ELSEVIER Desalination 162 (2004) 249-254

DF~ALINATION

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Influence of water hardness on removal of copper ions by ion-exchange-assisted electrodialysis

S.L. Vasilyuk*, T.V. Maltseva, V.N. Belyakov Vernadskii Institute of General and Inorganic Chemistry, National Academy of Sciences of Ukraine,

Palladina Avenue 32/34, 03142 Kiev, Ukraine email: membrane@lone, kar.net

Received 24 September 2003; accepted 15 November 2003

Abstract

The organic ion exchangers (Dowex HCR-S, Dowex 50WX-2) and inorganic ion exchangers with phosphate functional groups (zirconium phosphates ZrP-1, ZrP-2) were studied as transport media for ion-exchange-assisted electrodialysis of diluted solutions containing Cu (II) ions. The ion-exchange and electric conductive properties of ion exchangers containing Cu(II) ions were studied. The most conductive exchangers were chosen. The adsorption data showed a high selectivity of the zirconium phosphate towards copper ions in a concentration range from 0 to 0.5 mmol. Both exchangers were tested in ion-exchange-assisted electrodialysis of a 10 -z M solution, containing copper and hardness ions in a ratio of 1:9. It was found that efficiency of removal of the Cu(II) ions was better for the inorganic ion exchanger.

Keywords: Ion-exchange-assisted electodialysis; Zirconium phosphate; Copper; Hardness; Selectivity

1. Introduct ion

The removal of toxic multivalent ions from dilute water solutions can be successfully realized by the ion-exchange-assisted electrodialysis pro- cess, which is a combination of ion exchange with electrodialysis [1,2]. An ion exchanger is placed between membranes to concentrate the

* Corresponding author.

toxic ions. Adsorbed ions are able to move into electrode compartments in the bed of the ion exchanger under the applied potential (Fig. 1).

This method allows an increase in the effi- ciency of the electrodialysis process and to regenerate an ion exchanger without chemicals. The transport properties of an ion exchanger are an important point for the efficiency of the process. An organic [3] and inorganic exchangers

Presented at the PERMEA 2003, Membrane Science and Technology Conference of Visegrad Countries (Czech Republic, Hungary, Poland and Slovakia), September 7-11, 2003, Tatranskd Matliare, Slovakia.

0011-9164/04/$- See front matter © 2004 Elsevier B.V. All rights reserved PII: S0011-9164(04)00048-7

250 S.L. Vasilyuk et al. / Desalination 162 (2004) 249-254

5

"~ g ?

7 ~":

n

/ ,

Fig. 1. Electrodialysis cell. 1 central compartment; 2,3 cathode and anode compartments; 4 feed reservoir; 5 pump; 6,7 cathode and anode reservoirs; 8 membrane.

of the zirconium phosphate type were tested successfully in the process of copper removal [4]. The purpose of this study was to study the influence of the hardness ions (Ca 2+ and Mg z+) on the adsorption ability of some commercial organic (Dowex HCR-S, Dowex 50WX-2) and synthesised inorganic (amorphous zirconium phosphates (ZrP-1, ZrP-2) cation exchangers towards Cu(II) ions and measure a mobility of Cu(II), Ca 2+ and Mg 2+ ions in the bed of an ion exchanger. The influence of hardness ions on efficiency of metal removal was also investigated. It was very important for removal of toxic ions from real waste waters.

2. Theory

Migration of ions through a homogenous bed of an ion exchanger in the i-form can be de- scribed by the Nemst-Planck relationship [3]:

(1) Ni = zi Xi ui gradd~ + v Ci

where N~ is the flow of i-ions into the cathode compartment under the effect of the potential gradient. The first term on the right-hand side describes the migration of/- ion with concert-

tration Xi in the bed of the ion exchanger, charge number zi and mobility u~. The second term includes the volume of pore v and concentration of solution C~. This term accounts for the move- ment of ions within the ion-exchange particles. The apparent mobility of an ion, u~(app), can be obtained from the amount of the ions which is transported in the cathode compartment during a given time.

