Dyes have long been used in different types of industry such as dyeing, textiles, paper, plastics, leather, and cosmetics. The color stuff discharged from these industries is hazardous and has an environmental impact. The presence of dyes in water is causing problems such as reducing oxygen levels; interfering with the penetration of sunlight; retarding photosynthesis; and interfering with gas solubility. Various methods have been used to remove dyes from wastewaters such as chemical oxidation, biodegradation, electrocoagulation, photodegradation, solvent extraction, ultrafiltration, and adsorption. The adsorption technique proved to be an effective and attractive process for removing dyes from aqueous solutions in terms of the initial costs, ease of operation, insensitivity to toxic substances, high efficiency, easy recovery, and simplicity of design. Adsorption is a process that occurs when a gas or liquid solute accumulates on the surface of a solid, forming a molecular or atomic film. This accumulation is associated with three types of interaction among adsorbate - adsorbent - solvent: physical, chemical, and electrostatic interactions. These interactions are affected by the properties of adsorbents (surface area, pore size and distribution, and surface chemistry); by the nature of adsorbates (polarity, planarity, solubility, molecular size, and functional groups); and by the solution chemistry (adsorbate concentration, adsorbent mass, temperature, pH, and ionic strength). CNTs have already been widely studied for removing organic contaminants from aqueous solutions.

CNTs tend to aggregate together as bundles because of Van der Waals interactions. The aggregation of CNTs leads to a reduction in their surface area, while generating interstitial channels between nanotubes and grooves on the periphery of the nanotube bundles. The available sorption sites of CNTs bundles include the external surface, the interstitial and groove areas formed between the CNTs, and the inner pores of the tubes. Additionally, the adsorption sites for MWCNTs include the concentric channels between the nanotubes layers . This distance between the MWCNTs layers is too small for any organic molecule to fit into . The external surfaces and groove areas are generally available for adsorption. The inner cavities can provide a very large surface area and an effective pore volume for adsorption, but they need to have open ends. The accessibility of the interstitial channels depends on the size of neighboring nanotubes and, as a result, some channels can be wide enough to accommodate adsorbate molecules. The presence of amorphous carbon and metal catalysts could block the inner pores .The blocked inner pores can be opened up by acid treatment using hydrochloric acid to remove metal catalysts located at the end of the CNTs, or by using hydrogen peroxide, nitric acid, base, or heat treatment to remove the amorphous carbon . These purification treatments change the surface area and sometimes the surface chemistry of CNTs.

بواسطة: م.م. هالة ثامر محمد