Indian Journal of Chemistry Vol. 38A, November 1999, pp.1 092-1 099

Papers

Sorption characteristics of hydrous bismuth-thorium mixed oxides

B S Mathur & B Venkataramani ' Radi ation Chemistry and Chemical Dynami cs Division

Bhabha Atomic Research Centre, Trombay, Mumbai 400085, India

Received 15 April 1999; revised 6 July 1999

Hydrous mi xed ox ides havi ng better anion sorption properties cou ld be prepared by substituting the host metal ion in the hydrous ox ide with a cation of hi gher valency. Hydrous bismuth-thorium mi xed oxide (BT), whi ch is one such mixed ox ide, of di Ileren t com posi ti ons have been prepared by coprecipitation using NaO H and are characteri sed ror their chemica l compositi on , crystallin it y, surface area and the nature of Ihc surface groups by analysing the water sorption isotherms using the D' Arcy and Watt equation . The in trinsic dissociation constant s: IIK*,, ;, and the poi nt of zero charge, PZC, have been evaluated by analysing the pH-tit ration curves in ac id (in presence of CIO·~ . NO,' and Cn and alkaline (Na+) media. BT samples having more than 40 wt% of Th could not be prepared. With increase in the content of Th in BT, the fin al product becomes less crystalline and the surface area in creases; the amount of strong primary water sorpt ion si tes and the interacti on of water with these primary si tes also increase. An enhancement in the anion sorption compared to the expected value, based on the sorption characteri sti cs of the component hydrous oxides and the composition of BT, has been observed with a maximum around 20 to 30 wt % of Th. A broad minimum in pK*,,; and PZC has been observed between 10 to 30 wt% of Th. The inability to prepare hydrous bismuth -thori um mixed oxides over the entire range of composition, the variation of the surface chemica l and surface charge characteristi cs with the co mposition of BT have been explained on the basis of the structural charac teri sti cs of the component ox ides (that is, hydrous bismuth and thorium oxides) and the coordinati on tendenc ies of the central ca ti on (that is, bismuth and thorium).

The presence and pH-dependent dissoc iati on of surface hydroxy l groups are responsible for the development of surface charge on hydrous oxides ' . At pH above the point of zero charge (PZC), the hydrous ox ide is nega­ti ve ly charged and sorbs cations from the aqueous elec­trolyte solutions and at pH below PZC, it is positively charged and sorbs anion . The ion sorption property could be en hanced by suitably substituting the host metal ion in the hydrous ox ide with a cation of lower or hi gher va lency to give hydrous mixed oxides hav ing, respec­ti vely, either better cation or anion sorpti on characteri s­ti cs ' . Zeolites (or aluminosilicates) are typical examples of the cation-sorbing mixed ox ides2. Studies on hydrous mixed oxides having enhanced anion sorption, prepared on similar lines, have been reportedH

. It is expected that the induced ion sorption property will be governed predominantly by the ex tent to which the mixed oxide has been formed and to a lesser ex tent on conditions such as solution pH etc. Such mixed oxides, having in­trinsic anion sorption characteristics, have applications in nuclear industry, like purification of coo lant waters in the primary heat transport system, in spent fuel storage tank , and in other industrial and natural water systemsl.x.

StUdyin g the surface charge characteri stics of mi xed ox ides is also of interest from the fundamental view point~ · ' 2 .

Em'lier7, it was shown that among the di fferent hy­drous mixed oxides, hydrous bi smuth-thorium mi xed oxide has better anion sorption property. In Drder to get a better understanding, the surface chemical and surface charge characteristics (especially, in the presence of dif­ferent anions) of hydrous bi smuth-thorium mixed ox­ides (BT) of different compositions have been studied and the results are di scussed in the present paper.

Materials and Methods

Preparation a/hydrous mixed oxides

The hydrous bi smuth-thorium mi xed oxides (BT) were prepared from a 0.1 mol dm,3 solutions of bi smuth nitrate and thorium nitrate in I mol dm,3 HN03. Keep­ing the total volume constant, the volume ratios of the two metal ionic so lutions were varied to prepare BT samples of different compositions . The precipitation was effected us ing excess of I mol dm3 NaOH at 370 K.

MATH UR el {I I. : SORPTION STUDI ES ON HYDROUS BISMUTH-THORI UM MIXED OXIDES 1093

The precipitate was digested in hot condition for 2 h, aged for 24 h in the mother liquor at room temperature (298 K), late r filte red , washed free of alkali , dried in a ir (298 K) , rewashed and finally stored as air-dried samples . Simil ar procedure was adopted to prepare hydrous ox­ides of bi smuth (B) and thorium (T ).

