Generally X is dependent on the precipitation conditions; in very fast precipitations X approaches to unity
2.5. Factors which may influence the removal of radioisotopes by co- co-pvecipitation and adsorption
From the theory as presented in this chapter the conclusion is that there are various factors which may influence the radioisotope removal by coprecipitation and adsorption such as
- the concentration of the precipitants used - the cation/anion ratio of the precipitants - the order of addition of the precfpitants - the rate of addition, of the precipitants
41
- the pH
- the time of ageing of the precipitate - the temperature
- atmospheric COj
- the addition of foreign ions i.e. ions which do not belong to the precipitate forming ions
These factors may play a more or less important part in the research as described in the chapters III, IV and V.
With the exception of the temperature their influence on the finally obtained radioisotope removal was investigated.
I i
f*, i
42 %
• ' i
CHAPTER I I 1
THE INFLUENCE OF THE FORMATION OF VATERITE, CALCITE AND ARAGONITE IN THE LIME-SODA PHOCESS ON THE REMOVAL OF RADIOSTRONTIUM
ii
•X
,\ In this chapter the removal of radiostrontium from waste-'s
;; respectively surface-water by the lime-soda process will be
\ described.
; As was already communicated in the introducing chapter I the main .:• attention has been paid to a study about the influence of the three ..; calcium carbonate phases vaterite, calcite and araoonite to the, ' j removal of radiostrontium. As known the conventional water-treatment
3 process of lime-soda softening is not only used to remove water
•| hardness but is also employed for the purification of low-level if radioactive waste prior to discharge. Mostly excess of soda is I added in order to produce a negatively charged precipitate surface.
I Some examples of the application of this purification process for f radioactive liquid waste originating from nuclear power plants were
I already presented in chapter I.
'•% The method could perhaps also be used in case of an emergency situa-I tion, viz. a nuclear detonation, for the preparation of drinking
% water from radioactively contaminated surface water (compare chapter
I
I)-5 For our experiments use was made of the radioisotope Sr instead of
." nn or
t| 3 Sr (t, = 28 y ) . Besides the fact that O 0S r is less hazardous to
-i *
si w o r k with (the M P C - v a l u e differs a f a c t o r 1000 as compared with
'5- g n ™ . •
;>| Sr) the application of this radionuclide in the experiments is also I rather suitable because
I 90 90
fg - i t h a s n o r a d i o c h e m i c a l d a u g h t e r ( c o m p a r e S r - Y ) w h i c h m i g h t i complicate the radioactive measurements and
•*4 ' 90
|i - it emits easily measurable y-radiation in contrast to Sr which 1 is a pure $-emitter.
>a or nn
| The use of Sr instead of Sr has no consequences for the
experi-43
mental results because both radioisotopes have the same chemical pro-perties. The experiments as discussed in this chapter differ from process as carried out in the nuclear establishments in this way that no excess of soda was used but a stoichiometrvc amount of
precipi-tants.
As said durifui the formation of CaCO, from CaCl~ and Na-CO, and de-pending on the experimental conditions three modifications of CaCO-, vis. vaterite, cal cite and/or aragonite may be formed. Some physical properties of these modifications are described in paragraph 3.2.1.
Aranonite has the best binding properties with respect to strontium but is gradually transformed in the solution into calcite; calcite is the most stable form but has less binding properties for strontium.
Vaterite is the unstablest modification; it crystallizes via aragonite
•into calcite. The contribution of the three modifications to the radiostrontium decontamination has been studied..
From the literature it follows that the formation of vaterite, calcite and aragonite depends on many factors such as the order of addition of the orecipitants, pH, temperature, presence- and kind of impuri-ties. Some factors were briefly discussed in chapter II. A more de-tailed discussion is presented in the paragraphs 3.2.2. and 3.2.3.
Addition of seeds of aragonite or calcite before precipitation of CaCCL also influences the kind of modification formed during preci- . pitation. The precipitation is then faster because the period of nuclei' formation is eliminated (3.2.4). A description of the way of preparation of vaterite, aragonite and calcite necessary for seeding and adsorption is given in Appendix I.
To determine the relative amounts of vaterite, aragonite and calcite in the samples X-ray diffraction was used as described in Appendix II.
In the experimental part of this chapter (3.3) the results obtained for coprecipitation with and adsorption of radiostrontium by the various CaCO, polymorphs both in the absence as well as in the pre-sence of modification determining impurities are presented. The fac-tors as described in the theoretical paragraph 3.2 which influence the formation of the CaCO, modifications and therefore the
decontami-44
nation of radiostrontium are involved in these experiments.
Because aragonite has the best binding properties for radiostrontium in the experiments it was strived after to establish the experimental conditions for which a maximum amount of this modification is formed.
The results originally obtained in batch procedures were applied in some continuous, precipitation experiments; the results of these are described in paragraph 3.3.6.
Sometimes the effect of atmospheric CO, on the removal of radio-itrontium by the CaCO., precipitation process cannot be neglected.
Especially in the experiments where "Sr was bound by chemical ad-sorption the radiostrontium decontamination was influenced by at-mospheric C0~. Therefore the experiments had to be carried out in an
inert
N^-atmosphere-3.2. Theoretical aspects.
On account of 'the special character of the lime-soda process where vaterite, aragonite and calcite are used for decontamination purposes it is necessary, in order to have a clear insight in the experimental results, to consider some aspects of these modifications.
3.2.1. Seme properties of vaterite, aragonite, aalaite, strontianite and whiterite.
For the three CaCCL modifications vaterite, aragonite and calcite two kinds of phase transitions have to be distinguished:
- phase transitions in solution
- phase transitions in the solid state.
In solution vaterite is the most unstable CaCO, modification. Vate-rite is usually prepared at 30°C from CaCl2 and Na2C03 but converts easily to aragonite and calcite. Calcite is the most stable phase and is prepared generally at temperatures between 40° and 50°C.
Aragonite has a stability in between and changes gradually to cal-cite. Aragonite is prepared at temperatures higher than 50°C.
Consequently in solution vaterite and aragonite are mainly converted into calcite; under special conditions aragonite can be stabilized
45
(3.2.3).
In the solid state the reversed processes can also occur. During grinding of a vaterite sample and with the addition of some caicite crystals the following reactions occur:
25 hr _,_,._ 70 hr
vaterite calcite aragonite
After 70 hours an equilibrium mixture of calcite and aragonite is formed (BURNS and BREDIG, 1956; GAflHAGE and GLASSON, 1963). The degree of transition
calcite = aragonite
in the solid state is a function of the temperature and the pressure as shown in fig. 3.1 (JAMIESCN, 1953; Me DONALD, 1956).
6OO
i 400
200
0
coicite
- / /
/ / i
/ /
oracjonlte
1 t 5 10
m— pressure (Kbar)
15 20
Fig,3.1. Calcite •=. aragonite as a function of temperature and pressure in the solid state.
" - — — - - P,T-curve according to Me DONALD:
P = 16T + 2400 (P expressed in bars, T in °C)