In general, the mobility of an ion in the bed of the particles, u~, is defined bythe Nemst-Einstein equation:

u ~ : (D ~)/(z~RT) (2)

This equation links the diffusion coefficient D~ and the mobility of ions. The migration of ions under a potential gradient is therefore directly related to the diffusion coefficient of an ion inside the particles of the ion exchanger. The degree of association of ions with the surface groups and the extent of hydration of ions are important factors that influence the diffusion coefficient.

3. Experimental

3.1. Synthesis and characterisation of ion- exchange materials

3.1.1. Synthesis

Granulated inorganic cation-exchangers, namely zirconium phosphate (ZrP-1) and zirco- nium phosphate (ZrP-2) were synthesised using the sol-gel technique. The Zr:P ratio was 1.5:1.0 for ZrP-1 and 1.0:1.0 for ZrP-2. The dry ion- exchange materials were sieved, and particles with a size of 0.2-0.8 mm were chosen. The packed density of the dry ion exchangers ZrP-1 and ZrP-2 is 0.7 g*cm -3 and 1.1 g*cm -3, respectively.

3.1.2. Exchange capacity towards Cu(II) ions

To determine the cation-exchange capacity, 2 em 3 of the ion exchanger was equilibrated in

S.L. Vasilyuk et al. / Desalination I62 (2004) 249-254 251

100 cm 3 0.25 M CuSO4 solution during 24 h. The solution was filtered and the ion exchanger was washed with a large amount of water and dried at 343 K. The dry ion exchanger was treated with 20 cm 3 1.75 M sulphuric acid during 24 h to remove Cu(II) ions from the ion exchanger. The eluent was analyzed using atomic absorption spectroscopy, and the adsorption value was calculated.

3.1.3. Specific conductivity

The ion exchanger saturated with cations according to the above-mentioned procedure was washed carefully with a large amount of deio- nized water. It was placed into a cubic con- ductivity cell with two platinum electrodes. The resistance of the ion-exchange bed was measured using altering current. The Autolab system and Frequency Response Analyzer program were used. The frequency range applied was 1-50 kHz.

3.2. Adsorption from multicomponent solutions

The content of the calcium, magnesium and copper ions in a prepared water solution is given in Table 1. The volume of solution was 100 c m 3,

the weight of the ion exchanger was 2.0 g. The ion exchanger was equilibrated with a solution for 24 h, and initial and equilibrium solutions were analyzed using atomic absorption spectros- copy.

Table 1 Chemical composition of the solutions

3. 3. Ion-exchange-assisted etectrodialysis

3.3.1. Experimental set-up

The experimental set-up described in Spoor et al. [2] was used. The cell consisted of a three- compartment perspex cell divided by Nation- 117 membranes. Outer compartments contained plati- num electrodes; the ion-exchanger bed was closed in the central compartment. The electrolyte circuits, with a volume of approximately 100 cm 3 each, were connected to the electrode compart- ments. H2SO4 (0.05 M) was pumped into these circuits by a SP311 peristaltic pump. The model solution was pumped from a reservoir through the central compartment. The effective area of the membrane and electrode inside the cell is 14.4 cm 2 (3.8 cmx3.8 cm) while the volume of the anode, central and cathode compartments was 9.3 cm 3.

3.3.2. Experimental details

The system was kept at a constant tempera- ture, 298 K, using a thermostatic bath. The flow rate through the central compartment was kept constant (0.09 cm3*s-1). The cell current was kept constant by a B5-49 power supply (10 mA). The cell voltage was measured by a B7-34A voltmeter.

3. 3.3. Model solution

Deionized water was used to prepare the solutions. The model solution contained Cu(II) and hardness ions. The solution composition was: CuSO4:1'10 -3 mol*dm-3; CaC12:4"10 -3 tool* din-3; MgSO4: 5* 10 -3 tool*din -3.

Solution no. Content of ionic component, mg*dm -3

Ca > Mg > Cu >

1 120.8 101.6 51.3 2 60.4 50.8 51.3 3 90.6 76.2 51.3 4 30.2 25.4 51.3 5 15.1 12.7 51.3

4. Results

4. l. Characterization of ion-exchange materials

The maximal values of the static capacity 7 for Cu(II) ions are presented in Table 2. All ion exchangers under investigation possess an accept- able capacity towards Cu(II) ions. Specific

252 S.L. Vasilyuk et aL /Desalination 162 (2004) 249-254

Table 2 Properties of the ion exchangers

Ion exchanger Structure of the surface anion; degree of cross-linking, % (for organic only)

Exchange capacity towards Cu(II) ions, X, mol*m -~

Specific conductivity of Cu (II)-loaded form, K, mS*m -1

Dowex HCR-S [R-SO3]-.; 8 Dowex 50WX-2 [R-SO3]-n; 2

ZrP-1 [ZrO412-. [Zr(PO4):]2-.

