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The Advantage

By Leila Karimi lectrodialysis was developed in that are in contact with the mid-1950s to demineralize rinse . By applying a deter- Ebrackish water at commercial mined amount of voltage, the two end scale. In general, electrodialysis and generate the potential field are considered required for anions to be transported more appropriate for applications in toward the and the cations to Reducing costs via processes with low total dissolved be transported toward the cathode. preferential removal solids (TDS). Both processes have During electrodialysis, the cation a good degree of resistance against exchange membranes permeate cat- in electrodialysis scaling and fouling, which makes and block anions, while the anion the technology attractive in brackish exchange membranes do the opposite. water . Two different solutions therefore are Electrodialysis has been applied in generated in this process: one dilute fields such as brackish and and the other concentrate. desalination; production by con- In electrodialysis reversal, the elec- centrating seawater or using dis- trodialysis process is enhanced by charged from a (RO) periodically changing the direction of seawater desalination plant; recover- ion transport. This periodic change ing RO concentrate stream; and citric in polarity is done in order to pre- acid separation. vent scaling and fouling problems and supplies in the increase process efficiency. southwestern U.S. are highly depen- dent on groundwater sources. Due Ion Removal & Selectivity to an emerging water crisis, brackish During the electrodialysis process, groundwater continues to be consid- various ions are removed at differ- ered a resource for drinking water sup- ent rates, depending on the operat- plies. Depending on the location of the ing conditions inside the stack, their available brackish resource, however, specific charges and the selectivity there are different water chemistries to of the membranes in the stack. Rates contend with and concentrate issues also depend on the mobility of the to address. Electrodialysis may be an ions in the , which is affected appropriate desalination technology to by their hydrated radii in solution. utilize in these applications. Ions with a smaller hydrated radius have higher mobility, while ions with How It Works greater charge (divalent or greater) are Electrodialysis is a membrane affected by the electrical field more separation process in which ions are than monovalent ions. transported through semipermeable Another factor is the permeability membranes under the influence of of counter ions in a membrane that is an electric field from one solution to influenced by the selectiv- another in order to reduce the ionic ity, as well as the mobility and selectiv- content of water. The electrodialysis ity in the membrane phase. The mobil- stack is composed of alternately con- ity of different ions in the membrane structed cation and anion exchange is determined by the steric effects, the membranes to form dilute and size of ions and the cross-linking den- concentrate chambers. sity of the membrane.Furthermore, the Additionally, the two ends of the ion exchange selectivity is determined Scaling problem on anion exchange membrane stack are blocked with two electrodes mostly by electrostatic interactions.

10 I february 2013 The impact of ion mobility and permeability differs under various oper- ating conditions. At low applied voltage, the mem- brane plays an important role in selectivity of a type of ion, but that diminishes at higher voltage where the membrane does not play a significant role in the preferential trans- port of selective ions. The range and effect of oper- Pilot-scale electrodialysis stack ating conditions (as well as membrane type) on selectivity of specifics of counter ions, typically can interact with the counter ions. ions is determined experimentally based on the following: Depending on the water chemistry, and is impacted by water chemistry. • Electroselectivity: Defined by the the selectivity could be adversely The membrane type and concentra- Donnan Potential, this is the effect impacted by ions that interfere with tion of a certain ion has an impact of counter ion valences on ion the removal of the ions of interest. on the selectivity coefficient for the exchange equilibrium. The strength Because the cost of the electrodi- transport of divalent ions such as of the Donnan Potential is propor- alysis process is proportional to the ions. tional to the ionic charge. Highly amount of ions removed, selective Selective removal of charged ions have a stronger poten- removal of targeted ions can signifi- or calcium ions is feasible here, tial for exchange. cantly help reduce process costs. given the appropriate concentra- • Ionic hydration and swelling pres- tion ratio of the ion of interest using sure: Larger hydrated counter Further Research Required membranes that are modified by ions cause stronger swelling and In advanced water treatment the compound epichlorohydrin. greater swelling pressure. The ion approaches, electrodialysis provides Monovalent ions and multivalent exchange membrane thus pre- clear advantages over other technologies ions are removed selectively accord- fers counter ions with smaller when used in desalina- ing to operating conditions and ion hydrated ions due to the elasticity tion with low TDS. Still, due to the inter- exchange membrane types. of its membrane. actions associated with various water A combination of the membrane • Sieve action: In contrast to the chemistries, operating conditions and types, operating conditions and swelling pressure factor, the small membrane types, the selective removal water chemistry affects the prefer- counter ions cannot be replaced of ions continues to require research and ential removal of ionic species in the completely by large ions due to must be application specific. MT electrodialysis process. Depending sieve action. on the specifics of the operating con- • Ion interaction: Specific interactions Leila Karimi is a Ph.D. candidate in ditions such as pH, feed concentra- among the ions in the exchange chemical engineering and a research tion, applied voltage, temperature membrane, as well as association assistant for the Institute for and flow rate, selectivity in the vari- and complex formation by them in and the Environment at New Mexico ous membrane types is impacted. As the solution, also affect selectivity. State University. Karimi can be reached a result, under selective experimental at [email protected] or 575.646.3075. conditions, the removal of monova- Temperature and pressure also lent ions is greater than divalent and are important factors in selectivity, For more information, write in 1203 trivalent ions. specifically when organic and other on this issue’s reader service form on Selectivity also is driven by the special components are present and page 13.

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