ISSN 1995-0780, Nanotechnologies in Russia, 2015, Vol. 10, Nos. 7–8, pp. 558–564. © Pleiades Publishing, Ltd., 2015. Original Russian Text © L.N. Polyanskii, V.S. Gorshkov, D.D. Vakhnin, T.A. Kravchenko, 2015, published in Rossiiskie Nanotekhnologii, 2015, Vol. 10, Nos. 7–8.
Sorption-Membrane System for Deep Deoxygenation of Water L. N. Polyanskii, V. S. Gorshkov, D. D. Vakhnin, and T. A. Kravchenko Voronezh State University, Universitetskaya pl. 1, Voronezh, 394006 Russia e-mail:
[email protected] Received March 3, 2015; accepted for publication April 9, 2015
Abstract—We have studied the redox sorption of oxygen from flowing distilled water in a multistage sorptionmembrane cell with a metal (Cu)–sulfo cation exchanger (KU-23 in the H+ form) granular nanocomposite in the cathode compartment and a sulfo cation exchanger (KU-23 in the H+ form) in the anode compartments separated by a cation-exchange membrane (MC-40 in the H+ form). It is shown that the redox sorption of oxygen on the granular nanocomposite bed polarized with a current which is less than the limiting external diffusion one is complicated by internal steps (oxygen diffusion in the polymer matrix pores and the chemical oxidation of metal nanoparticles) and proceeds with a mixed diffusion-kinetic control. In the mode of limiting diffusion current polarization, the contribution of internal stages decreases and the process becomes steady due to the transition into the external diffusion region. A theoretical calculation shows that the process performed in series-connected multistage cells with polarization of each stage in the mode of limiting external diffusion current allows obtaining water with a dissolved oxygen content of I2 > ...> I H2 O , O 2
Fig. 1. Flow chart for a multistage electrolyzer with a filling of NC granular layer in the cathode compartment for the removal of water-dissolved oxygen: C are copper wire cathodes, A are platinized titanium anodes, N is a polymeric separation network, MC-40 is a sulfo cation-exchange membrane, KU-23 is a granular sulfo cation exchanger, Cu0⋅KU-23 is a nanocomposite, R are variable resistors, Rj is the resistance of j stage, and Ij is the amperage at the j stage.
by a MC-40 cation-exchange membrane. All ionexchange components were taken in the H+ form. The nanocomposite used in the cathode compartment was a granular macroporous sulfo cation exchanger with chemically deposited copper nanoparticles. The grain size was (9.0 ± 2.0) × 10–4 m and the macropore size was 80 ± 20 nm. A study of morphology of NC by scanning electron microscopy (Fig. 2a) showed that the mean size of deposited copper nanoparticle aggregates was 300–500 nm. X-ray phase analysis (Fig. 2b) revealed that these aggregates consist of smaller master crystallites with sizes of 30–50 nm. The transmission electron microscopy studies performed earlier [8] detected particular copper nanoparticles with sizes of 3–10 nm. The capacities for copper metal and hydrogen counter ions were 5.5 ± 0.5 and 1.25 ± 0.05 meq/cm3 of the bulk weight, respectively. NANOTECHNOLOGIES IN RUSSIA
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The concentration of water-dissolved oxygen was determined by an AKPM-1-01P (Alfa BASSENS) oxygen analyzer. The oxygen concentration in the starting water was maintained at the level of c0 = (2.4 ± 0.2) × 10–7 mol/cm3 by continuous saturation of the starting water with atmospheric air. The pH value of starting water was 5.7 ± 0.2. Water was passed through the cathode compartment of the electrolyzer at the linear flow rate of u = 0.23 × 10–2 m/s. Cathode polarization was performed using a B5-48 universal power supply. The amperage was recorded by a B7-58/1 multipurpose high-resistance voltmeter milliamperemeter. The electrical operational parameters of an electrolyzer filled with a granular NC layer in the cathode compartment were calculated using the model of external diffusion transfer of oxygen described in [6]. Within this model, upon galvanostatic polarization, the maximum permissible amperage on an n-stage device with a cross-sectional area S, total height L, 2015
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(a) I, arb. units 200
(b) (111)
150
100 (200)
50
1 μm
0
(220)
(311)
10 20 30 40 50 60 70 80 90 100 110 120 2θ, deg
Fig. 2. Microphotographs of grain scans (a) and X-ray diffraction pattern (b) of Cu0⋅KU-23.
and height of each stage of granular layer L/n is written as follows: −n 3χ S 1 − (1 + A) , i lim (0) R0 A the amperage for each j stage
I (L) =
)
(
(1)
−j
(2) I j = I com A L 1 + A L , N N the relative concentration of oxygen at water outlet from a layer with a height L
(
с(L) = 1+ AL c0 n
)
−n
,
(3)
3χ i lim (0) , χ is the portion of volume occuzFuR0c0 pied by NC grains; R0 is the grain radius; ilim(0) is the where A =
Table 1. Calculated values of polarization current I j specified for each j stage and relative oxygen concentrations c(L) / c0 at the outlet of each stage. Stage No. j 1 2 3 4 5 6 7
Steady current, Relative oxygen calculation by concentration at the outlet equation (2), of stage, calculation by 3 equation (3), c(L) / c0 I j × 10 , A 11.6 9.5 7.7 6.3 5.1 4.2 3.4
0.82 0.66 0.54 0.44 0.36 0.29 0.25
limiting diffusion current at the initial oxygen concentration c0 in water; Icom is the total current necessary for the removal of all oxygen entering the electrolyzer; z is the number of electrons being involved in the reaction; and F is the Faraday constant. The current distribution and relative concentration of oxygen over stages are given in Table 1.
