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Investigation of inorganic ion exchange membranes for electrodialysis
Desuiinatkwa - Ek.evier Publishing Company, Amsterdam - Printed in The Netherlands
ZNVESTIGATfON
OF
INORGANIC
ION
EXCHANGE
MEMBRANES
FOR ELECTRODIALYSIS
SUMMARY
The mixed oxides of Groups III, fV and V were investigated as selective ion exchangers for incorporation into ekctrodiatysis membranes. Aluminum vanadate precipitated from acid solution demonstrated reversibfe cation exchange properties and, if precipitated from a basic solution, it demonstrated reversible anion exchange properties. Rigid membranes formed by pressing and sintering techniques had high cation and anion selective properties and moderate resistivities_ Flexible meInbranes formed from aluminum vanadate exchangers and organic binders demonstrated only cation selectivity and higher resistivities than pure inorganic membranes. 1.o
INTRODUCTION
The objective of this research program was to investigate the basic properties of inorganic ion exchangers and, in particuiar, to study their fabrication into membrane form for use in efectrodiaiysis (I). Electrodialysis plants presently use organic ion exchange membranes that are subject to degradation at etevated temperatures, fouling, and high cost. Inorganic materials, due to their comparative thermal stability, lack of carbonaceous ingredients, and low cost, could presumabiy overcome these objections if fabricated into suitable ion exchange membranes. Inorganic membranes suitable for use in fuel cells were investigated in this laboratory for over two years (2). Inorganic ion exchange membranes were also prepared by this and other laboratories (3, 4). Inorganic ion exchangers (ahtminosiiicates) were originally discovered in certain soils containing clay minerals, and the first synthetic ion exchangers were also inorganic-the synthetic zeofites or permutits (5). The subsequent develop ment of the organic exchanger resins, with emphasis on practical applications, compfetely dominated the fielld of ion exchange chemistry, and it was not until recently that there has again been research on inorganic cation exchangers. The heteropolyacid salts, such as ammonium phosphomolybdate, aad &he acid salts
370
H. Ii. BlSHOP et al.
of hydrous oxides, such as zirconium phosphate, have proven to be particularly interestins (5). Much less attention has been given to inorganic anion exchangers. Hydrous oxides, particularly the quardrivalent metal oxides, such as ZrO,, SnO,, ThO,. and TiO,, have been investigated (6, 7). Hydrous cxides in which a second cation of higher valence than the parent cation is introduced into the structure with the resulting net positive charge being balanced by the presence of exchangeable anions have been prepared (8, 9). These hydrous oxides have been pictures (6) as positively charged inorganic polymers formed on the acid side of the iso-electtic point. Anion exchange capacity then becomes a function of the pH of the solution in which exchange takes place. The more acidic the solution, the greater the positive charge on the exchanger matrix due to absorbed hydrogec ions. 2.0
INORGANIC
ION EXCHANGERS
OF MIXED
METAL
OXIDES--THEORY
The effort to develop inorganic ion exchange solids suitable for membrane fabrication was directed toward mixed heavy metai oxides from Groups III, IV, Y and VI of the Periodic Table. The mixed oxides form solids of nonstoichiometric proportions, are characterized by a variety of crystal structures, and form complex ions with many anions. Aluminum, titanium, zirconium and hafnium vanadates, tungstates and molybdates and other mixed oxides of these groups were examined. These solid crystalline materials of Group IIC and Group IV mixed oxides will make cation exchangers of the zeolite type, where the cation exchange capacity is due fo the negative charge on the insoluble crysraliite. 0 1
‘7Al-O-S&-O1 0 [
0
-
t
M+ I 0
1
The negative charge is balanced by an exchangeable cation. The mixed oxides of Groups V and VI with Group III, or Group IV with Groups V and VI, should be anion exchange solids by the same principle in reverse. A positive charge now is distributed throughout the mixed oxide unit, which must be balanced by an anion. 0
0
I
I
l 0
I 0
--0--T&0-V--0 [
1 +
A-
Insoluble mixed oxides of aiuminum, titanium and zirconium are interesting Desalination, 6 (1%9) 369-380
ION E?CC?fANGER!3
IN ELECTftODfALYSlS
371
MEMBRANES
because of their ability to expand their vatenceshelt to form coordination complexes with negative ions. The insolubilizin~ atom is a Group V metal, such as vanadium, which serves to attach the compfexing atom through an oxygen link. In anion exchangeable complex ions on afuminur.!, the aluminum atom coordinates in an octahedral configuration and can absorb as many as six anions per aluminum atom. The vanadium ion is either tetrahedraily or octahe&aUy hybridized in its compounds depending upon the complexing agent and the conditions. In our investigation of unbalanced charge, non-stoichiometric crystals, tetrahedrally hybridized mixed oxides were prepared, and in the investigation of metal complexes as anion exchange solids octahedralty hyb~dized vanadium was prepared. Table I iIIu,:rates the formulations of vanadium over pH range I-14. TABLE
I
CONSI~T~.~ION
-
OF VANADtC
IONS IN SOLUTION
_--
PH
Number of vanadium atoms in iun Constitution of ion sodium satt by ~ystallization Color --B;trrf on data of
hNDER
14
12-10.6
S-9
7-6.8
I&-
&7,+
4 (W~VKh3l”-
LV&W-
NajV01
NarVzOt None
NOlK2
AND
JAHR.
i?%&C.
NaJkhVsOtd NORC
Na#bVsOls] Brown or red
6.&2.2 Pr?Xip. of (VzO&
2.2-I &OP’ Paie yellow
21t. (1933)49.
3.0 ALUlbtEWM VANADATE Initial results from this mixed oxide were the most encouraging, so this system was examined in some detail. A detailed description of other mixed oxides is given in Ref. (I).
