Journal of Radioanalytical and Nuclear Chemistry, Vol. 222, Nos 1-2 (1997) 75--80

Ion exchange/adsorption properties of crystalline compound of anatase and rutile Song Yinjie,* Jiang Liqiang,* Zhao Aimin,* Jin Qixin,** Song Dalkang*** *Department of Modern Physics, Lanzhou University, Lanzhou, 730000, Gansu Province, P. R. China **Lanzhou Radiation Technology Development Center, Lanzhou, 730030, P. R. China ***Department of Material Science, Lanzhou University, Lanzhou, 730000, P. R. China (Received January 15, 1997)

The crystallinely hydrous titanium dioxide, H4Ti30$, was synthesized by heating amorphous titanium hydroxide at 80 ~ for 12 hours. XRD, TGA and pH titration were employed to characterize the prepared crystal. The studies on acid, base and radiation stabilities of the crystal demonstrate the reliability of this type of ion exchange material for treating radioactive nuclear wastes. The uranyl ions are so much preferred by the crystal that the complexation of uranyl and cMoride ions in solution phase is completely destmcted. The uptake of uranium on the crystal is remarkably sensitive to the solution pH. Plot of log/KD of the uranium on the crystal vs. equilibrium pH generates a series of lines with the mean slope of 0.40, which is the verification of sophisticated loading mechanisms in HAJO2+ reaction.

Introduction Recently, extensive studies on ion exchange/adsorption properties of inorganic materials have been carded out from the point of view of analysis and separations, l adsorbents for the recovery of trace metals in natural waters, 2 especially of matrices for radioactive nuclear wastes, as due to their good thermal and radiation stabilities, their excellent selectivities with respect to a certain element or group of elements which can be discussed in terms of an ion sieve effect, steric factors, ion seize preference, an entropy effect and ion memory preference.6 Silica-titanium mixed-hydroxide gels is a good material for adsorption of uranium from sea-water7 because of cation exchange property o f silica and both cation and union exchange properties of titanium, g The sorption of uranyl ions on silica-titanium mixed-hydroxide gels was thoroughly investigated9 and the mechanisms of uranium uptake at various pH values were developed. But the thermodynamic functions of H/UO 2+ reaction on silica-titanium mixed-hydroxide gels show only a slight preference for UO~ + ions at both 25 and 65 ~ because of the strongly chemical bonds between H + ions initially on gels and oxo-surface oxides (SO-H). Ion exchange equilibrium of strontium ions on layered sodium titanate was reported by LEirro et al. 1~ Previous investigation 12 indicated that the uranium loading on both Anatase and Ruffle was of the same mechanism and crystalline material exhibited higher selectivity for uranium than amorphous one did. Based on the researches, this paper concerns easy preparation and characterization of Anatese and Ruffle compound, investigation on the acid, base and radiation stabilities of the crystal synthesized, interaction of the crystal with uranium.

0236-5731/97/USD 17.00 9 1997 Akadtmiai Kiad6, Budapest All rights reserved

Experimental Preparation of the crystal Amorphous titanium hydroxide prepared by means of direct precipitation9 was aged in a water-bath at the temperature of 8 0 + 2 ~ for 12 hours to generate crystalline hydrous titanium dioxide. In the process of bringing up of the crystal, a little amount of samples was withdrawn at certain time intervals for monitoring the growth of the crystal. The final product was filtered on a G2-glass filter crucible and thoroughly washed with deionized water. Then the crystal dried at 110 ~ for 4 hours was thermostated in desiccator containing silica-gels until constant weight was reached.

X-ray powder diffraction measurements The XRD patterns were recorded according to the step scanning procedure with Rigaku-2400 X-ray diffractometer made in Japan using Ni-filtered Cu I ~ 1 radiation under the conditions of 4 deg/min.

Thermogravimetric analysis The TGA curves of the crystal as function of temperature were obtained by an automatic thermal analyzer (Dupont 1090) under the reported conditions 13 by which the TGA curves of a- and ~-ZrP were recorded.

Radiation of the crystal The radiation dose rate was standardized by chemical method. The synthesized crystal was radiated under the dose rate of 0.793 and 1.902 kGy/hour for 605 hours, producing the total absorption dose of 480 and 1150 kGy, respectively, which are all remarkably greater than that commonly used for organic compounds radiation.

