Halloysite nanotube@carbon with rich carboxyl groups as a multifunctional adsorbent for the efficient removal of cationic Pb(II), anionic Cr(VI) and methylene blue (MB)

As a natural clay mineral with fascinating nanostructures, halloysite nanotubes (HNTs) have been widely used as raw materials for the fabrication of HNT-based adsorbents for environmental remediation. However, the current application of the HNT-based adsorbents always suffers from drawbacks such as the use of toxic chemicals during fabrication, relatively low adsorption capacity, and the limitation in the types of adsorbed pollutants. Therefore, in this study, a multifunctional HNT@carbon with rich carboxyl groups (HNT@CRC) was prepared using a facile and cost effective method, and the adsorbent was further used for the removal of pollutants with different features like Pb(ii), Cr(vi) and methylene blue (MB). Results show that the increased amount of carboxyl groups can enhance the adsorption performance of the modified HNT@CRC, and the HNT@CRC-8 exhibited superior and multifunctional adsorption ability for Pb(ii), Cr(vi) and MB with a maximum adsorption capacity of 184.95 mg g(-1) at pH 6.0, 140.87 mg g(-1) at pH 2.0 and 690.45 mg g(-1) at pH 5.5, respectively. By a combination of various techniques (e.g.: X-ray photoelectron spectroscopy and Fourier transform infrared spectroscopy) and an extensive comparative analysis, the carboxyl group on the surface of HNT@CRC-8 has been identified to play the most critical role in the efficient removal of Pb(ii), Cr(vi) and MB. Through this work, a new and cost effective method was proposed for the fabrication of a multifunctional HNT@CRC adsorbent with excellent performance for the efficient removal of a variety of pollutants with different features.