3, I. 1. Genetal Roth vanadyt chloride VOC& and vanadium pentoxide were used in this preparation. Vanadyl chloride hydrolyzes in water, but is soluble in dilute acid forming the vanadyi ion which is VO+ 2. Vanadium pentoxide is soluble in concentrated acids at 25°C to the extent of 34%. Upon standing, the acid solution of V,O, is reduced at 25°C partiaI1y to VO**. The following reaction takes place in acids VOCI, + 2H” I* VOi2 + 2HCl V,O, + 2H+ -+ 2VOL+’ f H,O Desa&iMrion, 6 (t9699) 369-380
372
H. K. BISHOP c?l d.
and upon heating V,O,
-I- 6H+ -I- 2Cl- -. 2VO+2 -I- 3H,O f Cl,
The proposed structure of poly pentavanadic OH
HO \
OH
OH
I I v---u--v--o-v--o-v--o---v 0
acid is shown befow.
0
OH
I
0
/
0
OH
Polyvanadic acid apparently has three readily replaceable hydrogen ions that are thought to be those in the middle of the molecule. The four terminal hydrogen atoms are nondissociable or of low dissociation constant. 3.1.2 Preparation Aluminum chloride hexahydratc, AICIJ - BH,O, was dissolved in water to make a solution of 0.282 molar in Al’j_ A solution of V,O, in 6M HCI, made up to 0.167 molar in V,05, was added. Complete precipitation would yield a 1: 1 mixed oxide of (N203) (V,O,). The precipitation of the (Al,O,), (V,O& was effected by adding 1 A4 KOH solution to the mixture until no more precipitate was obtained. Complete precipitation occurred at pH 6 and the colloidal solid was a dark green color. The solid precipitate was filtered, washed and then dried at 90°C. 3.1.3 Ion exchange capacity An approximation of the H+ capacity of this mixed oxide solid was obtained by eluting the washed and dried sample with 0.01 M HCi solution on an ion exchange column with an area of 1 cm 2. A sotubility determination had previously shown that the solubility was negligible. The pH of the etliuent acid was about 4 and the highest meq Hi/ml was 0.02 x 10e2. It is therefore seen that the CM+ in the final effluent can be neglected and that the H+ capacity is then the total volume of 0.01 M acid put through. This figure is 370.7 ml giving 370.7 x 0.01 = 3.71 meq of H* absorbed by the solid, or a capacity of 4.20 meqjg. 3.2 Aluminum
vanaa’kte 3.2.1 General
Base form
Aluminum nitrate is dissolved in water and base added until a precipitate forms and then redissolves. AI@JO,)J + Hz0 --+ AlW + 3NO,- + 2H* AtO* -f- OH- + Hz0 + Al(OH), (precipitate gelatinous) AI( + OH- + [AI(OH AI( -t- 30H- 3 [Al(OH)J3 Desalinatiott,6 (l%Q) 369-380
ION EXCHANGERS
IN ELECXRODIALYSIS
373
MEMBRANE3
The singly and triply charged complex aluminates are water soluble and exist as such in solutions containing hydroxyl ion. In basic solution, the vanadium also forms a series of complex ions :,nd heteropoly acid which are not so well chara#:terized as the alum-num complexes. The type of vanadate preser:t in basic solution depends to a large extent upon -,he pH of the solution (see Table I). V,Os + 60H-
--, 2[V0.,]-3
+ 3H20
The precipitation occurs at pH 9-7, therefore, the structures of the vanadium ions are probably [V,O,,]-* and p,O,,] -‘. The structure of the [v5016]-3 ion is shown as follows: o-
HO \
[ HO
/I
1 v-o---v-o-v-~v-o--v
0
I
0
o-
o-
I
I
I
t
OH
-3
/ .
0
0
I\
0
OH I
and the aluminate probably combines with this in some manner. 3.2.2 Preparation A 0.01 M solution of aluminum and vanadium (1: 1 was slowly precipitated with 1 M HCl to a final pH of 8 where no more precipitation occurred. The yellow solid was filtered, washed and dried at 90°C. The yield was 96%. 3.2.3 Colutnn e&ion of basic ahnrinum vanadate A sample of aluminum vanadate precipitated from basic solution was washed in a column with 0.01 M KOH to determine hydroxide ion pickup. After four passes, the yellow solid became colorless. Hydroxide ions were absorbed equivalent to a capacity of 8.91 meq/g. The unusually high capacity for hydroxide ion is explained by the formation of a complex aluminum ion coordinating with 6 hydroxyl ions. 4.0
FABRICATION
OF ION EXCHANGE MEMBRANES
4.1 Pressing and sintering The dried and powdered ion exchanger in the appropriate chemical form was generafly screened and then loaded into a die. Filling the die with powder appeared to be critical in reducing crack formation on sintering. A ~nifo~ fiU is desirable. It was found that careful drying was advisable both before and after pressing in order to obtain a good Sll and to prevent warping and cracking when sintered. Finer particle size (200 mesh and smaller) also produced better fills. The die filling is illustrated in Fig.1. Desalination,
6 (1%9) 369-380
Pressing was done in a 100 ton ptcss. After crack-free presd membranes were &t&cd, they were set upn ceramic tile and sin&red (F&2). Powdered atuncfum was used as hhricant for the pressed niembrane to gmvmt adhesion during sinfting and a two-in& square ceramic tiltswsis $ked an top af the memc brane. -i-be ~$2 cansistcnt results WCTCobtained by using a heat up the of three ta four hours and a sintering temperature of 600’ to looO”C for approximately faur hnars. Snitiaffy, membraries were bezited to MIT; however, it was found that siatcring at ftX.WC produced a strong membrane 4th equal ian seletiitity, The mcmbraney were then annct;rlcdin the furnace cFvernight although theannealing cycle was nut pos;itively necessary. The final sinfcrcd membrane shrunk 20 ta 30% and deveIapcd considerabfe strength.
