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Purification of a diluted nickel solution containing nickel by a process combining ion exchange and electrodialysis
DF_~ALINATION ELSEVIER
Desalination 162 (2004) 179-189
Purification of a diluted nickel solution containing nickel by a process combining ion exchange and electrodialysis Yu. S. Dzyazko*, V.N. Belyakov V.I. Vernadskii Institute of General & Inorganic Chemistry, Palladin Ave. 32/34, 03142, Kiev 142, Ukraine TeL +38 (044) 424-0462; Fax +38 (044) 424-3070; email: [email protected] Received 2 July 2003; accepted 3 September 2003
Abstract
The removal of nickel ions from diluted solution using some ion-exchange resins (Dowex MSC- 1, Purolite C 100E, KU 2-8 and Dowex HCR-S) was studied. The purification process involved the use of an electrodiatysis-type cell in which the centre compartment was filled with a packed bed of ion-exchanger particles. The Dowex HCR-S resin was shown to be the most suitable material for use in electrodeionization processes. The current efficiency under constant rate of nickel transport to the catholyte reached 14%. The energy consumption, which is needed for removal of 0.35 tool Ni 0I) from 1 m 3 solution containing 1 tool NiSO,, was estimated as 208 Wh. The apparent diffusion coefficients of Ni(lI) ions in the ion exchangers were determined with the electrornigration method. These values increased with decrease of the effluent pH and reached 1.30"10- lz (Dowex MSC- 1), 1.36-10-12(Purolite C 100E), 2.69.10-12(KU 2- 8), and 4.07-10 -12 (Dowex HCR-S) m2 s-1 at pH 2.3-2.5. The largest magnitude of the nickel diffusion coefficient was obtained for the Dowex HCR-S resin, which contains the maximum quantity of functional groups per volume unit. It was found that the mobility of sorbed ions determines the efficiency of the purification process. Keywords: Cation exchange; Desalination; Electrodeionizaton; Electrodialysis; Electroextraction; Migration
1. I n t r o d u c t i o n It is known that the combination o f electrodialysis and ion-exchange methods (electrodeionization) can be applied to the removal o f *Corresponding author.
Ni(II) ions [1-3] as well as monovalent [4-7] and divalent [7-10] cations from diluted solutions. A flexible ion-exchange resin containing a 2% cross-linking agent (divinylbenzene) was used for purification o f nickel containing solutions [1-3]. However, chemical stability o f this ion exchanger
Presented at the PERMEA 2003, Membrane Science and Technology Conference of Visegrad Countries (Czech Republic, Hungary, Poland and Slovala'a), September 7-11, 2003, Tatranskd Matliare, Slovakia. O011-9164/04/$- See front matter © 2004 Elsevier B.V. All rights reserved PII: S0011-9164(04)00041-4
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is rather low. A presence of oxidizing impurities in the solution to be purified results in destruction of the resin [11]. As a rule, the ion exchangers with a larger degree of cross linkage (8%) that possess a higher chemical stability are usually used for sorption purification of Ni(II)-containing solutions [12]. Electrodeionization includes two processes, which are carried out simultaneously: concentration of ions in the ion exchanger (sorption) and transport of ions through the ion exchanger and membrane system caused by a potential gradient [4,5]. The efficiency of usage of any ion exchanger in the electrodeionization process is mainly determined by a mobility of sorbed ions [13]. This parameter can be determined with an electromigration method described in Spoor et al. [14,15]. The purpose of this work was the quantitative estimation of Ni(II) ion mobility in strong acidic styrole-divinylbenzene ion exchangers containing an 8% cross linking agent and choice of the most effective ion-exchange materials for the electrodeionization processes of nickel removal from diluted solutions.
10 mol m -3 (macroporous ion exchanger) NiSO 4. In order to determine the amount ofsorbed Ni(II), an aliquot of ion exchanger (0.5 cm3) was treated with a 3 M H2SO4 solution (20 cm3). Concentration of Ni(II) in the effluent was determined with an atomic absorption method using a Puy UNICAM SP 9 spectrophotometer. 2. 2. Experimental set-up
The experimental set-up consisted of a threecompartment cell, three separate liquid lines, power supplier and measurement instrumentation (Fig. 1). Smooth platinum electrodes were used. The cathode and anode compartments of the cell were separated from the center compartment with a CMI-7000 cation-exchange and an AMI-7001 anion-exchange heterogeneous membranes (Membranes International). The use of heterogeneous membranes is possible as they are known to leak H2SO4 [14]. Moreover, anion-exchange membranes containing quaternary amino groups catalyse the split of water [16].
2. Experimental 2.1.1on exchangers
Strongly acidic styrole-divinylbenzene ionexchange resins containing an 8% cross linking agent, namely KU 2-8 (Chimiya), Purolite C 100E (Purolite International), Dowex HCR-S and Dowex MSC-1 (Dow Chemical) were investigated. Inert glass particles were also used for the comparison ofelectrodeionization and traditional electrodialysis. 0.5-0.8 mm fractions were taken. Characteristics of the materials such as a total ion-exchange capacity and water content were determined according to known methods [11]. For loading with nickel the ion exchangers (27 cm 3) were treated with a solution (2000 c m 3) containing 5 molm -3 (gel-like ion exchangers) or
Fig. 1. Experimentalset-up consistingof anode reservoir (1), pumps (2), eleetrodialysiscell (3), cathodereservoir (4), thermostat (5), feed reservoir (6), pressure sensors (7), reservoir for pH measurements (8), pH meter (9), waste container(10), power supplier(11), voltmeter(12) ammeter(13).
Yu.S. Dzyazko, KN. Belyakov / Desalination 162 (2004) 179-189
The center compartment was filled with an ion exchanger or glass particles. The effective area of the membranes and electrodes inside the cell was 15 cm 2 (1 cm x15 cm), the bed thickness was 1 cm. Deionized water or NiSO4 solution was pumped through the centre compartment using a NP-1M peristaltic pump according to the "oncethrough" operation. The flow velocity was 6.67,10 -7 m3s -]. The pH of the solution atthe cell outlet was measured using a EV-40 pH-meter. H2SO4 solutions (200 em 3) circulated through the electrode compartments using a Zalimp PP 2-15 peristaltic pump. Circulation velocity was 1.67"10 -5 m 3 s -1. An IPPT 65-49 power supplier was used to keep a constant cell voltage, controlled with a SCH-1312 voltmeter. The current was registered with an SCH-4311 ammeter. Constancy of the temperature in the system (298 K) was provided by means o f a UTU-2/77 thermostatic bath. The pressure drop through the ion-exchanger bed was measured using mercury pressure sensors placed both at the inlet and outlet of the centre compartment,
2. 3. Investigation of Ni(II) transport Fifteen cm a swelling resin or glass particles were placed into the centre compartment. All the experiments were carried out at -10 V. Nickel
181
concentration in the samples was determined with the atomic absorption method. After the finish of the experiments the membranes were withdrawn from the cell and carefully purified from the ion exchanger or glass particles mechanically. Thenthe membranes were treated with a 3 M H 2 S O 4 solution several times. The effluent was analyzed and the nickel amount deposited on the cation-exchange membrane was determined.