Characterisation of the hydrous mixed oxides

The che mical compos ition of the ox ide samples were dete rmined by di ssolving them in ac id and estimating the metal ion conte nt u.

X-ray powder diffracti on was used to characte ri se the crysta llinity of the diffe rent samples.

The s urfa ce a rea was d e te rmin e d us in g th e Qu antasorb surface area analyser after degass ing and drying the sample at 4 83 K for 4 h . The hydrous mi xed ox ides were found to be stable to thi s thermal treatment.

Water sorption isotherms

The nature and di stributi o n o f the so rpti o n s ites present on the ox ide samples were charac teri sed by anal ys ing the water sorption isotherms. The data were collected with the he lp of an isopiesti c set-up described earlier I4.1'i . Water sorption isotherms were dete rmined using H

2S0

1 so luti ons of known water acti vityl 6. Other

ex perimental deta ils a re g iven in earli e r publica ti ons I4.1'i

pH-fif m tioll

Surface charge characte ri stics of the hyd rous mi xed ox ides were evaluated by continu ous pH-titration us ing 0.7-0 .8 g of the a ir dri ed sample in 50 cm' of 0 .1 mol dm-} NaNO, with 0.1 mol dm} HNO} or NaOH. The pH of the slurry was measured with an Elico dig ital pH meter

(accuracy, ± 0 .01 pH units) . Afte r the additi on of the

ac id or alka li , the soluti on mi xture was stirred continu­ously fo r 10 min (within which time the equili brium was oht (l ined) before pH measure ment. pH-titrati ons we re a lso carried out in presence of 0 . 1 mol dm.i NaC I0 4 or NaC I with , respecti ve ly, 0 . 1 mol dm-.i HC I0

4 o r HC !.

Results and Discussion

Characterisation of the hydrous ox ides

Th e ph ys icoc he mi ca l c harac te ri st ic s o f th e BT samples a re g iven in Table I . T he X-ray powder pa t­te rns are shown in F ig . I .

It was not poss ible to prepare hydrous bi smuth-t ho­

rium mixed ox ides with th orium content more than 40

r t­(/)

Z UJ t­Z

1.B

•

r f-

(/)

Z UJ f­Z

5.9T4

20 40 60

29

Fi g. I - X-ray powder patterns of hydrous bismuth- thori ulll mi xed ox ides .

wt %. When the thorium content in the initi a l mi xture was above 40 wt%, the supern atant soluti on was ye l­low ish in colour, poss ibl y due to the fac t that bismuth got converted to bi smuthate in the presence of excess NaOH during precipitation.

It has been observed earlie r l4.15 as we ll as shown in

the present study (Fig. I ) that prec ipitated hyd rous bis­muth ox ide (B) is usually c rysta lline and that of tho ri um (T ) amorphous. With increase in thorium content, the

hydrous mixed ox ides became less c rys ta lline and more amorphous (sample BT5) li ke hydrous thoriu m ox ide (T ) (Fig. I ). The X- ray powder patte rn o f sampl e B c losely resembl ed that of Bi

20 } or B i

20

275 [card no.27-

50 , 18-244, 27-49 of ref. ( 17)]. The BT samples showed patte rns that were in between the crysta lline bi smuth and thorium ox ides [card no.35-1 03, of ref.( 17)].

The crysta lline ox ide of thorium (ThO) has fl uorite structure and that of bi smuth (Bi )O) is geomet ri ca ll y re lated to the flu orite structure (coordination numbers

o f M and ° in T h0 2 is 8:4 and that in Bi p } is 6:4 in the unit cell )I X and the two ox ides are, thus, structura ll y CO ln­

patible. The ionic radii of the metal io ns are a lso c lose

1094 INDI AN J C HEM, SEC. A, NOV EMB ER 1999

Tab le l - Phys icochemical charac teri stics o f hydrous bismuth-thorium mi xed ox ides.

Sample Chemi cal composit ion Surface area Interaction parameter* % (dry wt. )

PB

PT

B 85.2

BT l 77.0 4 . 1

BT2 71.7 8.9

BT3 59. 1 22.7

BT4 54.7 25.5

BT5 50.4 28.9

T 83 .9

PT

PB + P,.