375
fig_ Z 4 x 4 518 prtsxd cation (bottom)_
(Kynar) organic ux),ooo
and sintercd aluminam
vanadatc
membranes,
anio;l (top) and
binder. The membranes were prepared by casting mixtures of the inmaterial witb a dimethyl formamide (D&IF) solution of Kynar (MW to 60&m). The proportions were such as to make a final membrane
composition of 75:25 and 85: 15 inorganic ion exchanger to Kynar binder. ,Membranes were cured by dry heat or steam at 80 to lOO”C. Flexibility wasgood(Fig. 3). 5.0
TESTING OF 10X EXCHANGE MEMBRANES
5_1 Ion sefecrivity
Laboratory Transport Measurements ‘- Measurements of transport number was used as an indication of the value of an ion selective membrane in desalting of water. Transport numbers of these inorganic membranes were determined using the Hittorf method, The transport eeli consisted of two I5 mf compartments with a membrane mounted between and fitted with a silver-silver chloride electrode inserted horizontally in the ends. An exploded view of the transport cell is shown in Fig. 4. After assembly, current is passed through the cell and electrolyte at a Desahafion.
6 (I %9) 369-380
376
’
Fig_ 3. la~gc sheet of fkxibk
inorg&ii
ion a&angc
membrane 24’
H. K. BISHOP i?t id.
x
0.010’.
constant level of 2 milliamperes for 3 hours. Concentration changes in the anode and cathode chambers were measured. The transport numbers were cakulated from these v&es and the quantity-of electricity passedThe cation and anion transport numbers of various membranes in 0.t M KCI, 0.05 M KCI. and 0.1 M NaCl solution were measured and are tabufated in Table III Aluminum vanadate precipitated from acid was found to be a good cation selective membrane with K+ and Na+ transport numbers greater than 0.8. When mixed with 036-11 (20°& ceramic mixture and sintered at 1000°C. a membrane barrier was obtained which was almost perfectly cation selective, with a KS ion transport number of 0.98. Efficient anion exchange membranes were also obtained from aluminum vanadate precipitated from basic solution. Anion transport numbers of 0.80 to 0.94 were found. Membranes containing the 036-I I ceramic mixtures and sintered at 1ooO”C were mechanically strong and readily handled. Titanium vanadate, zirconium vanadate, and pressed zirconium phosphate were found to be somewhat cation selective with a Na’ and K+ transport number greater than 0.60. The transport number of Na” in 0.1 M NaCf is 0.38 and of Kc in 0.1 M KCL is 0.49. The transport numbers of the cast films are generally characteristic of the Desalitmtion,6 (1969) 369-380
10N EXCHANGERS IX ELECTRODIALYSIS
MEMBRAI*‘ES
377
Fig 4, DisasscmbW transport c&.
TABLE It
Aluminum wnadate (Acid) H- form Aluminum vanadatc (Acid) Na- form Aluminum vanadate (Acid) K- form Aluminum vanadatc (Acid) 100 Aluminum vanad3te (Acid)/03611 80/n, Aluminum vanadate (Acid)/O~ I I St&Q5
Cation tramp. no.
Anion transport no.
0.1 Af NaCI
0.84
0.16
I.820
0.1 M NaCl
0.67
0.33
O-05I
350
0.1 M NaCI
0.65
0.35
0.056
710
0.84
0.16
0.064
280
0.98
0.02
0.075
I30
0.05 M KC1 0. t hf KC1 0.05 AI KCI 0.1 M KCI 0.05 M KCI 0.1 MKCI
0.99
0.01
Technique of formation
Thickness, Membrane Uecrrolyre* resistonce cow. cm ohm-cm
Pressed&
0.05 1
940
0.05I
sioteled PlW+iCd& siotencd Prcs!zd& SilMCd
Ptxswd & sintered PlXW!d&. sinkred PlYSS&l& sin1eRd
L)csoliMtion.
6 (1969) 369-380
H. K. BEWOP
378
et a!.
. TABLE If
(continuedj
Membrutte composition
Techttique of fdfmation
Aluminum w&date (Acid)/0361 1 W/20 Aluminum vana&te fAcid)j03611 SO/20 Aluminum vanadate (Acid),/0361 1 SOQO Aluminum vanadate (Acid).!0361 t SS,llS Aluminum vanadate (Acid)!Kynar 85! 15 Aluminum vanadate
sintered Pressed& sinkred Pressed& sintered Cast & heated cast & heated
(Acid),‘0361
sintered
l 80120
Aluminum vanadate @a=) Aluminum vanadate
* I&al
concentrakm
Resscd&
Pressed& Pressed&
sintered Presscd& sintered P_xssed & sintered Pressed & sintcred Pressed& sintered Pressed& sintenxi Pressed& sintered Pressed & sintered Pressed& sinlered Pressed &sintered Pressed&
Thickness, Membrane Eiectro&te+ “m resistamx?~colzc* ohm-cm
I --.
0.075
266
0.075
295
.0.023
31,ooo
0.1 0.05 0.1 0.1
0.023
3t,600
0.1
o-075
0.064
‘IG
280
Anion tranrgori~
no.
no.