3. Results
3.1. Ion exchangers The characteristics of the ion-exchange resins are given in Table 1 where it is shown that the total sorption capacity per volume unit of wet resin is similar for all the materials except the Dowex HCR-S. This ion exchanger is characterized by the largest value of sorption capacity. The amount of water per volume unit of resin decreases within the order: Purolite C 100E > Dowex HCR-S = Dowex MSC-1 > KU 2-8. Partial loading of ionexchangers with Ni(II) caused a decrease in their volume. However, the decrease of water content was not considerable due to the increase in the amount of resin particles in the volume unit. It should be also noted that under approximately similar loading degree, the loss of volume
Table 1 Characteristics o f ion exchangers Ion exchanger
K U 2-8 Dowex HCR- S Purolite C 100 E Dowex MSC- 1
Structure
Gel-like Gel-like Gel-like Maeroporous
Hydrogen form
Ni(II) loaded form
Total ion-exchange capacity, mol m -3
Water content, %
Ni(II) cone,, m o l m -3
Water content, %
Decrease in volume, %
1600 1800 1600 1600
48 53 59 53
390 420 390 370
46 51 58 52
9 6 12 8
Yu.S. Dzyazko, V.N. Belyakov / DesalinatT"on 162 (2004) 179-189
182
for the studied ion exchangers was different. The largest shrinkage was observed for Purolite C 100E, and the minimal volume decrease was obtained for Dowex HCR-S. 3.2. Effect o f anode and cathode compartment HsSO 4 concentration on the Ni(11) transport In the first series of experiments the Ni(II) removal from Dowex MSC-1 resin was investigated. The centre compartment was filled with nickel-loaded ion exchanger. Deionized water was passed through the resin bed. The initial concentration o f acid solution in the electrode compartment was varied from 0.5 to 4 M. In all the cases the initial amounts of H2SO4 in the cathode and anode compartments were equal. The pH of the solution at the centre compartment outlet before energization is given in Fig. 2 as a function of initial acid concentration in the electrode compartments (Ca~,,1). It follows from the figure that an increase in Ca~,,z value caused a shift of pH o f the solution, which was formed in the centre compartment, to the more acidic field. Evidently it is a result o f leakage of acid through the anion-exchange membrane.
Since the investigated ion exchanger was strongly acidic, the Ni(II) amount in the solution at the center compartment outlet (C_~i)was rather low due to slight desorption. The C~i values were 1"10-2, 8"10 -3, 7-10 -3 and 5"10 -3 mol m -3 at effluent pH of 2.1, 2.2, 2.4 and 2.5, respectively. Under applied voltage Ni(II) ions migrated to the cathode compartment while no nickel was found in the anolyte and effluent. The dependencies of nickel content in the catholyte (nl, zi,cat) on time (t) are plotted in Fig. 3. It is seen that the amount of nickel in the catholyte increased with an increase in acid concentration in the electrode compartments. T h e nNi,cat VS. t C u r v e s c a n be fitted with following equation: (1)
nm, eat : a 1 + a 2 e -a3t
where al-a3 are the empirical coefficients. The current density (0 as a function o f time was plotted in Fig. 4 excluding the start-up effect. When the acid concentration in the electrode compartments was 4 M, the current increased with increase of regeneration time. In other cases the current decreased during the experiments. 0.4
4
0,2
o
Z 0.1
' "z= 0.2 ,.e
2 . 0
.
.
.
.
. . 2
4
0,0
0
C:,~.~x 10.3 [molm "3] Fig. 2. T h e e f f l u e n t p H b e f o r e ( o )
0.0 10000
20000
t Is] a n d after ( o )
energization (- 10 B); amount of Ni(II) deposited on the cation-selective membrane (m) as a function of initial H2SO 4 concentration in the electrode compartments. The center compartmentwas filled with Dowex MSC- 1 resin.
Fig. 3. Amount of Ni(II) in the catholyte over time of Dowex MSC-1 resin regeneration. Initial H2SO4 concentration in the eatholyte and anolyte was 0.5 (O), 1 (0), 3 (o), 4 (•) M or 0.5 M in the catholyte and 4 M in the anolyte 07).
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After energization, the pH of the solution formed in the centre compartment increased (Fig. 2) due to transport of hydrogen and sulfate ions to the cathode and anode compartments, respectively. No changes in the pH were found over time of the experiment. The Ni(II) amount deposited on the cationexchange membrane as hydroxide (n~a) vs. acid concentration in the electrode compartments is shown in Fig. 2. It follows from the figure that an increase in C~c,~z and a corresponding decrease of pH of the solution in the centre compartment causes a reduction of nni. At pH 2.2 (Ca~,~z corresponds to 4 M), no nickel deposit on the membrane surface was observed. The next experiment was devoted to investigation of Ni(II) transport through the Dowex MSC-1 resin under different concentrations of acid in the catholyte (0.5 M) and anolyte. In this case the initial H2SO4 concentration was 0.5 (catholyte) and 3 M (anolyte). The HNi°cat-t curve (Fig. 3), effluent pH and amount of deposited Ni(II) were practically identical to those for equal concentrations of the catholyte and anolyte (3 M). This justifies the assumption concerning the acid leakage through the anion-exchange membrane.
The nickel transport through Purolite C 100E, Dowex HCR-S and KU 2-8 was also studied. The initial H2SO4 concentration in the electrode compartments was 3 and 4 M. Other experimental conditions were similar to those described for Dowex MSC-1. The HNi,eat--t curves (Fig. 5) can be fitted with inversely exponential functions Similarly to (1). It can be seen that the transport rate increased with an increase in Cac, et, For all the ion exchangers the effluent pH reached 2.3 (Cac, ez =4 M) and 2.5 (Cac,,z= 3 M) and did not change during the experiments. Over time ofDowex HCR-S regeneration (Cac.e l = 3 M) the current increased, and no Ni(II) deposit on the cation-exchange membrane was found. Alternately, for KU 2-8 and Purolite C 100E regeneration under similar conditions, the current decreased. The n~ values reached 0.3 and 0.1 mmol, respectively. Alternately, under an increase in concentration of the acid in the electrode compartment and a corresponding decrease of the effluent pH, the current increased over time due to the absence of Ni(II) hydroxide deposition.
20
2 E
qT"
,
i 0
0
~
0
.....
10000
~
20000
t Is] Fig. 4. Current density over time ofDowex MSC-1 resin regeneration• H2SO4 concentration in the catholyte and anolyte was 0.5 (il,), 1 (o), 3 (e), 4 ( I ) M.
t 0000
20000
t[s] Fig. 5. Amount of Ni(II) ions in the catholyte over time of electroregeneration of Purolite C 100E (O, tt), KU 2-8
(ooV) and Dowex HCR-S (o) resins. Initial H2SO4 concentration both in the catholyte and anolyte was 3 (,O,,o,e) and 4 (re,V)M.
Yu.S. Dzyazko, V.N. Belyakov/ Desalination 162 (2004) 179-189
184
The pressure drop through the bed was registered to be approximately 4 kPa in all the cases.
3.3. Electrodeionization o f Ni(11)-containmg solution The purification o f the solution containing 1 mol m -3 Ni(II) was investigated using both ion exchangers, which were initially in a hydrogen form, and glass particles; 3 M H2SO4 solutions filled the electrode compartments. The Ni(II) amount in the catholyte and Ni(II) concentration in the solution at the cell outlet (CNi) as functions o f time are represented in Figs. 6 and 7, respectively. When Dowex HCR-S was placed in the centre compartment, the rate o f Ni(II) transport increased during the first 4 h and was stabilized within a 4-8 h interval. Analogous regularity was obtained for CNi. Altemately, the current decreased until 4 h o f the experiment and then remained constant (Fig. 8).
In the case o f KU 2-8, Dowex MSC-1 and Purolite C 100E, the nickel flux through the membrane also increased in the beginning o f the experiment. During 4-8 h, the flux slightly decreased while the nickel concentration in the effluent was stabilized (Dowex MSC-1 and Purolite C 100E) or diminished (KU 2-8). The current was found to decrease slightly over whole time.