0

0 .05

0. 11

0.28

0.32

0.36

1.0

m2/g

2

21

27

46

30

29

35

(K)

19

23

43

92

97

9

* Analys is o f the water sorpti o n isotherms us ing the OWE [Eq.( I)]

to each other ( 1.02 A for Th and 0 .96 A fo r B i)1 and

hence, they a re expected to form solid soluti ons in the mi xed oxide phase1x.I<J.

It has been observed 19.20 that when heterocations in

di fferent valent states, (+3 and +4), are present in the

flu orite structure , the solid solubility is limited as com­

pared to when the same cati on in two di fferent ox ida­

tion states are present in the mi xed oxide. M oreover, a continuous change from the face cente red cubic flu orite

structure to the C-type body cente red cubic structure has a lso been observed with the progress in the substituti on

of heterocations in +3 and +4 valence states l ,!2 1. Thi s

has been attributed to the low mobil ity of the cati ons in th e f luorite stru c ture l <J.20; w hen th e sa me e le me nt is

present in two diffe rent ox idation states, an ordering is

poss ibl e by e lectron transfer, whi ch is restri cted when di fferent cati ons are present in the flu orite structure l

').

Thus, a combinati on of fac tors, such as the oppos in g tendency o f precipitated hydrous bi smuth ox ide to form a c rysta lline produc t and hydrous thorium ox ide to form

an amorphous produc t, the limited solid so lubility of

heterocations in two di fferent ox idation states in the flu o­rite structure, the poss ible fo rmati on of bismuthate in

the presence of excess a lka li during prec ipitation could be the reasons fo r the inability to prepare the BT samples over the entire range of compos iti ons and the ir phys ico­chemical characte ri sti cs (Table I )

With inc rease in the content of thoriu m in the BT

samples the surface area inc reased and showed a max i­

mum (Table I ). Thi s is cons istent w ith the known infor­

mation that the surface area inc reases with decrease in the degree of c rysta llinity of a sample.

Water sorption

Water sorption isotherms of the hydrous ox ides and mi xed oxides are shown in F ig.2 . As mentioned earlier, sample B sorbed less wate r and showed tendency to fo rm multilayer. With inc rease in the amount of thorium in the BT sample, hydrophili c ity increased , the tendency

for multilayer formati on decreased and the wate r sorp­ti on isothe rms became c loser to tha t o f sampl e T (a Lang muir-ty pe sorp tion isotherm) (Fig.2). Thi s is con­sistent with the fact that the c rysta lline mate ri als are usu­all y hydroph obi c (so rb less wate r) and th e hydroph ili c­ity increases with the inc rease in the amo rphous nature of the sample I5.22 .

Analysis of the water sorption isotherms

The types of sorpti on sites present on the hyd rous mi xed oxides were evaluated by analys ing the wate r sorp­ti on isotherms employ ing the 0 ' Arcy and Watt equ ati on (OW E)2:l. The OWE has been successfull y used to characte ri se the di fferent types of sorption si tes and to obta in informati on regarding the ion-water and wate r­wate r inte ractions in diffe rent in organic and organi c ion

-~

MATHUR el al. : SORPTION STUDIES 0 HYDROUS BISMUTH-THORIUM MIXED OXIDES 1095

0.01 -' 1. B

' W 0

X '00 0

>-0:: 0 0.2 Cj\

? . 0 W ID 0::

0.1

° If)

0:: W t-<{

~ u.. °0.1S t-Z ::> 0 L 0.1 <:

0:5 Ow

2 . 611

3.8 T 2

1.0

1.0

0 .1 8.------~

4 .813

0 .1

a.l~

,~ o 0.5 1.0

Ow

0.1G 7.1

o o 0 .5 1.0 Ow

Fig.2 - Water sorption isotherms of hydrous bismuth-thorium mixed oxides .

exchangers and sorbents I4.15 .24-21i. The general form of

the OWE is2.l :

I Ki K: a k k' a ". ". IV I + Da + ... ( I ) ".

i=1 1 + Ki a 1- k a ... "

where w is the amount of water sorbed, gi g of dry ox ide and a

w is the water activity. The first term in Eq.( I )

refers to the Langmuir-type sorption sites, the second term to the sorption sites which can be approximated to a linear isothenn and the third term to multilayer forma­

tion. In Eq.( I ), K'j and k' are the primary and multilayer

sorption site densities, respective ly and K and k are the . . . InteractIOn parameters related to the heat of sorption in primary sorption site and in the mu ltilayer, I is the num­ber of different types of primary sites and 0 is a con­stant ass igned to the linear form of the sorption iso­therm23.

