Experimental Study on the Influence of Initial pH, Ionic Strength, and Temperature on the Selective Adsorption of Dyes onto Nanodiamonds

In the current work, the performance of untreated nanodiamonds (UNDs) and thermally oxidized nanodiamonds (ONDs), as adsorbents for selective adsorption of methylene blue (MB) and methyl orange (MO) from aqueous media, was examined. The adsorption isotherm, initial pH, ionic strength, and thermodynamic study were investigated in batch experiments. The equilibrium adsorption data were analyzed by Langmuir and Freundlich isotherm models, which indicated that the isotherms were well fitted with the Langmuir model for both dyes. Thermodynamic parameters indicated that the adsorption operation was a feasible, spontaneous, and physisorption process in experimental conditions. Meanwhile, the adsorption of MO onto the UND was exothermic and determined by enthalpy change, while the adsorption of MB onto the OND was endothermic and an entropy-controlling process. The adsorption selectivity study exhibited that OND can rapidly and selectively separate cationic MB dye from the MO and MB mixture. On the basis of the negatively charged surface of the OND with highly negative zeta potential, the fact that the OND opted to adsorb MB over MO was firmly dependent on the initial pH value and ionic strength, which suggested that the possible mechanism of adsorption such as electrostatic interaction, hydrogen bonding, and pi-pi stacking might be important. Finally, based on the mentioned considerations, thermally OND was an attractive candidate for rapid adsorption and separation of cationic dyes from aqueous solution.