Sorption of U(VI) compounds on inorganic composites containing partially unzipped multiwalled carbon nanotubes

Abstract

Unlike ion-exchange resins, inorganic sorbents possess high selectivity towards heavy metal ions and stability against ionizing radiation. However, sorption on these materials is rather slow. Moreover, sorption capacity strongly depends on the solution pH. In order to improve sorption properties of inorganic ion-exchangers, composites containing advanced carbon materials are obtained. Regularities of sorption of U(VI) compounds from low-concentrated aqueous solutions (up to 0.1 mmol dm–3 of uranium) on hydrated zirconium dioxide and zirconium hydrophosphate are considered. The sorbents were modified with partially unzipped multiwalled carbon nanotubes (PUMWCNTs). Sorption isotherms were obtained and analyzed. They obey Dubinin-Radushkevich model indicating sorption sites, a size of which is comparable with that of ions being sorbed. As found, the sorption mechanism is ion exchange. The effect of the solution pH on the sorption rate of U(VI) ions and capacity of inorganic ion-exchangers and their composites has been considered. Carbon additions increase sorption capacity of zirconium dioxide and zirconium hydrophosphate, when the initial pH of one-component solution is 3–4 and 5–7 respectively. Under these conditions, U(VI)-containing cations are removed from the solution practically completely. The rate of sorption obeys the model of chemical reaction of pseudo-second order, when uranium is removed from one-component solution. PUMWCNTs slow down sorption on zirconium dioxide and accelerate it on zirconium hydrophosphate. The dependence of the pseudo second order equation constants on the pH of U(VI) solutions was analyzed. The reaction of the first order occurs, when the solution contains also Ca2+ and Mg2+ ions. Regeneration was carried out using HNO3 and NaHCO3 solutions: the rate-determining stage of desorption is particle diffusion. It has been shown that one-component ZHP can be regenerated with a NaHCO3 solution practically completely. The most suitable solution for U(VI) desorption from ZHP-PUMWCNTs composite is a 1 M HNO3 solution.