0.99
0.01
0.80
0.20
M KC1 M KC1 M KCI M NaCl
0.83
0.17
0.83
0.17
MNaCt
0.9’
O-08
0.051
l.ulo
O-OSM KC1 0-t MKCl 0.1 M NaCl
0.05 1
5300
0.t
M NaCl
0.19
0.81
0.98
0.02
0.20
0.k
0.05 t
20
0.1
M Naei
0.06
.0.94
0.05 1
.350
0.f
M NaCl
0.26
0.74
0.084
593
0.01
0.99
0.082
51
0.05
0.95
0.075
800
M KC1 M KCt M KCt MKCI M KCI MKCI .Cf NaCl
0.01
0.99
0.66
0.34
O.OSL
2,300
0.05 0.1 0.05 0.1 0.05 0.1 0.1
0.05 1
5,100
0.1
M NaCt
0.7 I
0.29
O-05 1
2,750
0.1
M NaC(
0.64
0.36
O-054
ztso
0.1
M N&
0.60
0.40
sintered
cast&k hea?ed Cast& sintered Cast L steamed cast &
O-05 M KC1 0.1 M KCt 0.05 Icf KC1
Cation transjL
0.025
0.05 M KC1 0.1 M KCI
0.71
0.29
0.054
0.05 0.1 0.05 0.1 0.05 0.1
O&4
0.36
0.80
0.20
0.98
0.02
0.064 0.140
M M M M M M
on both sidej (singIe value) or for each side
KC1 KCt KCt KC1 KC1 KC1
(two
valu&,
,
-
.
+kw&ation,
..-.
.
6 (1969) 369-380 ‘.
. .
..’
.’
..
ION EXCHANGERS
IN ELECTRODIALYSIS
379
MEMBRANES
ion exchanger used and it was possible to obtain a transport number of 0.92 with a 0.14 cm (SO mil) film of zirconium phosphate in Kynar. Results using other ion exchangers with Kynar were not as good since the films made were thinner and optimum curing and post treatment conditions were not explored. Anion selective membranes with organic binders-could not be prepared. This pgnomenon was attributable to irreversible reactions between the anion exchange sites and acidic or polar groups of the binder or solvent. conductivity Conductivity measurements were made on inorganic ion exchange membranes using an impedance bridge (General Radio Type 1603 Z-X)_ The membrane was mounted in a cell of the same design as the tnnsference cell with silver-silver chloride electrodes. Measurements were made at 1 KC. Thecapecitativecomponent of the impedance was balanced out. The results in Table II show a variation in specific resistivity for apparently identical samples: however, specific resistivity of most of the membranes are in the range of 20-2000 ohm-cm. These resistivities are of such magnitude that operation of a multicompartment cell would be practicable. Commercial organic cation selective membranes were measured by the same technique in 0. I M KC1 for comparison purposes. Observed specific resistivity ranged from 200 to 1370 ohm-cm and are comparable to those obtained with the inorganic membranes_ Specific resistivities reported in the literature for organic membranes may be as low as 100 to 200 ohm-cm; however, the method of measurement is different and larger membrane areas are measured. Membranes including a Kynar binder exhibited generally higher resistivities than when the inorganic ion exchanger was used without binder. The steamed Kynar membrane had the lowest overaIl resistivity. S.2 Me&rune
6.0
CONCLUSIONS
The aluminum,
titanium and zirconium vanadates show good ion exchange vanadate precipitated from an acid solution is a cation exchanger and if precipitated from a basic solution it is an anion exchanger. Aluminum vanadate membranes can be prepared by pressing and sintering techniques and exhibit selective transport even after heating to 1000°C. Both cation and anion selective membranes can be prepared by this method with transport numbers of 0.98 for each. Nominal specific resistivities are 500-800 ohmcm, somewhat higher than organic membranes. These membranes are not suitable for scale-up to sizes required by most electrodialysis applications since their rigidity limits their size to 10 to 16 square inches per membrane. Flexible membranes composed of inorganic exchangers and organic binders capabilities, up to 8 meq/g. Aluminum
Desafina?ion,6 (1969)
369-380
380
H. K. BISHOP et d.
can be prepared. Such cation membranes exhibit selective transport numbers of 0240 IO 0.90, lower than those of the rigid membranes. Specific resistivities of flexible membranes reported here are higher than pure inorganic membranes. Afuminum vanadate of the anion exchange form did not produce flexible anion selective membranes when fabricated by the methods investigated on this program. Hydrous oxides are noted for their reactions with acidic organic materials by forming lakes in mordant dying. Possibly, acidic or polar groups of the binders or solvents used in membrane preparation reacted irreversibly with the hydrous oxide anion exchange site and destroyed their selectivity in ?he membrane form. Membranes containing inorganic cation exchangers are close to the development stage and shoufd be developed using readily available materials for cost considerations. A wide variety of inorganic cation exchangers and organic binders are avaiiable for this development effort. ACKNOWLEDGEMENT
The authors wish to acknowledge the helpful assistance of Dr. Carl Perger. This work is a portion of that supported under OSW Contract 1441-0001-613. RJTERENcEs A. GLT~ MD H. K. BISHOP. Investigation of inorganic ion exchange membranes for electrodialysis applications. OSW Contract No. 14-01-0001-613, Asrrqwwer Luboraror~ Report No. Shf-46-729-F (1967). C. BR~GER.er uf. Investigation of Zeoiite Membranes for Fuel CelIs, NASA Conttact NAS 7-150, Finat Report (1964). J. 1. BRMNAN ~59 R. S. BRAMAN, Inorganic ion exchange membranes, J. CoffoiJ. Sci., 20 tisasj 913. Invcstigzttion of Inorganic ion Exchange Membranes for Electrodialysis Applications. OSW Contract t4-01-ooO1-338, Asrropower Laborurory Report iV5. S&&45039-F (1965). C. 8. AMPH~, Inorgank ion l%chhangers, Elscvier, Amsterdam. 1964. K. A. KRAUS, ef al.. lon exchange properties of hydrous oxides, Paper 1932, Second United dVations Conference on Peaceful Uses of Atomic Energy, Geneva (1958). C. B. AMPHLETT, Synthetic inorganic ion exchangers and their applications in atomic energy. Paper 271. Second Unired Narionv Conference an Peaceful Uses of Aromic Energy. Geneva (1958). E. J. DW%LL AND J. W. SHEPARD, J. Php. Chem., 63 (1959) ~2344. I. SCHOEMTLD, Israel Atomic Energy Commission Report IA-7X (t%2).
1. G.
2. 3. 4. 5. 6. 7.
