Ind ian Journal o f Engineering & Materials Sciences Vol. 10, February 2003, pp. 75-81

Preparation of inorganic microfiltration membranes and their characterization

K S Seshadri", K B Lal ", R Kesavamoorth/, Pushpa Muthiah" , M Selvara/ & V Kri shnasaml

"Centralized Waste Management Facility, Bhabha Ato mic Research Ce ntre, Kalpakka m 603 102 . Indi a. bMaterial Science Divi sion, Indira Gandhi Centre for Atomic Research, Kalpakkam 603 102, India

<Department o f Chemistry, Anna University, Chennai 600025, India

Received 13 March 2002; accepled 8 Augusl 2002

A study on the dynamic growth o f titania and zirconi a layers on mi croporous alumina was carried out. The porosity of the membrane layers was esti mated from the amount o f water content in the pores. The thicknesses of the me mbrane layers were estimated from the membrane volume, me mbrane pore volume and the coated area o f the support. Results indicaled thai the porosity of the titani a and z irconia membranes were 0.31 and 0.38 respecti ve ly and the thickness o f the layers as 3 mi cron in a period o f 60 min. The membranes were found to be very good to retai n suspended panicles.

Membrane processes are gaining importance in the treatment of radioacti ve effluents in view of their advantages over other treatment methods. Typical advantages are the simultaneous purification, concentration, fractionation of macromolecules and fa irly low pressures for operation. Sol gel technique is one of the preparation methods for ceramic membrane, which involves hydrolysi s of alkoxide, peptisation of the hydrous oxide in suitable medium, adj ustment of sol viscosity by inclusion of a binder, filtration of the viscous sol through a microporous

support, gelation (at 20°C for 24 h) and sintering to different chosen temperatures. The porous morphology is governed by sintering temperature. The thickness of the membrane layer is governed by duration of filtration, area of the filtration, membrane volume and the membrane pore volume. The thickness of the membrane layer for membrane structures represents a compromise between the physical integrity requirement, on the one hand and the high flux requirement, on the other.

Experimental Procedure Analytical reagent grade tetrabutoxides of titanium

and zirconium procured from Merck-Schuchardt and Aldrich respectively were used as a precursor material for titania and zirconia. Polyvinyl alcohol of analytical reagent grade as a viscosity modifier was used as a binder. High refractory porous alumina tubes procured from BHEL, Bangalore was used as support. The tube was a cylindrical tube of 3 cm length and 1 cm internal diameter with internal surface area of 9.45 cm2

. The filtration of the viscous

sol through the support was carried out using the filtration set-up as shown in Fig. 1. The content o f the titania and zirconia were es timated spectrophotometrically using hydrogen peroxide and alizarin red respectivelyl. Analytical reagent grade chemicals of copper sulphate, potass ium ferrocyanid e, calcium chloride and tri-sodium phosphate were L1 sed for the preparation of suspended parti cles. Nephelometer of Elico make was used fo r est imating the turbidity of the suspensions before and after filtration .

The hydrous oxides of titanium and zirconium were prepared by complete hydrolysis with 100 mol or distilled water per mole of tetrabutox ide at room temperature with constant stirring". They were

Fig. I-Experimental set-up ((I) gas cy linder, (2) lank. (3) safelY valve. (4) pressure gauge, (5) stand, (6) o uter jackel . and (7 ) holder for porous tube)

SESHADRI el al.: PREPARATION OF INORGANIC MICROFILTRATION M EM BRANES 77

The weight of the support with the clogged materi al was measured (W/). It was immersed in boiling water fo r 6 h to completely fill up all the pores, taken up, outer surface wiped out and reweighed in air (W/) and in co ld water in a beaker without touching the sides or bottom of the beaker (W/).

Filtrati on was continued and membrane was allowed to grow. Then the clogged support with the membrane was taken out at various time durations, ge lled, sintered and weighed (W/"). Then it was immersed in boiling water for 6 h to completely fill up all the pores, outer surface wiped out and reweighed in air (W/") and in water (W./") . From these we ights, V"" the volume of the membrane and V""" the pore volume of the membrane are given by,

V", = W/" + W/ -(W/' + W/,) . . . ( I )

and

V"", = W/" + W/-(W/" + W/) ... (2)

Hence, a, the porosity of the membrane at various ti me intervals is given by,

. .. (3)

. . . (4)

The filtration was conducted fo r 60 min W/", W/"and W/' for the titani a and zirconia membranes are given as a function of time (Tables 1 and 2).