Elsevier Science B. V., Amsterdam Akad~miai Kiad6, Budapest

SONG YINJI~ et al.: IoN EXCHANGE/ADSORPTION PROPERTIES OF CRYSTALLINE ODMPOUND

pH titration The pH titration of the crystal was conducted as follows: 14 0.15 g crystals of known moisture content were weighed into a series of separate tubes, the same quantity of water and different amount of titrant (0.020 mol/l HCI or NaOH) were added to each tube and all of them were agitated on GUO-HUA HY-2 variable-speed reciprocal shaker made in China for 24 hours. Then the equilibrium pH of each solution was measured by means of pH S-3B acidimeter made by Shanghai Leici Instrumental Factory.

hours, indicating the easily preparation of the crystallinely hydrous titanium dioxide.

Thermogravimetric analysis Thermal effect often manifests differences in structure of inorganic ion exchange materials. TGA curves are likely to be sensitive indicators of these differences. The TGA curve, accompanied with its derivative one, for crystallinely hydrous titanium dioxide was obtained and constructed in Fig. 1. As shown in Fig. 1 the loss of all

Ion exchange/adsorption testing The uptake of uranium on the crystalline titanium dioxide was examined with respect to batchwise operation by shaking 0.15 g crystal with 10.0 ml of aqueous solution including uranium for 24 hours at 25 ~ The analytical concentration of uranium was determined by speclrophotometer with Arsenazo III as chromogen. The amount of uranium loaded by the crystal are determined with the difference between initial and equilibrium concentrations of uranium in solution.

Results and discussion

Synthesis of crystallinely hydrous titanium dioxide and its X-ray powder patterns The crystaUinely hydrous titanium dioxide was successfully prepared by aging of amorphous titanium hydroxide in water-bath thermostated at 80 + 2 ~ for 12 hours. The crystal obtained was identified by means of X-ray powder diffractometry and the results are summarized in Table 1. From standard card of TiO2, XRD patterns listed in Table 1 with the d/n values of 3.551, 2.382, 1.903, 1.698, 1.486 and 1.363 should belong to anatase while that with the d/n values of 2.481, 2.190 and 1.685 to mille. XRD pattern with the d/n value of 3.266 is attributable to both anatase and rutile, resulting in the highly relative intensity. The crystallinely hydrous titanium dioxide is a compound which is predominantly composed of anatase and is modified with a little amount of rutile according to its only half of the I/Io values compared with normal mille crystal. In addition, it is necessary to mention that the time required for growing of the crystal is greater than or equal to 8 Table 1. XRD patterns for the crystal synthesized

a/,

mo

d/.

tito

~.

tito

3.551 2.382 1.903 1.698 1.486 1.363

100 37 32 46 24 18

3.266

49

2.481 2.190 1.685

28 16 44

76

1

100-

2 -3

96-

-2

"~ t~

\,

92-

g

E 88-

-0

84-

---1

0

I 1~

I

I

200 300 Temperature, *C

-~"

I

4~

500

Fig. 1. TGA curve of the crystal synthesized

water takes place in the neighborhood of 58.7--->150.3 ~ generating a maximum at 100.6 ~ in derivative curve. From weights of the loss and crystal used for TGA measurement, the crystal can be formulated by 3TiO2 9 9 2H20 which is identical with Na4Ti,O2n§ reported by C~AP.FmI~ and LErrro.15 Moreover, this type of water should be regarded as non-lattice water and the exchangeable hydrogen ions are located between the layers of the crystal. 15

pH titration, exchange and apparent capacities The pH titration curve of prepared crystal using 0.020 mogl NaOH (Fig. 2) showed a lower pH jump with N'aOH added than blank run, indicating that the crystal H4Ti308 prepared behaved as a weakly acid-type exchanger. The amount of replaceable protons responsible for ion exchange reactions can be determined by the difference between blank run and the pH titration curve. Figure 2 also displayed exchange capacity thus determined as function of pH. From Fig. 2, the maximum exchange capacity is 0.52 mmol H+/g. In addition, the maximum apparent capacity of the crystal obtained with the method published9 is 1.20 mmol H§ corresponding to titration product H3.67Nao.33Ti308. The disagreement of two types of

SONG YIN,rI~et aL: ION IEXCIIANGE/AI)~,~RF'I"IONPROPI~TIESOF CRYSTALLINECOMPOUND

Apparentcapacity,mmolH+I g 0.2 0.3 0.4 0.5

0.1 I

|. 10...i~

.

I

I

I

0.6

I

I

.