*8. 9.
Desahation.
6 (1969) 369-380
ZNVESTIGATfON
OF
INORGANIC
ION
EXCHANGE
MEMBRANES
FOR ELECTRODIALYSIS
SUMMARY
The mixed oxides of Groups III, fV and V were investigated as selective ion exchangers for incorporation into ekctrodiatysis membranes. Aluminum vanadate precipitated from acid solution demonstrated reversibfe cation exchange properties and, if precipitated from a basic solution, it demonstrated reversible anion exchange properties. Rigid membranes formed by pressing and sintering techniques had high cation and anion selective properties and moderate resistivities_ Flexible meInbranes formed from aluminum vanadate exchangers and organic binders demonstrated only cation selectivity and higher resistivities than pure inorganic membranes. 1.o
INTRODUCTION
The objective of this research program was to investigate the basic properties of inorganic ion exchangers and, in particuiar, to study their fabrication into membrane form for use in efectrodiaiysis (I). Electrodialysis plants presently use organic ion exchange membranes that are subject to degradation at etevated temperatures, fouling, and high cost. Inorganic materials, due to their comparative thermal stability, lack of carbonaceous ingredients, and low cost, could presumabiy overcome these objections if fabricated into suitable ion exchange membranes. Inorganic membranes suitable for use in fuel cells were investigated in this laboratory for over two years (2). Inorganic ion exchange membranes were also prepared by this and other laboratories (3, 4). Inorganic ion exchangers (ahtminosiiicates) were originally discovered in certain soils containing clay minerals, and the first synthetic ion exchangers were also inorganic-the synthetic zeofites or permutits (5). The subsequent develop ment of the organic exchanger resins, with emphasis on practical applications, compfetely dominated the fielld of ion exchange chemistry, and it was not until recently that there has again been research on inorganic cation exchangers. The heteropolyacid salts, such as ammonium phosphomolybdate, aad &he acid salts
370
H. Ii. BlSHOP et al.
of hydrous oxides, such as zirconium phosphate, have proven to be particularly interestins (5). Much less attention has been given to inorganic anion exchangers. Hydrous oxides, particularly the quardrivalent metal oxides, such as ZrO,, SnO,, ThO,. and TiO,, have been investigated (6, 7). Hydrous cxides in which a second cation of higher valence than the parent cation is introduced into the structure with the resulting net positive charge being balanced by the presence of exchangeable anions have been prepared (8, 9). These hydrous oxides have been pictures (6) as positively charged inorganic polymers formed on the acid side of the iso-electtic point. Anion exchange capacity then becomes a function of the pH of the solution in which exchange takes place. The more acidic the solution, the greater the positive charge on the exchanger matrix due to absorbed hydrogec ions. 2.0
INORGANIC
ION EXCHANGERS
OF MIXED
METAL
OXIDES--THEORY
The effort to develop inorganic ion exchange solids suitable for membrane fabrication was directed toward mixed heavy metai oxides from Groups III, IV, Y and VI of the Periodic Table. The mixed oxides form solids of nonstoichiometric proportions, are characterized by a variety of crystal structures, and form complex ions with many anions. Aluminum, titanium, zirconium and hafnium vanadates, tungstates and molybdates and other mixed oxides of these groups were examined. These solid crystalline materials of Group IIC and Group IV mixed oxides will make cation exchangers of the zeolite type, where the cation exchange capacity is due fo the negative charge on the insoluble crysraliite. 0 1
‘7Al-O-S&-O1 0 [
0
-
t
M+ I 0
1
The negative charge is balanced by an exchangeable cation. The mixed oxides of Groups V and VI with Group III, or Group IV with Groups V and VI, should be anion exchange solids by the same principle in reverse. A positive charge now is distributed throughout the mixed oxide unit, which must be balanced by an anion. 0
0
I
I
l 0
I 0
--0--T&0-V--0 [
1 +
A-
Insoluble mixed oxides of aiuminum, titanium and zirconium are interesting Desalination, 6 (1%9) 369-380
ION E?CC?fANGER!3
IN ELECTftODfALYSlS
371
MEMBRANES
because of their ability to expand their vatenceshelt to form coordination complexes with negative ions. The insolubilizin~ atom is a Group V metal, such as vanadium, which serves to attach the compfexing atom through an oxygen link. In anion exchangeable complex ions on afuminur.!, the aluminum atom coordinates in an octahedral configuration and can absorb as many as six anions per aluminum atom. The vanadium ion is either tetrahedraily or octahe&aUy hybridized in its compounds depending upon the complexing agent and the conditions. In our investigation of unbalanced charge, non-stoichiometric crystals, tetrahedrally hybridized mixed oxides were prepared, and in the investigation of metal complexes as anion exchange solids octahedralty hyb~dized vanadium was prepared. Table I iIIu,:rates the formulations of vanadium over pH range I-14. TABLE
I
CONSI~T~.~ION
-
OF VANADtC
IONS IN SOLUTION
_--
PH
Number of vanadium atoms in iun Constitution of ion sodium satt by ~ystallization Color --B;trrf on data of
hNDER
14
12-10.6
S-9
7-6.8
I&-
&7,+
4 (W~VKh3l”-
LV&W-
NajV01
NarVzOt None
NOlK2
AND
JAHR.
i?%&C.
NaJkhVsOtd NORC
Na#bVsOls] Brown or red
6.&2.2 Pr?Xip. of (VzO&
2.2-I &OP’ Paie yellow
21t. (1933)49.
3.0 ALUlbtEWM VANADATE Initial results from this mixed oxide were the most encouraging, so this system was examined in some detail. A detailed description of other mixed oxides is given in Ref. (I).
