In the present invention nanotubes-based dye-adsorbent powder is used for adsorption and decomposition of synthetic dye under dark-condition without the use of any external power-source such as the applied potential-difference, microwave, ultrasonicator, and others to save the energy consumption, which results in a complete dye-decomposition on the powder-surface. The surface-cleaned dye-adsorbent powder has very high specific surface-area and dye-adsorption capacity comparable with those of the original dye-adsorbent powder and can be recycled for the next cycles of dye-adsorption and dye-decomposition under the dark-condition. In the present method, thenanotubes-based dye-adsorbent powder is stirred under the dark-condition in an aqueous dye solution containing a strong oxidizer to get simultaneous dye-adsorption and dye-decomposition on the powder-surface, or getting dye adsorption in one solution and dye decomposition in a separate solution by said adsorbent powder under dark condition.