From V"" V,}/" and the area of filtrati on. which is 9.45 cm2

, the th ickness of the membrane layers was computed. Fig. 5 shows the thi ckness of the membrane layer as a functi on of square root of time of deposition.

The amount of the coated material was determined gravimetrically at various intervals of time by finding the weight of the tube before coating the materi al anc! after coating the tube with the materi al from the start of the membrane layer formation, i.e., 35 min for titani a and 40 mi n fo r zirconia.l . The percentage rejection of the titani a and zirconi a materi als were computed from the concentrations of feed , and filtrated time to time. It is plotted as function of square root of time of filtration in Fi g. 6a for ti tani a and Fig. 6b fo r zirconi a.

The clogged support with the titani a and zirconia layers were gelled at 20°C and then sintered at 400°C and 470°C respecti vely to remove the binder. Figs 7

3.5

2.5

~ 1.5

~

~ . ~ zirrll lli a "- ti l :llli~J

o · :L~ o L' _ --'-_ __ .l-_--"--_--'-_ ------L

7 7. 1 7.2 7.3 7.4 7.5 7. 1,

Squ;m: mul of timt.', min

7.7 7.S

Fig. 5-Variati on of thickness o f the membra ne layers as function of coat ing time

Table I- Porosity of titani a membra ne layer as a function of coating time

S. No T ime W,1Il W2ITI W ITI , Vm Vprn T hick ness

(min ) (g) (g) (g) (cc) (CC) (pm)

50 5. 11 27 5.6747 3.80 14 0.00 13 0.0006 2.02

2 55 5. 114 1 5.6763 3.8025 0.00 16 0.0008 2.54

3 60 5.11 56 5.6779 3 .8036. 0.0020 0 .0009 3.06

Table 2- Porosity o f zirconi a membrane layer as a functi on of coating ti me

S.No Ti me W, m W2m W m , Vm Vpm Thickness

(m in ) (g) (g) (g) (cc) (CC) (pm)

50 5. 1499 5.8876 3.839 1 0 .0006 0.0004 1.05

2 55 5. 1530 5.89 11 3.84 17 0 .00 11 0 .0008 2.0 1

3 60 5. 1562 5.8946 3.8442 0.00 18 0 .00 11 3.07

78 I DIAN J. ENG. M ATER. SCI. , FEBRU ARY 2003

120 ,

100

~ 80

c + c kg/err. '

4- 3 kg/em'

* 4 kg/nn'

......... 5 kglcro~

O~~-L~--L-~-L~ __ L-~-L~~

o 5 10 15 ~O 25 30 35 40 45 50 55 60 Tilll~. Illill

Fig. 6a- Vari at ion of percent rejecti on of titani a as a function of time

UO~---------------------------1

100 .

'" 'g SO ~

ON

20

Ik!!/cm'

-I- 2k!!/cm'

* 3k!!/cm'

""* 4 k!!/cm'

-+- Skglcm'

5 10 15 20 25 30 35 40 45 50 55 60 Time. min

Fig. 6b- Vari ati on of percent rejection o f zi rconia as a function of time

of: '.0

.;;-0

>< !:o, . iii

~ "0

C 0

0-

Fi g. 7-Scanning e lec tron micrograph or titallia membrant:

6r--------l 5

4

3

2

~1

I !

I

\ I

\ i ;I;------l-------1-------L-_----L.]

500 1000 1500 2000 tvkan pore si7e. nm

Fig. 8-Pore size distribution of titania membrane

and 8 show the SEM of the titania laye r formed <.tfter 1 h duration of filtration and the corresponding pore size di stribution respectively . Figs 9 and 10 show the SEM of the z irconi a layer formed after I h durat ion of filtration and the cOlTesponding pore size di st ribution respecti vely. The liquid pe rmeabilities were obtained with water as permeate. Fig. I I shows the permeability as a function of pressure.

