatus</i> fungi, for the effective removal of Mn²⁺ ions from aqueous solutions through biosorption. Cleaned and dried fungal biomass was ground, and subjected to thermochemical modification using an alkaline medium (KOH) to enhance its sorption properties. Three fungal biosorbents were fabricated and tested: untreated mushrooms (original, AM), thermally modified mushrooms (TMM), and chemically modified mushroom (CMM)-based powders. Fourier transform infrared (FTIR) spectroscopy and scanning electron microscopy (SEM) techniques&#xa0;were used to characterize the biosorbents and confirm successful surface modification. The biosorbent performance in Mn²⁺&#xa0;removal was investigated under various experimental parameters, including contact time, Mn²⁺ concentration, pH, temperature, and biosorbent dosage. Bioadsorption efficiency was quantified using atomic absorption spectrophotometry&#xa0;(AAS). The results confirmed that the CMMs showed a substantial proficiency to remove manganese ions from wastewater, with its effectiveness being significantly dependent upon the employed experimental parameters. The CMM adsorbent exhibited a maximum adsorption capacity of 29.99&#xa0;mg/g at optimum conditions (pH = 4.0, T = 303&#xa0;K, and adsorbent dosage of 0.1&#xa0;g/L), corresponding to 99.99% adsorption efficiency. The observed results demonstrate that biosorption capacity is substantially enhanced by thermochemical modifications, establishing modified <i>Pleurotus ostreatus</i> as a&#xa0;highly effective-biosorbent for wastewater treatment. The thermodynamic study confirmed that the interactions between the Mn²⁺ ions and biosorbents are spontaneous, endothermic, and entropy-driven. Kinetic and isotherm analyses further confirmed that&#xa0;the dominant interactions between the Mn²⁺ ions and the heterogeneous surfaces of these biosorbents are physicochemical in nature.</p> Graphical Abstract <p></p>