1.0
~e, 0.5 r..)
o.o
0
toooo
20000 t Is]
30000
Fig. 7. Ni(II) concentration in the effluent over time of solution purification. The centre compartment was filled with a Purolite C 100E (O), Dowex MSC-1 (m), KU 2-8 (o), Dowex HCR-S (e) and glass particles (V). Initial H~SO4 concentration both in the catholyte and anolyte was 3 M.
2 300
2.6
20o
2,4
100
2.2
)
O~ o
10000
20000 t Is]
30000 0
Fig. 6. Ni(II) amount in the catholyte over time of the solution purification. The centre compartment was filled with Purolite C 100E ($), Dowex MSC-1 (=), KU 2-8 (o), Dowex HCR-S (e) and glass particles (V). Initial H2SO4 concentration both in the catholyte and anolyte was 3 M.
1oooo
20000
30000
"J 2.0
t is]
Fig. 8. Current density (e) and effluent pH (o) over time of solution purification. The centre compartment was filled with Dowex HCR-S resin. Initial H~SO4 concentration both in the catholyte and anolyte was3 M.
Yu.S, Dzyazko, KN. Belyakov / Desalination 162 (2004) 179-189
When the center compartment was filled with glass particles, the nickel transport rate as well as the current decreased gradually with increasing in time. Alternately the nickel concentration in the effluent remained constant. The dependence of the effluent pH vs. t for Dowex HCR-S is shown in Fig. 8. It follows that the acidity of the solution in the centre compartment decreased and was stabilized after 4 h. Similar regularities were obtained for other ion exchangers. In all the cases the effluent pH did not exceed 2.5. In the case of glass particles, the pH was 2.5 and did not change over time. The Ni(II) amount deposited on the cationexchange membrane reached 0.2 (Dowex HCR-S), 1.2 (KU 2-8), 2.16 (Dowex MSC-1), 1.88 (Purolite C 100E) and 3.15 (glass particles) mmol. From these results we conclude that the most suitable ion exchanger for purification of nickel-containing solutions is Dowex HCR-S resin.
185
Here the y coefficient is a ratio of charges of cation and anion. If ions are transported bo__th through the ion exchanger and solution, the NN val_ueincludes the flux through the ion exchanger
(4)
-~Ni= -N~i + (1 +?)NNi~
The flux through the ion exchanger can be represented as a sum of migration, diffusion and convection terms [19]. At the start of the experiment, which corresponds to t= 0, the diffusion term can be omitted [14,15]. According to Grebenyuk et al. [9,10], the convection term can also be neglected due to its ratherlow contribution to the total flux. Thus the Nm value is assumed to be determined only by migration at t=0. Under this condition the Ni(II) flux through the ion exch_anger is connected with a diffusion coefficient (Di) by a combination of the NemstPlanck and Nernst-Einstein equations [ 19]:
4. Discussion
The obtained results allow us to make some quantitative estimation of Ni(II) ion transport. Under the conditions of a forced convection and the absence of the ion exchanger between membranes, the nickel flux through the cationexchange membrane (n~) is determined by migration in the solution bulk and diffusion through the solution layer at the membrane surface. The equation of the material balance can be represented as [17]: (2)
RT
N~i~ = Y N ~ gr
(3)
(5)
The diffusion flux through the solution is described as:
(6) If the current reaches a limiting value or exceeds it, then C'Ni =..0. The g r a d e value, which corresponds to t = 0, can be determined according to the formula [17]: gradE=
Diffusion and migration components of the flux are connected with Eq. (3) [18]:
grad g
Eeatb~ - E ~°'~ - E eett
(7)
The contribution of potential drop through the electrode compartments was neglected due to high electric conductivity of the acid solutions.
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Yu.S. Dzyazko, KN. Belyakov / Desalination 162 (2004) 179-189
Similarly, the potential drops through the membranes were not taken into account since their thickness was much less than that of the ionexchanger bed. The Eo,thoe~-E~nod~ value determined with a voltametric method was found to be near -1.8 V, Thus the g r a d e value reached -820 V m -I ' The mass transport coefficient can be found from Eq. (8) [17]:
k~it~
1.52
(8)
The Ni(II) flux through the cation-exchange membrane can be determined from the experimentally obtained nNi,o,t- t dependencies as: =
1 dnNi,~t
(9)
The flux values, which were calculated as reference quantities o f the derivatives o f functions 0.1) at t = 0, are plotted in Fig. 9 vs. effluent pH. The N N magnitudes are seen to increase with an increase o f effluent acidity. At pH<2.5 no changes in values o f the initial fluxes were found. The total al=n=ount ofdesorbed nickel determined as/'/Ni, cat + n N reached 0.21, 0.15, 0.32 and 0.35 at the effluent pH o f 3.7, 3.2, 2.5 and 2.3, respectively. A decrease o f the initial Ni(II) flux as well as a decrease o f the amount ofdesorbed Ni(II) with an increase o f the effluent pH and diminishing o f the current over time are evidently caused by partial deposition of nickel hydroxide on the membrane. Moreover, the resistance o f heterogeneous membranes is known to increase due to their polarization [ 17]. These factors can result in a decrease of the potential gradient through the ion-exchanger bed. On the other hand, partial Ni(II) deposition on the cation-exchange membrane can cause a distortion o f the nNi,~,t-t dependence. Earlier the
independence o f the initial Ni(II) flux through the membrane on acid concentration in the catholyte and anolyte was found [14]. This disagreement can be caused by different experimental conditions. It should be noted that in the case o f the Dowex MSC-1 resin a decrease in acid concentration in the catholyte from 3 to 0.5 M at a constant anolyte concentration__does not influence either the effluent pH or N N. It is possible to estimate a Ni(II) flux through the solution. A maximum amount o f nickel in the effluent before energization was found for Dowex MSC-1 at the effluent pH o f 2,1 (it cor-responds to C~c,el = 4 M). Taking into account that the linear flow velocity was 6.7"10 -3 m s -1, the effective diameter o f the particles was 6.5"10 -4 m and also DNi = 6.49"10 -l° m 2 s -1 and u = 8.99.10 -7 m2s -1 at 298 K [20]; the Ni(II) mass transport coefficient was estimated as 3.93.10 -5 m s-1. Since CNi = 1"10-2 mol m -3 at t = 0, the NN~ value, which corresponds to the limiting current condition, reached 3.93-10 -6 mol m 2 s -1. If the current does not reach a limiting value, it is valid for NiSO4:
-3
V
~Y
~-5 -6
2
3 pH
4
Fig. 9. Initial Ni(II) flux through the cation-selective membrane for Purolite C 100E (<)),Dowex MSC-1 (0, o), KU 2-8 (27) and Dowex HCR-S (i) as a function of the effluent pH. Initial H2SO4 concentrations of the catholyte and anolyte are equal (0o~l,,V,m) or unequal (0.5 M in the catholyte and 3 M in the anolyte) (o).
Yu.S. Dzyazko, V.N. Belyakov / Desalination 162 (2004) 179-189
N~i : NNm:g~ _ ZNi F D~i C~i g r a d e RT
(1 O)
It follows from Eq. (10) that tl2e N ~ ~ value is 1.23"10 -6 tool m -2 s-1 at g r a d e = 820 V m -1. The obtained magnitude exceeds the diffusion flux, which corresponds to the limiting current. Thus the Ni(II) transport through the solution was determined by diffusion. As a result, the total nickel flux through the solution caused by diffusion and migration can be defined as 2kNiC~i. The apparent diffusion coefficients of Ni(II) ions in ion exchangers are plotted in Fig. 10. The D~ values obtained under similar conditions are seen to decrease in the following order: Dowex HCR-S > KU 2-8 > Dowex MSC-1 > Purolite C 100E. A quantitative estimation of the solution purification process using ion-exchange resins is rather difficult due to nickel hydroxide deposition, formation of the concentration gradient in the ion-exchanger bed and shrinkage of the resins. Under the electrodialysis using glass particles the Ni(II) amount reached 0.48 mmol in the catholyte and 1.26 mmol on the cationexchange membrane in 8 h. From these results
B
*$e.
