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A more energy efficient and sustainable method of desalinating water is needed due to increasing water shortages and contamination of current freshwater sources. Capacitive deionization (CDI), a new emerging technology, is a type of electric desalination that uses an applied voltage to pull the salt ions out of the salty solution and store the ions in porous carbon electrodes. CDI uses less applied energy than more commonly used methods of desalination like reverse osmosis and multi-flash distillation and has the added advantage of energy recovery. This report details experiments conducted to analyze the effect of different separation distances between the electrodes on salt ion adsorption for a high concentration solution under various flow rates and a 1 V voltage potential difference. The testing was performed in the Multiscale Thermal-Fluids Laboratory at The University of Texas at Austin using a uniquely fabricated CDI cell. Voltage, elapsed time, and electrical conductivity measurements were taken during the testing. Electrical conductivity was used to signify salinity of the solution. Two different separation distances were created by placing either one 2mm mesh between the electrodes or by using two 2 mm meshes between the electrodes. The results did not agree with the expectation that the one-mesh tests would adsorb twice the amount of salt ions as the two-mesh tests because of the differences in the electric field between the two types of tests. This is believed to be due to the high concentration tested. Future testing should include repeating these tests to verify the results and performing the tests for lower concentrations to see if they followed the expectation. / text

Surface functionalised silica materials have been prepared, followed by the extensive testing of their ability to remove metal ions from aqueous solutions. Modifications include ligand attachment and polymer grafting from the silica surface whilst the metals tested range from first row transition metals right through to the lanthanides and actinides. Characterisation of the materials produced has been of paramount importance for the understanding of the modification process and this is also extensively discussed. Atom transfer radical polymerisation (ATRP) has been used as the primary polymerisation method. Following polymerisation of 2-hydroxyethyl methacrylate (HEMA), post functionalisation was attempted. However, this was found to cause severe cross-linking and all attempts to attach ligands to this failed. Nonetheless, this process was transferred to grafting from silica surfaces and a novel approach to the characterisation of this material was implemented. (3-aminopropyl) triethoxysilane (APTES) was reacted with multiple forms of silica, primarily ZEOprep silica (average particle size 71.48 πm) and fumed silica (0.007 μm). This produced an amine coated surface to which 2-bromoisobutyryl bromide (BIBB) was attached, providing the required surface for radical polymerisation to proceed with a selected monomer. Solid State Nuclear Magnetic Resonance (SSNMR) has been utilised as the major characterisation technique for each step, leading to significant understanding of how this occurs. Thermogravimetric Analysis (TGA) and elemental analysis has supported this method at each stage whilst also enabling one to calculate the moles of APTES present, per gram of APTES-functionalised silica. For the ZEOprep silica this was calculated to be at up to 1.51 x 10-3 mol g-1 and for the fumed silica 1.63 x 10-3 mol g-1. As well as testing the selective nature of these materials, solutions of individual ions and radionuclides were used to measure the effectiveness of the materials for a specific ion. Rd values for these metals ions including solutions of Co(II), Ni(II), Cu(II), Cd2+, Eu(III) and [UO2]2+ have reached values ranging from 7.49 x 104 mL g-1 to as high as 2.17 x 109 mL g-1. These values are regarded as outstanding by other groups that have reported similar results and these are discussed in the report. This range includes values that were observed when competing Na+ and Ca2+ ions were present at 0.5 % and 1 % (w/w). pH testing was also investigated with the materials using a solution of europium ions to determine the most effective range and this was found to fall between pH 4 and 5. X-ray Photoelectron Spectroscopy (XPS) has been utilised to help gain an understanding of the binding between Cu(II) ions and APTES, suggesting that copper ions bind with oxygen atoms closer to the silica surface as well as the nitrogen atoms at the end of the ligand. Meanwhile STEM (Scanning Transmission Electron Microscope) has been used to show how effectively the surface area of the material is used by imaging the europium ions over a sample of APTES-functionalised fumed silica. Ligands and polymers have been focussed on to build a catalogue of functional materials and this has been achieved in collaboration with PhosphonicS Ltd. The most significant finding from these selective investigations was that uranyl ions were found to be the most readily removed. Cu(II) and Eu(III) ions were also removed relatively effectively whilst Co(II), Ni(II), Zn2+ and Cd2+ proved the most challenging but certainly not impossible. [UO2]2+ concentrations were reduced from 17.1 ppm to 1.6 ppm after 4 weeks with use of the ligand SEA (2-aminoethyl sulfide ethyl silica), even with six other metal ions present at similar initial concentrations and a starting pH of 4.67 by adding just 50 mg of the material to a 45 mL solution.