Electrodeionization: Technology and Applications

The R&D Notebook 10 A publication of the Lab Water Division of Merck Millipore 1. Introduction

Telis Dimitrakopoulos, Emmanuel Feuillas, Daniel Purifying water has long been a topic of concern, Darbouret and Stéphane Mabic whether for the production of drinking water, effluent Research & Development, Lab Water Division, treatment and water reclaim, or for the preparation Merck Millipore of water suitable for scientific experiments. Purifying consists of removing all, or selected, contaminants initially present in the water. Contaminants include ions, organics, particulates, microorganisms and gases. Abstract : As purification purposes can differ according to water In combination with reverse osmosis (RO), usage, volumes, and initial and desired water quality, electrodeionization (EDI) is gaining importance in the various purification methods have been developed. water purification market. EDI removes ions (organic Some technologies, such as distillation and reverse and inorganic) using semipermeable membranes and osmosis, have a broad spectrum of efficiency on the ion exchange resins that are continuously regenerated various classes of contaminants, while others target by an electric field. Used downstream from RO, a single contaminant type. Achieving ionic purity this self-regenerating module provides water with is an important objective for a number of industry resistivity > 10 MΩ·cm and a low organic level. This sectors and research areas, including organizations paper presents technical insights and also discusses active in microelectronics, analytical chemistry, several applications in which RO-EDI water is used. In clinical analyses, water reclaim, molecular biology particular, the benefits of using the combination of RO and cell culture, to name just a few. Therefore, several and EDI water as a pretreatment system for polishing technologies have been developed to specifically units are highlighted. Water quality in this study was address ion removal: electrodialysis,1,2 ion exchange characterized using a variety of analytical data. resins,3 electrodeionization,4,5 and filtration over grafted membrane.6 Most water treatments now combine contaminant-specific and broad range technologies to achieve optimum efficiency and robustness. A successful and increasingly utilized combination is RO-EDI, for both small-scale and process water treatment.7 This method owes its increasing importance to the development of robust RO membranes, and to a number of technological improvements that now have modified the original electrodeionization process. This paper describes the manner in which some of the technical challenges have been overcome. It also highlights the evolution of EDI technology (called Elix® technology) at Merck Millipore, a pioneer in EDI development for laboratory- scale water treatment. Expected RO- EDI water quality is described, with regard to ionic, organic, silica and boron levels. Various applications of RO-EDI water are discussed, and the performances of RO-EDI are compared to other currently used technologies. 2 - EDI technology

2.1 - Basic principle of electrodialysis

In the electrodialysis process, compartments are sandwiched between a series of anion- and cations- selective membranes (Figure 1). Anions can pass through the anionic membranes, and are rejected by the cationic membranes. Cations, on the contrary, can pass through the cationic membranes and are rejected by the anionic membranes. Thus, desalting (product) compartments and concentrate (reject) compartments are formed. At each end of this “sandwich” is a pair of electrodes, to which an electrical field is applied to attract ions. During the process, electrolysis of water produces hydroxide (OH-) and hydronium (H+) ions.

Figure 2 : Schematic of the first EDI module

Overall, this first EDI design produced high quality purified water, typically 5 to 15 MΩ·cm at 25 °C. Although the module was both successful and reliable, its design presented several disadvantages. For example, if the initial feed water contained a high concentration of alkaline earth ions (> 300

mg/L as CaCO3), additional pretreatment, such as water softening, was required. Otherwise, significant

CaCO3 precipitation would be deposited onto the anion-membrane side of the EDI concentrating compartment. Also, the cathode-electrode surface