0
2
3
4
pH Fig. 10. Ni(II) diffusion coefficient in Purolite C 100E ((>), Dowex MSC-1 ( , ) , KU 2-8 (V), Dowex HCR-S (") resins as a fi,mction of the effluent pH. The initial H2SO4 concentrations inthe electrode compartments were equal.
187
the nickel flux was estimated as 8.40"10 -5 mol m-2s-1. The obtained value corresponds to the NN magnitude calculated for the limiting current condition at C>;i =1 m o l m -3. A small disagreement can be caused by the inaccuracy of estimating the effective particle size on the one hand and a change in a form of transported Ni (II)ions on the other hand. Replacing of glass particles with Dowex HCR-S ion-exchange resin allows us to improve the efficiency of the solution purification and decrease the amount of nickel deposited on the membrane. Under the constancy of the Ni(II) transport rate, the Ns value was estimated as 1.28.10 -5 tool m -2 s -~. Taking into account that the current after stabilization reached 50 mA, the energy consumption, which is needed for removal of 0.35 mol Ni (II) from 1 m 3 solution containing 1 mol NiSO4, was estimated as 208 Wh. Use of other ion exchangers, which are characterized by lower Ni(II) mobility, is ineffective. It is possible to suggest that in the case of Dowex MSC-1 and Purolite C 100E, the nickel is transported practically only through the solution. A decrease in quantity of deposited nickel under replacing of glass particles with ion exchangers can be explained as follows. OHions, which are generated on the membrane surface due to water split, are neutralized by hydrogen ions transported from the ion exchanger. Significant deposit formation in the case of solution purification is caused by a relatively large Ni(II) content in the solution, on the one hand, and over limiting current conditions, on the other hand.
5. Conclusions It was found that an increase in effluent acidity due to leakage of the acid through the anion-selective membrane caused a decrease in the Ni(II) amount deposited on the cationselective membrane. As a result, the nickel
Yu.S. Dzyazko, V.N. Belyakov / Desalination 162 (2004) 179-189
188
flux through the cation-exchange membrane increases. The apparent diffusion coefficients o f Ni(II) ions in the ion exchangers containing an 8% cross-linking agent were determined with the electromigration method. The Dra increased with the decrease o f the effluent pH. The values o f Ni(II) diffusion coefficients obtained at pH 2.32.5 reached 1.30.10 -12 (Dowex MSC-1), 1.36. 10 -12 (Purolite C 100E), 2.69.10 -12 (KU 2-8) and 4.07.10 -12 (Dowex HCR-S) m 2 s <. This is the same order o f magnitude as self-diffusion coefflcients o f divalent cations through an ionexchange resin o f the same type [21]. The largest value o f nickel diffusion coefficient was found for the Dowex HCR-S, which contains the maximum quantity o f functional groups per volume unit. All the investigated ion exchangers were tested in the process o f Ni(II) removal from diluted solution. The Dowex HCR-S resin was found to be the most suitable material for use in electrodeionization processes. The current efficiency under a constant rate o f nickel transport to the catholyte reached 14%. The energy consumption, which is needed for removal o f 0.35 mol Ni(II) from 1 m 3 solution containing 1 mol NiSO4, was estimated as 208 W h. Thus the mobility of sorbed ions determines the efficiency o f the purification process. Further development o f a technology for nickel removal from diluted solutions using Dowex HCR-S resin is possible and connected with optimization o f the electrodeionization process, 6.
--
C~r~ CNi
--
--
S
- - Membrane (electrode) effective area,
Concentration of Ni(II) in the solution at the membrane surface, mol m -3 DN~ - - Ni(II) diffusion coefficient in the solution, m 2 s-: /~ - - Ni(II) diffusion coefficient in the ion exchanger, m 2 s -1 - Effective particle size, m E - - P o t e n t i a l drop through the ionexchanger bed, V Eceu - - Applied cell voltage ,V Eca~o~ - - Cathode potential, V E~o~ - - Anode potential, V F - Faraday constant, A s mol-: - - Current density, A m -2 i - - Mass transport coefficient o f Ni(II), kNi m s-: 1" - - Ion-exchanger bed thickness, m NN - Ni(II) flux through the membrane, mol m -2 s -1 - - N i ( I I ) flux through the ion-ex/¢Ni changer, mol m -2 s-1 NNi ~ - Ni(II) flux through the solution due to diffusion, mol m -2 s -1 NN~gr - - Ni(II) flux through the solution due to migration, mol m -z s -1 /'/Ni, cat - - Amount o f Ni(II) in the catholyte, tool nN - - Amount of Ni(II) deposited on the membrane, mol R - Gas constant, J mo1-1 K-:, V C mol-: K-1 m2
t T ZNii
S y m b o l s
Cao., l
c 'r~i
Concentration of acid in the electrode compartments, m o l m -3 Concentration of Ni(II) in the ion exchanger, mol m -3 wet resin Concentration o f ionic species in the solution bulk, mol m -3
- - Time, s - - Temperature, K - - Charge o f Ni(II) ion
Greek Y V
- - Ratio o f charged modules o f cations and anions - - Viscosity o f the solution, m 2 s -1 - - Linear flow velocity, m s-:
Yu.S. Dzyazko, KN. Belyakov / Desalination 162 (2004) 179-189
Acknowledgements This work has been supported financially by the N A T O SfP 972490 programme entitled "Removal o f Nickel and Copper from Dilute Solutions b y Ion-Exchange Assisted Electrodialysis". The authors wish to express their gratitude to Dr. L.J.J. Janssen, Dr. P.B. Spoor (Technical University o f Eindhoven, the Netherlands) and Dr. L. Koene (TNO Institute o f Industrial Technology, Den Helder, the Netherlands) for fruitful discussions o f the results.
References [1] P.B. Spoor, L. Koene, W.R. ter Veen and L.J.J. Janssen, Chem. Eng. J., 85 (2001) 127-135. [2] P.B. Spoor, L. Koene, W.R. ter Veen and L.J.J. Janssen, J. Appl. Electrochem., 32 (2002) 1-10. [3] P.B. Spoor, L. Grabovska, L. Koene, L.J.J. Janssen and W.R. ter Veen, Chem. Eng. J., 89 (2002) 193202. [4] E. Glueckauf, Brit. Chem. Eng., 4 (1959) 646--651. [5] G. Ganzi, J. Wood and C. Griffin, Enviromental Progress, 11 (1992) 49-53. [6] E. Dejean, E. Laktionov, J. Sandeaux, R. Sandeaux, G. Pourcelly and C. Gavach, Desalination, 114 (1997) 165-173.