The conventional chemi cal trea tmen t methods employ 20 ppm Cu2

+ as copper sulphate and 30 ppm

SESHADRI el af.: PREPARATION OF INORGANIC MICROFILTRATION MEMBRANES 79

Fig. 9- Scanning e lectron micrograph of zirconia membrane

8,------------------------------.

6

~ 5 ,...

'0

X 4 ~ . . ~ '3

-::J

§ 3

2

500 1000 1500 2000 Meal1 pore size, 11111

Fig. IO-Pore size di stribution of zirconia membrane

f) U (JO

·H10~1

"'-

10f)O

-

~ LOIlO M l' lIliu.UH! S

:.: It HIO --Zll Culll~ ---+- Tiuwi oJ

0 U

Pn,:s~ lIrl: . kg/l:lll

Fig. II - Permeability o f titania and zirconi a membranes at various pressure

160

140

120

c: .9 100 t> .., .~

.., 80 Oil

3 5 u t;

60 a..

40

20

x

2 4 6

x

I kg/em'

+ 3kglcm'

*

8 10 12 14 16 18 20 Time. min

Fig. 12a-Percentage retention of copper ferrocyanide th rough titania membrane

120 ------------------ --

100 r ----' . ~ -------

k'

/ -I. 3 kg-em: I -;t:-- 5kg/cn~~ I

I

40

20

2 4 6 8 10 12 14 16 18 20 "j'iJJl<':. ll1ill

Fig. 12b- Percentage retention of coppe r ferrocyani de through zirconia memhrant!

80 INDIAN 1. ENG. MATER. SCI., FEBRUARY 2003

[Fe(CN)6t as potass ium ferrocyanide as the optimum dosages at pH 8.5 for the effective removal of radioactive cesium. Similarly for the effective removal of radioactive strontium, 75 ppm Ca2

+ as ca lcium chloride and 130 ppm P04

3- as sodium tri

phosphate at pH 10.5 are observed to be the optimum dosing. These optimum dosings are called plant scale dosings. The suspensions at 10-50% plant scale dos ings were passed through titania and zirconia membranes at pressures I, 3 and 5 kg/cm2 using filtration set-up as shown in Fig. I . Since the retention of the precipitate on the membrane will retain the radionuclide and hence the radioactivity, it is imperative to study the percentage retent ion of the precipitate. The calibration curve for the estimation of concentration of the filtrate was obtained by measuring the turbidity of suspensions of calcium phosphate and copper ferrocyanide prepared at various plant scale dosings. The percentage retention of the precipitate was es timated from the turbidity of the fi Itrate and the calibration curve. Figs 12a and 12b show the variation in the percentage retention of the copper ferrocyanide precipitate at various operating pressures and time intervals through titania and zirconia membranes respectively .

Figs 13a and 13b show the variation in the percentage retention of the calcium phosphate precipitate at various operating pressures and time intervals through titania and zirconia membranes respec tively.

Results and Discussion Pore size distribution

From SEM and the corrsponding pore size di stribution of the porous support (Figs 2 and 3) we observe that pore size of the support ranges between 1000 to 9000 nm with average pore size as 5400 nm and pore density 2.6 xI06/cm2.

Porosity of the clogged support (W/ -W/ ) gives the sum of the volume of the

support (VJ and the volume of the c logged material (Ve). (W/ -W/) gives the volume of the pores present in the clogged support (Vp,J. Hence, the porosity of the clogged support is g iven by,

... (5)

The values of W/ , W/ and W/ in grams were found to be 5.1427 , 5.8800 and 3.8325 respectively for zirconia clogged support and 5.1078, 5 .6692 and

100 ~------------------------------~

80

.\ Il:gC~- 1

+ 3kg!cm-

Jc;---'-__ ....L __ -'----'-__ ~ __ L-__ ____L ___ L _

2 4 6 8 10 12 14 16 18 20 Time. min

Fig . l3a-Percentage retentio n o f calcium phosphate th roug h titania membrane

120r-------------------------------,

100 ~~-~ -----.

k--------'-_____ ---L-_____ L

S 10 Time, mill

tS 20

Fig. l3b-Percentage retention of calc ium phospha te th roug h zircon ia membrane

3.7978 respectively for tit ani a clogged support. It gives rise to porosity of the zirconia clogged support as 0.36 and 0.31 for titania clogged support.