In the present analys is, the water sorpti on isotherms were analysed by putting i = I in Eq.(I). The total amount of water sorbed on the mixed oxide, n , at a = I (which represents the fully hydrated state) ca; be gi~en in terms of water associated with the strong primary sites, n [con­tribution from the first term in Eq .( I)), water ass~ciated

-J> '

1. 2

C7"I ,::>0 .8 o E E

0.0.4 c:

3.0

~ o

2.0 E E

1.0

Fig.3 - Sorption sites on hydrous bismuth-thorium mixed ox ides .

with the weak sites, n [contribution from the second li

term in Eq.( I )] and that present in the multi layer, n [con-tribution from the third term in Eq.( I )] . Thus, atn~ = I and using Eq.( I ), we have W

. . . (2)

A computer program based on non-linear least squares analysis was employed to fit the equation to the water sorption data. Deta ils are g iven in earlier publicati ons 1).24.

The amount of strong and weak sorption sites (n and no' respectively) present on the hydrous mixed o; ides BT as a function of their composition are shown in Fig.3. T he values of the interaction parameter, K, for the water in the primary strong sorption sites on BT samples are given in Table I.

With increase in the amount of thorium, the amount of s~rong sorption sites on BT samples increased (Fig.3). WhIle both Band T samples did not show the presence of weak sorption sites, the hydrous mixed oxides (BT samp les) had weak sorption s ites and their a mount showed a maximum with composition (Fig.3). With in­crease in the thorium content in BT sample the sorbed ,:ater interacted more strongly with the primary sorp­tion sites, as indicated by the values of K (Table . 1 ). As ~entioned earlier, these are related to the hydrophobic­Ity and hydrophilicity, respectively, of hydrous bismuth and thorium oxides.

1096 INDIAN J CHEM , SEC. A, NOVEMBER 1999

q , Inmolj9 0 .6 0.4 0.2 0 0.2 0.4 0 .6 O.B

AC I O_ ALKALI 10

B

J

1 o 6Tl

6

~! · 8T4

I J D.

4

/ 2

AC I Of-l ..... AlKALI

0 .2 0 0.2 0.4 0.6 q , mmo l/g

Fig.4 - Typi cal pH-titrati on curves of hydrous bismuth-thorium mixed oxides. (0) BTl ; (e ) BT4.

pH-titration

Typical pH- titration curves are shown in FigA. The surface hydroxyl groups (-OHsLlrr) on hydrous oxides and mixed oxides are in vo lved in the foll owing protonation and deprotonation reactions during the pH-titration .

.. (3)

alkaline region: -OHh",1I -0 ' {SUI I ) + H\,,'n' ... (4)

The equilibrium constants for these reac ti ons are given by:

K", = N,," m,/N(//, =" (C-q)lq .. (5)

K"l = N,,- .m,/N,," = Ii. q/(C-q) .(6)

where mil' N Olh+ N OH and N o- are the mo larity of H+ in the solutIon and the mole fraction of proto­nated (-OH\ (sLlr!))' undissociated (-OH(SlIl!)) and di ssoc i­ated (-O'(SlIl!)) surface hydroxy l groups on the oxides, re­specti ve ly ; C is the capac ity of the ox ide, h is the equi­li brium H+ concentration in so lution and q is the amount of the oxide in the -O'(sLlr !') or -OH\surn form. The ion sorption capac iti es of the oxides were determined as the limiting values of H+ consumed in the ac id reg ion or the OH' consumed in the alkaline region of the pH- titrati on and are shown in Fig. 5 as a functi on of composition for different ani ons and Na+. These va lues ag reed well with the amount of Na+, CI ' and NO.1 ' sorbed, eva luated by estimating their concentrations in the initi al and equilib­rium solutions flame photometrically, volumetricall y and spectrophotometrically, respective ly 11.

The intrinsic dissociation constants (p K*) were com­puted from the pH-titrati on foll owing the graphical

_/\ / \

1. 5 I \ '\

Fig.5 - Ion sorption capac it ies of hydrous bismuth-thoriu m mi xed oxides as a functi on or composi ti on.

(0) CJ-; (e ) CJ- - ca lculated: (~) NO,-; (. ) NO,' : -ca lcul ated; (D ) Clo.,' ; (_ ) ClO; -

I I ca lculated; (- 0 - ) Na+; (- e - I Na+ - calcul ated.

