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In this study, a novel nanoadsorbent material is developed using plant waste dead bark of Azadirachta indica plant and activated with phosphoric acid. By using FTIR, XRD, UV–vis, FESEM, BET, and TEM analytical tools, the structural and morphological features of the prepared nanoadsorbent were investigated, and the adsorption capability was tested for the removal of a cationic dye methylene blue (MB) from an aqueous solution. N2 sorption study confirmed that the nanoadsorbent structure is composed of 2–50 nm diameter size of pores and illustrated a type IV isotherm with H3 type of hysteresis loop. Batch experiments performed for adsorption isotherm investigation revealed that the sorption equilibrium was achieved within 80 min at pH = 11 when used 30 mg of adsorbent in 50 mL of dye solution (30 mg/L) at room temperature. In 80 min of contact time, 99.49% adsorptive removal of MB dye was achieved at optimized adsorption operational parameters. The adsorption process of MB dye was exhibited better agreement with the Langmuir isotherm model, and the utmost adsorption capability was estimated to be 106.92 mg/g at 303 K. The kinetics of adsorption process was well followed to the pseudo-second-order kinetics and the adsorption of MB dye molecules on the developed nanoadsorbent was found to be directed by the intra-particle diffusion (IPD) process but it was not a sole directing factor. The thermodynamic investigation of adsorption study revealed that the process was spontaneous and endothermic, and increased randomness at the adsorbate-adsorbent interface. The recyclability study performed in four successive cycles established the stability and reusability of adsorbent which retained nearly 90% adsorption ability after fourth run of reuse. [ABSTRACT FROM AUTHOR]