|Highly effective electrodia1ysis for selective elirnination of nitrates from drinking water |
K. Kesorea.*, F. Janowskia, V.A. Shaposhnikb
\'Insritute of Technical Chemisrry and Macro-molecular Chemistry, Martin-Luther-Universily Halle/Wittenberg, Schlossberg 2, 06108 Halle (Saale), Gennany
bDepartment of Analyrical Chemistry, Facult), of Chemisrry, Voronejh State Universily, 1 Universily Place, 394000 \'Voronejh,
Received 17 August 1995; received in revised fonI1 25 September 1996; accepted 26 September 1996
This paper considers a new and highly effective process for selective elirnination of nitrates from drinking water through electrodialysis. It is based on coupled use of a modified anion exchange membrane with a nitrate-selective anion exchange resiDo The latter is placed in the desalination compartment and constitutes a part of a new type of ion-conducting intermembrane spacer. A highly preferential transport of nitrate anions against chlorides and sulphates is observed to take place at low current densities. The results obtained prove that electrodialysis is a practical solution to the problem of selective elirnination of nitrates from drinking water.
Keyworos: Electrodialysis; Ion exchange membrane; Ion-conducting spacer; Nitrate; Drinkjng water
Increasing nitrate concentration in drinking water has become a matter oí public concem and sometimes oí panic algO, whenever it appears on front pages oí daily newspapers. The problems oí nitrates in drinking water are well known and an excellent documentation oí the nitrate\'-nitrite toxicity is given in . Because oi its links to several health hazards, limits have been set up regulating the maximum all?wed leveloí nitrates in drinking water. The European Union allows a max- imum oí 50 mg l-I nitrate in drinking water, but recommends a concentration of lower than 25 mg l-I nitrate for treated water . Indeed, the German Health Authority insists on a nitrate leveI of less than 10 mg l-I for water used in the preparation ofbabies\' food.
The elimination of nitrates from drinking water is a costly and difficult processo Among different methods used for this purpose, the ion exchange option has been applied for longo But because of its unecological aspects  and relatively high cost, it has been taken over by microbiological denitrification only since the last 15 years. This process is capable of eliminating nitrate totally from the concemed polluted water, changing it into nitrogen gas. Thus, it does not just shift the problem to another pollution as in the case of ion exchangeo However, microbiological denitrifica- tion has the big dIawback of requiring chemicals (ethanol or similar carbon source and phosphorus) for the process functioning. The nitrate/nitrite-nitro- gen conversion is strongly temperature-dependent. Deviation from the strict operating regime mar result in nitrite-contamination of the treated water. Another aspect rendering this method unattractive is that it adapts badly to non-continuous feed supplies and different feed loads. Furthermore, lhe biomass pro- duced poses another problem.
As an altemative, this work proposes electrodialysis as a better approach to the problem of nitrate- contaminated drinking water. lt is a very promising method belonging to the reagentless membrane tech- nology, which is considered by specialists as the leading separation technique of the next century . Electrodialysis does not require chemicals and is a simple-to-operate membrane process, which can be easily and fully automated. lt adapts immediately to non-continuous feed supplies and different loads, and temperature changes have a negligible effect on this processo Ali these characteristics together with its high re~o..\':ry !a~~ nf more than 97% make electrodialysis a specially attractive method, both ecologically and economically, for the elimination of nitrates from drinking water. The less than 3% of the total plant capacity rejection as concentrated effluent can either be used as fluid fertilizer or treated very effectively through a small capacity microbiological denitrifica- tion unit. lnteresting disposal possibilities are inves- tigated in .
However, electrodialysis itself, in its traditional version, is a general method for the overaIl deminer- alization of the feed. Nitrate-removal is a side-effect of the primary desalination processo As such, traditional electrodialysis becomes more feasible and practical as a separation technique if the water to be treated has severa! contaminants or high salinity. But deminera- lized water is not suitable for drinking. Therefore, attention must be paid to the fact that the other ions present in ground- and surface water are not removed during the production of drinking water, as they are needed for the normal physiological functioning of the human body. Also, sulphate and chloride ions compete permanently against nitrate anions in the elimination processo reducing the efficiency of the method. There- fore, there is need for selective and effective removal of nitrates from drinking water, without altering the main parameters for potability.