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[7] S. Ezzahar, A. Chefif, J. Sandeaux, R. Sandeanx and C. Gavach, Desalination, 104 (1996) 227-233. [8] K, Basta, A. Aliane, A. Lounis, R. Sandeaux, J. Sandeaux and C. Gavach, Desalination, 120 (1998) 175-184. [9] V.D. Grebenyuk, N.A. Linkov and V.M. Linkov, Water SA, 24 (1998) 123-127. [10] V.D. Grebenyuk, R.D. Chebotareva, N.A. Linkov and V.M. Linkov, Desalination, 115 (1998) 255-263. [11] M. Marhol, IonExchangers in Analytical Chemistry, Academia, Prague, 1982. [12] V.D. Grebenyuk0 G.V. Sorokin, S.V. Verbich and L.H. Zhiginas, Water SA, 22 (1996) 381-384. [131 P.B. Spoor, L. Koene and L.J.J. Janssen, J. Appl. Electrochem., 32 (2002) 369-377. [14] P.B Spoor, W.Rter VeenandL.J.J. Janssen, J. Appl. Electroehem., 31 (2001) 523-530. [ i 5] P.B Spoor, W.R ter Veen and L.J.J. Janssen, J. Appl. Electroehem., 31 (2001) 1071-1077. [16] V.I. Zabolotzkii, N.V. Sheldeshov andN.P. Gnusin, Electroehimiya, 22 (1986) 1676-1682 [in Russian]. [171 F. Walsh, A First Course in Electrochemical Engineering, Alresford Press, London, 1993. [18] K.J. Vetter, Electroehemishe Kinetik, Springer, Berlin, 1961. [19] F. Helfferieh, Ion Exchange, Dover, New York, 1995. [20] R. Parsons, Handbook of ElectrochemicalConstants, London, Butterworth Scientific, 1959. [21] G.E. Boyd and B.A. Soldano, J. Amer. Chem. Soc., 75 (1953) 6091--6099.
Desalination 162 (2004) 179-189
Purification of a diluted nickel solution containing nickel by a process combining ion exchange and electrodialysis Yu. S. Dzyazko*, V.N. Belyakov V.I. Vernadskii Institute of General & Inorganic Chemistry, Palladin Ave. 32/34, 03142, Kiev 142, Ukraine TeL +38 (044) 424-0462; Fax +38 (044) 424-3070; email: [email protected] Received 2 July 2003; accepted 3 September 2003
Abstract
The removal of nickel ions from diluted solution using some ion-exchange resins (Dowex MSC- 1, Purolite C 100E, KU 2-8 and Dowex HCR-S) was studied. The purification process involved the use of an electrodiatysis-type cell in which the centre compartment was filled with a packed bed of ion-exchanger particles. The Dowex HCR-S resin was shown to be the most suitable material for use in electrodeionization processes. The current efficiency under constant rate of nickel transport to the catholyte reached 14%. The energy consumption, which is needed for removal of 0.35 tool Ni 0I) from 1 m 3 solution containing 1 tool NiSO,, was estimated as 208 Wh. The apparent diffusion coefficients of Ni(lI) ions in the ion exchangers were determined with the electrornigration method. These values increased with decrease of the effluent pH and reached 1.30"10- lz (Dowex MSC- 1), 1.36-10-12(Purolite C 100E), 2.69.10-12(KU 2- 8), and 4.07-10 -12 (Dowex HCR-S) m2 s-1 at pH 2.3-2.5. The largest magnitude of the nickel diffusion coefficient was obtained for the Dowex HCR-S resin, which contains the maximum quantity of functional groups per volume unit. It was found that the mobility of sorbed ions determines the efficiency of the purification process. Keywords: Cation exchange; Desalination; Electrodeionizaton; Electrodialysis; Electroextraction; Migration
1. I n t r o d u c t i o n It is known that the combination o f electrodialysis and ion-exchange methods (electrodeionization) can be applied to the removal o f *Corresponding author.
Ni(II) ions [1-3] as well as monovalent [4-7] and divalent [7-10] cations from diluted solutions. A flexible ion-exchange resin containing a 2% cross-linking agent (divinylbenzene) was used for purification o f nickel containing solutions [1-3]. However, chemical stability o f this ion exchanger
Presented at the PERMEA 2003, Membrane Science and Technology Conference of Visegrad Countries (Czech Republic, Hungary, Poland and Slovala'a), September 7-11, 2003, Tatranskd Matliare, Slovakia. O011-9164/04/$- See front matter © 2004 Elsevier B.V. All rights reserved PII: S0011-9164(04)00041-4
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is rather low. A presence of oxidizing impurities in the solution to be purified results in destruction of the resin [11]. As a rule, the ion exchangers with a larger degree of cross linkage (8%) that possess a higher chemical stability are usually used for sorption purification of Ni(II)-containing solutions [12]. Electrodeionization includes two processes, which are carried out simultaneously: concentration of ions in the ion exchanger (sorption) and transport of ions through the ion exchanger and membrane system caused by a potential gradient [4,5]. The efficiency of usage of any ion exchanger in the electrodeionization process is mainly determined by a mobility of sorbed ions [13]. This parameter can be determined with an electromigration method described in Spoor et al. [14,15]. The purpose of this work was the quantitative estimation of Ni(II) ion mobility in strong acidic styrole-divinylbenzene ion exchangers containing an 8% cross linking agent and choice of the most effective ion-exchange materials for the electrodeionization processes of nickel removal from diluted solutions.
10 mol m -3 (macroporous ion exchanger) NiSO 4. In order to determine the amount ofsorbed Ni(II), an aliquot of ion exchanger (0.5 cm3) was treated with a 3 M H2SO4 solution (20 cm3). Concentration of Ni(II) in the effluent was determined with an atomic absorption method using a Puy UNICAM SP 9 spectrophotometer. 2. 2. Experimental set-up
The experimental set-up consisted of a threecompartment cell, three separate liquid lines, power supplier and measurement instrumentation (Fig. 1). Smooth platinum electrodes were used. The cathode and anode compartments of the cell were separated from the center compartment with a CMI-7000 cation-exchange and an AMI-7001 anion-exchange heterogeneous membranes (Membranes International). The use of heterogeneous membranes is possible as they are known to leak H2SO4 [14]. Moreover, anion-exchange membranes containing quaternary amino groups catalyse the split of water [16].
2. Experimental 2.1.1on exchangers
Strongly acidic styrole-divinylbenzene ionexchange resins containing an 8% cross linking agent, namely KU 2-8 (Chimiya), Purolite C 100E (Purolite International), Dowex HCR-S and Dowex MSC-1 (Dow Chemical) were investigated. Inert glass particles were also used for the comparison ofelectrodeionization and traditional electrodialysis. 0.5-0.8 mm fractions were taken. Characteristics of the materials such as a total ion-exchange capacity and water content were determined according to known methods [11]. For loading with nickel the ion exchangers (27 cm 3) were treated with a solution (2000 c m 3) containing 5 molm -3 (gel-like ion exchangers) or
Fig. 1. Experimentalset-up consistingof anode reservoir (1), pumps (2), eleetrodialysiscell (3), cathodereservoir (4), thermostat (5), feed reservoir (6), pressure sensors (7), reservoir for pH measurements (8), pH meter (9), waste container(10), power supplier(11), voltmeter(12) ammeter(13).