would become coated with CaCO3, due to locally high pH (> pH 10.5) at the surface of the cathode resulting from hydroxide ion formation. The presence of hydroxide ions at the cathode electrode converted 2- any dissolved CO2 to CO3 ions. Figure 1 : Principle of electrodialysis In addition, when Ca2+ ions were present in the feed water, this inevitably would coat the cathode 2.2 - EDI module electrode with CaCO3. Consequently, the electron flow was inhibited, which eventually halted regeneration The first known apparatus and method for treating of the ion exchange material as well as purified water liquids by EDI was described by Walters et al. in production. 1955,8 and in 1957, a U.S. patent was granted for an electrodialysis device filled with ion exchange Furthermore, the design of this EDI module was quite resins between the anion- and cation-exchange 9 sensitive to CO2 levels in the feed water, and the membranes. Figure 2 shows the internal arrangement module’s electrical resistance was rather high, which of the first EDI module built for the laboratory market. meant that a large power supply was required. Also, if the water softener was not properly maintained, scale In this EDI module, the mixed beds of anion and cation formation in the form of CaCO3 could develop inside ion exchange resins increase electrical conductivity the concentrating compartments and cathode between the membranes and act as a path for ion electrode, thus halting system operation. transfer. The electrolysis of water and generation of hydroxide and hydronium ions allows the continuous Figure 3 presents the design of the second-generation regeneration of the ion exchange resins to occur. EDI (Elix®) module. Two main differences distinguish Consequently, what is referred to as EDI technology this design from that of the previous module shown is a combination of electrodialysis and ion exchange in Figure 2: the introduction of mixed bed resins in technologies, whereby purified water is produced the concentrating (waste) compartments, and the continuously. EDI usually operates in combination inclusion of carbon beads in the cathode compartment. with RO pretreatment. The RO pretreatment uses a semi-permeable membrane to remove salts and other impurities, and the resulting permeate is used as feed 2 water for the EDI module. 2.3 - Technology improvement via fragmented electrodes These improvements made it possible to develop an EDI module requiring a smaller power supply, due mainly The third and latest generation of EDI design, shown in to the considerable reduction of the EDI module’s Figure 4, has addressed some of the aforementioned electrical resistance and to significant improvement issues. The improvements described previously for in the transfer rate of weakly charged ions from the second generation of EDI modules (see Figure 3) the desalting compartment to the concentrating were retained in the current EDI design. Two main compartment. improvements differentiate the third generation of EDI design: 1) Each of the fluidic compartments has a defined flow rate, and when placed in parallel with other fluidic spacers, these compartments form the EDI module, and 2) The position of the electrodes with respect to the flow path was repositioned 90°, whereby the electrodes were aligned with the sub- compartments of a given fluidic spacer, as shown in Figure 4. A factorial experimental design was implemented for the qualification and validation of this EDI design.11 The ionic mass balance (see section 3.1.2 below) was used as a key parameter.

Figure 3 : Improved EDI (Elix®) module

The addition of carbon beads to the cathode compartment of an EDI module made it possible to use non-softened RO permeate water to feed the EDI module, and thus eliminate the need for water softeners.

The presence of the carbon beads in the cathode compartment served to increase the surface area of Figure 4 : Schematic of the fragmented electrode EDI module the cathode electrode more than twentyfold, leading design to the rapid diffusion of OH- ions generated at the The main advantage of this design is scalability, surface of the cathode electrode. which means that although feed pressure and power requirements for the EDI 20 and EDI 100 modules are Therefore, pH was reduced from 10.5 to about 8.5, and the same, the modules produce different amounts water softeners were no longer needed, as they were of purified water. In addition, this scalability factor previously, when Ca2+ concentrations were below 300 makes it much easier to manufacture similar modules mg/l as CaCO3, before the RO pretreatment. A U.S. for OEM customers without the need for these new patent has been issued on the use of carbon beads in OEM modules to undergo intensive qualification. the cathode compartment of EDI modules.10 Although this development of EDI technology represented a The concept of the three segmented electrodes considerable improvement in comparison with the (see Figure 4), enables the optimum DC amperage previous generation shown in Figure 2, three main to deionize and regenerate the ion exchange resins weaknesses still remained: 1) CO2 was limited to a within each sub-compartment of the EDI module, maximum of 40 mg/L in the feed water, 2) The fluidic and a patent has been filed on this design.12 As the design was not scalable and each of the modules was purified water flows through each sub-compartment, unique with respect to flow rate, and 3) The applied it requires less DC amperage. Another advantage direct current (DC) amperage was globally optimized of the segmented electrode design is that the EDI and self-regulated by the EDI module itself. module can operate with higher CO2 concentrations in the feed water (up to 65 mg/L of CO2), with no effect on the quality of the product water.11

3 3 - Performances of EDI modules: Production of pure water with consistent purity

Several parameters are considered when evaluating water purity. Monitoring is recommended using the industry’s accepted norms and standards for the water’s ionic purity, overall concentration in organics, and the presence of specific elements such as silica and boron. Water quality consistency is a major criterion for efficiency and long-term performance. Data presented in this section focus on the efficiency of EDI technology for the aforementioned contaminants. Expected performances and water quality of the RO EDI combination also are reported, since EDI is used together with RO.