Studies on electrodialysis as a method for the elimination of nitrate from drinking water have been carried out by several workers [6-8]. In order to render electrodialysis into a selective nitrate-removal pro- cess, Eyal and Kedem  have attempted to bring out selectivity for the elimination of nitrates during electrodialysis by the introduction of polyfunctional groups (tertiary and quatemary ammonium groups) in an anion exchange membrane. An enhanced nitrate- eliminating process has been reported. In an analogous work by Indushekar et aI. , a nitrate-specific anion exchange membrane had been prepared from chlor- omethylated polysulphone by partial arnination with a secondary arnine followed by a tertiary arnine for the completion. But the above-mentioned workers have considered only binary nitrate/chloride solutions.
Oldani et aI. have algo focussed on the same pro- blem [li] with an aim to optimize electrodialysis for nitrate-removal. They have carried out a relatively large comparative study on eight different, commer- cially available anion exchange membranes and two experimental ones, developed by Kedem (Weizman Institute of Science, Rehovot, Israel), for the electro- dialytic elimination of nitrates from synthetic drinking water at low current densities. The best performances were found for ACS anion exchange membrane of Tokuyama Soda. However, for low sulfate-removal the membrane showed a more or less selectivity for monovalent anions, implying a relatively high chlorides transport, which is usually not wished during the elimination processo Miquel and Oldani have reported on a successful selective electrodialysis process  for the removal of nitrates from drinking water, commercially known as NlTREM. Other attempts in the same field are described in [13,14], but the membranes reported are selective to mono- valent anions.
In order to adapt traditional electrodialysis from an overall desalting process to a selective denitrating one, a novel process has been tested in this work. At first, a modified anion exchange membrane has been synthe- sised from a normal, highly basic anion exchange membrane, MA41, containing the quatemary tri- methylammonium functional group, described in Table 1. The modified membrane was then used in conjunction with an original intermembrane spacer during electrodialysis of nitrate-containing solutions for preferential nitrate-eliInination from the feed.
For modification, samples of the anion exchange membrane, MA41, of size 28 cm by 8 cm were first brought in the basic forro using 10% sodium hydrox- ide solution and then left in 3% acrylic acid solution for 24 h. After washing them to neutrality with bidis- tilled water, they were treated with 20% sodium persulphate solution (initiator of the polymerization reaction of sorbed acrylic acid) for 30 Inin at a tem- perature of 70°C with constant stirring. The treated membranes were then cooled to 40°C and kept at that temperature for 0.5 h. Next, the membrane stripes were taken out and put in a newly prepared 10% sodium persulphate solution. The whole of the above-mentioned process was repeated once again. The end-result of the above-mentioned treatments was the fixation of functional groups from the acrylic acid. i.e. the carboxylic groups. This implies the presence of negatively charged carboxylic ions algO, besides the positively charged trimethylammonium groups on the surface layers of the anion exchange membrane, ren- dering it into a Inixed anion and cation exchanger. If the modification was carried out with the aim of completely changing the ionogenic groups, then the membrane would have exhibited mostly an opposite function; but a liInited concentration of the fixed ions with opposite charge with respect to the original fixed ions on the membrane surface can become an electro-static barrier for multi-charged negative ions at the first instance. In this way, it was aimed to work out a method primarily, thereby using the new property oi the modified membrane, for the separation oi ions oi the same charges but different magnitudes . In our case, such a problem arises during the separation oi nitrates and sulphates.
The energy needed for the electromigration oi ionic species during electrodialysis is supplied by an exter- nal source oi DC. This energy is small in comparison with that needed for the pumping system. It is well known that the use oi ion-conducting intermembrane spacers intensifies significantly the process oi mass transier, decreasing the electrical consumption. A series oi experiments carried out in this area has proved that the energy needed for the electrornigration oi ions can be decreased by almost three times through the use oi a new type oi ion-conducting intermem- brane spacer, especially when treating ieeds oi low salinities . As the ionic species get depleted in the desalinating compartment during electrodialysis, these spacers provide a constant electro-conductivity, thus avoiding earlier polarization (hence, water dis- sociation). They also promote turbulences in the flow, effecting a good mixing oi the ieed inside the com- partment.