Yu.S. Dzyazko, KN. Belyakov / Desalination 162 (2004) 179-189
The center compartment was filled with an ion exchanger or glass particles. The effective area of the membranes and electrodes inside the cell was 15 cm 2 (1 cm x15 cm), the bed thickness was 1 cm. Deionized water or NiSO4 solution was pumped through the centre compartment using a NP-1M peristaltic pump according to the "oncethrough" operation. The flow velocity was 6.67,10 -7 m3s -]. The pH of the solution atthe cell outlet was measured using a EV-40 pH-meter. H2SO4 solutions (200 em 3) circulated through the electrode compartments using a Zalimp PP 2-15 peristaltic pump. Circulation velocity was 1.67"10 -5 m 3 s -1. An IPPT 65-49 power supplier was used to keep a constant cell voltage, controlled with a SCH-1312 voltmeter. The current was registered with an SCH-4311 ammeter. Constancy of the temperature in the system (298 K) was provided by means o f a UTU-2/77 thermostatic bath. The pressure drop through the ion-exchanger bed was measured using mercury pressure sensors placed both at the inlet and outlet of the centre compartment,
2. 3. Investigation of Ni(II) transport Fifteen cm a swelling resin or glass particles were placed into the centre compartment. All the experiments were carried out at -10 V. Nickel
181
concentration in the samples was determined with the atomic absorption method. After the finish of the experiments the membranes were withdrawn from the cell and carefully purified from the ion exchanger or glass particles mechanically. Thenthe membranes were treated with a 3 M H 2 S O 4 solution several times. The effluent was analyzed and the nickel amount deposited on the cation-exchange membrane was determined.
3. Results
3.1. Ion exchangers The characteristics of the ion-exchange resins are given in Table 1 where it is shown that the total sorption capacity per volume unit of wet resin is similar for all the materials except the Dowex HCR-S. This ion exchanger is characterized by the largest value of sorption capacity. The amount of water per volume unit of resin decreases within the order: Purolite C 100E > Dowex HCR-S = Dowex MSC-1 > KU 2-8. Partial loading of ionexchangers with Ni(II) caused a decrease in their volume. However, the decrease of water content was not considerable due to the increase in the amount of resin particles in the volume unit. It should be also noted that under approximately similar loading degree, the loss of volume
Table 1 Characteristics o f ion exchangers Ion exchanger
K U 2-8 Dowex HCR- S Purolite C 100 E Dowex MSC- 1
Structure
Gel-like Gel-like Gel-like Maeroporous
Hydrogen form
Ni(II) loaded form
Total ion-exchange capacity, mol m -3
Water content, %
Ni(II) cone,, m o l m -3
Water content, %
Decrease in volume, %
1600 1800 1600 1600
48 53 59 53
390 420 390 370
46 51 58 52
9 6 12 8
Yu.S. Dzyazko, V.N. Belyakov / DesalinatT"on 162 (2004) 179-189
182
for the studied ion exchangers was different. The largest shrinkage was observed for Purolite C 100E, and the minimal volume decrease was obtained for Dowex HCR-S. 3.2. Effect o f anode and cathode compartment HsSO 4 concentration on the Ni(11) transport In the first series of experiments the Ni(II) removal from Dowex MSC-1 resin was investigated. The centre compartment was filled with nickel-loaded ion exchanger. Deionized water was passed through the resin bed. The initial concentration o f acid solution in the electrode compartment was varied from 0.5 to 4 M. In all the cases the initial amounts of H2SO4 in the cathode and anode compartments were equal. The pH of the solution at the centre compartment outlet before energization is given in Fig. 2 as a function of initial acid concentration in the electrode compartments (Ca~,,1). It follows from the figure that an increase in Ca~,,z value caused a shift of pH o f the solution, which was formed in the centre compartment, to the more acidic field. Evidently it is a result o f leakage of acid through the anion-exchange membrane.
Since the investigated ion exchanger was strongly acidic, the Ni(II) amount in the solution at the center compartment outlet (C_~i)was rather low due to slight desorption. The C~i values were 1"10-2, 8"10 -3, 7-10 -3 and 5"10 -3 mol m -3 at effluent pH of 2.1, 2.2, 2.4 and 2.5, respectively. Under applied voltage Ni(II) ions migrated to the cathode compartment while no nickel was found in the anolyte and effluent. The dependencies of nickel content in the catholyte (nl, zi,cat) on time (t) are plotted in Fig. 3. It is seen that the amount of nickel in the catholyte increased with an increase in acid concentration in the electrode compartments. T h e nNi,cat VS. t C u r v e s c a n be fitted with following equation: (1)
nm, eat : a 1 + a 2 e -a3t
where al-a3 are the empirical coefficients. The current density (0 as a function o f time was plotted in Fig. 4 excluding the start-up effect. When the acid concentration in the electrode compartments was 4 M, the current increased with increase of regeneration time. In other cases the current decreased during the experiments. 0.4
4
0,2
o
Z 0.1
' "z= 0.2 ,.e
2 . 0
.
.
.
.
. . 2
4
0,0
0
C:,~.~x 10.3 [molm "3] Fig. 2. T h e e f f l u e n t p H b e f o r e ( o )
0.0 10000
20000
t Is] a n d after ( o )
energization (- 10 B); amount of Ni(II) deposited on the cation-selective membrane (m) as a function of initial H2SO 4 concentration in the electrode compartments. The center compartmentwas filled with Dowex MSC- 1 resin.
Fig. 3. Amount of Ni(II) in the catholyte over time of Dowex MSC-1 resin regeneration. Initial H2SO4 concentration in the eatholyte and anolyte was 0.5 (O), 1 (0), 3 (o), 4 (•) M or 0.5 M in the catholyte and 4 M in the anolyte 07).
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After energization, the pH of the solution formed in the centre compartment increased (Fig. 2) due to transport of hydrogen and sulfate ions to the cathode and anode compartments, respectively. No changes in the pH were found over time of the experiment. The Ni(II) amount deposited on the cationexchange membrane as hydroxide (n~a) vs. acid concentration in the electrode compartments is shown in Fig. 2. It follows from the figure that an increase in C~c,~z and a corresponding decrease of pH of the solution in the centre compartment causes a reduction of nni. At pH 2.2 (Ca~,~z corresponds to 4 M), no nickel deposit on the membrane surface was observed. The next experiment was devoted to investigation of Ni(II) transport through the Dowex MSC-1 resin under different concentrations of acid in the catholyte (0.5 M) and anolyte. In this case the initial H2SO4 concentration was 0.5 (catholyte) and 3 M (anolyte). The HNi°cat-t curve (Fig. 3), effluent pH and amount of deposited Ni(II) were practically identical to those for equal concentrations of the catholyte and anolyte (3 M). This justifies the assumption concerning the acid leakage through the anion-exchange membrane.
The nickel transport through Purolite C 100E, Dowex HCR-S and KU 2-8 was also studied. The initial H2SO4 concentration in the electrode compartments was 3 and 4 M. Other experimental conditions were similar to those described for Dowex MSC-1. The HNi,eat--t curves (Fig. 5) can be fitted with inversely exponential functions Similarly to (1). It can be seen that the transport rate increased with an increase in Cac, et, For all the ion exchangers the effluent pH reached 2.3 (Cac, ez =4 M) and 2.5 (Cac,,z= 3 M) and did not change during the experiments. Over time ofDowex HCR-S regeneration (Cac.e l = 3 M) the current increased, and no Ni(II) deposit on the cation-exchange membrane was found. Alternately, for KU 2-8 and Purolite C 100E regeneration under similar conditions, the current decreased. The n~ values reached 0.3 and 0.1 mmol, respectively. Alternately, under an increase in concentration of the acid in the electrode compartment and a corresponding decrease of the effluent pH, the current increased over time due to the absence of Ni(II) hydroxide deposition.
20
2 E
qT"
,
i 0
0
~
0
.....
10000
~
20000
t Is] Fig. 4. Current density over time ofDowex MSC-1 resin regeneration• H2SO4 concentration in the catholyte and anolyte was 0.5 (il,), 1 (o), 3 (e), 4 ( I ) M.
t 0000
20000
t[s] Fig. 5. Amount of Ni(II) ions in the catholyte over time of electroregeneration of Purolite C 100E (O, tt), KU 2-8
(ooV) and Dowex HCR-S (o) resins. Initial H2SO4 concentration both in the catholyte and anolyte was 3 (,O,,o,e) and 4 (re,V)M.