3.1 - Ion removal

3.1.1 - Resistivity Resistivity, which remains one of the simplest and most relevant parameters for monitoring overall ionic purity, is measured continuously during the development of purification technologies. Throughout the development of the EDI modules and verification of their performances over time, resistivity of several EDI modules was recorded. Two examples representative of the behavior of EDI modules will serve as illustrations here: an EDI module producing 35 liters of water per hour, and an EDI module producing 10 liters per hour (Figure 5).

While variations within a range of 4 to 5 MΩ·cm are observed, it is noticeable that in both cases, the resistivity remains constantly above 10 MΩ·cm during the test periods (six to nine months). Variations may stem from the quality of the RO water feeding the EDI

module and from the level of CO2 in the water (major parameter).

3.1.2 - Ionic mass balance and theoretical electrical current required for purifying water. Figure 5 : Resistivity monitoring of RO-EDI water over time RO feed The ionic mass balance is a good way to monitor the water range: conductivity = 10 μS/cm to 20 μS/cm, [CO2] = 30 to operation of an EDI module. A given EDI module must 2+ 2+ 60 mg/l, [Ca ] = 1.6 to 3 mg/l as CaCO3, [Mg ] = 1.6 to 3 mg/l as be kinetically balanced: the total concentration of CaCO , [SiO ] = 0.08 to 0.20 mg/l ions exiting a module should equal at least the total 3 2 concentration of entering ions, meaning that ions are not retained by the module.11 The only sources of ions entering a module are the RO feed water, as well as the total number of ions already bound to the ion exchange resins at any given time; the sources of ions exiting the EDI module are the product (purified) water and the concentrate (waste) water. For an EDI module, the DC amperage required to purify water is largely dependent on the concentration of dissolved

CO2 in the RO feed water and the ionic load that must be removed from the desalting compartments.11,13 This

ionic load is the sum of ionic species and CO2 present in the water. The theoretical amperage is determined by ionic-mass-balance considerations. One faraday of electrical charge is necessary to remove one equivalent of ions from the desalting compartments. The minimum amperage required to accomplish this is directly proportional to the ionic load present.

4 3.1.3 - Ionic levels measured using ion chromatography ICP-OES (ppb) ICP-MS (ppt) Ion chromatography (IC) of RO-EDI water (Figure 6) - 2- LQ Tap Elix® LQ Elix® demonstrates that levels of all major anions (Cl , SO4 , Element Isotope - - 3- + + 2+ 2+ ppb water water ppt water NO3 , NO2 , PO4 ) and cations (Na , K , Ca , Mg ) initially present in tap water are below 1 μg/L [1 part per billion Li 6707 1 4.4 < 1 7 0.3 < 0.3 (ppb)].14 Na 5889 4 12 000 < 4 23 3 194.5 Mg 2852 1 12 100 < 1 24 2 2.07 Al 1670 1 1.2 < 1 27 1.5 10.8 K 7664 10 881.5 < 10 39 1 67.3 Ca 1840 10 129 900 < 10 40 5 29.8 Ti 3349 1 < 1 < 1 48 3 54.9 Mn 2576 0.5 < 0.5 < 0.5 55 0.5 1.35 Fe 2599 1.5 5.3 < 1.5 56 1 7.04 Co 2286 1.5 < 1.5 < 1.5 59 0.4 0.47 Ni 2316 1 < 1 < 1 60 1 6.23 Cu 3247 2 < 2 < 2 63 0.6 19.76 Zn 2138 0.8 < 0.8 < 0.8 64 3 56.68 Sr 4077 0.5 227.8 < 0.5 88 1 103.4 Mo 2020 1.5 5.8 < 1.5 95 1 < 1 Ag 3280 1.5 < 1.5 < 1.5 107 0.2 13.6 Cd 2144 0.5 < 0.5 < 0.5 111 0.5 < 0.5 Ba 2335 0.5 48.2 < 0.5 138 0.075 206.3 Pb 2203 8 < 8 < 8 208 0.25 5.6

Table 1 : Elemental concentrations measured using ICP-OES and ICP-MS in water purified with an Elix® system (RO-EDI water). LQ = Limit of Quantification.

