Toward the Development of an Electromechanical Ion-Specific Conveyer Belt

Abstract

Bromide (Br−) is ubiquitous both in natural waters and those impacted by anthropogenic activities, such as, the discharge of treated or untreated wastewaters, releases from coal-fired power plants and hydraulic fracturing operations, and sea water intrusion. The development of methods for the extraction and purification of Br−, particularly from Br− -rich brines, is regarded as of great economic interest owed to the uses of bromine in many industrial processes. In fact, the removal of Br- from drinking water is beneficial from a public health perspective, as it can lead to the formation of regulated disinfection byproducts (DBPs) during water treatment. Although various alternatives have recently been proposed to remove halide anions, such as bromide, from drinking water, none of them have proven sufficiently effective for large-scale application (see e.g. Ref. 1).The present contribution describes a strategy for the removal and purification of bromide from an aqueous solution containing chloride in amounts that closely mimic those found in natural brine sources. As will be described, this tactic relies on the selective electrosorption of Br− on Au, an inert metal electrode material from such a synthetic brine, and its subsequent desorption into a second aqueous solution devoid of Br−.The all glass multi compartment cell employed for these experiments ‘shown schematically in the figure above consists of a central cylindrical cup filled with 0.01 HClO4 supporting electrolyte, which houses the reference, RE, and counter electrodes connected on either side to two identical cups of smaller diameter, C1 and C2 are separated from the central cup by glass frits so that diffusion between C1 and C2 is negligible. At the start of a regular experiment, C1 is filled with a solution containing 28 mM KCl and 12.5 mM KBr, the synthetic brine, in the supporting electrolyte, and C2 is filled with only supporting electrolyte. In the initial stage of this protocol, a gold foil is immersed in C1, which together with the RE and CE would form a conventional 3-electrode cell and a potential of 1.3 V vs RHE is then applied to the Au foil using a potentiostat to induce anion adsorption for ca.10 s. which is sufficient for Br-(aq) and Cl-(aq) to achieve their equilibrium coverages under the specific conditions of the experiment, as represented by the blue arrow. Based on the results of independent optical measurements, Br-(aq) would be the predominant ion adsorbed on the Au surface at this high potential. Subsequently, the Au foil is emersed from the solution without disconnecting the potentiostat, immersed into C2, and a potential of 0.2 V applied, which is just positive of the onset of hydrogen evolution but negative enough for adsorbed Br- to fully desorb, as represented by the red arrow. This process is then repeated tens of times, at the end of which the composition of the solution in C2 is analyzed using the methylene blue (MB) spectroscopic method described by Uraisin, et al. (2) to determine specifically Br-(aq), and a slightly modified Mohr’s turbidimetry technique, which employs K2CrO4 as the indicator, to determine the total amount of Br-(aq) and Cl-(aq).Four independent runs involving 50 to 75 sequential transfers were performed in order to assess the efficiency of this process, and found to yield an increase in the Br-(aq) concentration from an initial 30% in C1, up to higher than 75% in C2. Overall, the efficiency of this process can be increased by a corresponding increase in the area of the electrode. Alternatively, one can envision a continuous process involving a segmented electrode in the form of a belt and appropriate potential controls, that would be continuously emersed and immersed into C1 and C2, acting as a virtual ionic conveyer. Various aspects of this unique technology and its prospects for large scale implementation will be discussed. Acknowledgements This work was supported by a grant from NSF, CHE-1412060 References Sanchez-Polo, J. Rivera-Utrilla, E. Sahli, U. von Gunten, Ag-Doped Carbon Aerogels For Removing Halide Ions In Water Treatment, Water Res., 41, 2007,1031-1037Uraisin, T. Takayanagi, M. Oshima, D. Nacapricha, S. Motomizu, Kinetic-Spectrophotometric Method For The Determination Of Trace Amounts Of Bromide In Seawater, Talanta, 68, 2006, 951-956 Figure 1