A novel electrochemical ion exchange system and its application in water treatment

Transcription

1 Journal of Environmental Sciences 011, 3(Supplement) S14 S17 A novel electrochemical ion exchange system and its application in water treatment Yansheng Li 1,, Yongbin Li 1, Zhigang Liu 1,TaoWu, Ying Tian 1 1. College of Environmental and Chemical Engineering, Dalian Jiaotong University and Liaoning Key Laboratory for Environmental Science and Technology, Dalian 11608, China. hjx@djtu.edu.cn. Dalian Development Centre of Environmental Technology, Dalian 11604, China Abstract A novel electrochemical ion exchange system with porous cylinder electrodes is proposed for treatment of wastewater. This system can be used for desalination without the costly ion-exchange membrane and extra chemical reagents. Since the electrodes are completely uniform and no ion-exchange membrane was used in this system, it can be operated by switching anodes and cathodes flexibly for eliminating the scaling on the surface of electrodes. The strong base ion-exchange resin grains placed among the anode and cathode have played as supporting electrolyte, which is capable for the treatment of wastewater with low conductivity. The concentrated and neutralized anolyte containing chlorine is effective for disinfection and contaminants removal. Under the experimental conditions, the removal percentage of total dissolved salts was 83% and the removal percentage of chemical oxygen demand was 9% without consumption of extra chemical reagents. Key words: flow-through; novel electrochemical ion exchange system; titanic porous filter cylinder electrode, desalination, disinfection Introduction Electrochemical ion exchange (EIX) is an advanced ion-exchange process for removing metal ions from wastewater (Janssen and Koene, 00). In this system, ion exchange media is incorporated into the electrodialysis stack, forming an integrated membrane/electrode sandwich structure, in which the ion exchange resin grains are placed in the central part of the system (Andrew, 1996; Manosso and Forbicini, 009). The advantage of EIX over the traditional ion-exchange is the use of electrons as the only reagent, thus reducing the waste volumes. However, ion exchange membranes used in EIX are very costly which limit its extensive application. The efficiency of EIX process is not satisfactory because of concentration polarization and electrode scaling (Konstantinos and Konstantinos, 008). To overcome these problems, we proposed a new electrochemical ion exchange system (NEIXS) by introducing porous cylinder electrodes (PCF) as the electroosmotic unit. In new EIX system, anolyte and catholyte are selectively flow from PFC, thus avoiding the use of ionexchange membranes. Polarity of the electrodes can be flexibly switched, thus eliminating the electrodes scaling for long-time operation. The proposed system utilizes electrochemical redox and ion-exchange simultaneously to obtain the neutralized electrolyzed oxidizing water, desalted water and a side-product of hydrogen. 1 Materials and methods 1.1 Titanic porous filter cylinder electrode and electroosmotic unit The properties of the porous filter cylinder are shown in Table 1. The electroosmotic unit was constructed with three anodes and cathodes (area m ) in hexagon within an insulated polymethyl methacrylate shell. The distance of anode and cathode is 5 mm. The electrolyzed oxidizing water (containing gases) and electrolyzed reducing water were pressed to flow out of the anode and cathode, respectively (Fig. 1). The strong basic anion exchange resin grains were packed among the electrodes. The flux of the simulated wastewater is 7.5 L/hr. The voltage of the NEIXS is controlled at 5 V. The ratio of electrolyzed oxidizing water and electrolyzed reducing water is 1:1. COD was determined by bichromate method. Concentration of Cl was determined by titration with silver nitrate solution. A power supply (WSA-H 100V/30A, Shenzhen Leteac Technology Co., Ltd., China.) was used to maintain constant DC voltage. Conductivities were measured by a conductometer (DDS-IIA, Shanghai San-xin Instrumentation Inc., China). PH of water was measured by an acidimeter (DELTA30, Meltler-Toledo Instruments Co., Ltd., China). Temperature was held at room temperature during all experiments. * Corresponding author. hjx@djtu.edu.cn

2 Supplement A novel electrochemical ion exchange system and its application in water treatment S15 Table 1 Properties of the porous filter cylinder Speciality Dimension Porous Porosity Penetrability of Compression Most pressure (mm) dimension (μm) factor (%) gas (L/(cm min Pa)) resistance (MPa) difference (MPa) Value Φ NA-Form D113 Outflow Anode Cl Cl Cl OCl Cl CO Contaminant O Cl Inflow H OH Outflow Fig. 1 Schematic representation of the new electrochemical ion exchange system. : neutralized electrolyzed oxidizing water; : desalted water. 1. Cation exchange treatment of electrolyzed oxidizing and electrolyzed reducing water The electrolyzed oxidizing water out of anode was pumped into Na-form D113 bed (Table ). The electrolyzed reducing water out of cathode was pumped into H-form D113 bed. Results and discussion.1 Treatment of the simulated wastewater The properties of water samples, including the feed, electrolyzed oxidizing water, electrolyzed reducing water and the desalted water are shown in Table 3. The removal percentage of total dissolved solid for the desalted water is 83% and the removal percentage of COD for the desalted water is 9%. This result demonstrates that the novel EIX can remove contaminants effectively without chemical reagents. Figure shows variation for neutralized electrolyzed oxidizing water, desalted water from NEIXS versus time. It can be seen that of neutralized electrolyzed oxidizing Cathode H-FormD113 water gradually decreased, while of desalted water gradually increased with time. The change of can provide information about ion exchanger transforming degree for switching polarity operation periodically. Figure 3 shows bacteria concentration in the feed and desalted water samples. It can be observed that there are groups of bacteria in the feed sample. When water passes through the electrode, the bacteria can be perished by directly touching the electrodes. As a result, no bacteria exist in culture dish.. Mechanism of ions migration and COD removal The strong base anion exchange resins placed between anode and cathode have played an important role for gathering anions, adjusting ions migration and being as supporting electrolyte. There are three routes for anions migration: (1) ions migrate alternatively through solution or ion-exchange resins, () ions migrate through ionexchange resin grains, (3) ions migrate in solution (Yeon et al., 003). However, there is only one route for cations migration, that is, through solution. Furthermore, the diffusion coefficient for ions migration in ion-exchange resins is much higher than that in solution. As a result, the amount of anions in anode affinity is much higher than the cations in cathode affinity. Chlorine ions thus gathered in anode district and formed concentrated chlorine water. The relevant reactions are shown as follows: Cl e Cl (1) + Cl + Cl + HClO () e + 1 O (3) Organic matter - ne Intermediate + CO (4) The organic substances around the anode were oxidized directly or indirectly into carbon dioxide or intermediate. The hydroxide ions were concentrated and passed through the filter cylinder cathode. Hydrogen formed by water electrolysis penetrated into the inner part of PCF by the force of electric fields and flow fields. In this case, concentration Table Properties of D113 weak acid cation exchanger Group Exchange capacity Diameter Water Density Expansibility Upper limit of (mmol/ml) (mm) content (%) (g/ml) (%) temperature ( C) COO ( H Ca) Table 3 Properties of the feed and the electrolyzed oxidizing and electrolyzed reducing water Cl concentration Conductivity COD (mg/l) (μs/cm) (mg/l) Simulated wastewater Electrolyzed oxidizing water Electrolyzed reducing water Desalted water

