<p>The purification of REEs from leaching liquor of monazite ore; which contains contaminated iron and manganese, are significant important for REEs applications. Herein, this work investigated the sorption and separation of Fe³⁺ and Mn²⁺ from both aquatic solution and REEs of monazite liquor using carbon-based on lignocellulosic cotton stalk material (CCS) as a green biosorbent at optimum conditions. The prepared CCS was characterized by different physicochemical and morphological techniques as FTIR, SEM, TEM, and specific surface area measurement, which was 831 m² g^− 1 with total pore volume; 0.431 m³ g^− 1, micro-pores percentage; 93.27%, and meso-pores percentage; 6.73%. The sorption investigations like pH, stirring time, initial metal-ions concentration, temperature, and adsorbent weight were carried out to identify the best reaction parameters. The obtained results were analyzed using different kinetic and isotherm patterns. The pseudo-second order kinetic model surpasses on the other kinetic models in the sorption process, indicating the chemisorption reaction. The sorption isotherm matched with Langmuir model, depending on the highest R² and lowest of error functions. The maximum monolayer sorption capacities of Fe³⁺ and Mn²⁺ ions onto CCS are 531.9 and 680.27&#xa0;mg/g, respectively at optimum sorption conditions (pH 2.6 for Fe³⁺ and pH 5.5 for Mn²⁺, stirring time 30&#xa0;min. at 25&#xa0;°C). The sorption approach was exothermic, randomness and spontaneous in nature. Lastly, CCS can be highly separated Fe³⁺ and Mn²⁺ from REEs of monazite leachate with separation factors reached 228.947 and 2.5735, respectively. Consequently, the low-cost CCS sorbent was utilized for purification of REEs from Fe³⁺ and Mn²⁺ pollutants of monazite ore liquor.</p>