3.1.5 - Resistivity and pH The resistivity measured at the outlet of the EDI module attains 15 to 17 MΩ·cm in many cases, but usually will not reach 18.2 MΩ·cm, Figure 6 : Ion chromatograms of RO-EDI water the maximum theoretical resistivity of ultrapure water at 25 °C. The difference of approximately 1-3 MΩ·cm can be explained by several factors. Firstly and most significantly, traces of ions still remain, as ICP-MS measurements show (section 3.1.4). Although each ionic 3.1.4 - Metal analysis using ICP-OES species is present at a very low concentration, the overall level of ionic Metals were analyzed using ICP-OES and ICP- 15 contamination can contribute significantly to the 1-3 MΩ·cm. A similar MS instruments (Table 1). It can be noted that situation could be reproduced by adding the equivalent of 1.5 to 5 ppb concentrations in RO-EDI water are below the of NaCl to the water. quantification limits of the ICP-OES instruments for ll Another possibility which may account for the 1-3 MΩ·cm is that this elements. More accurate values were obtained using shift in resistivity could arise from hydronium or hydroxide ions. A an ICP-MS instrument. Levels range between below 1 resistivity reading of ≥ 15 MΩ·cm implies that the concentration of part per trillion (ppt) (ng/L) and 200 ppt, consistent -7 + + 2+ hydronium remains below 1.2 x 10 M, or that the concentration of with data obtained using IC for Na , K , Ca , and -7 2+ hydroxide stays below 2 x 10 M (for the purposes of calculation, it is Mg . assumed that each ion would be the only ion present).16 This means that the pH of RO-EDI water with a resistivity level ≥ 15 MΩ·cm would be between 6.9 and 7.3. Methods adapted to the measurement of the pH of ultrapure water should be used to measure the pH of RO-EDI water.

5 3.2 - TOC data

Organic molecules, referred to as total organic or Boron species (borate and boronic acid) are moderately oxidizable carbon (TOC), are a major concern in water rejected by the RO membrane (40-50 % on average) purification. Not only do the levels have an impact on and can be further removed by EDI. With 13.2 ppb of many applications, but also high levels of organics will boron measured in RO water (= feed for EDI), 2.3 ppb impair purification processes. In particular, organics were detected in EDI water. can bind by affinity to ion exchange resins and create a coating on the resin beads. The ion exchange sites of the resin are then no longer available, and the Tap feed water Elix® system water resin capacity decreases. The TOC level, measured in Reactive silica [ppb] Reactive silica [ppb] parts per billion (ppb), was monitored for several EDI 9 350 < 10 modules (Table 2). 7 100 8 While the EDI technology cannot be expected to 13 000 9 have any effect on neutral molecules, charged organics (such as carboxylic acids, sulfonates and Table 3 : Reactive silica concentration (ppb) in tap and Elix® (RO- ammoniums) behave similarly to inorganic ions EDI) water. and are, therefore, rejected in the concentrate compartments. Consequently, the TOC level decreases slightly from RO water to EDI water, and in normal operating conditions, the TOC in RO-EDI water would be consistently below 30 ppb measured in-line, or 50 ppb measured off-line.

Inlet TOC [ppb] Outlet TOC [ppb] 61 30 51 28 Inlet Permeate Concentrate 44 33 47 11 Figure 7 : Profiles of silica concentration (reactive) over time 64 27 38 26

Table 2 : TOC values (ppb; off-line measurement) for several EDI modules. Inlet is RO water.

3.3 - Silica and boron

Silica and boron are two elements that are difficult to remove with ion exchange resins because they are only weakly ionized at neutral pH. Indeed, silicic acid and boronic acid, which are the major species measured in water, have pKa values of 9.5 and 9.2, respectively. Silica and boron species are the first to break through from ion exchange beds.