3 S16 Journal of Environmental Sciences 011, 3(Supplement) S14 S17 / Yansheng Li et al. Vol Time (min) Fig. Conductivity (μs/cm) Conductivity Time (min) of, versus time by the new electrochemical ion exchange system (NEIXS) Feed Fig. 3 Comparison of bacteria concentration in the feed and the desalted water. Cl CO O +e H OH - -e Cl - Contaminant -e H O Cl - Cl CO Contaminant Contaminant Cl - CO Cl H O H O H OH -H O O Contaminant OH - CO H+ O O Cl Cl - CO Cl Cl - Contaminant OH - OH - H H Fig. 4 Mechanisms of ions migration process. and ohm resistance can be reduced remarkably. Hydrogen evolution from water electrolysis is shown as follows: + + e H + + OH (5) + e H (6) Figure 4 shows the ions migration from bulk solution to the inner part of PCF during the electroosmosis process. The of and can be controlled by adjusting operation time. The gases generated by electrode reactions were brought into the inner part of PCF with water flow, avoiding concentration polarization and decreasing ohm resistance. As a result, current efficiency can be significantly improved..3 Mechanism of ion exchange The neutralization reaction between the electrolyzed reducing water and the H-form D113 resin is as follows: R-COO + OH R-COO (7) was adsorbed on ion-exchange resin, realizing to desalt from the feeds. The Na-form D113 ion-exchange resin was regenerated into H-form D113 with the elec-

4 Supplement A novel electrochemical ion exchange system and its application in water treatment S17 50% 83% NaCl 9% COD Fig. 5 Feed 50% 17% NaCl Schematic representation of material equilibrium. 8% COD trolyzed oxidizing water for reusing. The reaction is shown as follows: R-COO R-COO (8) Since the electrodes are completely uniform and no ionexchange membrance was used in this system, this NEIXS can be operated by switching anodes and cathodes flexibly, eliminating the scaling on the surface of electrodes..4 Material equilibrium The material equilibrium of this NEIXS was calculated according to the ratio of electrolyzed oxidizing water and electrolyzed reducing water as shown in Fig. 5. As mentioned in Section., NaCl were concentrated in anode district to reaching 83% (mass ratio) with the help of strong base ion-exchange resin and electric fields. Organic contaminants were oxidized in anode with COD removal percentage of 9%. 3 Conclusion and expectation We proposed a NEIXS with porous cylinder electrodes instead of -dimensional plane electrodes in conventional EIX system. This system can be used for desalination without the costly ion-exchange membrane and extra chemical reagents. The mechanism of desalination is the ingenious combination of the electrolyzed reducing water and neutralization reaction with H-form D113. Since the electrodes are completely uniform and no ion-exchange membrane was used in this system, it can be operated by switching anodes and cathodes flexibly for the eliminating the scaling from the surface of electrodes. The strong base ion-exchange resin grains placed among the anode and cathode have played as supporting electrolyte, which is capable for the treatment of wastewater with low conductivity. The concentrated anolyte containing chlorine is effective for disinfection and contaminants removal. References Andrew T, Electrochemical ion exchange. Membane Technology, 75: 6 9. Janssen L J, Koene L, 00. The role of electrochemistry and electrochemical technology in environmental protection. Chemical Engineering Journal, 85: Konstantinos D, Konstantinos O, 008. Continuous capacitive deionization-electrodialysis reversal through electrostatic shielding for desalination and deionization of water. Electrochimica Acta, 53: Manosso H C, Forbicini C A L G de O, 009. Treatment of wastes containing cesium ions by electrochemical ion-exchange (EIX). Journal of Radioanalytical and Nuclear Chemistry, 79(): Yeon K H, Seong J H, Rengaraj S, Moon S H, 003. Electrochemical characterization of ion-exchange resin beds and removal of cobalt by electrodeionization for high purity water production. Separation Science and Technology, 38():

5 ID Title Pages A novel electrochemical ion exchange system and its application in water treatment 4