I I

method of Schindler and Gamsjaeger27 as the y-inter­cept of the pKai (= -log lO Kai ; i= I or 2) vs q plots. Simi­lar approach has been suggested and adopted by other in ves tigators als02x

, ". Detailed procedure is gi ven e lse­where2~ .

The point of zero charge (PZC) is the pH where the net charge is zero or -OH+,( . and -0' . are equal.

.!.slIrl) (sud)

PZC is calculated from the re lati on:

PZC = Y2 (pK*" + pK*,) ... (7)

The variation in pK*," and PZC (for experiments in­vo lving NaN0.1 ) with compos ition of BT samples are shown in Fig. 6.

Noh and Schwarz.12 and Kal lay and Za lac.13 have shown that the pH of the neutral sa lt so luti on in contac t

MATHUR el al.: SORPTION STUDIES ON HYDROUS BISMUTH-THORIUM MIXED OXIDES 1097

l , 1 O~

8 , \ I

\ "

,'" -/ I

I I , I

'~ i ."/PZC , ./.~ , , ... ' I

~ '" ..... ----~~ ,ft· ~ : ~ I

.- 6 : d .0 \ I r

~ ~,~\$_ _-----t! l" I ~-~_~----~ I.l I 'Y I U 4, . I I . I

I ~

" \~'u.., 9 f )i \ \ ~~ / " 9 "-\ '\ -0... ______ L~ -Ji-"-'4. ' ... ' '0. ___ ~,..... ,.../

............. A._--- ...... ." 2 -

o 0.4

•

1.0

Fig.6 - pK*,,, and PZC of hydrous bismuth-thori um mixed ox ides as a function of composition.

CO) pK*,. , - NO)'; CM pK*", - ClO.'; CD ) pK",., - CI; ce) pK* , - Na+. .. - I

( - 0 - ) pzc - NaNO). using Eq. (7); ( - e - ) PZC-

I I NaNO). method of Schwarz and Kallay [32-331.

wi th the hydrous oxide could be taken as the PZC of the ox ide and the method has been used to eva lu ate the ther­modynami c para m e te rs of th e protonation and deprotonati on reactions31

. The PZC es timated by thi s approach for NaNO, are shown in Fig.6; the va lues for NaNO, compared well with those evaluJled from the pH- tit ;ation data {us ing Eq.(7) I (Fig.6). As the empha­sis in the present investigation was from the po int of view of enhanced anion sorption of hydrous mixed ox­ides, only the acidic patt of the p H-titration of BT samples in the presence ofCIO'4 and CI ' were studied and evalu­

ated (Fig.6) .

Assuming that the constituent ox ides in the mixed ox ide behave independently, the sorption capacities of BT samples were calculated from the sorption values of the B and T samples taking into account the composi­tion (Table I) in the hydrous mixed oxide (termed "cal­culated sorption capacity") and are shown in Fig.5. It is seen that the calculated values varied linearl y with the composition and the experimenta lly evaluated capaci­ti es for the anion showed a max imum and were higher than the calculated values, indicating the effect of for­mation of hydrous mixed oxide as envi saged. The maxi­mum in these curves (Fig.S ) poss ibly re lates to the opti ­mum composition of the best hydrous mixed oxide. The cation (Na+) sorpti on more or less matched with the ca l­culated values (Fig.S). Thi s is because the cation sorp­ti on is not expected to be altered due to the formati on of hydrous BT mixed ox ide. Effort s are in progress to verify thi s aspect for other cations.

The ani on sorption of the hydrous mixed ox ides , in general fo llows the sequence: Ct' > NO,' > CI0

4' (Fig.S).

This shows the role of the nature of the constituent ox­ides on the property of the mi xed ox ides . Bismuth ox ide sorbs Ct ' specifically7.34 forming bi smuth oxychl oride (see Fig.S , for sample B). Hence, with decrease in the amount of bi smuth in BT, the capac ity for CI ' decreased (a negative slope in capac ity - composition curve) and the maximum in the capacity-composition curve occurred at a low bi smuth content (Fig .S). The trends for 0.,­and CIO ' were s imilar, exhibiting a pos iti ve s lope in the

4

capac ity-compositi on curves, with the maxi ma shifting to higher thorium content (Fig.S). An enhanced sorp­tion of NO ' over the expected capac ity values (Fig.S)

3

was observed, re fl ec ting the prefe rence of No.,' over CI0

4' by the BT sampl es.