Yu.S. Dzyazko, V.N. Belyakov/ Desalination 162 (2004) 179-189
184
The pressure drop through the bed was registered to be approximately 4 kPa in all the cases.
3.3. Electrodeionization o f Ni(11)-containmg solution The purification o f the solution containing 1 mol m -3 Ni(II) was investigated using both ion exchangers, which were initially in a hydrogen form, and glass particles; 3 M H2SO4 solutions filled the electrode compartments. The Ni(II) amount in the catholyte and Ni(II) concentration in the solution at the cell outlet (CNi) as functions o f time are represented in Figs. 6 and 7, respectively. When Dowex HCR-S was placed in the centre compartment, the rate o f Ni(II) transport increased during the first 4 h and was stabilized within a 4-8 h interval. Analogous regularity was obtained for CNi. Altemately, the current decreased until 4 h o f the experiment and then remained constant (Fig. 8).
In the case o f KU 2-8, Dowex MSC-1 and Purolite C 100E, the nickel flux through the membrane also increased in the beginning o f the experiment. During 4-8 h, the flux slightly decreased while the nickel concentration in the effluent was stabilized (Dowex MSC-1 and Purolite C 100E) or diminished (KU 2-8). The current was found to decrease slightly over whole time.
1.0
~e, 0.5 r..)
o.o
0
toooo
20000 t Is]
30000
Fig. 7. Ni(II) concentration in the effluent over time of solution purification. The centre compartment was filled with a Purolite C 100E (O), Dowex MSC-1 (m), KU 2-8 (o), Dowex HCR-S (e) and glass particles (V). Initial H~SO4 concentration both in the catholyte and anolyte was 3 M.
2 300
2.6
20o
2,4
100
2.2
)
O~ o
10000
20000 t Is]
30000 0
Fig. 6. Ni(II) amount in the catholyte over time of the solution purification. The centre compartment was filled with Purolite C 100E ($), Dowex MSC-1 (=), KU 2-8 (o), Dowex HCR-S (e) and glass particles (V). Initial H2SO4 concentration both in the catholyte and anolyte was 3 M.
1oooo
20000
30000
"J 2.0
t is]
Fig. 8. Current density (e) and effluent pH (o) over time of solution purification. The centre compartment was filled with Dowex HCR-S resin. Initial H~SO4 concentration both in the catholyte and anolyte was3 M.
Yu.S, Dzyazko, KN. Belyakov / Desalination 162 (2004) 179-189
When the center compartment was filled with glass particles, the nickel transport rate as well as the current decreased gradually with increasing in time. Alternately the nickel concentration in the effluent remained constant. The dependence of the effluent pH vs. t for Dowex HCR-S is shown in Fig. 8. It follows that the acidity of the solution in the centre compartment decreased and was stabilized after 4 h. Similar regularities were obtained for other ion exchangers. In all the cases the effluent pH did not exceed 2.5. In the case of glass particles, the pH was 2.5 and did not change over time. The Ni(II) amount deposited on the cationexchange membrane reached 0.2 (Dowex HCR-S), 1.2 (KU 2-8), 2.16 (Dowex MSC-1), 1.88 (Purolite C 100E) and 3.15 (glass particles) mmol. From these results we conclude that the most suitable ion exchanger for purification of nickel-containing solutions is Dowex HCR-S resin.
185
Here the y coefficient is a ratio of charges of cation and anion. If ions are transported bo__th through the ion exchanger and solution, the NN val_ueincludes the flux through the ion exchanger
(4)
-~Ni= -N~i + (1 +?)NNi~
The flux through the ion exchanger can be represented as a sum of migration, diffusion and convection terms [19]. At the start of the experiment, which corresponds to t= 0, the diffusion term can be omitted [14,15]. According to Grebenyuk et al. [9,10], the convection term can also be neglected due to its ratherlow contribution to the total flux. Thus the Nm value is assumed to be determined only by migration at t=0. Under this condition the Ni(II) flux through the ion exch_anger is connected with a diffusion coefficient (Di) by a combination of the NemstPlanck and Nernst-Einstein equations [ 19]:
4. Discussion
The obtained results allow us to make some quantitative estimation of Ni(II) ion transport. Under the conditions of a forced convection and the absence of the ion exchanger between membranes, the nickel flux through the cationexchange membrane (n~) is determined by migration in the solution bulk and diffusion through the solution layer at the membrane surface. The equation of the material balance can be represented as [17]: (2)
RT
N~i~ = Y N ~ gr
(3)
(5)
The diffusion flux through the solution is described as:
(6) If the current reaches a limiting value or exceeds it, then C'Ni =..0. The g r a d e value, which corresponds to t = 0, can be determined according to the formula [17]: gradE=
Diffusion and migration components of the flux are connected with Eq. (3) [18]:
grad g
Eeatb~ - E ~°'~ - E eett
(7)
The contribution of potential drop through the electrode compartments was neglected due to high electric conductivity of the acid solutions.
186
Yu.S. Dzyazko, KN. Belyakov / Desalination 162 (2004) 179-189
Similarly, the potential drops through the membranes were not taken into account since their thickness was much less than that of the ionexchanger bed. The Eo,thoe~-E~nod~ value determined with a voltametric method was found to be near -1.8 V, Thus the g r a d e value reached -820 V m -I ' The mass transport coefficient can be found from Eq. (8) [17]:
k~it~
1.52
(8)
The Ni(II) flux through the cation-exchange membrane can be determined from the experimentally obtained nNi,o,t- t dependencies as: =
1 dnNi,~t
(9)
The flux values, which were calculated as reference quantities o f the derivatives o f functions 0.1) at t = 0, are plotted in Fig. 9 vs. effluent pH. The N N magnitudes are seen to increase with an increase o f effluent acidity. At pH<2.5 no changes in values o f the initial fluxes were found. The total al=n=ount ofdesorbed nickel determined as/'/Ni, cat + n N reached 0.21, 0.15, 0.32 and 0.35 at the effluent pH o f 3.7, 3.2, 2.5 and 2.3, respectively. A decrease o f the initial Ni(II) flux as well as a decrease o f the amount ofdesorbed Ni(II) with an increase o f the effluent pH and diminishing o f the current over time are evidently caused by partial deposition of nickel hydroxide on the membrane. Moreover, the resistance o f heterogeneous membranes is known to increase due to their polarization [ 17]. These factors can result in a decrease of the potential gradient through the ion-exchanger bed. On the other hand, partial Ni(II) deposition on the cation-exchange membrane can cause a distortion o f the nNi,~,t-t dependence. Earlier the
independence o f the initial Ni(II) flux through the membrane on acid concentration in the catholyte and anolyte was found [14]. This disagreement can be caused by different experimental conditions. It should be noted that in the case o f the Dowex MSC-1 resin a decrease in acid concentration in the catholyte from 3 to 0.5 M at a constant anolyte concentration__does not influence either the effluent pH or N N. It is possible to estimate a Ni(II) flux through the solution. A maximum amount o f nickel in the effluent before energization was found for Dowex MSC-1 at the effluent pH o f 2,1 (it cor-responds to C~c,el = 4 M). Taking into account that the linear flow velocity was 6.7"10 -3 m s -1, the effective diameter o f the particles was 6.5"10 -4 m and also DNi = 6.49"10 -l° m 2 s -1 and u = 8.99.10 -7 m2s -1 at 298 K [20]; the Ni(II) mass transport coefficient was estimated as 3.93.10 -5 m s-1. Since CNi = 1"10-2 mol m -3 at t = 0, the NN~ value, which corresponds to the limiting current condition, reached 3.93-10 -6 mol m 2 s -1. If the current does not reach a limiting value, it is valid for NiSO4:
-3
V
~Y
~-5 -6
2
3 pH
4
Fig. 9. Initial Ni(II) flux through the cation-selective membrane for Purolite C 100E (<)),Dowex MSC-1 (0, o), KU 2-8 (27) and Dowex HCR-S (i) as a function of the effluent pH. Initial H2SO4 concentrations of the catholyte and anolyte are equal (0o~l,,V,m) or unequal (0.5 M in the catholyte and 3 M in the anolyte) (o).