The combination of RO and EDI technologies has shown to be a very efficient method for silica removal.17 All forms of silica are well-rejected by the semi-permeable RO membrane, and the silicic acid is further removed during the EDI process.

Levels expected after RO-EDI treatment are reported in Table 3. Silica measurements over a 40-day period are shown in Figure 7. While concentrations in the tap water vary, the silica level in RO-EDI water remains fairly constant.

6 4 - EDI module in a water purification system RO-EDI water is used principally: (Elix® system) - As a feed to ultrapure water purification units (polishing systems, such as Milli-Q® systems; The EDI module is never used as a stand-alone device in here, the Elix® system would be considered to be a a laboratory environment, but instead constitutes an pretreatment system) integral part of a global water purification system. As - As a water source for various applications requiring described in this paper, a representative global water ISO® 3696 Grade 218 water purification system would employ RO technology to remove the bulk of contaminants, and then use the 5.1 - RO-EDI used as a pretreatment technology RO permeate water as feed water for the EDI module. In addition to these two major technologies, a Using good water quality to feed the polishing units pretreatment cartridge would be installed upfront to has proven to be very valuable in ensuring good protect the purification media from oxidants present and consistent high purity water quality. There is a in tap water (chlorine, chloramines, or fluorine, for definite improvement in polishing unit performance instance). This cartridge also would contain a filter to when RO-EDI water is used instead of water that has remove large particles that would otherwise clog the been purified with ion exchange resins only (service RO membrane. Finally, a germicidal UV lamp may be deionization or SDI).19 In particular, it was shown that included to decrease the level of bacteria in the water it is very important to decrease the level of organics delivered by the system. when the final water is used for sensitive applications such as liquid chromatography - mass spectrometry It is important to be able to monitor the water quality, (LC-MS).20 as well as various parameters indicative of the system performance and operation. Resistivity (the inverse In the example selected (Figures 9a, 9b), water purified of conductivity) of water is continuously monitored using ion exchange resins circulates in a loop and at the outlet of the EDI module, and provides an feeds a polishing unit (Milli-Q® system) combining indication of the water quality. a UV photooxidation process and purification packs composed of mixed bed ion exchange resins and Monitoring conductivity is a way of obtaining activated carbon. information on the performances of the RO (percentage of ionic rejection) and EDI technologies. The major peaks seen on the HPLC chromatogram This parameter is used not only to diagnose potential and mass spectrum (ESI+ mode) were attributed to issues, but also to determine when the RO membrane water contamination. After rinsing the purification must be changed. packs with RO-EDI water, LC-PDA (photodiode array) chromatograms and MS spectra of the water delivered The flow schematic of an Elix® system combining the by the polishing unit were recorded. purification and monitoring technologies described earlier is shown in Figure 8. Analysis of the results clearly highlights that the major contaminant peak on the LC chromatogram 5 - Examples of RO-EDI (Elix®) water applications disappears when the pretreatment system is switched from SDI to the RO-EDI combination. As explained previously, the combination of RO and EDI technologies constitutes a very efficient method The comparison of the total mass plots (ESI+ mode) for providing good water quality with regard to ionic shows a tenfold lower contamination with RO-EDI and organic levels. water. It is interesting to observe that the effect on the final water quality stems from the pretreatment step.

Figure 8 : Flow schematic of an Elix® system

7 In this case, the same results that were highlighted using an analytical method also were observed during system qualification. (Figure 10).

Two experiments were run in parallel. Polishing units equipped with UV photo-oxidation lamps in combination with ion exchange resin and activated carbon (Milli-Q® water purification systems) were used in both cases.

Results of the experiment showed that using RO-EDI as a pretreatment method provides consistency in the performances of the polishing unit over time, and the level of TOC remains at a 5 ppb level.

On the contrary, SDI, even when combined with distillation, leads to important variations in the final high purity water.

In addition, when SDI is used, it is virtually impossible to anticipate whether the organic purity of the water will be very good or very bad at any given moment.

Using SDI is not an efficient method for removing neutral organic molecules, and distillation is only moderately efficient for organic removal.

Organic molecules with low boiling points, boiling points close to the boiling point of water, or with very hydrophilic structures can be found in the distilled water. When the level is too high, these organics cannot be eliminated by the polishing unit.