Such a new type oi ion-conducting spacer was produced at the Chernical Science Research Institute oi the Voronezh State University in Russia to deal with the problem of nitrated drinking water. lt was assembled manually, though an industrial production does exist there. The spacer was made up of fine stripes of the cation exchange membrane MK40 (Azotnoi lndustries, Russia), described in Table I, twisted into a net-type material. The net was cut to fit into the desalting compartment between the cation and anion exchange membranes, as shown in Fig. I. Slots of the nets were filled very carefully with the nitrate-selective anion exchange resin Wofatit SN35L (Bitterfeld AG, Germany). This is a macroporous resin with triethylammonium as the functional group. lt has a total exchange capacity of not less than 1.0 meq cm-3 (soaked), humidity of 40-50% and a granular size of 0.2-0.3 mm. The membrane MK40 was used as the cation exchange one throughout the experiments in this work. For comparison of the elimination process with the ion-conducting spacer, electrodialysis of identical solutions were carried out under the same conditions with an inert spacer made from perforated PVC.
The experiments were carried out at an ambient room temperature with a five-compartment labora- tory-scale electrodialyzer, schematically represented in Fig. 2. Synthetic drinking water, containing lhe potassium salts of nitrate, chloride, sulphate and hydrogen-carbonate anions, with a concentration of 1.5 meq 1-1 per component, was supplied continu- ously with lhe help of a multi-channel peristaltic pump. AlI lhe five compartments of lhe electrodialyzer were fed with it equally, from bottom to topo A galvanostatic regime was kept throughout, maintain- ing a constant current density from a stabilised DC supplier. An ammeter, connected in series to lhe circuito allowed to read lhe value of the CUrTem density across the membranes. Though a smaller intermem- brane distance would have been better for the mass- transfer process , it was, nevertheless, kept at 1 mrn so as to make roam for the spacer. Electro- dialysis of solutions was conducted within an interval of current densities, with the inert and ion-conducting intermembrane spacers placed in the desalting com- partrnents 2 and 4, respectively.
After attaining a constant potential difference at the electrode ends, measured with the help of a high- resistance voltmeter, the diluates and concentrates were collected separately and analysed for their ionic contents. Desalination took place in compartrnents 2 and 4, whereas in compartment 3 there was a con- centration of anions Eram compartment 4 and of cations Eram 2. Concentrations of nitrate and chIoride anions were deterrnined through spectrophotometry, sulphate through turbidity, hydrogen-carbonate with the help of a CO2-sensitive electrode, and potassium through atomic absorbtion spectrophotometry. The relative errors of the analytical methods were 3% for the nitrates determination, 2% for the chIorides, 5% for the sulphates, and 5% for the hydrogen-car- bonates.
Some other parameters of the experiments were:
Membrane surface-area: 20 cm2
Linear feed-tlow rate: 0.7 cm s-
Active feed-tlow path: 20 cm
Intermembrane distance: 0.1 cm
3. Results and discussion
The results of the electrodialysis trials are expressed as plots of fluxes, j, of the anions from the desalting compartrnent through the anion exchange membrane against current densities, i, flowing through mero.
Fig. 3 shows the results of the electrodialysis with the inert net as intermembrane spacer. It is found that fewer sulphate anions are transported than chlorides and nitrates but fluxes for the last two components are almost the garoe. The anion exchange membrane here possesses a general selectivity to monovalent anions, brought about by the modification of its surfaces. The negatively charged carboxylic groups fixed on the membrane surface exert a greater electrostatic repul- sion to the double-charged sulphate anions. The high- est flux is that for the hydrogen-carbonate cations. The removal of this anion is beneficial in the sense that it causes certain softening of the feed. In arder to avoid scaling of membranes by the precipitation of insoluble hydrogen-carbonates in the concentration compart- ment, electrodialysis reversal is commonly used and an antiscaling agent may be added to the effluent stream when needed.