..P'-.--Blank/ ~ ru~

IJ \1: q

"~

e/ I

2

I

VHc, ~

0

I

I

2

I

4

V.aoH

6

-12 I

8

10 I

Fig. 2. pH titration curve of prepared crystal and relationship of apparent capacity and pH

capacities exhibits that titration reaction can not be singly described by normal ion exchange mechanism.

Acid, base and radiation stabilities of the crystal Figure 3 shows the progression of changes during treating the crystallinely hydrous titanium dioxide with

,J

J

20

I

1

40

60

I

8O

20 Fig. 4. XRD pattems of: (a) the crystal prepared; (b) and (c) radiated with ~ absoq~tion dose of 480 and 1150 kGy, respectively

various concenlrafions of HC1 and NaOH. As shown in Fig. 3, the crystal prepared p0ssesses-an excellent acid stability within an investigated concentration of hydrochloric acid, but is partly destroyed by 2 moUl NaOH and only a little amount of irregular crystals are existed in the system. The uptake of uranium on the irregular crystals proceeds the same mechanisms as the uptake of uranium on amorphous titanium dioxide9 and is of the remarkable difference from that on the crystal synthesized. XRD was employed to justify the radiation stability of the crystal. It can be seen from Fig. 4 that the same crystalline structure is completely possessed by the material even if the total absorption dose is up to ll50kGy. This finding manifests the reliable radiation stability of the crystallinely hydrous titanium dioxide which is the most important for inorganic ion exchange materials to act as the substances for treating the radioactive nuclear wastes.

Ion exchange/adsorption properties

J I

20

I

I

40

6O

I

8O

20 Fig. 3. XRD of the crystal equilibrated at: (a) 2 mol/1HCI; Co) the crystal used; (c) 1 moFl NaOH and (d) 2 mol/l NaOH

The batchwise operation was used to test the ion exchange/adsorption properties of the crystaUinely hydrous litanium dioxide and some results are demonstrated as follows. Effect of pH on ion exchange~adsorption behavior: Effect of pH on ion exchange/adsorption behavior of the crystal prepared was carried out at 25.0 ~ initial uranium concentrations of 0.57.10 -2 to 3.44.10 -2 mol/l. The pH value was adjusted with HCI and/or NaOH. Figure 5 77

SONG YINJIE ~ aL: ION EXCHANGE/ADSORlqlON PROPERTIES OF CRYffI'ALLINE COMPOUND

e- 6

0.3-O"~

~

0.2--

i

.!

9 5

o~o

o 3

o11

e-

0.o 0

I

I

I

1

2

4

6

8

pH

Fi~. 5. The relationship of uranium loading and j~H 1, 2, 3, 4, 5, 6 2 c~o~+ =0.57, 1.15, 1.72, 2.30. 2.87, 3.44.10-Zmol/l, respectively

/

2.0--

1,6--

2

/I.',I.-

1.2 m

1-->4, particularly the abrupt increase occurs at pH about 3. This finding may be atlributale to partly hydrolysis of uranyl ions which makes the valence of uranium in aqueous solution go downwards. Furthermore, the pH at which the abruptly increased uranium,uptake occurs shifts to the lower pH as increasing initial uranium concentration in solution. This parallels the relativity of uranyl hydrolysis with uranyl concentration in aqueous solution and (3) when the equilibrium pH greater than about 4.5, almost all uraniums arc transmigrated to the solid phase largely as form of precipitation. As can be seen in Fig. 6, at the range of pH 1--->4,plot of log K D vs. equilibrium pH gives a series of good lines with the mean slope of 0A0 + 0.02 except line 1 with that of 0.86, reversely indicating that the normal ion exchange mechanism is invalid in the system. In other words, innegligible uranium are loaded onto the solid phase though the very little amount of hydrogen ions are replaced into the solution. The abrupt increase of log Ko at pH - 4 should possess the same reason as that just mentioned above. Ion exchange/adsorption isotherms: It is a laborious work for a researcher to obtain a satisfied ion exchange isotherm because uranium uptake on the crystal is strongly sensitive to solution pH. So the good way to obtain ion exchange/adsorption isotherms at a given pH value naturally runs to the profile of a series of uranium uptake vs. pH curves. The isotherm of uranium uptake on the crystal obtained by this method, together with log K o, is shown in Fig. 7, which indicates that the uranyl ions are remarkably preferred by the crystal synthesized at pH 3.50. Assuming that all uranium in the crystal presented as form of UO~+ ions at this pH value, the fraction of uranyl ions in solid phase Xuo~§ calculated on the basis of theoretical exchange cap~ify of 14.5 mmol H§ for HaTi308 and