3, I. 1. Genetal Roth vanadyt chloride VOC& and vanadium pentoxide were used in this preparation. Vanadyl chloride hydrolyzes in water, but is soluble in dilute acid forming the vanadyi ion which is VO+ 2. Vanadium pentoxide is soluble in concentrated acids at 25°C to the extent of 34%. Upon standing, the acid solution of V,O, is reduced at 25°C partiaI1y to VO**. The following reaction takes place in acids VOCI, + 2H” I* VOi2 + 2HCl V,O, + 2H+ -+ 2VOL+’ f H,O Desa&iMrion, 6 (t9699) 369-380
372
H. K. BISHOP c?l d.
and upon heating V,O,
-I- 6H+ -I- 2Cl- -. 2VO+2 -I- 3H,O f Cl,
The proposed structure of poly pentavanadic OH
HO \
OH
OH
I I v---u--v--o-v--o-v--o---v 0
acid is shown befow.
0
OH
I
0
/
0
OH
Polyvanadic acid apparently has three readily replaceable hydrogen ions that are thought to be those in the middle of the molecule. The four terminal hydrogen atoms are nondissociable or of low dissociation constant. 3.1.2 Preparation Aluminum chloride hexahydratc, AICIJ - BH,O, was dissolved in water to make a solution of 0.282 molar in Al’j_ A solution of V,O, in 6M HCI, made up to 0.167 molar in V,05, was added. Complete precipitation would yield a 1: 1 mixed oxide of (N203) (V,O,). The precipitation of the (Al,O,), (V,O& was effected by adding 1 A4 KOH solution to the mixture until no more precipitate was obtained. Complete precipitation occurred at pH 6 and the colloidal solid was a dark green color. The solid precipitate was filtered, washed and then dried at 90°C. 3.1.3 Ion exchange capacity An approximation of the H+ capacity of this mixed oxide solid was obtained by eluting the washed and dried sample with 0.01 M HCi solution on an ion exchange column with an area of 1 cm 2. A sotubility determination had previously shown that the solubility was negligible. The pH of the etliuent acid was about 4 and the highest meq Hi/ml was 0.02 x 10e2. It is therefore seen that the CM+ in the final effluent can be neglected and that the H+ capacity is then the total volume of 0.01 M acid put through. This figure is 370.7 ml giving 370.7 x 0.01 = 3.71 meq of H* absorbed by the solid, or a capacity of 4.20 meqjg. 3.2 Aluminum
vanaa’kte 3.2.1 General
Base form
Aluminum nitrate is dissolved in water and base added until a precipitate forms and then redissolves. AI@JO,)J + Hz0 --+ AlW + 3NO,- + 2H* AtO* -f- OH- + Hz0 + Al(OH), (precipitate gelatinous) AI( + OH- + [AI(OH AI( -t- 30H- 3 [Al(OH)J3 Desalinatiott,6 (l%Q) 369-380
ION EXCHANGERS
IN ELECXRODIALYSIS
373
MEMBRANE3
The singly and triply charged complex aluminates are water soluble and exist as such in solutions containing hydroxyl ion. In basic solution, the vanadium also forms a series of complex ions :,nd heteropoly acid which are not so well chara#:terized as the alum-num complexes. The type of vanadate preser:t in basic solution depends to a large extent upon -,he pH of the solution (see Table I). V,Os + 60H-
--, 2[V0.,]-3
+ 3H20
The precipitation occurs at pH 9-7, therefore, the structures of the vanadium ions are probably [V,O,,]-* and p,O,,] -‘. The structure of the [v5016]-3 ion is shown as follows: o-
HO \
[ HO
/I
1 v-o---v-o-v-~v-o--v
0
I
0
o-
o-
I
I
I
t
OH
-3
/ .
0
0
I\
0
OH I
and the aluminate probably combines with this in some manner. 3.2.2 Preparation A 0.01 M solution of aluminum and vanadium (1: 1 was slowly precipitated with 1 M HCl to a final pH of 8 where no more precipitation occurred. The yellow solid was filtered, washed and dried at 90°C. The yield was 96%. 3.2.3 Colutnn e&ion of basic ahnrinum vanadate A sample of aluminum vanadate precipitated from basic solution was washed in a column with 0.01 M KOH to determine hydroxide ion pickup. After four passes, the yellow solid became colorless. Hydroxide ions were absorbed equivalent to a capacity of 8.91 meq/g. The unusually high capacity for hydroxide ion is explained by the formation of a complex aluminum ion coordinating with 6 hydroxyl ions. 4.0
FABRICATION
OF ION EXCHANGE MEMBRANES
4.1 Pressing and sintering The dried and powdered ion exchanger in the appropriate chemical form was generafly screened and then loaded into a die. Filling the die with powder appeared to be critical in reducing crack formation on sintering. A ~nifo~ fiU is desirable. It was found that careful drying was advisable both before and after pressing in order to obtain a good Sll and to prevent warping and cracking when sintered. Finer particle size (200 mesh and smaller) also produced better fills. The die filling is illustrated in Fig.1. Desalination,
6 (1%9) 369-380
Pressing was done in a 100 ton ptcss. After crack-free presd membranes were &t&cd, they were set upn ceramic tile and sin&red (F&2). Powdered atuncfum was used as hhricant for the pressed niembrane to gmvmt adhesion during sinfting and a two-in& square ceramic tiltswsis $ked an top af the memc brane. -i-be ~$2 cansistcnt results WCTCobtained by using a heat up the of three ta four hours and a sintering temperature of 600’ to looO”C for approximately faur hnars. Snitiaffy, membraries were bezited to MIT; however, it was found that siatcring at ftX.WC produced a strong membrane 4th equal ian seletiitity, The mcmbraney were then annct;rlcdin the furnace cFvernight although theannealing cycle was nut pos;itively necessary. The final sinfcrcd membrane shrunk 20 ta 30% and deveIapcd considerabfe strength.