Water purification systems dedicated to trace ion analysis also provide their best performances when good pretreatment technologies are utilized.21 In addition, as the water feeding the polishing unit in these systems is already quite pure, the lifetime of the cartridges (ion exchange resins and activated carbon) in the polishing unit is extended.

There are several ways to set up an Elix® system as a pretreatment system. Laboratory-scale Elix® systems, delivering water at a flow rate of a few liters per hour, are connected to 30 - 100 liter reservoirs. When greater volumes of water are required, larger systems are used to deliver up to 100 liters of water per hour.

In these cases, water distribution loops can be installed to ensure water delivery for an entire building, a floor, or simply for two or three laboratories. The polishing units are then connected to this loop.

Figures 9a & 9b : Importance of pretreatment for LC-MS analysis: LC-MS of water from a Milli-Q® system fed with SDI or RO-EDI water.

8 Figure 10 : Impact of pretreatment on polishing system performances: TOC level in water delivered by a Milli-Q® system fed with SDI-distillation water or RO-EDI (Elix® system) water.

5.2 - Applications requiring water purified using 6 - Comparison with other purification solutions RO-EDI technology (Elix® system) Other purification technologies can be used to remove In general, water that has been purified using RO-EDI ions and, to a certain extent, organics from water. technology is suitable for use with analyses at the Several parameters were selected to compare the arts per million (ppm) or high ppb levels, and also may efficiency of distillation and deionization with the be used to fulfill several other needs in a laboratory: RO-EDI combination. - Buffers and media preparation, in particular for microbiology, titration (unless trace levels are 6.1 - Ion concentrations targeted), pH or general analytical chemistry - Non-sensitive sample preparation requiring Data reported in Tables 4 and 5 show, respectively, the extensive washing concentrations of cations and anions in water purified - Chemical reactions run in water using RO-EDI, double distillation, and deionization - Stability testing chambers and autoclave feeding (SDI). Results show that ion levels are fairly similar - Histology in the three types of water, with a logical order of - Glassware washing. Although washing can be purity: deionization ≈ RO-EDI > double distillation. done manually, Elix® systems can also be fitted to In all cases, the ionic concentration is at the low- to dishwashers to deliver water on demand. sub-ppb level. - With additional basic ion exchange polishing, the water can be used in clinical chemistry and as feed Ion exchange resins are expected to remove the water for weatherometers. ions very efficiently, while the water becomes recontaminated by air while cooling during the distillation process.

9 Double Double Dis- Tap Elix® SDI Ion Distilled Tap [ppb] Elix® [ppb] tilled [ppb] [ppb] [ppb] [ppb] [ppb] Sodium 12 905 < 1 < 1 < 1 859 20 58 Ammonium 168 < 1 7 < 1 977 14 47 Potassium 1 713 < 1 < 1 < 1 Table 6 : Comparison of TOC level (ppb; off-line measurement) in Magnésium 2 525 < 1 < 1 < 1 water purified using different purification technologies; Elix® = Calcium 70 501 < 1 < 1 < 1 RO-EDI

Table 4 : Concentration of major cations in tap water and water A large portion of the charged organic molecules purified using various technologies. Elix® = RO-EDI; SDI: service are retained by the ion exchange resins. Conversely, deionization. neutral organics (small and large molecules) and particulates have no affinity for the ion exchange sites and, therefore, flow directly through the SDI Double Tap Elix® SDI bottle. Ion Distilled [ppb] [ppb] [ppb] [ppb] Important variations in organic levels are expected Fluoride 103 < 1 < 1 < 1 due to the changes in tap water composition (Figure Chloride 30 033 < 1 2 < 1 11). In addition, some organic molecules will coat the resin, thus reducing ion removal efficiency. Nitrite 18 < 1 < 1 < 1 Lastly, leaching of the monomers and additives used Nitrate 26 508 < 1 < 1 < 1 to synthesize the polymeric beads may occur upon regeneration of the resins. Sulfate 31 424 < 1 < 1 < 1

Table 5 : Concentration of major anions in tap water and water purified using various technologies. Elix® = RO-EDI; SDI: service deionization.

6.2 - TOC levels

The TOC level was measured in RO-EDI and double distilled water (Table 6). Data highlight that the concentration of organics is lower in RO-EDI water.