Replacing the inert net in the desalting compart- ment with an ion-conducting one. consisting of a net made out of the cation exchange membrane MK40, with granules of the nitrate-selective anion exchange resins (Wofatit SN35L) in its slots, changes the posi- tions of the fluxes as shown in Fig. 4. There is an abrupt increase in the transport of nitrate anions against those of chloride and sulphate. As such, this improvement in the electrodialysis process can be used for selective removal of nitrates from drinking water.
Initially, the solution rnixture was passed through the electrodialysis cell in the absence of an electric field until the exit solution was of the same content as the feed. In this way, the simple sorption of anions in the desalting compartment through ion exchange was excluded, meaning that the decrease in the anionic content ofthe feed during electrodialysis occurs solely due to the passage of an electric current through the system. However, bringing the ion-exchanger in the equilibrium state before conaucting electrodialysis does not influence the results much. because a non- equilibrium process takes place during it, whereby the ion-exchanger is loaded and regenerated momentarily, almost simultaneously. The initial state of the ion-exchanger is disturbed already at low current densities.
In a much simplified representation, the actual transport of a given anion from the desalination com- partment mar be considered. roughly looking, as the sum of the rnigrations of the anion from the bulk of the feed into the anion exchange resin, then from it to the anion exchange membrane surface in the direction of the anode, and finally through the membrane to the concentrating compartment. Heteropolar contacts of the ion-conducting spacers pIar an important role in the transfer of íons in the desalinating compartment. Such contacts of the anion exchange granules Wofatit SN35L with the cation exchange membrane generate hydroxyl anions due to irreversible water dissociation occurring at these points. An analogous process takes place at the contacts of the cation exchange membrane net with the anion exchange membrane, liberating hydrogen íon. The hydroxyl anions regenerare the anion exchange granules, thus making way for the rnigration of nitrate and other anions. Sirnilarly, the hydrogen íons regenerare the cation exchange mem- brane neto These two processes allow ion exchange reaction to take place with the íons of the feed. The hydroxyl and hydrogen íons liberated after the ion exchange combine with each other to give water molecules. As such, the heteropolar contacts of the spacer act like regenerator for the íon exchange mate- rial inside the desalinating compartment at small current densities.
Electrical conductivities of íons in ion exchange materiaIs is smaller than those in solutions because of the higher viscosity of amorphous solid phases. Never- theless, the instantaneous regeneration process of the anion exchange resin in the desalinating compartment brings about an effective transport medium for the directed rnigration of anions to the anion exchange membrane surface. The sorbed anions are forced to move out of the resin. Because of the different affi- nities possessed by the íon exchange resin to the different anions, a differentiation in their transport is experienced. Being nitrate-selective, the anion exchange resin of the new intermembrane spacer assists the selective transport medium in the rnigration of nitrates towards the surface of the anion exchange membrane. Consequendy, there is a highly preferential nitrate-Ieakage over chloride ions. The plots show that, for small values of current densities -for exam- pie, at 0.5 mA cm -2 -there is a relatively high amount of nitrate ion transported, corresponding to a total of 43.5% desalination of the feed.
However, the absolute sulphate flux algO increases with the use of the active spacer. This is explained first through the selectivity series of the selective anion- exchanger, which has the following arder:
nitrate > sulphate > chloride.
Accordingly, sulphate anions from the feed are sorbed in preference to the chloride ones. The fluxes of anions obtained show that the above series of selectivity holds in the small current density interval until it reaches 1 mA cm-2. Hence, an enhanced and preferential sulphate transportation against chlorides takes place from the feed bulk to the anion exchange membrane surface, by the anion exchange resin, in the direction of the anode. But the modified membrane itself being more or less perrnselective to monovalent anions, alIows the arriving sulphates to be transported only at a second position after the nitrates.
Secondly, the active spacer intensifies the mass-transfer process as a whole , implying a deeper desalination of the feed. The strongly suppressed chloride rnigration is an interesting result and needs further investigation to support the explanation that the nitrate-selective resin dorninantly contrais the rnigration of the anions, according to its selectivity series at low current densities and in the considered conditions. However, due to lack of adequate literature on coupled trans- port during electrodialysis, it is difficult to follow lhe complete transport behaviour exhibited by the system.