375
fig_ Z 4 x 4 518 prtsxd cation (bottom)_
(Kynar) organic ux),ooo
and sintercd aluminam
vanadatc
membranes,
anio;l (top) and
binder. The membranes were prepared by casting mixtures of the inmaterial witb a dimethyl formamide (D&IF) solution of Kynar (MW to 60&m). The proportions were such as to make a final membrane
composition of 75:25 and 85: 15 inorganic ion exchanger to Kynar binder. ,Membranes were cured by dry heat or steam at 80 to lOO”C. Flexibility wasgood(Fig. 3). 5.0
TESTING OF 10X EXCHANGE MEMBRANES
5_1 Ion sefecrivity
Laboratory Transport Measurements ‘- Measurements of transport number was used as an indication of the value of an ion selective membrane in desalting of water. Transport numbers of these inorganic membranes were determined using the Hittorf method, The transport eeli consisted of two I5 mf compartments with a membrane mounted between and fitted with a silver-silver chloride electrode inserted horizontally in the ends. An exploded view of the transport cell is shown in Fig. 4. After assembly, current is passed through the cell and electrolyte at a Desahafion.
6 (I %9) 369-380
376
’
Fig_ 3. la~gc sheet of fkxibk
inorg&ii
ion a&angc
membrane 24’
H. K. BISHOP i?t id.
x
0.010’.
constant level of 2 milliamperes for 3 hours. Concentration changes in the anode and cathode chambers were measured. The transport numbers were cakulated from these v&es and the quantity-of electricity passedThe cation and anion transport numbers of various membranes in 0.t M KCI, 0.05 M KCI. and 0.1 M NaCl solution were measured and are tabufated in Table III Aluminum vanadate precipitated from acid was found to be a good cation selective membrane with K+ and Na+ transport numbers greater than 0.8. When mixed with 036-11 (20°& ceramic mixture and sintered at 1000°C. a membrane barrier was obtained which was almost perfectly cation selective, with a KS ion transport number of 0.98. Efficient anion exchange membranes were also obtained from aluminum vanadate precipitated from basic solution. Anion transport numbers of 0.80 to 0.94 were found. Membranes containing the 036-I I ceramic mixtures and sintered at 1ooO”C were mechanically strong and readily handled. Titanium vanadate, zirconium vanadate, and pressed zirconium phosphate were found to be somewhat cation selective with a Na’ and K+ transport number greater than 0.60. The transport number of Na” in 0.1 M NaCf is 0.38 and of Kc in 0.1 M KCL is 0.49. The transport numbers of the cast films are generally characteristic of the Desalitmtion,6 (1969) 369-380
10N EXCHANGERS IX ELECTRODIALYSIS
MEMBRAI*‘ES
377
Fig 4, DisasscmbW transport c&.
TABLE It
Aluminum wnadate (Acid) H- form Aluminum vanadatc (Acid) Na- form Aluminum vanadate (Acid) K- form Aluminum vanadatc (Acid) 100 Aluminum vanad3te (Acid)/03611 80/n, Aluminum vanadate (Acid)/O~ I I St&Q5
Cation tramp. no.
Anion transport no.
0.1 Af NaCI
0.84
0.16
I.820
0.1 M NaCl
0.67
0.33
O-05I
350
0.1 M NaCI
0.65
0.35
0.056
710
0.84
0.16
0.064
280
0.98
0.02
0.075
I30
0.05 M KC1 0. t hf KC1 0.05 AI KCI 0.1 M KCI 0.05 M KCI 0.1 MKCI
0.99
0.01
Technique of formation
Thickness, Membrane Uecrrolyre* resistonce cow. cm ohm-cm
Pressed&
0.05 1
940
0.05I
sioteled PlW+iCd& siotencd Prcs!zd& SilMCd
Ptxswd & sintered PlXW!d&. sinkred PlYSS&l& sin1eRd
L)csoliMtion.
6 (1969) 369-380
H. K. BEWOP
378
et a!.
. TABLE If
(continuedj
Membrutte composition
Techttique of fdfmation
Aluminum w&date (Acid)/0361 1 W/20 Aluminum vana&te fAcid)j03611 SO/20 Aluminum vanadate (Acid),/0361 1 SOQO Aluminum vanadate (Acid).!0361 t SS,llS Aluminum vanadate (Acid)!Kynar 85! 15 Aluminum vanadate
sintered Pressed& sinkred Pressed& sintered Cast & heated cast & heated
(Acid),‘0361
sintered
l 80120
Aluminum vanadate @a=) Aluminum vanadate
* I&al
concentrakm
Resscd&
Pressed& Pressed&
sintered Presscd& sintered P_xssed & sintered Pressed & sintcred Pressed& sintered Pressed& sintenxi Pressed& sintered Pressed & sintered Pressed& sinlered Pressed &sintered Pressed&
Thickness, Membrane Eiectro&te+ “m resistamx?~colzc* ohm-cm
I --.
0.075
266
0.075
295
.0.023
31,ooo
0.1 0.05 0.1 0.1
0.023
3t,600
0.1
o-075
0.064
‘IG
280
Anion tranrgori~
no.
no.
0.99
0.01
0.80
0.20
M KC1 M KC1 M KCI M NaCl
0.83
0.17
0.83
0.17
MNaCt
0.9’
O-08
0.051
l.ulo
O-OSM KC1 0-t MKCl 0.1 M NaCl
0.05 1
5300
0.t
M NaCl
0.19
0.81
0.98
0.02
0.20
0.k
0.05 t
20
0.1
M Naei
0.06
.0.94
0.05 1
.350
0.f
M NaCl
0.26
0.74
0.084
593
0.01
0.99
0.082
51
0.05
0.95
0.075
800
M KC1 M KCt M KCt MKCI M KCI MKCI .Cf NaCl
0.01
0.99
0.66
0.34
O.OSL
2,300
0.05 0.1 0.05 0.1 0.05 0.1 0.1
0.05 1
5,100
0.1
M NaCt
0.7 I
0.29
O-05 1
2,750
0.1
M NaC(
0.64
0.36
O-054
ztso
0.1
M N&
0.60
0.40
sintered
cast&k hea?ed Cast& sintered Cast L steamed cast &
O-05 M KC1 0.1 M KCt 0.05 Icf KC1
Cation transjL
0.025
0.05 M KC1 0.1 M KCI
0.71
0.29
0.054
0.05 0.1 0.05 0.1 0.05 0.1
O&4
0.36
0.80
0.20
0.98
0.02
0.064 0.140
M M M M M M
on both sidej (singIe value) or for each side
KC1 KCt KCt KC1 KC1 KC1
(two
valu&,
,
-
.