RESULTS AND DISCUSSION Dynamic output curves for the redox sorption of oxygen from water and the water pH during the electrochemical polarization of the Cu0 KU-23(H+) nanocomposite in a seven-stage electrolyzer are shown in Fig. 3. Polarization was performed by the steady current I exp ( L ) = 42 × 10–3 A, which is 90% of the maximum possible one I ( L ) corresponding to limiting diffusion currents at the outlet of each stage according to Eq. (2). In the initial period (t < 50 h) of the experiment, the oxygen level at the outlet increases analogously to that as observed upon redox sorption on the NC layer in the absence of polarization (Fig. 3a, curve 1). As far as chemical oxidation of copper nanoparticles proceeds and the contribution of internal steps decreases, there is a slip of oxygen into water flowing out of the granular layer. When polarization is continued (50–150 h), the degree of redox sorption possesses the constant value, coinciding with that calculated theoretically by the experimentally specified current I exp ( L ) . Throughout the experiment, the water pH at the outlet of electrolyzer remains close to that at the inlet (Fig. 3b). The hydrogen ions necessary for the reduction of oxygen I st O 2 + 4H + + 4e − ⎯⎯⎯ → 2H 2O
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Fig. 3. Output curves for the redox sorption of oxygen c(L) / c0 – t (a) and water pH (b) at the outlet of cathode-polarized granular layer Cu0⋅KU-23(H+) in a seven-stage electrolyzer with a total height L: (1) currentless curve; (2) current-carrying curve; and (3, 4) relative contents of oxygen calculated by the experimental current I exp ( L ) = 42 × 10–3 A and by the maximum steady current I ( L ), respectively. The polarization currents on stages are 90% of the maximum possible ones.
enter in an amount equivalent to the current passing from the anode compartment where water molecules are oxidized and pass as counter ions over the fixed groups of the KU-23 polymer matrix and MC-40 ionexchange membrane into the cathode compartment. Output curves for the redox sorption of oxygen from water and the pH change during the electrochemical polarization of the Cu0 KU-23(H+) nanocomposites at currents that are 70% of the maximum possible one are shown in Fig. 4. Despite a considerable decrease in the current, the relative oxygen concentration in water at the outlet of the electrolyzer remains as low as in the case of polarization, with the current being 90% of the maximum possible one (Table 2). This suggests a significant role of the internal stages of oxygen diffusion in the polymer matrix pores and chemical oxidation of metal nanoparticles, that is why the process occurs with a diffusion kinetic control. Figure 5 shows the scans of nanocomposite grains over the electrolyzer stages after its polarization in the NANOTECHNOLOGIES IN RUSSIA
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mode of diffusion kinetic control for 150 h. It is seen that, in the frontal part of the granular layer (at the inlet of oxygen-containing water), grains are oxidized almost completely, while at the outlet only near-surface regions of grains are oxidized. The photographic images given confirm that oxygen is reduced not only under the action of current, but also by dispersed copper in the nanocomposite. With time, the reaction centers of the material undergo oxidation and its chemical activity decreases, which is why the process reduces itself to the electrochemical reduction of oxygen. The oxygen concentration at the outlet of the granular layer becomes steady under such conditions. The resulting experimental data confirm the applicability of the theoretical model developed earlier for multistage electrochemical devices. Based on this approach, changing the parameters of electrolyzers, such as number of stages, cross section of the stage, and water flow rate, it becomes possible to design a device for the deep deoxygenation of water in the open (flow-through) system. 2015
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(b)
8 6
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Fig. 4. Output curves for the redox sorption of oxygen c(L) / c0 – t (a) and water pH (b) at the outlet of the cathode-polarized granular layer of Cu0⋅KU-23(H+) in a seven-stage electrolyzer with a total height L: (1) currentless curve; (2) current-carrying curve; and (3, 4) relative contents of oxygen calculated by the experimental current I exp ( L ) = 33 × 10–3 A and by the maximum steady current I ( L ), respectively. The polarization current on stages are 70% of the maximum possible ones.