ZrP-2 [Zr(PO4)212- n

530 295

375

420

42 260

25

243

conductivity differs for all exchangers. It can be seen that the degree of cross-linking of the organic ion exchanger by divinylbenzene was mainly influenced by the value of the specific conductivity of the ion exchanger: it is low for the Dowex HCR-S with 8% of the cross-linking and high for the Dowex 50WX-2 with 2% of the cross-linking. The inorganic ion exchanger ZrP-2 possessed higher specific conductivity in com- parison with the ZrP-1. This can be explained by the fact that the ZrP-1 cation exchanger as oxi- phosphate contains a large amount of the func- tional groups another type (hydroxyl) that are characterised by low mobility of protons. That is why the proton conductivity of the ZrP-1 sample is low. The most conductive exchangers, namely, Dowex 50WX-2 and ZrP-2, were chosen for further investigation.

4. 2. Adsorption from multicomponent solutions

The results are presented in Figs. 2-4. It can be seen from the curves in Fig. 2 that the addition of the hardness ions strongly influenced the adsorption of the Cu(II) ions. Both types of cation exchanger had decreased exchange capacity towards the copper ions while the concentration of the hardness ions increased. The organic ex- changer possessed a greater exchange capacity in comparison with the inorganic exchanger in the presence of hardness ions. As we can see from Fig. 3, which represents simultaneous adsorption

0.05:

0.04

0103

~, 0.02

,~: o.o!

O.OO , , , o 2: ~ :~ :8 lO

c <Ca~+ • M ~2~,~t j!m~o!:.d~ -~

Fig. 2. Adsorption of Cu(II) on the Dowex 50WX-2 (1) and ZrP-2 (2) as a function of the initial summary concentration of the hardness ions (initial solution concentration of the Cu(II) ions is 0.8 retool*din-3).

0.13

o:1o

0:0511

0.00

• 1 ̧

~ 7 1 ! ~i ̧

7 ~ : L: '

I 2, 3 4

Fig. 3. Adsorption of the Ca :+ (1, 3) and Mg 2+ (2, 4) ions on the Dowex 50WX-2 (1, 2) and ZrP-2 (3, 4) as a function of the equilibrium solution concentration of each of the hardness ions (initial solution concentration of the Cu(II) ions is 0.8 mmol*dm-3).

S.L. Vasilyuk et al. / Desalinmion 162 (2004) 249-254 253

100

80 ̧

4 0

Zo:

O 0

0

2

1

2: 4 6 8 io

C (:ca ~+ ÷ Mg~+)~r/r~not,am "3

Fig. 4. Surface fraction of the Cu(II) ions adsorbed on the Dowex 50WX-2 (1) and ZrP-2 (2) as a function of the initial solution concentration of the hardness ions (initial solution concentration of the Cu(II) ions is 0.8 mmol* din-3).

of the hardness ions, zirconium phosphate adsorb ones too much low then organic ion-exchanger. This can be explained by the fact that zirconium phosphate possesses a high selectivity towards Cu(II) ions, especially at low concentrations of the solution. If we express the part of adsorbed copper in general ion-exchange capacity as a function of hardness ions concentration (Fig. 4), we can see that zirconium phosphate is more selective adsorbent towards ions Cu(II) in diluted solutions. Copper ions, which are adsorbed on the zirconium phosphate, occupied a main part of the surface of the cation exchanger.

O. 10

0.08 ~ 2 3 +i 0.06

o.o4

O. 02

1

O.O0 " ' @ ' ~ , I • *

1:0 11 112 t3 14 15

t : l i

Fig. 5. Amount of Me 2+ in the catholyte as a function of electrodialysis time. 1 Cu2+; 2 Ca>; 3 Mg2+; ion exchanger: Dowex 50WX-2.

