Highly carboxylate functionalized halloysite nanotube as an efficient and recyclable adsorbent for methylene blue

Halloysite nanotubes (Hal) have drawn intensive attentions because of their unique nanotubular structure and large surface area. However, limited adsorption capacity and poor recyclability severely impeded their practical application for water remediation. Herein, to overcome the deficiencies of pristine Hal, we deliberately designed and prepared a series of anhydride and carboxylate functionalized Hal (Hal@PDM-COO-) via self-stabilized precipitation polymerization. Owing to the merits of both nanotubular structure of Hal and high-density carboxylate ions in the shell layer, the Hal@PDM-COO- possessed a remarkable pH-dependent adsorption performance towards methylene blue (b -MB). As the solution pH increased from 3 to 11, the sorption capacity increased remarkably from 179.1 to 459.1 mg/g. Furthermore, all sorption processes were better-fitted with pseudo-second-order and Langmuir models, exhibiting a maximum sorption amount of 302.1 mg/g at pH = 7. These results indicate that electrostatic attraction between positive b -MB molecules and negative carboxylate groups plays a dominant role during sorption process, and carboxylate groups distribute homogeneously in the shell layer of Hal@PDM-COO-. More importantly, Hal@PDM-COO- possessed outstanding recyclability, and the removal rate of b -MB kept >95% after five recycle runs. Considering the highly enhanced adsorption capacity, rapid adsorption rate and excellent reusability, the Hal@PDM-COO- showed significant potential for wastewater treatment.