+kw&ation,
..-.
.
6 (1969) 369-380 ‘.
. .
..’
.’
..
ION EXCHANGERS
IN ELECTRODIALYSIS
379
MEMBRANES
ion exchanger used and it was possible to obtain a transport number of 0.92 with a 0.14 cm (SO mil) film of zirconium phosphate in Kynar. Results using other ion exchangers with Kynar were not as good since the films made were thinner and optimum curing and post treatment conditions were not explored. Anion selective membranes with organic binders-could not be prepared. This pgnomenon was attributable to irreversible reactions between the anion exchange sites and acidic or polar groups of the binder or solvent. conductivity Conductivity measurements were made on inorganic ion exchange membranes using an impedance bridge (General Radio Type 1603 Z-X)_ The membrane was mounted in a cell of the same design as the tnnsference cell with silver-silver chloride electrodes. Measurements were made at 1 KC. Thecapecitativecomponent of the impedance was balanced out. The results in Table II show a variation in specific resistivity for apparently identical samples: however, specific resistivity of most of the membranes are in the range of 20-2000 ohm-cm. These resistivities are of such magnitude that operation of a multicompartment cell would be practicable. Commercial organic cation selective membranes were measured by the same technique in 0. I M KC1 for comparison purposes. Observed specific resistivity ranged from 200 to 1370 ohm-cm and are comparable to those obtained with the inorganic membranes_ Specific resistivities reported in the literature for organic membranes may be as low as 100 to 200 ohm-cm; however, the method of measurement is different and larger membrane areas are measured. Membranes including a Kynar binder exhibited generally higher resistivities than when the inorganic ion exchanger was used without binder. The steamed Kynar membrane had the lowest overaIl resistivity. S.2 Me&rune
6.0
CONCLUSIONS
The aluminum,
titanium and zirconium vanadates show good ion exchange vanadate precipitated from an acid solution is a cation exchanger and if precipitated from a basic solution it is an anion exchanger. Aluminum vanadate membranes can be prepared by pressing and sintering techniques and exhibit selective transport even after heating to 1000°C. Both cation and anion selective membranes can be prepared by this method with transport numbers of 0.98 for each. Nominal specific resistivities are 500-800 ohmcm, somewhat higher than organic membranes. These membranes are not suitable for scale-up to sizes required by most electrodialysis applications since their rigidity limits their size to 10 to 16 square inches per membrane. Flexible membranes composed of inorganic exchangers and organic binders capabilities, up to 8 meq/g. Aluminum
Desafina?ion,6 (1969)
369-380
380
H. K. BISHOP et d.
can be prepared. Such cation membranes exhibit selective transport numbers of 0240 IO 0.90, lower than those of the rigid membranes. Specific resistivities of flexible membranes reported here are higher than pure inorganic membranes. Afuminum vanadate of the anion exchange form did not produce flexible anion selective membranes when fabricated by the methods investigated on this program. Hydrous oxides are noted for their reactions with acidic organic materials by forming lakes in mordant dying. Possibly, acidic or polar groups of the binders or solvents used in membrane preparation reacted irreversibly with the hydrous oxide anion exchange site and destroyed their selectivity in ?he membrane form. Membranes containing inorganic cation exchangers are close to the development stage and shoufd be developed using readily available materials for cost considerations. A wide variety of inorganic cation exchangers and organic binders are avaiiable for this development effort. ACKNOWLEDGEMENT
The authors wish to acknowledge the helpful assistance of Dr. Carl Perger. This work is a portion of that supported under OSW Contract 1441-0001-613. RJTERENcEs A. GLT~ MD H. K. BISHOP. Investigation of inorganic ion exchange membranes for electrodialysis applications. OSW Contract No. 14-01-0001-613, Asrrqwwer Luboraror~ Report No. Shf-46-729-F (1967). C. BR~GER.er uf. Investigation of Zeoiite Membranes for Fuel CelIs, NASA Conttact NAS 7-150, Finat Report (1964). J. 1. BRMNAN ~59 R. S. BRAMAN, Inorganic ion exchange membranes, J. CoffoiJ. Sci., 20 tisasj 913. Invcstigzttion of Inorganic ion Exchange Membranes for Electrodialysis Applications. OSW Contract t4-01-ooO1-338, Asrropower Laborurory Report iV5. S&&45039-F (1965). C. 8. AMPH~, Inorgank ion l%chhangers, Elscvier, Amsterdam. 1964. K. A. KRAUS, ef al.. lon exchange properties of hydrous oxides, Paper 1932, Second United dVations Conference on Peaceful Uses of Atomic Energy, Geneva (1958). C. B. AMPHLETT, Synthetic inorganic ion exchangers and their applications in atomic energy. Paper 271. Second Unired Narionv Conference an Peaceful Uses of Aromic Energy. Geneva (1958). E. J. DW%LL AND J. W. SHEPARD, J. Php. Chem., 63 (1959) ~2344. I. SCHOEMTLD, Israel Atomic Energy Commission Report IA-7X (t%2).
1. G.
2. 3. 4. 5. 6. 7.
*8. 9.
Desahation.
6 (1969) 369-380