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The textile industry causes lots of pollution due to its discharge of untreated coloured effluents into water bodies, impacting the environment. The present study includes a slow pyrolysis technique to produce magnetic biochar derived from waste areca nut husk (ANH)) biomass to adsorb methylene blue dye. The biochar and biomass were characterised via proximate analysis, ultimate analysis, bulk density, heating value, extractive content, biochemical analysis, thermogravimetric analysis (TGA), Fourier transform infrared spectroscopy (FTIR), SEM, BET surface area, pH, water holding capacity (WHC) and X-ray diffraction (XRD). A semi-batch reactor was used to produce biochar (ANHB) at 600 and 800 ^oC at 10 ^oC min^{- 1} heating rate and 45 min holding time in an inert atmosphere. The produced biochar was magnetised by blending aqueous biochar suspensions with aqueous Fe³⁺/Fe²⁺ solutions. Further, magnetised biochar is employed to eliminate methylene blue (MB) dyes at different pHs, contact times, temperatures, dosages and concentrations. Biochar derived at 800 ^oC (ANHB800) gave increased carbon content (62.93%), heating value (33.02 MJ/kg), and BET surface area (112 m²/g) over biochar derived at 600 ^oC. The results of the acid treatment biochar (ANHBA800) demonstrated that 5M H₂SO₄ causes a BET surface area increase (265 m²/g) and a ash content decrease (9.96%). However, when magnetic biochar was produced at 800 ^oC it shows an additional increase in BET surface area upto 385 m²/g. The MB dye absorption analysis confirmed 85.47% adsorption at 0.3 g/l dosage, 100 ppm concentration, 30 ^oC, 60 min contact time, and pH 7. The adsorption capacity was 785.34 mg/g when fit by the Langmuir isotherm model. Magnetic nanoparticles enhance active sites, electrostatic interactions, and recovery, improving efficiency, cost-effectiveness, and sustainability in dye removal. The adsorption kinetics results suggested that the pseudo-second-order model best explains the experimental data with an R² value of 0.994. Additionally, the adsorption isotherm studies were best fitted by the Langmuir model adsorption conforming monolayer adsorption of MB on biochar surface.