I o. ~f ,,' ~/' "0.8--

0.20

0.4--

o.16-

o.0 0

I

I

I

I

I~

1

2

3

4

5

pH

Fig. 6. The linear relationship of log KD and pH. The symbols are same as

those in Fig. 5

demonstrates the amount of uranium loaded by 0.15 g crystals as function of equilibrium pH of aqueous solution. As shown in Fig. 5, (1) the negligible uranium is loaded by the crystal when the equilibrium pH of the solution less than I due to noticeable resistence of H+ ions in solution to I-IAJC~2+ reaction; (2) ion exchange/adsorption abilities of uranium on the crystal are promoted in the pH range of 78

=

0.12-E E

j

e

5

~

-4

X.J"

-3

-2

9-= O.08--

o E "E 0,O4-

0.0

~

o,,._,_..~ o , _ . _ . . . _ o ~ _

~

1

I

t

0.4

0.8

I 1.2

I 1.6

2.0

ceuo=, 2+ x 10"= mol / I

Fig. 7. Isotherm of uranium uptake on the crystal a t pH 3.50

SONG YINJ]IE et aL: ION KXCHANGE/ADSORIrI'IONPROPEIITIES OF CRYSTALLINE COMPOUND

t~ ,e=.

d "6 E

0.30

9 9

9

9

9

9 qb~ e - - , - - - - o

0.20q

~'" o.3o-I o ~

J

(a) 0.0

0.4

0.8

1.2

1.6

|~ 2.0

C ~=c=, mol / I

i' ' z'4V I

20

I

Fig. 9. Effect of ionic strength on uranyl loading; O, 9 Cuo~2 0 = 1.15, 2.30.10 -2 mol/l

I

40

60

I

80

20

ions on the ion exchange/adsorption of uranyl ions on the crystal. This result shows that the selectivity of the crystal for m'anyl ions is preferential enough to invalidate the complexation of chloride and uranyl ions in solution9 and demonstrates the possibility of removing uranyl ions from the media of highly salt concentration.

Fig. 8. XRD pauems_..,~ the crystal with various amount of uranyl ions. a, b, c - XUCrt+= 0.0, 0.085, 0.108, respectively

Conclusions

standardized with monovalent ions, is up to 0.20 at the equilibrium concentration of uranium of 2.0.10 -2 mol/l, corresponding to incompletely exchange product,

H3.2(OO2)0.4Ti3Os. XRD was again used to trace the effect of uranyl loading on the structure of crystallinely hydrous titanium dioxide and the results are edited in Fig. 8. It has been found that the rigidly crystalline structure of the crystal is reliable for uptake of uranyl ions though d/n value slightly decreases (from 3.551 to 3.523) at Xuo]+ = = 0.108. The decrease of d/n value may be attributable to the stronger chemical bonds between the lattice oxygen and UO~ + ions than that between the lattice oxygen and H+ ions located initially in the layers of the crystal. Previous investigation 9 indicated that hydrous titanium dioxide gel displays slight preference for uranyl ions because of strongly chemical bonds between H+ ions initially on gel and oxosurface oxides. But the opposite is just the case in using crystallinely hydrous titanium dioxide to load uranyl ions from aqueous solution. This is why that the crystallinely hydrous titanium dioxide obviously exhibits more excellent selectivity for uranium than amorphous one does. This is the focus of the paper. Effect of ionic strength on the ion exchange property of the crystal: The ionic strength of bulk solution was conveniently characterized by the concentration of sodium chloride. The initial concentrations of uranyl ions were 1.15.10 -2 and 2.30.10 -2 mol/l and the corresponding equilibrium pH value was 1.66_+0.01 and 1.165:0.01, respectively. The temperature thermostated was 25.0_ + 0.2 ~ Figure 9 shows the negligible effect of sodium

The synthesized crystallinely hydrous titanium dioxide H4Ti30 s is the compound of anatase and rutile. The theoretical ion exchange capacity of the crystal is 14.5 mmol H§ The total loading capacity for Na § ions determined with pH titration curve is 1.20 mmol H§ The amount of replaceable protons responsible for ion exchange reactions, determined by the difference between the blank run and pH titration curve, is 0.52 mmol H§ The crystal prepared is reliably stable at the range of 2 mol/l HCI to 1 mol/l NaOH and at the total absorption doses of 1150 kGy. The loading of uranium on the crystal is largely sensitive to solution pH. The normal ion exchange mechanism is invalid for ff/UO22+ reaction at the pH range investigated. The uranyl ions possess much more selectivity for the crystal than hydrogen and sodium ions do and the very preferential selectivity completely destructs the complexation of chloride and uranyl ions in the solution. Thus it is in the hope that the crystallinely hydrous titanium dioxide can be used for removing uranyl ions from the media with highly salt concentrations. It is confirmed that, from selectivity point of view, the crystallinely hydrous titanium dioxide possesses more excellent properties than amorphously one does.

