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Abstract In this study two deep eutectic solvents (DESs) were prepared using ethylene glycol (EG) and two different ammonium-based salts. The potential of these DESs as novel agents for CNTs functionalization was examined by performing a comprehensive characterization study to identify the changes developing after the functionalization process. The impact of DESs was obvious by increasing the surface area of CNTs to reach 197.8 (m2/g), and by adding new functional groups to CNTs surface without causing any damage to the unique structure of CNTs. Moreover, CNTs functionalized with DESs were applied as new adsorbents for the removal of methyl orange (MO) from water. The adsorption conditions were optimized using RSM-CCD experimental design. The kinetics and the equilibrium adsorption data were analyzed using different kinetic and isotherm models. According to the regression results, adsorption kinetics data were well described by pseudo-second order model, whereas adsorption isotherm data were best represented by Langmuir isotherm model. The highest recorded maximum adsorption capacity (q max) value was found to be 310.2 mg/g. Graphical abstract Image 1 Highlights • DESs were used as functionalization agents for CNTs. • DES-CNTs combination was introduced as new adsorbent for Methyl orange (MO). • The dispersion of CNTs was enhanced after functionalization with DESs. • The highest maximum adsorption capacity of DES-CNTs adsorbent was 310.2 mg/g. [ABSTRACT FROM AUTHOR]