Figure 6 shows the flow chart for series-connected flow-through sorption-membrane electrolyzers with nanocomposite filling and additional filters for the fine purification of water from impurities. The flow chart consists of sequential filters: 1 is a mechanic filter for the removal of solids from water, 2 is a sorption charcoal filter for the removal of organic and inor-
ganic impurities, 3 is ion-exchange mixed-bed filter (MBF) for water demineralization, 4–8 are flowthrough sorption-membrane electrolyzers with a filling of granular nanocomposite layer (Fig. 1) for the electrochemical reduction of water-dissolved oxygen, 4'–8' are power supplies, and 9 is an ion-exchange MBF filter.
Table 2. Experimental parameters of electrochemical polarization of the granular layer of a Cu0 KU-23(H+) nanocomposite in the sorption-membrane electrolyzer. The layer height is L = 10.5 × 10–2 m, the number of electrolyzer stages is n = 7, the height of each stage is 1.5 × 10–2 m, the total current is Icom = 63 × 10–3 A. The experiment time is 150 h. Steady current Maximum, calculation by equation (1) I ( L ) × 103, A
Relative oxygen concentration at the outlet of granular layer c(L) / c0
Specified experimentally
I
exp
( L ) × 10 A 3,
Calculation by I ( L )
Calculation by I exp ( L )
Experiment
48
42
0.25
0.30
0.29 ± 0.05
48
33
0.25
0.47
0.31 ± 0.05
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a
metals, complex equipment, and high-intensity current loads as were used in [2–5].
Cu0
Cu2O
b
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c
1 mm Fig. 5. Scans of nanocomposite grains over electrolyzer stages after 150 h operation: (a) frontal layer (at the water inlet to electrolyzer), (b) outlet layer, and (c) starting sample. The polarization currents are 70% of the maximum possible ones.
The use of a combination of several sorption-membrane electrolyzers proposed where each particular layer is polarized with its limiting oxygen diffusion current will allow one to achieve a low oxygen level in flow-through systems. Calculation by Eq. (3) showed that, at the water flow rate selected in this work, the oxygen content in water after passing through a 35-stage granular layer with a total height of 0.55 m is no more than 10 μg/L (10 ppb). It was found that such a high degree of water deoxygenation can be achieved without the application of expensive reagents and
Upon the galvanostatic polarization of multistage sorption-membrane electrolyzer filled with a copper– KU-23 ion exchanger NC where each particular layer is polarized by its own prelimiting current, the largest part of the material is polarized in the mixed diffusionkinetic control mode and the oxygen level at the layer outlet is influenced by internal stages (oxygen diffusion in the polymer matrix pores and chemical oxidation of metal nanoparticles). The process on nanocomposites occurs in the presence of hydrogen counter ions, which are continuously supplied as a result of electrode processes in the anode compartments, followed by a transfer to the cathode compartment through cation-exchange membranes. As far as metal nanoparticles undergo chemical oxidation in the nanocomposite, its chemical activity decreases and the process comes down mainly to the electrochemical reduction of oxygen, which results in the stabilization of the output oxygen concentration. The theoretical calculation showed that the process performed in series-connected multistage electrolyzers with the polarization of each stage in the mode of limiting the external diffusion current allows obtaining water with a dissolved oxygen content of less than 10 ppb.
13 11 4′
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Fig. 6. Flow chart for deep deoxygenation of water by the electrochemical method in the open flow-through system: (1, 2, 3, 9) sequential filters numbered according to regulations, (4–8) flow-through seven-stage electrolyzers (filters) with direct current power supplies (4'–8 ', 10) oxygen analyzer, (11) pH meter, (12) valves, (13) source-water tank, and (14) water-collection tank NANOTECHNOLOGIES IN RUSSIA
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ACKNOWLEDGMENTS This work was financially supported by the Russian Foundation for Basic Research (project no. 14-0800610_a) and the Ministry of Education and Science of the Russian Federation under terms of the state task to higher education institutions for 2014-2016 (project no. 675).
3. 4. 5.
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by A. Basile and F. Gallucci (Wiley, 2011), Ch. 24, pp. 531–548. S. Wenxin, C. Chongwei, Z. Liye, Y. Shuili, and Y. Xia, Frontiers Chem. Eng. Chin., No. 3, 107–111 (2009). G. Bianchi and G. Faita, “Electrochemical deoxygenation process for corrosion control in deionized waters,” US Patent No. 4830721 (1988). D. A. Kirpikov, O. Yu. Pykhteev, E. Yu. Kharitonova, Yu. V. Tsapko, I. V. Chistyakov, and V. S. Gurskii, RF Patent No. 2494974, Byull. Izobret. No. 28 (2013). V. S. Gorshkov, L. N. Polyanskii, L. A. Shinkevich, and T. A. Kravchenko, Russ. J. Phys. Chem. A 86, 1881 (2012). V. S. Gorshkov, L. N. Polyanskii, and T. A. Kravchenko, Russ. J. Phys. Chem. A 88, 305 (2014). M. Yu. Chaika, T. A. Kravchenko, E. V. Bulavina, V. S. Gorshkov, and A. B. Yaroslavtsev, Russ. J. Phys. Chem. A 85, 1065 (2011).
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