The trends for pK"i and PZC with composition for BT (Fig. 6) showed a broad minima, similar to that ob­served for substituted magnetites20.35 . Attempts have been made to corre late and pred ict the PZC of composi te ox­ides and those o f component ox i des·) " ~ . In genera l, a linear re lat ionship has been observed between the su r­face c hemical and PZC of the composite ox ide and the weight fraction of the pure components, when the con­stituent ox ides do not interact with eac h other, li ke fo r example, S i(IV) - AI(III), Ti(IV) - AI(III) mi xed ox­id es') " ~·3(, . In their ex tens ive studi es Morimoto and Kittaka35J7 have observed that these mode ls do not ad­equately expla in the surface charge characteri st ics (the isoelectri c po int , IEP or PZC) of spine l- type ox ides (mixed coord inated ox ides), where in the orig in of the

1098 INDI AN J CHEM , SEC. A, NOVEMBER 1999

surface charge is due to the tetrahedrally coordinated multivalent ion. It also depends on the influence of the hydroxyl group present on the octahedrally coordinated metal ion on the dissociation tendency of the hydroxyl groups present on the tetrahedrally coordinated metal ion o5 .37 .

Morimoto el a/}X and Connell and Dumes ico(, have studied a variety of mixed oxides from the point of view of development ofBrbnsted acid sites (surface hydroxyl groups), which are important in catalytic reactions . Morimoto et al.,3x have found that the hydroxyl groups that bridge the silicon and aluminum atoms in the silica­alumina mixed oxide function as strong water sorption sites and those hydroxyl groups attached to iildividual cations as weak sorption sites. Connell and Dumesic36

also concluded (based on sorption of probe molecules) that the hydroxyl groups bridging the tetrahedral alumi­num and tetrahedral silicon alone could function as Brbnsted acid sites. Mixed oxides containing cations that have different coordination tendencies (example, Mg and Si , Band Si, Zn and Si) do not give rise to Brbnsted acid sites36

. Thus, the surface charge and surface chemi­cal characteristics of the mixed oxides depend on the coordination tendency of the cations of the component oxides and the structure of the final compound.

It was mentioned earlier that though the crystalline ox ides of bismuth and thorium are structurally compat­ible (in the fluorite unit cell) the coordination tenden­cies of the cations are different 1x

• Moreover, the solid solubility is limited because of the presence of heterocations in two oxidation states I9

.2 1

• In the case of hydrous bismuth-thorium mixed oxide, hydroxyl groups bridging the bismuth and thorium atom would be formed so long as the fluorite structure accommodates these cat­ions. As the constituent oxides (B and T) are themselves basic in nature1.2·7.39, the resultant hydroxyl groups would also be basic and one observes an increase in the sorp­tion of anions (Fig.S). The formation of the bridging hydroxyl groups in the mixed oxide is also reflected in the increase in the amount of primary strong water sorp­tion sites (n ) (Fig.3) and an increase in the value of the

r interaction parameter, K, for the water molecules present in the primary sorption sites (Table I)

When the composition exceeds the solid solubility limit (in the case of hydrous bismuth thorium mixed oxide, it is between 20 to 30 wt% of thorium) contribu­tion from the constituent oxides is also seen in the sur­face chemical and charge charaCteristics. This is re­flected in the appearance of a maximum in the weak sorp-

tion sites (n) - composition curve (Fig.3 ), decrease in the anion sorption capacities (Fig.S) and the minima in

pK*"i or PZC - composition cu rves (Fig.6).

The present study, thus, indicates that in the case of mixed oxides, where the cations have si mil ar cOOl'dina­tion tendency (fo r example, tetrahedral coordination in Si (IV) - AI (III) mixed ox ides, octahedral coordination in Ti (fIl ) - AI (III ) mixed oxides, etc. ) the surface charge and surface chemical characteristics will have a linear re lationship with the composition . Such a rel ation is not observed for mixed oxides where the cations of the component oxides have different coordination tenden­cies , as in the case of substituted magnet ites26

.35 and hy­drous bi smuth-thorium mixed oxides (Figs 3, Sand 6).

Acknowledgment The authors wish to thank Dr. T. Mukherjee, Head,

Radiation Chemistry and Chemica l Dyna mics Di vision, and Dr. l.P. Mitta l, Director, Chemistry Group , BARC for their constant encouragement during the course of this investigation.

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MATHUR el al.: SORPTION STUDIES ON HYDROUS BISMUTH·THORIUM MIXED OXIDES 1099

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