Carbon nanotube membranes for the purification of water from uranium

Abstract

Carbon nanotubes (CNT) are unique materials with inner diameters as small as one to several nanometers. There has been a growing interest to use CNTs to develop the next generation of membranes to use the inner diameters to selectively sieve chemicals based on their sizes. Scientists have shown that combining the fast transport of water through CNTs and the uniform structures of CNTs results in an improved and efficient membrane. We applied CNT membranes to the separation of uranium from water. Uranium is found in the environment in multiple oxidation states, with the primary oxidation state in aqueous solution being U(VI). The uranyl cation U(VI)O22+ readily complexes with carboxylic acids, carbonate, phosphate, or sulfate ions and in these forms is soluble and easily transported. With this in mind, we investigated how CNT membranes separated uranium from water. Complexation of uranium with citric acid led to large complexes that could not permeate the membranes, but the uranium in water without citric acid permeated. Raman spectroscopy confirmed the presence of uranium species, [(UO2)2(HCit)2]2- and [(UO2)3O(Cit)3]5-, and provided further insight into the size of the uranyl-citrate complexes. In my thesis I will describe our hypothesis that the presence of a sodium ion in the water led to the formation of a supramolecular complex that is much larger than the inner diameter of a CNT. This work can offer insight on complex aqueous systems involving uranium, and further our understanding of CNT membrane technology.