Yu.S. Dzyazko, V.N. Belyakov / Desalination 162 (2004) 179-189
N~i : NNm:g~ _ ZNi F D~i C~i g r a d e RT
(1 O)
It follows from Eq. (10) that tl2e N ~ ~ value is 1.23"10 -6 tool m -2 s-1 at g r a d e = 820 V m -1. The obtained magnitude exceeds the diffusion flux, which corresponds to the limiting current. Thus the Ni(II) transport through the solution was determined by diffusion. As a result, the total nickel flux through the solution caused by diffusion and migration can be defined as 2kNiC~i. The apparent diffusion coefficients of Ni(II) ions in ion exchangers are plotted in Fig. 10. The D~ values obtained under similar conditions are seen to decrease in the following order: Dowex HCR-S > KU 2-8 > Dowex MSC-1 > Purolite C 100E. A quantitative estimation of the solution purification process using ion-exchange resins is rather difficult due to nickel hydroxide deposition, formation of the concentration gradient in the ion-exchanger bed and shrinkage of the resins. Under the electrodialysis using glass particles the Ni(II) amount reached 0.48 mmol in the catholyte and 1.26 mmol on the cationexchange membrane in 8 h. From these results
B
*$e.
0
2
3
4
pH Fig. 10. Ni(II) diffusion coefficient in Purolite C 100E ((>), Dowex MSC-1 ( , ) , KU 2-8 (V), Dowex HCR-S (") resins as a fi,mction of the effluent pH. The initial H2SO4 concentrations inthe electrode compartments were equal.
187
the nickel flux was estimated as 8.40"10 -5 mol m-2s-1. The obtained value corresponds to the NN magnitude calculated for the limiting current condition at C>;i =1 m o l m -3. A small disagreement can be caused by the inaccuracy of estimating the effective particle size on the one hand and a change in a form of transported Ni (II)ions on the other hand. Replacing of glass particles with Dowex HCR-S ion-exchange resin allows us to improve the efficiency of the solution purification and decrease the amount of nickel deposited on the membrane. Under the constancy of the Ni(II) transport rate, the Ns value was estimated as 1.28.10 -5 tool m -2 s -~. Taking into account that the current after stabilization reached 50 mA, the energy consumption, which is needed for removal of 0.35 mol Ni (II) from 1 m 3 solution containing 1 mol NiSO4, was estimated as 208 Wh. Use of other ion exchangers, which are characterized by lower Ni(II) mobility, is ineffective. It is possible to suggest that in the case of Dowex MSC-1 and Purolite C 100E, the nickel is transported practically only through the solution. A decrease in quantity of deposited nickel under replacing of glass particles with ion exchangers can be explained as follows. OHions, which are generated on the membrane surface due to water split, are neutralized by hydrogen ions transported from the ion exchanger. Significant deposit formation in the case of solution purification is caused by a relatively large Ni(II) content in the solution, on the one hand, and over limiting current conditions, on the other hand.
5. Conclusions It was found that an increase in effluent acidity due to leakage of the acid through the anion-selective membrane caused a decrease in the Ni(II) amount deposited on the cationselective membrane. As a result, the nickel
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flux through the cation-exchange membrane increases. The apparent diffusion coefficients o f Ni(II) ions in the ion exchangers containing an 8% cross-linking agent were determined with the electromigration method. The Dra increased with the decrease o f the effluent pH. The values o f Ni(II) diffusion coefficients obtained at pH 2.32.5 reached 1.30.10 -12 (Dowex MSC-1), 1.36. 10 -12 (Purolite C 100E), 2.69.10 -12 (KU 2-8) and 4.07.10 -12 (Dowex HCR-S) m 2 s <. This is the same order o f magnitude as self-diffusion coefflcients o f divalent cations through an ionexchange resin o f the same type [21]. The largest value o f nickel diffusion coefficient was found for the Dowex HCR-S, which contains the maximum quantity o f functional groups per volume unit. All the investigated ion exchangers were tested in the process o f Ni(II) removal from diluted solution. The Dowex HCR-S resin was found to be the most suitable material for use in electrodeionization processes. The current efficiency under a constant rate o f nickel transport to the catholyte reached 14%. The energy consumption, which is needed for removal o f 0.35 mol Ni(II) from 1 m 3 solution containing 1 mol NiSO4, was estimated as 208 W h. Thus the mobility of sorbed ions determines the efficiency o f the purification process. Further development o f a technology for nickel removal from diluted solutions using Dowex HCR-S resin is possible and connected with optimization o f the electrodeionization process, 6.
--
C~r~ CNi
--
--
S
- - Membrane (electrode) effective area,
Concentration of Ni(II) in the solution at the membrane surface, mol m -3 DN~ - - Ni(II) diffusion coefficient in the solution, m 2 s-: /~ - - Ni(II) diffusion coefficient in the ion exchanger, m 2 s -1 - Effective particle size, m E - - P o t e n t i a l drop through the ionexchanger bed, V Eceu - - Applied cell voltage ,V Eca~o~ - - Cathode potential, V E~o~ - - Anode potential, V F - Faraday constant, A s mol-: - - Current density, A m -2 i - - Mass transport coefficient o f Ni(II), kNi m s-: 1" - - Ion-exchanger bed thickness, m NN - Ni(II) flux through the membrane, mol m -2 s -1 - - N i ( I I ) flux through the ion-ex/¢Ni changer, mol m -2 s-1 NNi ~ - Ni(II) flux through the solution due to diffusion, mol m -2 s -1 NN~gr - - Ni(II) flux through the solution due to migration, mol m -z s -1 /'/Ni, cat - - Amount o f Ni(II) in the catholyte, tool nN - - Amount of Ni(II) deposited on the membrane, mol R - Gas constant, J mo1-1 K-:, V C mol-: K-1 m2
t T ZNii
S y m b o l s
Cao., l
c 'r~i
Concentration of acid in the electrode compartments, m o l m -3 Concentration of Ni(II) in the ion exchanger, mol m -3 wet resin Concentration o f ionic species in the solution bulk, mol m -3
- - Time, s - - Temperature, K - - Charge o f Ni(II) ion
Greek Y V
- - Ratio o f charged modules o f cations and anions - - Viscosity o f the solution, m 2 s -1 - - Linear flow velocity, m s-:
Yu.S. Dzyazko, KN. Belyakov / Desalination 162 (2004) 179-189
Acknowledgements This work has been supported financially by the N A T O SfP 972490 programme entitled "Removal o f Nickel and Copper from Dilute Solutions b y Ion-Exchange Assisted Electrodialysis". The authors wish to express their gratitude to Dr. L.J.J. Janssen, Dr. P.B. Spoor (Technical University o f Eindhoven, the Netherlands) and Dr. L. Koene (TNO Institute o f Industrial Technology, Den Helder, the Netherlands) for fruitful discussions o f the results.
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