The surface of the solid

nr

In the case of adsorption of the radioisotope Sr on the BaSO. surface the magnitude of this surface area will be of influence The magnitude of this surface per unit of weight is dependent on the . size of the particles forming the surface. As mentioned in chapter II the size of these particles depends on the concentration of the precipitants used. At larger precipitant concentrations smaller par-ticles are formed than at low concentrations. In the former case a larger BaSO. surface results. More discussion about this aspect is given in paragraph 4.3.7.1.

Another factor which is of influence on the magnitude of the surface area is the ageing time of the precipitate. A longer time of ageing

140

leads to a smaller surface (4.3.7.2).

For the determination of the specific surface (F/in) of a BaSO.

sample the BET-method was used.*** The specific surface area (m^/g)

pr

was calculated from the amount of ' Kr adsorbed on the BaSO» preci-pitate supposed to adsorb in a monomolecular layer (=V expressed as A counts/x mg of solid). As a reference substance a standard V - A 190 ,

sample with known surface area was used (30137 counts/m ) .

Also the BET constant c was determined. For a given adsorption gas

qc

( Kr) and sample (BeSO.) c should have a constant value. However

§ mostly the same order of magnitude for c is satisfactory because the

•f- physical properties of all BaSO» samples are not quite analogous ij (e.g. the different particle sizes).

•4 From the values of F/m the particle diameter was calculated according

51 to f = 6/c.m/F where p means the density of the solid (oc cn =4.5

m

£ g/cm ) . This expression applies for spherical particles.

••I

::| ••.,'•. ^. 1. The influence of the /•}'•;:•:'[•:":.znt ;*Gn^t-:Kt-2\2tzori _•>: ike

sur--Ê face size of BaSO ..

I For a constant time of ageing (19 h) the surface area of f BaSO. has been determined as a function of varying concentrations of

'"*• 8 5

* BaClo and of Na,SO,,. Sr was removed from the solution by adsarpvior

•••IB C C. H

-ii Procedure:

:| Just like in experiments as described previously a dilution factor fl of 5 was used for the preparation of BaSO., e.g. 100 ml X M

§ 100 ml X M Na?SO. were added to 300 ml of water. After a time of

| contact of 19 hours the samples were filtered and washed with

demi-"I nsralized water. The samples were dried till constant weight at tem-1 perature of tem-1tem-10°C.

§

Results.

The results of the surface determinations are shown in table 4.6.

"J§ ***The surface determinations were carried out by Ir. A.F.J. van

|f Tuijl of the Interuniversitair Reactor Instituut (IRI).

is ana :;*2s

size c^ 3dSC 28

BaCU and Na,SO4

concentration 1 M 0.5 0.1 0.05

specific surface area

F/m 9.61 m2/g 6.39

1.71 1.14

BET constant

c 110

94 172 96

mean particle size

F

0.14-i

0.21 0.77 1.17

A decrease of the precipitant concentration leads to a lower value for the specific surface area. The mean particle size which is in-versely proportional to the specific surface area increases with decreasing concentration.

With the aid of the results given it is possible tc make a rough calculation of the number of Sr-atoms present on the BaSO. surface.

Therefore some adsorption experiments were carried out on 0.2 g of

85

BaSO» in which the removal of Sr was determined at varying concen-trations.

From the removal percentage, the surface of the solid and the total 2+ -?

amount of Sr -ions originally present in the solution (8x10 mg Sr/mCi = 8x10" mg Sr/ uCi) the number of Sr -ions n<- bound by

~ i I-A /_\ _ r _ 2

BaSO can be calculated as:

nSr = NAv x

100 - C/C,

ToU

^{mSr (4.1)}

In this equation H^ = number of Avogadro, 100 - C/CQ = percentage of Sr removed by BaSO-, ÜL = weight of Sr-atoms totally added (g), M = atomic weight of Sr.

Substitution of the da.ta in equation 4.1 leads to:

- fi y - Ö X

100 - C/Co ,-8

= 0.56 x 1 0131 3 x Sra(Js(%) (4.2)

142

From this expression it follows that the number of Sr +-ions present on 1 m BaSO^, (n<. JF) can be calculated as:

* 0.56 x 101 3 x

= 0.56 x 1 0i 3 x (4.3;

•Ï

1

-S

'si

For varying concentrations of precipitants n, and n<. /F according to the equations 4.2 and 4.3 were calculated. The results are pre-sented in table 4.7.

'i. '. . 'Cli'i'ni'r,.ii : ..•>: The value for n<. in the table means the total number of Sr +-ions i.e. the addition of radioactive and carrier strontium present at 0.2 gram of BaSO^ surface.

In connection with this it should be remembered that only 1/1900 part of the total amount of Sr -ions is radioactive (see paragraph 3.3.1).

As said the size of the surface is determined by the particle size which is dependent on the süpersaturation i.e. the precipitant con-centrations in the solution (compare 2.2.1). From the calculated values for the BaSO. surface arsa and particle size at varying pre-cipitant concentrations the number of BaSO^ particles forming the solid can be calculated.

From the mean particle diameter r (table 4.6) the surface area of one particle can be calculated as: u.(r/2) . Division of the surface cal-culated for 0.2 gram of BaSO^ (F from table 4.7) by 7r.(r/2) results in the number of BaSO^ particles present in the solid ( nB a S 0 ).

From the number of Sr-atoms adsorbed on 0.2 gram of BaSO- (n<. ) and the number of BaSO^ particles (ngasQ ) present in the same sample»

can be calculated (table 4.8).

the ratio nS r/ nB a S £ )

From table 4.8 it appears that in case of a precipitant concentration of 0.05 M 160 Sr-atoms are adsorbed on one BaSQ. particle. At a 1 M

11

concentration much more BaSO^ particles are present (1250.10 ) re-sulting in a degree of occupation per BaSO^ particle of only 1.3.

4.3.7.2. The influence of the ageing time of BaSO on the aurfaae size.

Froaedure:

The BaSO^ samples were prepared in the same way as described in paragraph 4.3.7.1. Constant precipitant concentrations of 0.05 M were used but now the ageing time was varied between 2 minutes and 19 hours.

Results.

As discussed in chapter II a longer time of ageing should lead to a smaller surface and larger particles. The results are presented in table 4.9.

A deviated value for the specific surface area is observed after an ageing time of 8 hours.

This cannot be caused by a maximum surface area formed after such a long time of ageing. In chapter II it was discussed that the size of the surface area reached a maximum during the period of crystal

144

. ~C-e

I

ing time a of

ire a xnz and I. Ik

-J2V~-~2 .

28 2

time of ageing A 2 min B 1 h C 8 D 19

specific surface area

F/m 1.73 m2/g 1.73 2.34 1.14

BET

In document THE REMOVAL OF RADIOSTRONTIUM BY PRECIPITATION (pagina 148-153)