Fast and efficient removal of chromium (VI) anionic species by a reusable chitosan-modified multi-walled carbon nanotube composite

Chromium pollution has posed an increasing problem to the aquatic environment worldwide, especially in industrial areas and mining fields. To date, the removal of Cr (VI) has been achieved by using various techniques such as chemical precipitation, solvent extraction, and electrodialysis; however, these methods typically take significantly longer reaction time and expensive, complex preparations. Herein, we demonstrate a simple polymer crosslinking method for efficient and fast removal of Cr(VI) anionic species via adsorption in aqueous solutions. Specifically, we modified carboxylated multi-walled carbon nanotubes (MWCNTs-COOH) with chitosan to enable enhanced adsorption of Cr (VI) in acidic aqueous solutions (pH= 2), resulting in maximum adsorption capacities of 142.9 +/- 0.9 mg g(-1), 151 +/- 1 mg g(-1) and 164 +/- 2 mg g(-1) at 293 K, 303 K and 313 K, respectively. The adsorption process was exothermic, following the Langmuir isotherm model. We found that the adsorption of Cr (VI) by the composite is caused primarily through physical electrostatic adsorption as well as chemical redox reactions. Furthermore, the chitosan/MWCNTs-COOH composite was demonstrated to be successfully applicable in multiple Cr (VI) adsorption cycles, with none, or limited, loss of performance (98-100% adsorption up to the 4th cycle). Nonetheless, this chitosan-modified MWCNTs-COOH composite is capable of enhancing the adsorption capacity, as well as shortening the adsorption reaction time needed for efficient Cr (VI) removal to occur. Overall, our work presents a simple and rapid methodology in designing and fabricating new materials that will find significant utility in a number of environmental applications, such as wastewater treatment and chemical waste management.