0,10

&06 +L . . . . 2

0:04

0 .02 o ~

0.00 "~ : • I • ~ ~ :11 12 i 3 :: i4 1:5

t : h

Fig. 6. Amount of Me > in the catholyte as a function of electrodialysis time. 1 Cu2+; 2 Ca>; 3 Mg>; ion exchanger: zirconium phosphate ZrP-2.

4. 3. Ion-exchange-assisted electrodialysis

It was found that during the first 3-4 h after the beginning of the experiment, the amount of ions in the cathode compamnent was near zero. After an "accumulation" period the copper, cal- cium and magnesium ions appeared in the cathode compartment. The data are presented in Figs. 5 and 6.

The dependencies of the amount of ions in the cathode compartment on time are linear during the first few hours after appearance. The apparent

diffusion coefficients (Dapp) were calculated for all ions under investigation. The comparison of the data obtained for organic and inorganic ion exchangers shows that apparent diffusion coeffi- cients differ from each other by about a factor of 3-20. The row of the diffusion coefficients for ions seems to be the same with the adsorption and hydration rows. The degree of interaction of ions with the surface groups and the extent of hydra- tion of ions are important factors that influence the diffusion coefficient (Table 3).

254 S.L. Vasilyuk et al. / Desalination 162 (2004) 249-254

Table 3 Apparent diffusion coefficients

Ion exchanger Diffusion coefficient, D,pp, m 2. s-1

Cu(II) Ca 2+ Mg 2÷

Dowex50WX-2 0.21x10 -n 0.13x10 -11 0.24x10 -H ZrP-2 0.01xl0 -11 0.03x10 -H 0.08x10 -1I

from solutions with a concentration less than 10 mmol/l, i f hardness ions are present. The inorganic ion exchanger possesses high selecti- vity towards copper ions in dilute solutions with hardness ions.

The degree of copper removal from a model solution copper from an inorganic ion exchanger is 10 times higher than with an organic ion exchanger at similar parameters of removal.

Table 4 Degree of removal of each cation after 8-10 h

Ion exchanger Degree of removal, %

Ca 2+ Mg 2+ Cu ~-+

Dowex 50WX-2 30.5 20.2 2.9 ZrP-2 37.9 35.3 21.3

It can be suggested that the apparent diffusion coefficients obtained can be rich if a higher value of the cell voltage and gradient of electrical potential is applied.

The degree of removal o f the cations under investigated is near constant after "accumulation" period (about 8 h). Table 4 gives the degree o f purification of the model solution after a period of the time, which can be related to "accumu- lation" o f the ions inside the particles of the ion exchanger. The data show that at a flow velocity 0.09 cm3*s -1 and cell current o f 10 mA, the extent o f the purification towards Cu(II) ions is much higher for zirconium phosphate. This can be explained by the high selectivity of the inorganic ion exchanger.

5. C o n c l u s i o n s

Organic as well as inorganic ion exchangers can be used in purification processes of this type

6. S y m b o l s

A

Dap p

F/k

t m X n

/¢

Adsorption of the metal cation, mmol*g -1 Apparent diffusion coefficient for Cu 2+ ions in the bed of an ion exchanger, m 2 s -~ Amount of the metal ions in the catholyte, mol Time, h Metal cation concentration in the bed, mol*m -3 Specific electrical conductivity, m S , m -1

R e f e r e n c e s

[1 ] P.B. Spoor, W.R. ter VeenandL.J.J. Janssen, J. App. Electrochem., 5 (2001) 523-530.

[2] P.B. Spoor, W.R. Ter Veen and L.J.J. Janssen, J. App. Eleclrochem., 10 (2001) 1071-1077.

[3] M. Mahmoud, L. Muhr and F. Lapicque, Investi- gation on copper cation removal from dilute solutions by ion-exchange assisted electrodialysis, 53 ra Meeting of the ISE and GDCh-Fachgr. Angew. Electrochemie, Dusseldorf, Germany, 2002.

[4] S. Vasiluk, T. Maltseva, V. Belyakov and F. Lapicque, Ion-exchange assisted electrodialysis for non-ferrous metal removal with zirconium phosphate usage, 53 ra Meeting of the ISE and GDCh-Fachgr. Angew. Electrochemie, Dusseldorf, Germany, 2002.