At the current density of 0.5 mA cm-2, the nitrate- and sulphate-ion fluxes are 3.4x 10-9 and 1.94x 10-9 eq cm-2 S-I, respectively, but there is almost no transportation of chloride anions. A nitrate transport, about 1.75 times higher than that for lhe sulphate ones takes place here, corresponding to 65% elimination of the total nitrate-content of the feed. The nitrate concentration is reduced in just one cycle of electrodialysis from 93 to 32 mg I-I, a value satisfy- ing the lirnit, 50 mg I-I, set-up by the health autho- rities. As seen in Fig. 4, further elirnination of nitrate ions is still possible; but at a much higher current density, a greater desalination of the feed would algO take place. In order to achieve the needed degree of denitration, the tteated feed has to be passed through a second cycle (a second stack) of electrodialysis at the needed current density. The same principIe mar be adopted algO to treat feeds with much higher loads of nitrates.
With the new spacer, there is algO a beneficial large elimination of hydrogen-carbonate anions at very low current densities. But as the latter increases, lhe transport is decreased considerably because the hydrogen ions liberated from the water dissociation in the desalting compartment combine with hydrogen- carbonate anions to give carbonic acid, which breaks up to give carbon dioxide and water. Consequently, the flux of hydrogen-carbonate anions through the anion exchange membrane algO decreases. The high elirnination of hydrogen-carbonates observed during this work is a beneficial side-effect, which has not been achieved by other workers mentioned before.
Till a current density of I mA cm-2, there are separate fluxes for the four anions of the feed. As the current density increases, alI ions available at the ion exchange material surface are carried away equally and no distinguished ion selectivity is expected. Therefore, it is advisable to conduct the denitration process at low current densities, whereby the intrinsic membrane and resin (spacer) properties like nitrate selectivity would not be blanked out and the current efficiency remains high.
This study shows that a safe and practical method to eliminare selectively and effectively nitrates from drinking water is easily achieved through an improved process of electrodialysis at low current densities. This improvement results from the use of a modified anion exchange membrane coupled with a new intermem- brane spacer containing a nitrate-selective anion exchange resin. It has been shown that treated water with a nitrate concentration of ca. 30 mg I-I is easily obtained from a feed containing 93 mg I-I nitrate just in one cycle of electrodialysis. with roam for further denitration. This process is ecologically and econom- ically very much feasible and well advantageous over the other existing denitrating processes because of its negligible requirement in chemicals. The use of elec- trodialysis reversal and an antiscaling agent (when needed) would prevent scaling of the membranes. A small capacity microbiological post-treatrnent of the concentrated reject would render the combined pro- cess 99% effective.
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Speciflc electrical resistivity in 0.6 M NaCI solution (O cm) Transpolt number in 0.01 M NaCI solution, not less than
Total static exchange capacity, relative to 0.1 M HCI or NaOH, of dried membrane (meq g-1 Humidity content (%)
Density, (g cm-3) Dry:
Bursting strength (kg cm-\'J. not less than
Change In volume during swelling (%) along lhe Length:
Fig. I. An element of lhe new ion-conducting intermembrane spacer (drawing with lhe kind pennission fiom Mr. Mils and Mr. Zubets of lhe Voronejh State Unive~ity).
Fig. 2. A schematic representation of lhe electrodialyzer. 1-5 are lhe compartments numbers; c. concentration; co, feed concentra- tion; AEM, anion exchange membrane; CEM, cation exchange membrane; and ;+\' stands for cation and \'-\' for anion.
Fig. 4. Dependences of fluxos of anions throUgh lhe modified anion exchange membrane MA41 against currcnt densitics during electrodialysis of soIution mixture containing potassium saiu of chloride, nitrato, hydrogen-carbonate and sulfale, 1.5 meq I-I each, with lhe desalinating compartrnent filled ~ith cation exchange membrane net (MK40) and nitrate-selectivc anion exchanger Wofatit SN35L.