Off-line measurement usually displays values below 50 ppb in water produced by an Elix® system, while in-line monitoring provides values below 30 ppb.

The results observed here can be explained considering the benefits and limitations of each technology or Figure 11 : Variability of SDI water quality combination of technologies.

Distillation is not fully efficient for organic removal. Organic molecules with low boiling points, boiling 6.3 - Other parameters points close to water, or with very hydrophilic structures can be found in the distilled water (TOC When other parameters such as maintenance, level typically ranging from 30 to 100 ppb). power and water consumption, and ease-of-use are considered, the benefits of the Elix® system over The advantage of a membrane-based technique such distillation are even more evident. In comparison with as RO in the pretreatment step is the rejection of a SDI water, Elix® system water not only has a higher larger spectrum of contaminants: ions, organics and purity level in terms of organics, but also has lower particulates. More than 85 % of organics generally bacteria, silica and boron levels. are removed, depending on the size and polarity of the molecules.

Although no category of contaminants is entirely removed, RO significantly reduces the overall load of contaminants. EDI diminishes the levels of charged molecules remaining in the water after RO treatment: organic ions such as carboxylic acids, sulfonates or ammoniums.

10 7 - Experimental section 8 - Conclusions

Instrumentation and Chromatography Conditions. In combination with RO, EDI technology is beginning Ion analysis was performed using a Dionex® DX-500 to occupy a more prominent place in both the ion-chromatography system (Dionex Corporation, process water treatment and laboratory-scale water Sunnyvale, CA, U.S.A.) with a GP50 Gradient Pump, an purification system markets. RO removes the bulk of AS40 Autosampler and a CD20 Conductivity Detector. the contaminants and produces water suitable for EDI. The EDI technology further reduces the levels of For the anion chromatography, an IonPac® AG17 inorganic as well as organic ions. Due to its capacity Guard (4 x 50 mm) and AS17 Analytical column (4 x for removing a wide range of contaminants (ions, 250 mm) were used with an Anion Self- Regenerating particulates and organics), the RO-EDI combination Suppressor® (ASRS®-ULTRA, 4 mm) at a flow rate of is becoming a preferred water treatment method, 1.5 mL/min. capable of providing good water quality both in terms of resistivity and TOC level. In addition, thanks to The eluent, KOH, was generated by an EG40 Eluent RO-EDI’s low running cost, constant operation and Generator; and a concentration gradient was applied. minimal maintenance, this method surpasses the distillation process for laboratory use. For the cation chromatography, an IonPac CG12 Guard (4 x 50 mm) and CS12 Analytical column (4 x EDI is a sustainable technology that combines the 250 mm) was used with a Cation Self-Regenerating strengths of several key purification technologies or Suppressor® (CSRS®-ULTRA) at a flow rate of media. Several ways to improve this technology have 1.0 mL/min; the eluent was 20 mM methanesulfonic been identified and are currently being evaluated. acid (Fluka, Buchs, Switzerland). Moreover, all of the steps required to assemble these EDI modules are performed at our centralized product Polyetheretherketone (PEEK™ polymer) 0.125 mm manufacturing site and specific Quality Control tests ID tubing was used in all of the chromatography are performed after key steps in the production process paths. Instrument control and data collection to increase product reliability. This approach enables were accomplished using a personal computer and Merck Millipore’s EDI (Elix®) technology to play a key PeakNet® Version 5.1 software. role in instrumentation dedicated to the production of ultrapure water for laboratory applications. TOC. The off-line TOC measurements were made using a Sievers® 2244 AP analyzer (Sievers Instruments, Inc., Boulder, CO, U.S.A.), combining acid treatment, persulfate acidification and UV oxidation.

To eliminate the risk of contaminating the sample through air exposure, sealed containers were used.

For comparative studies (distillation, Elix®, SDI): All purification systems were fed with tap water from municipal water supplies in Saint-Quentin-en- Yvelines (France). Sampling from all systems was made following identical procedures.

Water was not stored after collection. Samples were collected while systems had been running for a minimum of one hour, and suitable labware was used for each sampling.

For IC, 100 mL samples were collected in thoroughly rinsed polyethylene bottles; organic-free glass containers were used for TOC measurement.

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