(Iv versus ( plots From Fig. 4, it is observed that there is dev iati on

from linearity in the plo t of Ilv aga in st t from 35 min

SESHADRI et al. : PREPARATION OF INORG AN IC MICROFILTRATION MEMBR ANES 8 1

and 40 min for titania and zi rconia respectively indicating the start of the membrane growth and hence further continuati on of filtration resulted in membrane growth at these timings.

Porosity of the membrane layers

The porosity of the membrane at vanous time interval s is given by ,

cx= VplIlVII/ + vtlll , = (W/"+ W/ -(W/"+ W/,) }/{ (W21l1+WJc-

(W11l1+W/) }

From the Tables I & 2, it is observed that the porosity of the titania and zi lcon ia membranes are around 0.3 1 and 0.38 respectively.

Thickness of the membrane layers The thickness of the tltania and zirconia membrane

layers, I"" was est imated using Vm, Vpm and Am, the area of the coated surface as,

... (6)

at various intervals of time. Am is the internal surface area of the cylindrical tube of 3 cm length and 1 cm inner diameter which is 9.45 cm2

. From Fig. 5, it is observed that thickness vari es with square root of time of deposition. Many researchers4

-6 reported

si m; lar observation.

The percentage rejection

The % rejection is defined as,

% Reject ion ={ [feed] -[filtrate)/[feed]} x IOO ... (7)

where [feedJ and [filtrate] represent the concentrati ons of the feed and filtrate respecti vely . Fig. 6 shows that % rejection increases with time, reaching almost 100% at 60 min in the filtration of the so ls. Because of the clogging and consequent reductioll in the pore size of the support, the amount of the material in the filtrate came down with time. Hence, the percentage rejection of the materi al increased with time.

Surface morphology and pore size distribu tion From Figs 7 and 8, it is observed that the titani a

membrane layer formed shows pore size varying between 250 and 1900 nm with mean pore size as 650 nm and pore density 1.47x 107/cm2. From Figs 9 and 10, it is observed that the zi rcon ia membrane layer formed shows pore size varyi'1g between 250 and 1350 nm with mean pore size as 543 nm and pore densi ty 1.47x 10 7 /cm 2

.

Perllleability The pore size of the membrane is manifested in the

permeabilities and separation characteristics of the

membrane. Fig. II SllOWS the linearity of the nu x wi th pressure for both titania and zircon ia membranes.

Retention of suspended matter Cs- 137 and Sr-90 are considered to be the mllst

important fi ssion products present in the rad ioactive effluent. The treatment of these rad ioisotopes is imperative in view of the hi gh fission yields and half lives of 30 and 28 years respectively. which are comparable with the life span of man. The problem of handling huge sludge volume and the effective separation of solid from liquid needs the introducti on of membranes.

From Figs 12a, 12b, 13a and 13b. it i~ observed that % retention increases with increase in pressure and also with increase in time of filtrati on. Th is is du(: to deposition of more suspended particl es with press ure and time of filtration leading to formation of secondary layer. In the pressure range 1-5 kg/cm" the increase in % retention is linear with pres~ ure. With time, the percentage retention increases linearl y and then attains saturation. The saturation in retaining the precipitate is attained in about 15-20 min in both copper and calcium precipitating systems on titani a and zirconia membranes .

The percentage retention was more with copper ferrocyanide system than with calcium phosphate system at any operating pressure owi ng to the relatively lesser settling nature of the later than the fonner. The percentage reten tion of the prec i pi tate was found to be more with zirconia membrane than with titania membrane.

Conclusions The thickness of the ceramic membrane layers

formed by the sol gel process could be determined gravimetrically successfully . Since thi ckness governs the flux of the filtration process it can be tailormade by adjusting the particle size of the so l and time of filtration. Also the permeability and the retent ion of suspension were found to be satisfactory.

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5 Hind SR. TraIlS Chelll Soc (Eng) . 22 (1923) 90 6 Carman P C, TrailS Chem Ellg (Lollr/ell). 16 ( 1938) 16X