Supported by Natural Science Fund of China General Company of Nuclear Industry.

79

SONG YINJIlg et

al.: ION EXCHANGFJADSORPTIONPROPERTIESOF CRYSTALIJNECOMPOUND

References 1. F. SAGARA,W. B. N~G, I. YOSHIDA,K. U),~o, Separ. Sci. Technol., 24 (1989) 1227. 2. A. RUVARAC, Group IV hydroxides, synthetic inorganic ion exchangers, in: Inorganic Ion Exchange Materials, A. CL~A~mLD (Ed.), CRC Press Inc., Boca Raton Florida, 1982, p. 152. 3. M. Z. MISAK,Main features of hydrous oxides as ion exchangers, in: New Developments in Ion Exchange, M. ABE,T. KATAOKA,T. SuzuIo (FAs), Proc. Intern. Conf. on Ion Exchange, ICIE'91, Tokyo, Japan, Oct. 2-4, 1991, p. 193. 4. D. K. BI-IATT^C~ARYYA,N. C. DUTTA,Uptake of several tracer cations and the separation of carrier-free 140Ba and also 125rainfrom ll5cd using a zirconium(IV) oxide column, in: Recent Developments in Ion Exchange, Vol. 2, P. A. WILLIAMSand M. J. HUDSON"(F.As), Proc. Intern. Conf. Ion Exch. '90, North East Wales Institute, Wrexham, Clwyd0 UK, July 9-11, 1990, p. 67. 5. S. ZOUAD,C. LtX)s-NESKOVIC,E. GARNIER,J. JEANJEAN,M. F~aOROFF, Recovery of fission products by fixation on inorganic adsorbents, in: Recent Developments in Ion Exchange, vol. 2, P. A. WmUAMSand M. J. HUDSON, (Eds), Pro,:. Intern. Conf. Ion Exch. '90, Noah East Wales hstimte, Wrexham, Clwyd, UK, July 9-11, 1990, p. 67.

80

6. M. An~, Ion-exchange selectivities of inorganic ion exchangers, in: Ion Exchange and Solvent Extraction, Vol. 12, J. A. MAR~SKY, Y. MARCUS(Eds), Marcel Dekker Inc., New York, 1995, Chap. 9, p. 381. 7. S. KAN~O, S. OKUDA,M. NAKAMURA,Y. KUnO, Chem. Lett. (1980) 1621. 8. S. KANEKO,K. TSUKAMOTO,H. OHTAKLIon exchange properties of silica-containing mixed-oxide gels, in: New Developments in Ion Exchange, M. ABE, T. KATAOKA,T. SuzuI~ (F_As),Proc. Intern. Conf. Ion Exch., ICIE'91, Tokyo, Japan, Oct. 2-4, 1991, p. 193. 9. SoNG Yn~Jm,ZHANGHtn, LI ~)UANn, YANGQIAOL~, J. Radioanal. Nucl. Chem., !90 (1995) 155. 10. J. Lima,, A. ~ , J. Radioanal. NucL Chem., 118 (1987) 1. 11. J. I.~rv3, R. HAR.Un~, A. M. GmARD, J. Chem. SOc. Dalton Trans. (1989) 101. 12. JIN QIXlN, SONG YI~JIE, ZHAO AIMIN, Adsorption of uranium on crystalline TiO2 and the compusited modification SiO2-TiO 2 which have reliable radiation stability (in Chinese), to be published. 13. Song YncJ~, ZHANGHtn, Y ~ G Q~oLn~, Za~o AIMI~,L Radioanal. Nucl. Chem., 198 (1995) 375. 14. W. R~MAN, H. WALTON, Ion Exchange in Analytical Chemistry, Pergamon Press Inc., Headington Hill Hall, Oxford, 1970, p. 23. 15. A. C L B ~ , J. I~HTO, J. Solid State Chem., 73 (1988) 98.