Commercial ion exchange resins are formed from cross-linked polymers, which commonly have styrene/divinylbenzene or acrylate backbones. They can be either macro- or microporous and are produced on a large scale by a patented technology known as “spray-jetting”, which allows the formation of small beads, ~300 microns in diameter and of a narrow size distribution. Specific organic functional groups are then grafted to the polymer in multi-step synthetic processes. These can be designed to be selective to certain ions in aqueous solution, leading to purification and separation applications.
This research article is a collaborative effort between researchers at the University of Manchester and Dr Mark Ogden of the department of chemical and biological engineering and features the use of “Purolite S930+”, a resin bearing iminodiacetic acid functional groups. This resin is commercially available for decontamination of heavy metals from industrial wastewater. However here, a completely unique application is envisaged; that of selective uranyl uptake for geological uranium capture and recovery. This is an essential process for nuclear fuel production and consumes massive amounts of borehole water. This of course is a scarce resource in many countries and one line of thinking is to get round this problem by substituting seawater. However, this requires the ion-exchange resin to work well in challenging “hypersaline” conditions.
This study investigated the ability of S930+ to extract uranyl ions from a simulated aqueous feed solution, containing high levels of salt, iron, copper and sulfate. Encouragingly, the resin was found to have greater affinity for uranyl than the other two cations in the system and furthermore and crucially, there was negligible suppression of the uranyl uptake even at extremely high chloride concentrations (up to 6 M). Although careful control of the feed pH would be necessary, it is thought that there is real applied potential in an ion-exchange system of this nature in the uranium mining industry.
From a more fundamental perspective, the researchers were also able to perform Extended X-Ray Absorption Fine Structure (EXAFS) work at the Diamond Light Source in Oxfordshire. EXAFS is a type of spectroscopy that irradiates a sample with X-rays and measures the electron binding energies on the different atoms making up the sample. Through this, an accurate prediction could be made of how the uranyl cations were interacting with the resin functionality and it was revealed that a tridentate coordination mode was occurring (as seen in the accompanying diagram), with the other coordination sites being filed by a sulfate anion. This is the first time such behaviour has been properly qualified.
Original research article: “Extraction of uranium from non-saline and hypersaline conditions using iminodiacetic acid chelating resin Purolite S930+”, J.T.M. Amphlett, C.A. Sharrad and M.D.Ogden, Chemical Engineering Journal, 342, 133-141.
Article by Tom Robshaw; a 2nd year EPSRC Polymers, Soft Matter and Colloids CDT PhD student.